m

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THE

EVOLUTION

OF THE

VERTEBRATES

AND

THEIR KIN

PATTEN

k

THE /<'

EVOLUTION

OF THE

VERTEBRATES

AND

THEIR KIN

BY

WILLIAM PATTEN, PH. D.

PROFESSOR OF ZOOLOGY, AND HEAD OF THE DEPARTMENT OF BIOLOGY
IN DARTMOUTH COLLEGE, HANOVER, N. H.

WITH 309 ILLUSTRATIONS

PHILADELPHIA
P. BLAKISTON'S SON &, CO.

1012 WALNUT STREET

1912

COPYRIGHT, 1912, BY P. BLAKISTON'S SON & Co.

Printed by

The Maple Press

York. Pa.

INTRODUCTION.

It is many years since a sustained attempt has been made to unite the various
branches of the animal kingdom into a natural, coherent system, or genealogical
tree, that would indicate the rise and decline of the important functions and
organs, map out the highways of organic evolution, and assign in geological terms
the approximate dates and surroundings for the critical events in structural in-
novation and readjustment that have marked its progress.

The most important problem involved in such an undertaking is to discover
which one, if any, of the many existing invertebrate phyla forms the trunk line of
descent from the lowest vertebrates to the ccelenterates, and through them to the
protozoa.

The vertebrates abruptly make their appearance as fully formed fishes, at
the close of the Silurian or the beginning of the Devonian. They were evidently
more highly organized than any of the invertebrate types that had appeared up to
that time, and they must have arisen, either by a marked transformation of some
of the known, preexisting types, or from some extinct and totally unknown ones.
On either supposition, the apparent absence of transitional forms is surprising,
since the relatively large size, distinctive form, and well developed skeleton of
primitive vertebrates, under the known conditions, should leave behind some
recognizable traces of their predecessors in the form of fossils.

The real missing links in the graded series of animal forms that most concern
the morphologist belong, therefore, to the Silurian period. There the main
trunk of the animal kingdom, upon which the whole vertebrate stock rests, is
lost, leaving without reason or warning a vast unknown abyss beside which the
gap between man and his immediate predecessors sinks into microscopic in-
significance.

On the one side are the vertebrates, including a long series of animals, from
the lowest fishes to man. All of them agree in their fundamental plan of struc-
ture and mode of development; the principal organs of any member of the series
may be surely identified in the others, and the general trend of evolution in the
phylum is clearly indicated. The fishes, for example, are the lowest members of
the series, and they are followed by the amphibia, reptiles, and mammals. Com-
parative anatomy shows the gradual evolution of form and structure in this
series as a whole, and its evidence is corroborated, in the main, by the embryonic
development of any member of the series, while the geologic, or historic record
harmonizes with, and confirms the testimony of the other two. In fact, the
vertebrates clearly constitute a common stock, a single phylum of the animal

v

VI INTRODUCTION.

kingdom. It has many side branches, it is true, but comparative anatomy,
embryology, and paleontology are in substantial agreement as to what kind of
animals and what organs and functions came first in time, what were the most
highly developed, and what was the general trend of evolution.

Even the simplest vertebrates, that stand at the beginning of this long
series, were very highly organized animals, for all the fundamental systems of
organs well known to us in man, such as the sensory, nervous, skeletal, circulatory,
and excretory, were there fully established and highly efficient. But there is little in
the structure or development of these organs that gives us any positive information
as to their previous history, condition, or origin, the very information essential
to a true understanding of their meaning.

On the other hand, when we look below the vertebrates for the main highway
of evolution, we are bewildered by the multiplicity of doubtful trails that appear
to have neither beginning nor end, that lead as readily in one direction as another.
Each invites us onward; but if followed, suspicion soon grows to conviction that
we have been deceived, that some" other road is after all the right one.

The familiar cry, "This w r ay," "I have it," that rose when an enthusiastic
pioneer struck the annelid, tunicate, balanoglossus, or some other promising trail,
would for a time rouse great expectations; but it always ended in disappointment,
and gradually created an attitude of indifference, and the feeling that the solution
of this great problem was forever beyond our reach. The conviction grew that
one or more large classes of animals that once constituted the living trunk of
the genealogical tree during Silurian, or pre-Silurian times, were entirely extinct,
and had left no traces whatever behind.

With the historic record of the most important period in the evolution of the
higher animals completely destroyed, the problem did indeed appear hopeless.
What was lacking in actual records was supplied by the speculative biologists,
and they did their work so well, and reiterated it so often, that it finally passed for
the truth, and its central idea, that the vertebrates had their origin in the
annelids, in some more or less roundabout way, became a dogma.

We then witnessed the interesting phenomenon, common enough but always
profitable to contemplate, as lightning that strikes near by, that those who would
not make the annelid theory a part of their creed, and who continued the search
in other directions for a substantial body of facts to build upon, were branded
as morphological heretics and speculators, or as the victims of a too vivid imagina-
tion; and it was always the most "orthodox" and persistent speculator that
waved the hottest brand.

But when the annelid dogma passed the period of productivity without off-
spring, even the orthodox biologists lost all hope of solving the real problem of the
origin of vertebrates, as well as many other large problems in invertebrate phy-
togeny, and turned their attention toward the Eldorado of cytology, heredity.
and experimental evolution, where "results" were easy and promised to carry far.

Sterility has often turned devotion to contempt, and it is not suprising that the

INTRODUCTION. Vll

biologist, whose theories were unfruitful, wrecked his wrath on the temple of
morphology and condemned its triune god to the consolation of his more credu-
lous colleagues; "Paleontology," he cried, "is mute, Comparative Anatomy
meaningless, and Embryology lies."

But perhaps the fault was ours. We did not understand, because of igno-
rance and over confidence. It is not fifty years since the doctrine of evolution has
been generally recognized, and during the latter half of that period surprisingly
little persistent, or concerted work has been done on the larger problems of
phylogeny, and there is but little to justify the too common attitude that the pos-
sibilities of morphology are exhausted. Much disconnected fragmentary work
has been done, but how little is known about the evolution of any one organ or
system of organs; how very few animals, if indeed there are any, whose structure,
development, and paleontological record are known with even approximate
fullness or accuracy. What large class of animals is not separated from its next
of kin by a gap too wide to be bridged by any known forms ? Are these gaps due
merely to a hiatus in the available records, or in our knowledge of them, or are
they realities, representing periods of unusually rapid transformation due to
sudden changes in the methods, or conditions of growth? If the gaps between
the vertebrates and ostracoderms, and the ostracoderms and arachnids appear to
be wide ones, are they really any wider than those between the fishes and amphibia,
the reptiles and mammals, or the ccelenterates and arthropods? Are not the
evidences of genetic relationship of the same nature and value in one case as in
the other? Is not the paleontological record more precise and complete than we
have supposed ? Will not embryology be less enigmatic under a new interpreta-
tion ? If the arachnids are indeed the next of kin to the ostracoderms, and through
them to the vertebrates, is that after all so incredible ? With this gigantic column in
position, will not the remaining branches readily fall into their natural positions,
and the entire genealogical tree of the animal kingdom take on the convincing
symmetry and coherency of reality, of a living, growing organism that contains
the story of its own creation ?

These are some of the problems bound up in the evolution of the vertebrates.
Clearly it is not merely a question of constructing a convenient and more or less
satisfactory genealogy of the animal kingdom. The whole philosophy of creative
evolution is involved in the answer. We must face these problems fairly, without
prejudice and without arrogance (surely the record of past achievements affords
no grounds for that attitude), and with a full recognition of their significance.
Facts are stubborn things that will not be ignored, that call out for recognition,
and for their proper location in a well ordered scheme, if not in one, then in some
other that is better.

The problem is of the utmost importance to the biologist, for the answer
should determine the location of severallarge classes of animals, now completely
isolated; it will enable us to reconstruct the broad outlines of the genealogical

Vlll INTRODUCTION.

tree of the animal kingdom where the main branches emerge from the darkness
of the pre- Cambrian period; it will furnish us the only means by which we can
hope to solve some of the most important problems in vertebrate morphology,
such as the meaning of vertebrate cephalogenesis, of concrescence, germ layers,
gastrulation, and the structure of the oldest fossil representatives of the vertebrate
series. The answer to such problems cannot be found till after we have dis-
covered the immediate ancestors of the vertebrates and the broad outlines of their
structure, for when the point of departure is determined, and only then, can we
determine which is the base and which is the summit of a series of changes, which
the primitive, which the derived; in short, the direction in which evolution is

moving.

The arachnid theory of the origin of vertebrates has made slow progress.
This is not surprising since it has had to contend against the fixed ideas of the
specialist working in some narrow field of vertebrate or invertebrate morph-
ology, and who is unfamiliar with the multitude of facts and details, intricate in
themselves and in their bearings, upon which the arachnid theory rests. It has
also had to contend against the indifference of the newer school of biologists, who
look on morphology as an exhausted field, and who attach an exaggerated import-
ance to experimental, or statistical work, or to the minute structure of cells, or to
the analysis of protoplasm.

This is largely due to a common misconception of the real aim of the mor-
phologist; for it is evident that tracing the identity of structure under the disguise
of new forms is only the beginning of the morphologist's work. His real problem
is to measure the rate of these changes, and to seek out the underlying causes.
Hence, a great morphological problem, such as the origin of vertebrates, is essen-
tially a problem in experimental evolution, an experiment performed on the largest
scale of any in the history of organic evolution. But here the problem presents
itself in a different form from the ordinary laboratory experiment. There the ex-
perimenter fixes the conditions, as nearly as possible, and then records and meas-
ures the events as they appear. Here the morphologist records and measures the
events, and from them tries to discover the conditions. I believe I have discovered
the main events in this experiment of Nature, and I have recorded it, in terms of
systematic zoology, in a genealogic tree of the great arthropod-vertebrate stock.
This discovery enables us to see clearly some of the factors that have brought
about the results. They are mainly internal factors, insignificant in themselves,
but acquiring such immense transforming power by persistent and prolonged
action that it is unnecessary to invoke the agencv of such factors as external

j *._j

environment, natural selection, and heredity. At most, it seems to me, these
factors can account only for the superficial details of the essentially completed
body. Morphology teaches us that the foundations of anatomical structure are
automatically created by the processes of growth and organic readjustment, and
that they remain essentially unmoved by external conditons.

For almost a quarter of a century the problem of the evolution of the verte-

INTRODUCTION. IX

brates has been to me a stimulus and a guide. What appears to be an approxi-
mate solution of it has been tested and tested again, and elaborated in itself the
severest test of all by many methods and from many points of view, for it has
seemed to me the one great problem that must be solved before the biologist can
approach the problems of creative evolution on a reasonably secure footing. To
gain this end, I have given the best I had; whether that is much or little is of no
consequence, except in so far as it is a guarantee of serious endeavor and of good
faith. That I am conscious of many difficulties and imperfections need not be
emphasized. I would gladly make them less. But to be overconscious of the
one, unsteadies the hand and draws the eye away from the open waters, and too
long a delay over the inevitable defects means to be surprised by the night, and
still unprepared.

CONTENTS.

PAGE

INTRODUCTION . .

HISTORICAL SKETCH v

LIST OF AUTHOR'S PAPERS ... xi

CHAPTER I. xiv

OUTLINE OF THE ARACHNID THEORY 1-26

I. Its scope and relation to other theories, i. II. Nature of the evidence to be pre-
sented, 3; A. cephalogenesis in arthropods, 3; B. embryology, 4; C. arachnid
cephalogenesis prophetic of the vertebrate head, 5; D. paleontology. III. The
process of cephalization in the arthropods, 7 ; A. The grouping and the increase in
number of metamers, 7; B. origin of the linear arrangement of unlike cephalic
functions, 8. IV. The subdivisions of the incipient vertebrate head in arachnids,
ii ; mesoderm, 12. i. The procephalon, 12; insects, 13; arachnids, 13; sense
organs, 13; olfactory lobes, hemispheres and optic ganglia, 13; rostrum, 14; exter-
nal boundaries of the procephalon in the adult, 14. 2, 3. The dicephalon and the
mesocephalon, 15; endocranium, 16; oral arches, 16; taste buds, slime buds and
cranial ganglia, 17; segmental sense organs, 17; the diencephalon and the mesen-
cephalon, 18; the suprastomodaeal commissure and the cerebellum, 19. 4. The
metencephalon, or vagus region, 19; vagus appendages, 19; vagus neuromeres, 19;
vagus nerves, 20. 5. The branchiocephalon, 20; mesoderm, 20; neuromeres, 21;
nerves, 21; the endocranium, 21; the mesoderm, 22; comparison, 23; the vascular
area and concrescence, 24; the new mouth, cephalic navel or haemostoma, 25; the
closure of the old mouth or neostoma, 26; conclusion, 26.

CHAPTER II.

OUTLINE OF THE ARACHNID THEORY; CONTINUED 27-40

I. Comparison of adult arthropods with adult vertebrates, 27. I. Orientation of
neural and haemal surfaces, 27. II. Comparison of adult arthropods and verte-
brates, 29; bunodes, 29. III. Comparison of arthropod and vertebrate embryos,
33; form controlling factors in the early stages, 34; the gustrula, ccelenterate, or
trochosphere stage, 34; transition from radiate to bilateral symmetry, 35; telopore,
35; germ wall, 35; concrescence of the germ wall, 35; the nervous system, 38; the
primary sense organs, 38; cornua, 38; vertebrate stages, 38; the auditory pit, 39;
the heart, 39; cornua, 40; the oral arches and the haemostoma, 40; cranial flex-
ure, 40.

CHAPTER III.

EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS 4i~52

I. Meaning of the term brain, 41. II. The sternodceal nerves, 42. III. The
framework of the nervous system, 43. IV. The differentiation of the peripheral
nerves, 45; factors that modify the arrangement of peripheral nerves, 46; segrega-

xi

Xll CONTENTS.

PAGE

tion of nerve fibers, 46; elimination, 47; union, historic factor. V. Xeuromeres
and metamarism, 49. VI. Primitive sense buds, 50.

CHAPTER IV.

THE SUBDIVISIONS OF THE BRAIN . . 53~/o

I. The prosencephalon, or forebrain, 53; the procephalic lobes, division into three
metameres; acilius, the hemispheres, optic ganglia and ocelli; arachnids, 54; the
olfactory lobes of the first segment; the hemispheres of the second; the forebrain
flexure. II. The diencephalon, 57; diencephalic flexure, cheliceral nerves and
ganglia, minute structure in limulus, 58; the large basal association neurones,
the large association neurones on the hemispheres, the cortex neurones; the cheli-
ceral lobes; the forebrain and cheliceral commissures; the stomodaeal ganglia and
the supra-stomodaeal commissure, 60; nerves to labrum; association with the coxal
taste organs; comparison with vertebrates, 60; summary, 64; III. The mesen-
cephalon, 65; the oral and hyoid arch neuromeres; mesenccele, 66; comparison of
thoracic neuromeres of arachnids with the midbrain neuromeres of vertebrates, 66.
IV. The metencephalon, or vagus neuromeres, and V. the branchiencephalon,
67; the vagus zone in arthropods, its special sensory characters; the branchience-
phalon, 69; the branchiencephalic neuromeres, number, segregation of their nerves
into branchial, cardiac, intestinal, hypobranchial, or branchiothoracic, 70.

CHAPTER V.

MINUTE STRUCTURE OF THE BRAIN AND CORD OF CERACHNIDS 7 l ~93

Methods, 71. I. The branchial neuromeres of limulus, 71; development, 71;
commissures, 72; peripheral nerves, 72; cell clusters, 73; nerve roots, 76; branchial
nerve roots, haemal roots, 77; commissures, 79; the neuropile centers, 79. II.
The cephalic neuromeres, 80; haemal commissures, 81; haemal roots, 81; cranial
ganglia, 81; motor neurones, 84; gustatory nerves and tracts, 84. III. Longitudi-
nal tracts, 85; longitudinal haemal tracts, 85; longitudinal neural tracts, 86; the
lateral or pedal ganglion tracts, 86; the general cutaneous tracts, 87; comparison
with vertebrates, 87. IV. Commissures; summary, 90. V. The neuroccelia, 92.
VI. The neuroglial 93,

CHAPTER VI.

PERIPHERAL NERVES AND GANGLIA . 94-109

I. Components of a neuromere, 94. II. Nerves of the diencephalon and mesen-
cephalon, 94; A. neural nerves, 94; the flabellum, 94; the cranial ganglia, 95; the
ganglia of the cord, 97; the haemal nerves, 97; the lateral line nerve of the chele-
ceral neuromere, 97. III. Nerves of the metencephalon, 98; limulus, neural nerves,
98; chilarial, opercular, haemal nerves; scorpion, 101; neural and haemal nerves,
longitudinal abdominal, 104; general cutaneous, io5;cardiac and hypobranchial
longitudinal abdominal, 104; general cutaneous, 105; cardiac and hyppobranchial
nerves. V. Relation of vagal and hypobranchial nerves in arachnids to those in
vertebrates, 107.

CHAPTER VII.

GENERAL AND SPECIAL CTUANEOUS SENSE ORGANS .... 110-124

I. General cutaneous sense organs, no; temperature organs, no; free nerve ends,
in. II. Special cutaneous sense organs, in; the gustatory organs of limulus,

CONTENTS. Xlll

PAGE
in; reactions to stimuli, 112; structure, 113; the flabellum, 113; the branchial

warts, 115; the slime buds, 116; the auditory organ, 120; lateral line organs of
vertebrates; summary, 121.

CHAPTER VIII.

LARVAL OCELLI AND THE PARIETAL EYE . 125-148

I. The different kinds of eyes in arthropods and vertebrates, 125; eyes of orthro-
pods, 125; larval ocelli, 125; frontal ocelli or stemmata, 125; parietal eye, lateral
eyes, cerebral eyes of vertebrates, 126; parietal eye, lateral eye, olfactory organ,
127. II. The eyes as segmental sense organs, 127; procephalic and thoracic sense
organs. III. The larval ocelli of insects, 128; larval ocelli, stemmata. IV. The
parietal eye, 129; the parietal eye of the scorpion, 129; the parietal eye of limulus,
131; development; palial fold; epiphysis; anterior neuropore; change of position;
endoparietal and ectoparietal eye; nerves; retinal cells; the parietal eye of bran-
chipus, 136; the parietal eye of apus, 138; the parietal eye of vertebrates, 139;
petromyzon, 140; parietal eye vesicle, 140; ganglia, 142; asymmetry; comparison
with arthropods; the lenses of the parietal eye, 144; location of the placodes, 146;
minute structure, 140; summary, 147.

CHAPTER IX.

THE COMPOUND EYES OF ARTHROPODS AND THE LATERAL EYES OF VERTEBRATES . 149-159
I. Compound eyes of arthropods, 149; A. serial location, 149; B. development,
150. II. Lateral eyes of vertebrates, 151; location, 151; origin of the chroid fis-
sure and the blind spot, 151; the retinal-cell pattern, 153; the retinal ganglion, 153;
the lens, 153; origin of the imperfections of the vertebrate eye, 154. III. The
optic ganglia, 154; location, 154; parietal eye ganglia, 155; lateral eye ganglia,
optic lobes, 155; minute structure in limulus, 155. IV. Comparison with verte-
brates, 157; summary and conclusion, 158.

CHAPTER X.

THE OLFACTORY ORGANS OF ARTHROPODS AND VERTEBRATES . 160-173

I. The olfactory organ of limulus, 160; structure in adult, 160; gross structure;
minute structure; development of olfactory organ and nerves, 162; olfactory
placodes; lateral olfactory nerves; median olfactory nerve; summary, 163. II.
The olfactory lobes of arachnids, 164; development; scorpion and spiders; limulus;
development; structure. III. The olfactory organs in phyllopods, frontal organs,
p. 165; branchipus, p. 166; apus, 167. IV. Comparison of olfactory organs of
vertebrates and arthropods, 168; i. number of placodes, 168; 2. number of nerves,
169; 3. structure and termination of the nerves, 169; 4. origin of the olfactory
ganglia, 170; 5. position of placode cells, 170; 6. serial location of the placodes and
their migration, 170; 7. olfactory lobes, 171; 8. function, 171; summary and con-
clusion, 172.

CHAPTER XL

FUNCTIONS OF THE BRAIN . . . . 174-194

Parti. Introduction, methods, 175; description of experiments, 176-186. Part

II. Summary of experimental and anatomical results, gustatory reflexes, structure
of gustatory apparatus, 186; the nerve-muscle chewing apparatus, 188; experi-

XIV CONTENTS.

PAGE
mental results, the swallowing reflexes, 189; course of the nerve impulses in the

gustatory, chewing and swallowing reflexes, 189. II. The crossed thoracic
reflexes, 190. III. The crossed and uncrossed abdomino-thoracic reflexes, 190.
IV. Locomotion, 191. V. Equilibrium, 191. VI. Respiration, 191; respiratory
reflexes, 192; comparison with vertebrates, 193. VII. The cerebral hem-
spheres, 194.

CHAPTER XII.

THE HEART . ... 195-209

I. Location of the heart, 195. II. The development of the heart, 195; compari-
son of vertebrate and arachnid heart. III. The circulation, 198; arachnids, 198;
comparison with vertebrates, 198, direction of blood currents, aortic arches, carotids,
circle of Wellis, aortae, cardinals, curvature of heart, split posterior end, reduction
of cardiac area, and moulding effect of blood stream on structure of heart walls.

IV. The cardiac nerves and ganglion, 200; the median cord or ganglion, 200;
the cardiac plexus, the lateral cardiacs, pericardials, segmental cardiacs. V. The
minute structure of the cardiac ganglion, 202; small multipolar or motor cells,
giant bipolar cells, small bipolar cells, motor terminals, 205; sensory terminals,
cardiac ganglia in vertebrates. VI. Experiments on the heart, 205. VII. Sum-
mary and conclusion, 208.

THE NERVOUS SYSTEM AND SENSE ORGANS OF VERTEBRATES AND ARACHNIDS. GEN-
ERAL SUMMARY OF CHAPTERS I-XII 209-214

CHAPTER XIII.

THE EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS . 215-248

I. Primary causes of differential growth, 215. II. Morphological interpretation
of the early stages, 219. III. Embryology of limulus, 222; i. cleavage, 223; com-
parison with vertebrates, 223; 2. the germ disc or primitive cumulus, 224; 3. for-
mation of metameres, 225; 4. the gastrula, 227; 5. the germ wall, 228; 6. the meso-
derm, 230; the sources and kinds of mesoderm: (a) procephalic, (b) postoral, (i)
axial cord, (2) mesoblastic somites, (3) lateral plates; the fibre cells, 232, give rise
to: (a) inter-tergal, branchial section of branchio-thoracic, and numerous scat-
tered, muscles; (b) to semi-amoeboid wandering cells which persist in adult stages;
Vascular area, 236; pellucid area. IV. The cephalic navel, dorsal organ, or neos-
toma, 238. V. Concrescence and the caudal navel or blastopore, 243.

CHAPTER XIV.

THE OLD MOUTH AND THE NEW; LOCOMOTOR AND RESPIRATORY APPENDAGES 249-273
The salient features of the mouth and appendages in arthropods and vertebrates,
249; the argument, 250. I. The closing of the old mouth, 251. II. The new
mouth, 253. III. The jaws or oral arches, 255; development of the oral arches
in the frog, 257; conclusion, 260. IV. The gill arches and the external gills, 261.

V. The gill sacs, the thyroid and the thymus, 263. VI. The gut pouches, 266.
VII. The locomotor appendages, 263; conclusion, 271.

CHAPTER XV.

VARIATION AND MONSTROSITIES .... 274-288

Problem stated. I. Invaginated appendages, 275. II. Asymmetry, 276. III.
Degeneration, 277; A. median fusion and antero-posterior degeneration, 277;

CONTENTS. XV

PAGE

B. hour-glass embryos, 279; C. acephalic and acaudal embryos, 280; D. final
stages of degeneration, 281; IV. Double embryos, 281. V. Triple embryos,
284. VI. Summary and conclusion, 287.

CHAPTER XVI.

THE DERMAL SKELETON 289-305

The five kinds of skeletal structures in arthropods and vertebrates, 289. I. The
dermal skeleton of vertebrates, 289; the minute structure of the dermal bones in
the ostracoderms, 290; tremataspis, 290; pteraspis, 293; ateleaspis, 295. II.
Dermal skeleton of limulus, 296; minute structure, 297. III. Summary and com-
parison, 302.

CHAPTER XVII.

THE ENDOCRANIUM, BRANCHIAL AND NEURAL CARTILAGES 306-322

I. The endoskeleton of arachnids, 306. II. The neural arches, 307. III. The
branchial cartilages, 307; development of branchial cartilages in limulus, 308;
minute structure, 309. IV. The endocranium, 312; apus, 312; mygale, 313; lim-
ulus, 314; scorpion, 317; telyphonus, 319. V. Summary and comparison, 319.

CHAPTER XVIII.

THE MIDDLE CORD, THE LEMMATOCHORD AND THE NOTOCHORD 323-336

I. The middle cord of insects, 324; the lemmatochord of lepidoptera, 326. II.
The middle cord of the scorpion; A. neural sinus, merochord and bothroidal cord
of the adult, 328; B. development of the lemmatochord, 329; merochord, 330;

C. development of the neural sinus, neuroglia and canalis centralis, 331. III.
The middle cord of limulus, 334. IV. Summary and comparison, 335.

CHAPTER XIX.

THE OSTRACODERMS AND THE MARINE ARACHNIDS ... . 337-347

Nature of the problem. I. The marine arachnids and their origin, 338. II.
The ostracoderms, 341; historical review, 342.

CHAPTER XX.

THE OSTRACODERMS 348-380

Subdivisions of the body, 349; the cephalic appendages, 350; jaws, 350; skeleton,
351; trend of development of the exoskeleton, 352; the eyes, 355; olfactory organ,
356; auditory organ, 356; cutaneous sense organs, 356. I. Aspidacephali, 358;
cephalospidae, 358; trematospidae, 359; exoskeleton, oral region, lateral eyes,
marginal and postorbital openings, lateral line organs, appendages, ateleaspidae,
363. II. The anaspida, 364; ccelolepidce, birkeniidse. III. The pteraspida, 364;
pterospidae, psamostaedae. IV. Antiacha, 367; exoskeleton, 367; atrial frill, 371;
gills, 371; viscera, 372; jaws, 373; hyoid arches, 375; mouth, 375; eyes, olfactory
organs, sensory grooves, 376; cephalic appendages, 376; preservation, 377; loco-
motion, 379; food, 379.

CHAPTER XXI.

THE VERTEBRATES. . . 3^ >

I. The cyclostomata, 383. II. The elasmobranchii and holocephali, 384. III.
The arthrodira, teleostomii dipnoi and amphibia, 386.

XVI CONTENTS.

PART II. THE ACRANIATA.

CHAPTER XXII.

PAGK

THE CRANIATES AND THE ACRANIATES 393-407

Statement of the problem, 393. I. The craniates, 395. II. The acraniates, 396;
metamerism, 398; appendages, nervous system, 399; degeneration, attachment,
mantle, 400; skeleton, heart and circulation, 401; sexual organs, development, 401;
A. molluscs and annelids, 403; trochosphere, gastrula, blastopore; B. craniates,
403; modification of the gastrula, telopore, concrescence; C. acraniates, 405; mes-
entocoel, telopore, mouth, 406; the naupula, ccelom, 407.

CHAPTER XXIII.

THE ClRRIPEDS, TUNICATES, AND ECHINODERMS . . 408-430

I. The cirripeds, 408; the nauplius and the naupula, the metamorphosis, 410;
appendages, alimentary canal, 411; coelom, excretory organs, 412; sexual organs,
degeneration, 413; the old mouth and the new. II. The tunicates, 415; meta-
morphosis, heart and vascular system, 417; eyes, 418; the old mouth and the new.
mantle, 419; comparison with cirripeds, summary, 420. III. The echinoderms,
421; larva, 422; ciliated band, 423; cephalic appendages, 424; attachments, de-
velopment, 425; teloccel, mesoderm and ccelom, 426; thoracic appendages, 427;
excretory organs, disc, vertebrate, 428; asymmetry, summary, 430.

CHAPTER XXIV.

THE ENTEROPNEUSTA, PLEROBRANCHIA, POLYZOA-BRACHIOPODA PHORONIDA AND

CHALTOGNATHA . . . . 431-453

IV. The enteropneusta and echinoderms, 431; the enteropneusta and the arthro-
pods, 432; structure and development, 433; cleavage, 433; gastrula and teloccele,
433; mesoderm and ccelom, 434; metamorphosis, 435; cephalic caecum, 435;
late larval and adult stages, 435; endocranium, 436; muscles, 437; ccelom, 437;
nervous system, 437 V. The pterobranchia, 439. VI. The polyzoa, 440; ento-
procta, 441; conclusion, 443; ectoprocta, 443. VII. The brachiopoda, 445.
VIII. The phoronida, 446; fusiform cells, 447. IX. The chaetognatha, 448; de-
velopment, 448; adult, 449; integument, 449; excretory organs, 449; trunk, 450;
head, 450; endocranium, 450; nervous system, 450; cephalic sense organs, 452;
lateral eyes, 452; parietal eye, 452; olfactory organ, 452; conclusion, 453.

CHAPTER XXV.

SUMMARY AND CONCLUSION . . 454-472

I. The evolution of a creative environment, 454. II. Crises in organic evolution,
456; A. the evolution of metamerism and bilateral symmetry, 457; B. asymmetry
as a creative factor, 458; C. chiten and the exoskeleton as creative factors, 439;
D. the increasing volume of the yolk sphere as a creative factor, 461 ; E. the increas-
ing volume of the brain as a creative factor, 461; F. the creation of a new environ-
ment for the eyes, 462; the significance of a natural system of classification, 463;
the various aspects of evolution, 468.

EXPLANATION OF THE LETTERING . 473

INDEX .... ... 483

HISTORICAL SKETCH.

The resemblance between vertebrates and arthropods first attracted my
attention in 1884. In my paper on the development of the phryganids, it was
stated, page 594, that a wonderful analogy, if not homology, exists between the
structure and mode of growth of the medullary plate, the neural and gastrular in-
vaginations, and the neurenteric canal of insects and the corresponding structures
in vertebrates.

Three years later, a resemblance between the minute structure of the
compound eyes of arthropods and the retina of vertebrates was recognized.

In 1888, it was shown that the imagination of the procephalic lobes, supposed
by writers of that period to give rise to a two-layered compound eye, in reality
gave rise to the optic ganglion only, while the eye itself consisted of a single layer.
Further study of the developing brain and eyes of Acilius, Buthus, and Limulus,
showed that in many arthropods the procephalic lobes underwent a complex
process of imagination, accompanied by the overgrowth of a neural crest, or
palial fold, the result being the formation of a vesicular forebrain, and the transfer
of the ocelli, located on the outer margin of the lobes, to the end of a tubular or
epiphysial outgrowth of the membranous roof of the forebrain vesicle.

Here were revealed, for the first time, all the steps in the transformation
of an invertebrate type of eye into the type of eye so characteristic of ver-
tebrates. This apparently simple fact was in reality the result of very complex
conditions, and it seemed incredible that they could be duplicated except in
animals belonging to the same stock.

These discoveries, therefore, appeared so profoundly significant that I deter-
mined to follow the clue to the end, to see whether further analysis of the eyes, the
brain, and other systems of organs would not confirm the obvious conclusion to be
drawn from them. The results proved to be so surprisingly in accord with them,
that in the following year, 1889, a definite theory was formulated, and a prelim-
inary sketch, or outline, of it was published under the title " On the Origin of
Vertebrates from Arachnids."

This theory has formed the basis of all my subsequent work, and as far as it
went, is practically the same as the one presented here. In that paper it was
maintained that the vertebrates are descended from the arachnid division of the
arthropods, in which were included the typical arachnids, the trilobites, and
merostomes. The ostracoderms were regarded as a separate class, uniting
the arachnids with the true vertebrates. Limulus and the scorpion were the
types most carefully studied, because they were the nearest and most available

xvii

XV111 HISTORICAL SKETCH.

living representatives of the now extinct merostomes, or giant sea scorpions, that
were regarded as the arachnids standing nearest to the ostracoderms.

Other evidence and conclusions were as follows: i. In the arachnids a forebrain
vesicle is formed by the same process of marginal overgrowth as in the vertebrates.
From the floor of the vesicle arise the forebrain and optic ganglia; from the
membranous roof, a tubular outgrowth is formed that contains a parietal, or pineal
eye, similar in structure, mode of origin, and innervation to the pineal eye of
vertebrates. 2. The kidney-shaped compound eye of arachnids has been trans-
ferred to the walls of the cerebral vesicle in vertebrates, giving rise to the retina,
which still shows traces of ommatidia in the arrangement of the rod-and-cone
cells. Its original shape is temporarily retained in vertebrates, but gives rise
ultimately, by adaptive exaggeration, to the choroid fissure. 3. The arachnids
have a cartilaginous endocranium similar in shape and location to the primordial
cranium of vertebrates. 4. They have an axial, subneural rod comparable with
the notochord. 5. In arachnids, the brain contains approximately the same num-
ber of neuromeres as in vertebrates. It is also divided into similar regions, each
one having a similar number of neuromeres, a similar distribution of nerves, and a
similar relation to cranial ganglia and sense organs, to those in vertebrates. 6.
The segmental sense organs (median and lateral eyes, olfactory and auditory
organs) are comparable with those in vertebrates. The coxal sense organs are
associated with special sensory nerves and ganglia, comparable with the cranial
dorsal-root nerves and ganglia (suprabranchial sense organs) of vertebrates.
7. The basal arches of the appendages are comparable with the oral and branchial
visceral arches in vertebrates. 8. The tendency toward concentration of neuro-
meres has narrowed the passage way for the stomodeum and modified the mode of
life in the arachnids. This ultimately led to its permanent closure, the infundi-
bulum and adjacent nerve tissues in vertebrates representing the remnants of
the old stomodseum with its nerves and ganglia. 10. The progressive degeneration
of haemal thoracic muscles, the fusion of thoracic metameres, the position of the
oral, or neural surface, in swimming and crawling, were identified with corre-
sponding conditions in vertebrates, n. The eye muscles of vertebrates arose
from a special group of haemo-neural muscles belonging probably to the first
two or three thoracic segments. 12. The process of gastrulation in vertebrates
and arachnids is confined to the procephalic lobes, in the place where at a later
period the primitive stomodaeum appears. The so-called "gastrulation" of verte-
brates and arachnids is an entirely different and independent process, that is, the
process of adding by apical or teloblastic growth a segmented, bilaterally sym-
metrical body to a primitive radially symmetrical head. 13. The arachnids
resemble the vertebrates in more general ways, as in the minute structure of
cartilage, muscle, nerves, digestive, and sexual organs.

In the following paper, '93, the structure of the forebrain of Limulus, with its
lobes and cavities was compared in detail with the brain of vertebrates. The
coxal sense organs w r ere described and shown to be gustatory organs comparable

HISTORICAL SKETCH. XIX

with the suprabranchial organs of vertebrates. The remarkable structure of the
olfactory organs in Limulus was also described for the first time.

In 1894 it was shown that the exoskeleton consisted of a complicated, and
for an invertebrate, a very remarkable system of chitenous trabeculse resembling
a primitive form of dermal bone.

In 1896 was published a paper on the "Variations in the Development of
Limulus." It was undertaken in the hope that it might throw some light on the
normal development, or give some indications of the kind of variations that have
led to the higher types.

In 1899 and 'oo, in cooperation with Mr. Redenbaugh and Miss Hazen, a de-
scription of the peripheral nervous system, endocranium, and coxal glands was
published. The work was begun w r ith the purpose of furnishing a detailed
account of the various systems of organs in arachnids as a basis for further com-
parisons with vertebrates.

In 1901 advantage was taken of a six months leave of absence from college
duties to study the principal collections of ostracoderms in European museums.
It was rarely possible to make use of such collections for anything more than a
superficial examination. An effort was therefore made to obtain material that
could be sectioned, or used in any manner that seemed desirable, in order to get
at the anatomical structure. A valuable collection of Tremataspis and Thy-
estes was obtained in the island of Oesel in the Baltic Sea, and a few cephalaspids
and pteraspids were obtained by gift and purchase in England. In the next four
or five years an effort was made to obtain ostracoderms in the vicinity of Dal-
housie, N. B., Canada, at first with little success. Finally, I obtained a very large
number of specimens in a beautiful state of preservation, from which it was
possible to work out the anatomy in great detail. The structure of the eyes, jaws,
and internal organs afforded a striking confirmation of our conclusion that the
ostracoderms form a new class of animals standing between the vertebrates and
arachnids.

In 1888, '89, and '90, Gaskell published his first papers on the Origin of the
Central Nervous System of Vertebrates. The basis of his theory was that "the
central nervous system of a crustacean ancestor had grown round and enclosed
the alimentary canal." " The ventricles of the brain were the old cephalic stomach
and the canalis centralis of the spinal cord, the long straight intestine which led
originally to the anus." The vertebrate develops a new heart, alimentary canal,
and other organs to take the place of those enclosed in the central nervous system.

In its conception and mode of analysis of the conditions in the vertebrates and
arthropods, this theory is entirely different from, and wholly irreconcilable with my
own. In my judgment, the foundations on which it is built are totally wrong.
The fundamental error, which is inextricably interwoven in all his conclusions,
making a detailed criticism of them unnecessary, is the assumption that the neural
surface of an arthropod is the same as the haemal surface of a vertebrate. In this
confusion of opposite surfaces, which is like starting on a voyage of discovery with

XX HISTORICAL SKETCH.

the notion that north is south, and east is west, the nerve cords are transferred
from one side of the body to the other, turning them literally upside down and
inside out. annihilating the most fundamental systems of organs, such as the
heart and entire alimentary canal, and necessitating the creation "de novo" of
whole systems of organs to take their place. In this process the axes of growth
and differentiation are reversed, or ignored, and no attempt is made to reconcile
these assumptions with the actual conditions that are so familiar in the embryonic
development of both vertebrates and arthropods.

LIST OF THE AUTHOR'S PAPERS CONCERNING THE EVOLUTION OF THE VERTEBRATES

1884. The Development of Phryganids with a preliminary note on the development of
Blatta Germanica. Quart. J. M. Sc., Vol. XXIV, X. S.

1886. Eyes of Molluscs and Arthropods. Mitth. aus. d. Zool. Stat. zu. Neapel. Vol. VI.

1887. Eyes of Molluscs and Arthropods. Journ. Morphol. Vol. I, April n.

1887. Studies on the Eyes of Arthropods. I. Development of the Eyes of Vespa,
with Observations on the Ocelli of some Insects. Journ. of Morphol., Vol. I,
April ii.

1888. Studies on the Eyes of Arthropods. II. Eyes of Acilius. Journ. of Morphol.,
Vol. II.

1888. Segmental Sense Organs of Arthropods. Journ. of Morphol., Vol. II.
1890. On the Origin of Vertebrates from Arachnids. Quart. Journ. Micr. Sc.,
Vol. XXXI, Part 3.

1893. On the Morphology and Physiology of the Brain and Sense Organs of Limulus.
Quart. Journ. Micr. Sc., Vol. XXXV, No. 137.

1894. On Structures Resembling Dermal Bones in Limulus. Anat. Anz., Bd. 9, No. 14.
1896. Variations in the Development of Limulus Polyphemus. Journ. of Morphol.,

Vol. XII, No. i.
1896. The Visual Centres of Arthropods and Vertebrates. Morph. Soc. Sc., Vol. V,

P- 43 !
1899. Patten Wm., and Redenbaugh, W. A. The Endocranium of Limulus, Apus,

and Mygale. Journ. Morphol., Vol. XVI, No. i.
1899. Patten, Wm., and Redenbaugh, W. A. The Nervous System of Limulus

Polyphemus. Ibid.
1899. Gaskell's Theory of the Origin of Vertebrates. Am. Naturalist, Vol. XXXIII,

April, pp. 360-9
1900 Patten, Wm., and Hazen, A. P. The Development of the Coxal Gland,

Branchial Cartilages, and Genital Ducts of Limulus. Journ. Morphol., Vol. XVI,

No. 3.

1901. On the Origin of Vertebrates, with Special Reference to the Ostracoderms.
Address before the V. International Congress of Zoologists, Berlin.

1902. On the Structure and Classification of the Tremataspidat. Amer. Nat., Vol.
XXXVI, No. 425.

1903. On the Structure and Classification of the Tremataspidas. Mem. Acad. Imp.
Sci. St. Petersbourg, Vol. XIII, No. 5.

1903. On the Appendages of Tremataspis. Amer. Nat., Vol. XXXVII, No. 436.

1903. The Structure of the Ostracoderms. Science, Vol. XVII, No. 430.

1903. On the Structure of the Pteraspidas and Cephalaspidae. Am. Nat.

1904. New Eacts Concerning Bothriolepis. Biol. Bui., Vol. VII., July.

HISTORICAL SKETCH. XXI

1904. The Structure of Bothriolepis, with Exhibition of Specimens of Devonian Fishes
of Canada. Read before the Am. Soc. of Zoologists, Phila.

1907. On the Origin of Vertebrates. I. The Conditions Controlling the External
Morphology of Primitive Vertebrates. Lantern Slides. Read before Section
VII, General Zool, VII. International Zool. Congress, Boston, August.

1907. On the Origin of Vertebrates. II. The Interpretation of the Structure of Echino-
derms, Ascidians, Balanoglossus, and Cephalodiscus. Lantern Slides. Ibid.

1907. International Congress, Boston, Mercator Projections of Vertebrate and Arachnid.
Embryos.

Exhibits. A. Collection of Bothriolepis from the Devonian rocks of New
Brunswick.

B. Fifty Models Illustrating the Structure and Embryology of Primitive Verte-
brates and Related Forms. Reviewed in Amer. Nat., Vol. XLI, No. 490.

THE

EVOLUTION OF THE VERTEBRATES
AND THEIR KIN.

CHAPTER I.

OUTLINE OF THE ARACHNID THEORY.

In the two following chapters we shall present a brief outline of the arachnid
theory, showing the broad foundations upon which it rests and the relation of
the principal organs in the arachnids to those in the vertebrates.

I. ITS SCOPE AND RELATION TO OTHER THEORIES.

The arachnid theory, like every other large problem in descent, should be
based on comparative physiology, anatomy, embryology, and paleontology,
and should be constructed in accordance with the established principles of these
sciences. This particular theory has the additional task of reconciling, eliminating,
or absorbing the claims of strongly entrenched rival theories, some of which con-
tain certain elements of truth, It is important, therefore, to at once determine
which supplies the greatest volume of evidence; which draws its evidence from
the widest fields; which can eliminate the others, or include the others within
itself.

We shall show that in these respects the arachnid theory stands in a class
by itself, for it is the only one that is securely built on the natural science trinity
of structure, function, and historic sequence. It not only has its own distinctive
merits upon which it claims recognition, but it is the only theory that can either
eliminate the others, or incorporate them within itself, where they become rein-
forced and revitalized.

The essential features of the annelid theory, for example, are included in
the arachnid theory, because both arachnids and annelids agree in the funda-
mental nature of their metameric structure. But w r hen standing alone, the anne-
lid theory ceases to be of value as a working hypothesis, or as a touchstone to
solve the problems of vertebrate morphology, becausp we find no traces in the
annelids of those illuminating modifications of metamerism so characteristic of
the arachnids, and that afford us the required data for filling in, and explaining,
the enormous gap between the unspecialized metameres of an annelid and the
groups of highly specialized metameres in the head of a vertebrate. The annelid

OUTLINE OF THE ARACHNID THEORY.

theory, therefore, in the form in which it is generally understood, could be in-
corporated into the arthropod theory, but it is evident that the conditions could
not be reversed, for no resemblance of annelids to vertebrates could either elimi-
nate, or account for, the resemblance of arthropods to vertebrates.

The tunicate, echinoderm, balanoglossus, amphioxus, etc., theories have
similar inherent weaknesses, indicating that they must be subordinated to
some larger view. The baffling resemblances between the embryonic stages of
these forms and vertebrates do not help us to explain vertebrate cephalogenesis,
or to account for the origin of the most characteristic vertebrate structures; and
so long as their own origin is unknown, and they have no fixed location in a
general system of classification, they can throw no light on the origin of verte-
brates, or on the still broader problems of the origin and inter-relations of the
other great subdivisions of the animal kingdom.

All this is changed, however, as soon as we recognize that the echinoderms
tunicates, balanoglossus, and cephalodiscus are degenerate offshoots of a common
arthropod-vertebrate stock. In the light of this interpretation, the arachnid
theory not only recognizes and explains the resemblance of the echinoderms,
tunicates, and other acraniates to the vertebrates, but it fixes approximately
their position in the animal kingdom, and elucidates the salient features of their
morphology. It supplies, in the evolution of the arthropod cephalothorax, the
key to the analysis of the vertebrate head. It unites the apex of the arthropod
stock with the base of the vertebrate stock, and welds the entire series of seg-
mented animals into one homogeneous group. It shows that the great verte-
brate-ostracodenn-arthropod phylum forms the main trunk of the genealogical
tree of the animal kingdom; that, emerging from unsegmented, ccelenterate-like
animals, as though driven by some mysterious internal power, moves with aston-
ishing precision, through broad, predetermined channels from which neither
habit, nor environment, nor heredity, can cause it to diverge toward its goal.
And finally it lays before us in their historic order the critical events of these age-
long periods, the succession of structural and functional changes that have fol-
lowed them, and that have in turn given rise to still other changes of form and
new conditions of growth. It thus reveals to us, as only the true science of mor-
phology can reveal, the important agents that have directed the course of evolution,
and that have determined the organic forms, or shapes, in which it is expressed.

The arachnid theory thus not only unites and- harmonizes these apparently
conflicting views as no other interpretation can, but it will, in my judgment, go
a long way toward restoring morphology to its former dominant position as the
expounder and prophet of the biological sciences. Morphology reduced to a
barnyard science, without its vast resources in comparative anatomy, its per-
spective in geological time, and its world-wide laboratory of Nature, is robbed of
its chief glory and power.

One naturally looks on the arthropods as the probable ancestors of the
vertebrates, because they are the most highly organized of segmented invertebrates

NATURE OF THE EVIDENCE. 3

and because the histological structure of their muscles, nerves, sense organs,
cartilages, etc., closely resembles that of the vertebrates. This view was, there-
fore, the first to be entertained by the older anatomists (Leydig and Dohrn);
but in more recent years it has not been regarded with favor.

So far as I have been able to determine, most zoologists of to-day, who make
any attempt to justify their deep rooted prejudice against the arthropod theory,
base their objections on the a priori ground that the arthropods, being highly
specialized animals, cannot have given rise to the vertebrates, because the verte-
brates must have come from some generalized type. This objection clearly has
but little weight, for the general application of such a law would exclude the
possibility of any evolution. Every animal is a specialized one when compared
with its ancestors, and at the same time a generalized one when compared with
its descendants. Even the most primitive vertebrate is a highly specialized
animal, and its immediate ancestors were also highly specialized. It is clear,
therefore, that in order to solve our problem we must discover not some gener-
alized ancestor but a specialized one, and the only evidence of value in deter-
mining whether we have found the right one or not, is the degree to which its
particular kind of specialization agrees with that of a vertebrate.

II. NATURE OF THE EVIDENCE TO BE PRESENTED.

Our problem then is a perfectly simple one in principle, although it is one
that involves an enormous amount of detail in its application. We have merely
to strip off the superficial disguise of our hypothetical arachnid ancestors and see
whether either their underlying structure, their mode of growth, the general
direction and historic sequence of their evolution, does or does not harmonize
with the assumption that they are the ancestors of the vertebrates. We venture
to state at the outset, that in our judgment they do harmonize with this assump-
tion, and so fully and in such detail as to leave no other conclusion open than that
the vertebrates arose from arachnid-like arthropods.

A. Cephalogenesis in Arthropods. We shall show, with the aid of com-
parative anatomy, that the process of cephalizing the anterior part of the body,
that is, the transformation of a large number of independent metameres into a
compact, organized group of unlike structures that may be called a "head," is
the dominant process in the evolution of arthropods, and that this process has
already definitely established in the higher forms the more characteristic features
of the vertebrate head. The process is initiated in primitive arthropods either
by the division of the anterior part of the body into regions, or by the addition
from time to time of distinct groups of like metameres, or tagmata. The succes-
sive appearance of new groups of metameres at the tail end of the body marks
.distinct epochs in the evolution of the arthropods, and they constitute the under-
lying basis for the characteristic subdivision of the body into pre-oral, oral, tho-
racic, vagus, abdominal, and caudal regions. We shall call them the procephalon,

4 OUTLINE OF THE ARACHNID THEORY.

dicephalon, mesocephalon, metacephalon, and branchiocephalon. Each region
usually consists of a certain number of metameres, modified, or specialized, in a
very constant and definite manner in respect to its sense organs, nerves, and other
characters. In the higher arachnids they unite in various ways to form larger
aggregates, such as the cephalo-thoracic-branchial region. In the vertebrates
they have become still more compactly united to form the head, the subdivisions
of which still consist, as nearly as may be determined, of the same number of
metameres, modified in the same characteristic manner as the corresponding
subdivisions of the arthropod trunk and cephalothorax. (Figs, i, 3 and 5.)

1)1.0..

' gust.o
die.enc.
mesc-n.c

meten.

FIG. i. Plan of a marine arachnid, based in part on Limulus. Designed to show the principal body regions
and their characteristic organs. A, Neural, or oral surface; B, hsemal, or cardiac surface.

B. Embryology. We shall show that arachnid and vertebrate embryos,
from the very beginning of their development, are fundamentally alike in structure
and mode of growth, and that this likeness is continued through successive,
parallel stages, up to a point where the arachnid stages cease; then the vertebrate
embryo, entering on its particular phases of development, carries them to com-
pletion. We shall show that the similarity between them consists: a. in the
origin of the germ layers; b. in the general form and segmentation of the neural
plate, its flexures, mode of enclosure, and the location of its principal parts; c.

CEPHALOGENESIS. 5

in the serial location and subsequent migrations of the primary cephalic sense
organs (median and lateral eyes, olfactory, and auditory organs); d. in the
degree of development of the cephalic mesoblast, and in the direction and extent
of its growth in the several regions; e. in the development of the heart;/, in the
concrescence of the so-called "lips of the blastopore," and in the growth of the
margins of the embryonic area (" germ wall ") ; g. in the formation of the head fold.
C. Arachnid Cephalogenesis Prophetic of the Vertebrate Head. We
shall show that the continuation, or the exaggeration, of the processes already
initiated in the arachnids inevitably leads to the establishment of the conditions
now seen in the vertebrates. For example: a. The further withdrawal of the

noto.

C

per c.

card.g.

FIG. 2. Semi-schematic cross-sections of a marine arachnid, showing location of principal organs. .4, abdominal
region; B, branchial region; C, mesocephalic, or thoracic region.

principal alimentary and urogenital organs of the arachnids into the postcephalic
regions, would produce the condition in vertebrates, b. The continued enlarge-
ment and closer union of the thoracic neuromeres, and their more precocious
development during embryonic periods, aided possibly by the further overgrowth
of the labrum and optic ganglia, would lead to a further narrowing, and over-
growth of the passage for the esophagus, and ultimately to the permanent closure
of the old mouth, as in vertebrates, c. The continued increase in the size of the
yolk sphere, the absence of mesodermic structures on the haemal side of the tho-

6 OUTLINE OF THE ARACHNID THEORY.

racic and cephalic region, and the increasingly precocious development of the fore-
brain, would inevitably lead to the formation of a more pronounced head fold,
with a disproportionately shortened or diminished haemal surface, and would force
the bases of the more anterior oral appendages forward and haemally till they
meet on the opposite side of the head, thus giving rise to thepremaxillary, maxillary,
and mandibular arches of the vertebrate head. d. This shortening of the ante-
rior haemal surface of the head inevitably draws the heart, with its neighboring
muscles and nerves derived from the vagal and branchial segments, farther for-
ward into the head region, thus producing that remarkable forward dis-
location of the heart, hypobranchial muscles and nerves so familiar in vertebrates.
(Figs. 17, 19, 33, 77.) e. Finally the readjustment of the whole head, in response
to these changes, leads to that new condition of architectural stability that marks
the true vertebrates.

The preliminary stages that lead up to these readjustments were, no doubt,
gradual and more or less tentative, for they did not in themselves create sufficiently
altered conditions to upset the balance of organic power. But the later stages
of the readjustment, especially the final stages in the transfer of the oral arches to
the haemal side, appear to have been rapidly accelerated for a period and then
checked by their approaching reunion on the haemal side of the head and by the
creation there of a new condition of organic stability.

The closing of the old mouth, the formation of a new one, the transfer of the
oral arches to the haemal side, and the appearance of true gill clefts must have
taken place during the embryonic, or larval period, the increasing volume of the
yolk sphere making such a cataclysmic metamorphosis possible. Hence it is
probable that the transition from the arthropod to the vertebrate type will never
be completely bridged by the discovery of new animals. The gap between the
two classes is a real one, representing a comparatively short period of rapid trans-
formation from the old condition in the arthropods to a new, approximately
stable condition in the vertebrates.

D. Paleontology. Nevertheless, we shall show that the wide gulf which now
separates the arachnids and vertebrates, in some important respects, was bridged
in early paleozoic times by a large and varied class of animals known as the
ostracoderms. They constitute the only great class of animals that have flour-
ished for a comparatively short period and then become totally extinct; a
fact that in itself testifies to the unstable, transitory character of their anatomical
structure.

Heretofore it has been assumed that the ostracoderms were highly specialized
vertebrates, in spite of the fact that they possessed a very simple and primitive
structure, and were the first vertebrate-like animals to appear on the geological
horizon. They were contemporaneous with the highest and most dominant
type of arthropods then in existence, the marine arachnids, or sea scorpions, of
the Silurian period. There is a striking resemblance between these early verte-
brates and the contemporaneous arachnids, not only in their form and general

PALEONTOLOGY. 7

appearance, but in the minute structure of their exoskeleton, the character of
their appendages, the arrangement of their median ocelli, and in the structure
of their jaws. (Figs. 232 to 265.) For a long time the ostracoderms were
supposed to be jawless fishes, but a special investigation of this point was made
and it was demonstrated that Bothriolepis, the best known member of the class,
possesses well developed maxillae and mandibles, quite unlike those of typical
vertebrates, but precisely like those demanded by the arachnid theory.

Thus, in the light of the arachnid theory, these ancient and remarkable
animals, that have been repeatedly mistaken for arthropods and for vertebrates,
but which are neither wholly; which have withstood the keen scrutiny of Agassiz,
Huxley, Ray Lankester, and Smith Woodward, take on a new meaning. We can
now clearly see that they belong neither to the vertebrates nor to the invertebrates,
but form a class by themselves, intermediate between the two; presenting on the
one hand, in their appendages, jaws, eyes, skeleton and gills, affinities with the
marine arachnids, and on the other, in their tail, dermal skeleton, and dorsal fins,
affinities with true vertebrates.

III. THE PROCESS OF CEPHALIZATION IN THE ARTHROPODS.

If we trace the evolution of cephalization in the arthropods and analyze the
causes that have brought it about, we shall see that it reaches its highest expression
in the arachnids and that it was brought about by the same kind of changes that
have taken place in the vertebrates.

A. The Grouping and the Increase in Number of Metameres. The domi-
nant process in the evolution of the arthropods is the spasmodic generation of new
groups of terminal metameres, the gradual specialization of each group, and its
more intimate union with the older, more anterior members of the series. The in-
crease in the total number of metameres, from the first three that are characteristic
of the nauplius, to the seven found in the ostracods, eleven in the cladocera, and
the twenty-one or -two so commonly present in the higher forms, goes hand and
hand with the specialization and union of the more anterior groups into an increas-
ingly complex organic unit that in the vertebrate sense may be properly called a
"head."

While this process, in a variable degree, occurs in all arthropods, it is only
in the arachnids that it takes place in the particular manner that is characteristic
of vertebrates. In the more typical representatives of that class, the first fifteen
or sixteen metameres are divided into unlike groups that have a similar sequence,
consist of a similar number of metameres, and present a similar morphological
and physiological specialization of organs to that seen in the corresponding regions
of the vertebrate head.

It is evident, therefore, that the ancestral history of the vertebrate head is con-
tained in the first fifteen or sixteen arachnid metameres, and that in the arachnids

8

OUTLINE OF THE ARACHNID THEORY.

we may study this process of cephalization in detail. At one end of the body we
may observe the birth of new, independent metameres, and at the other the gradual

Pr. C I) i.C.

Ms. C.

Br. C

st.co.

A

Pt.C DiC. Ms C. Mt.c.

E r. C.

st.co.

st g:

Pr.C. DiC. MS.C Mt.C.

B

Br.C.

D

FIG. ,5. Diagrams showing the five characteristic body regions of arthropods, and their progressive concen-
tration to form the head of a vertebrate. The principal points illustrated are: a, The early location of the prin-
cipal functions; b, the concentration of the cardiomeres in the branchial region; c, the enlargement and concentra-
tion of the anterior cephalic neuromeres; d, the change in position of the optic ganglia and oral arches; e, the
closure of the old mouth and the formation of the new one;/, the transfer of locomotor organs from the nieso-
cephalon to the postbranchial metameres. /I and B, Insect; C, arachnid; D, vertebrate.

decline of metamerism, and the incorporation of the old metameres, as specialized
subordinate parts, into a new and more highly organized unit.

B. Origin of the Linear Arrangement of Unlike Cephalic Functions. It is
frequently assumed that the primitive vertebrate head consisted of a considerable

ORIGIN OF THE ARRANGEMENT OF CEPHALIC FUNCTIONS. 9

number of like metameres, each one complete in itself, that is, having all the
organs of an ideal metamere. This assumption is untenable. A considerable
number of cephalic, or anterior metameres, even approximately complete or
perfect, rarely, if ever, occur in any animal outside those pictured in text-book
diagrams. It is certain that no such condition occurs in the arachnids. While
it may be assumed that metameric growth tends to produce a linear series of like
parts, it is clear that it does not do so in reality. The first products of apical
growth must necessarily differ from the last, because different conditions are cre-
ated by apical growth at each successive stage of its progress. The actual result,
therefore, is a linear sequence of unlike structures and functions for a given number
or generation of metameres. This particular sequence becomes unbalanced and
remodelled with the appearance of the next generation. But on the whole a
definite linear succession of unlike organs becomes established at a very early
period in the evolution of segmented animals; and it follows a logical, inherently
necessary order, that is never completely lost or disguised.

With the elongation and increase in size of the primitive trunk the ingestive,
gustatory, locomotor, cardiac, and respiratory functions become more localized,
their position being determined, in part, by the necessary conditions for their
activities, and in part by the historic order in which they became established; for
the location of any new function is limited to the territory that is not already pre-
empted by other organs. For that reason we find that the most essential organs
are the first to develop, and they arise from the oldest parts of the body, that is,
from the more anterior and median neural surface; the" organs of more recent
origin arise on the haemal and caudal sides of the older ones.

The primary sense organs, i.e., the parietal and lateral eyes, the olfactory
organs, and the coordinating centers (forebrain) are already definitely located in the
procephalon of the nauplius, which probably represents, in part, the remnants
of a trochosphere. These organs are, therefore, of very great antiquity. They
retain their original position throughout the entire range of the arthropod-verte-
brate phylum, and by the root-like extension of their nerve fibers establish re-
lations with the new metameres as fast as they are formed.

Hence the primary sense organs and the primary coordinating centers are
located at the anterior end of the body, not, as is frequently asserted, because the
body moves head first, or because of any necessary correlation between the location
of the brain and sense organs (Parker), but because the head is the oldest part of
the animal, and because these particular sense organs and nerve centers were, in
a historic sense, the first ones to be definitely established, taking their origin back
to a period when the primitive head was the whole body.

With the appearance of the first postcephalic metameres, arose the first
gustatory organs, and the first swimming, grasping, crushing, and chewing ap-
pendages. They were necessarily located immediately behind the primitive
head, in the oral region. With the addition of another generation of metameres,
the body became heavier and larger, and the appendages on the new metameres

10

OUTLINE OF THE ARACHNID THEORY.

were used as supplementary swimming, or respiratory appendages, or for crawling
or walking, and the circulatory organs appeared in the haemal region. (Fig. 308.)
The internal organs, such as the stomach, digestive glands, gut pouches,
organs of excretion and generation, establish their relations to the rest of the body,
if at all, through the circulation. They are less dependent on location, or on

ol.o.

FIG. 4. Diagrams showing location of principal organs in Bothriolepis and a primitive vertebrate. A, B, Haemal;
view; C, neural view. .-1, Bothriolepis; B, C, primitive vertebrate.

specialization in form, for effective action, hence they are eventually crowded into
the more posterior metameres, or they atrophy and new ones arise farther back
to take their places. (Figs. 307, 308.)

In the typical arachnids, a definite linear arrangement of unlike functions, in
accordance with the above principles, is established at an early period. The
order is essentially the same as that in the vertebrate head, and is as follows:
olfactory, coordinating, visual, swallowing, gustatory, auditory, locomotor, equili-
bratory, cardiac, and respiratory. (Figs. 5, 57 and 114.)

In the posterior cephalic regions, the digestive, excretory, and genital organs
are closely associated with, or overlap, the branchial and cardiac organs, this
arrangement forming a conspicuous feature in the arachnids. It appears to be
retained, to a large extent, in Bothriolepis and other ostracoderms. (Fig. 5.) In
the vertebrates, this arrangement is further modified by the atrophy of the pre-
branchial locomotor appendages, by the formation of new ones behind the gills,

THE SUBDIVISIONS OF THE HEAD. II

and by the gradual transfer of the digestive and urinogenital system still farther
back into the newly developing trunk.

We need not follow in detail the further progress of these changes in the
higher vertebrates; the atrophy of the gills and the development of the lungs
behind them; the atrophy of head kidneys, and the development of new ones
farther, and then again farther back; and the final shifting of locomotor func-
tions to the pelvic appendages, are all familiar manifestations of the same
process.

Thus the evolution of the arthropod-vertebrate stock consists: i. in the
successive generation of groups of like metameres, each group being from the
beginning somewhat different from the preceding one; 2. in the subsequent en-
largement, diminution, or elimination of segmental organs and the consequent re-
adjustments that follow these changes; the result always leading toward a more
successful linear coordination of unlike organs, the process attaining its highest
expression in man. Hence, broadly speaking, the progress of organic evolution
in segmented animals may be measured by the extent to which the linear coordi-
nation of unlike organs replaces the linear succession of like metameres.

IV. THE SUBDIVISIONS OF THE INCIPIENT VERTEBRATE HEAD IN

THE ARACHNIDS.

The five main divisions of the anterior part of the body in the arachnids are
as follows: (Figs. 3, 5, 14-21.) i. The procephalon, or primitive head, consists
of three pre-oral segments, the principal organs contained in it being the rostrum,
olfactory lobes, cerebral hemispheres, the visual and the olfactory organs. 2.
The dicephalon consists of two or three metameres immediately surrounding the
mouth, and includes the stomodaeum with its appropriate nerve centers, the leg-
jaws, principal gustatory organs, and the anterior part of the endocranium. 3.
The mesocephalon consists of three or four posterior thoracic metameres and in-
cludes the principal locomotor appendages, auditory and excretory organs, and
the posterior part of the endocranium. 4. The metacephalon, or vagus region,
consists of from two to four greatly modified metameres, the appendages being
either very small and standing close to the median line, or absent, or converted
into sense organs. The neuromeres and their ganglia are large, but very compact.
Other components of the metameres are absent, or small and degenerate. The
whole region forms a highly specialized, constricted intermediate zone lying be-
tween the mesocephalon and the next following division. 5. The branch io-
cephalon consists of four or five metameres, in which are located the principal
respiratory organs, branchial cartilages, and the heart.

The Brain. The structure and grouping of the neuromeres reflect the condi-
tions characteristic of these subdivisions of the body, thus laying the foundations
for the subdivisions of the brain in vertebrates. In the latter, the original appearance

12

OUTLINE OF THE ARACHNID THEORY.

of the arachnid brain is modified by the closure of the old mouth, and by the loca-
tion of the optic ganglia over the diencephalic and mesencephalic neuromeres,
instead of over the prosencephalic ones, to which they really belong. (Figs. 46,
47, 57 and 58.)

The Mesoderm. (Fig. 138, .4 and B). The procephalic mesoderm is scanty
and in the early embryonic stages forms a single, thin-walled coelomic chamber.
In the dicephalon and mesocephalon, six pairs of coelomic chambers are formed,
constituting true somites, or head cavities; but segmented lateral plates are con-

oc.

ol.o. pa.ey. Olfactory.
Pro.C. -- - . ^"TTN^ Coord. ra ev
R^oiy^aOk Visual. Py '

ol.o.

V

FIG. 5. Diagrams showing the probable relations between the subdivisions of the head and trunk, and the location
of the principal organs in an insect, merostome and ostracoderm (Bothriolepis) seen from the neural side.

spicuously absent. In the metacephalon and branchiocephalon, distinct somites
and lateral plates are developed in each metamere.

The Middlecord, or Icmmatochord (notochord of vertebrates), extends through
the posterior sections of the head. In the older stages it may terminate in an
enlargement in the mesocephalon, but it never extends beyond the dicephalon,
ending abruptly just behind the stomodseum (infundibulum).

Let us examine these subdivisions of the future head more carefully.

i. The Procephalon.

The procephalon is the primitive head. In the adult arachnids, it is, exter-
nally, an irregular, ill defined area of ectoderm within which lie the rostrum, and
the primitive visual and olfactory organs. (Figs. 140-155, p.c.) In the early
embryonic stages, it is represented by the procephalic lobes, from which the fore-

THE PROCEPHALON.

; Y. mt.

Br.C.

brain with its olfactory lobes, hemispheres, and its appropriate sense organs are
derived. (Figs. 14-21.) The structure of the procephalic lobes, their main
divisions, and the relations of the three sets of primary sense organs to them,
are practically identical throughout the arthropod series. In the higher arachnids,
their structure and mode of development, and that of their associated sense organs,
resembles that of the vertebrates.

In Insects (Acilius), the procephalic lobes consist of three segments, each one
containing a neuromere, an optic ganglion, a segment of the marginal plate, and
two pairs of segmental sense organs, or
ocelli. (Fig. 14.) Three infoldings occur
on the margins of the lobes, between the
optic plate and the optic ganglia, iv 1 3 ,
but they soon close without involving the
marginal sense organs, and without form-
ing a common cerebral vesicle.

/;/ tJie Araclmids (scorpion), the lobes
are at first similar to those of Acilius; later
they are depressed, and a thin marginal
fold, or neural crest, advances over them,
converting the entire forebrain into a hol-
low vesicle that for a long time opens to
the exterior through an anterior neuropore.
(Figs. 15, 16, 18, 46, 47, an. p. and eph.)

Sense Organs. Meantime the anterior
pair of marginal sense organs move forward
and unite in the median line to form the
anlage of the olfactory organ (Limulus).
(Figs. 38, 39, 141, 142, 153, ol.o.) The two
pairs of sense organs on the second seg-
ment (ocellar placodes, parietal eyes) are
ingulfed in the palial overgrowth and carried
to the middle of the roof of the forebrain
vesicle. Here a tubular outgrowth is for-
med, on the end of which the ocellar placodes are located, after the manner
of a typical parietal eye. (Figs. 46, 47, 57, 141, 142, pa.e.) In the arach-
nids, the sense organs of the third segment (lateral eye) lie for a time on the
outer margin of the neural crests, but later they move away from them,
so they are not ingulfed in the palial overgrowth. (Fig. 16, A.I. e.) The lateral
eyes of insects, crustaceans, and arachnids appear to belong to the fourth neuro-
mere (antennal or cheliceral), that is, to the first metamere of the next division
of the head.

Olfactory Lobes, Hemispheres and Optic Ganglia. During the formation of
the palial overgrowth, the first forebrain segment becomes deeply infolded to

FIG. 6. Mesonacis (Olenellus) vermontana
(Hall). Lower Cambrian, showing body regions,
and groups of like metameres, or tagmata.

14 OUTLINE OF THE ARACHNID THEORY.

form the olfactory lobes. The cerebral hemispheres arise from the median part
of the second segment, the optic ganglia of the parietal eye (ganglion habenula),-
from the lateral margin of the second segment, and the ganglion of the lateral
eyes (tectum opticum), from the lateral lobes of the third or fourth segment.
(Figs. 15, 46, 47, ol.l.)

The Rostrum (labrum) in insects arises as a pair of small cephalic appen-
dages, on the very anterior median margin of the cephalic lobes. (Fig. 14.) In
the arachnids it forms an unpaired, immovable process, which in the later stages
lies on the anterior margin of the mouth. (Figs. 15, 17, 18, 43, 47.) It differs
from all other arthropod appendages, in that it receives its nerves from ganglia

FIG. 7. Primitive Crustacea seen from the neural surface, showing various arrangements of the procephalic sense
organs. A, Sida; B, Limnadia larva; C, Branchipus larva.

situated on the median side of each nerve cord, that is, from the stomodaeal ganglia
and commissure, which are situated near the fourth, or first post-oral, segment.
(Figs. 38 and 39, st.g.)

External Boundaries of the Procephalon in the Adult. The margins of the
ectodermic area covering the outer surface of the forebrain, after the palial over-
growth is formed, mark the boundaries of the primitive head. The latter becomes
greatly distorted by the forebrain flexure, which carries the anterior part of the
forebrain round the end of the egg onto the future haemal surface, while the pos-
terior part is drawn a long way backward by the caudad migration of the mouth
and rostrum. (Figs. 3, 17, 43, 44, 46.) It thus happens that the neural surface
of the procephalon is the only one that is actually developed. The haemal surface
is not formed from procephalic tissue, but by the extension of the lateral and
anterior margins of the procephalic lobes around the anterior end of the ovum,

THE DICEPHALON AND MESOCEPHALON. 15

and by their union there with the haemal end of the first thoracic metameres.

jFig. 17-)

In this way the original area of the procephalic ectoderm has been greatly
extended. In the adult Limulus, it is divided into two isolated parts: that which
has been carried onto the haemal surface of the carapace, and that which remains
on the neural surface. (Figs. 141-155.) The latter portion may be approxi-
mately defined as an elongated area, with the olfactory organ at its anterior end
and the apex of the rostrum at its posterior end; it is drawn out laterally by the
migration of the lateral eyes toward the posterior haemal surface. (Fig. 153, pr.c.)

In the scorpion, there is a neural and haemal section of the procephalon, as in
Limulus. (Comp. Figs. 16, 17, 18,43.) The original neural surface of the em-
bryonic procephalon has been doubled over in the adult so that its anterior edge
lies on the haemal surface, directed backward instead of forward. (Figs. 17, 22.)

FIG. 8. Primitive crustaceans (Cladocera).
A, Neural surface; B, haemal.

FIG. 9. Same in side view and in median
section.

It is important to bear these facts in mind, since where these changes have
taken place, the linear arrangement of the segmental sense organs appears to be
the reverse of what it is when the procephalon remains largely on the neural
surface, as it does in many phyllopods and vertebrates. (Figs. 7, 8, 9, 34.)

2-3. The Dicephalon and the Mesocephalon.

The dicephalon and the mesocephalon include the first six or seven post-oral
metameres, frequently spoken of as the thorax. It is generally divided into two
regions. The anterior one, the dicephalon, consists of two or three circum-oral
metameres whose appendages may be smaller than the others, and specially
modified to serve as leg-jaws for testing, holding, tearing, or crushing food, and
conveying it to the mouth. It includes the stomodaeum and the stomodaeal
ganglia, the latter being intimately associated with the gustatory and swallowing
reflexes. The posterior division, or mesocephalon, comprises three or four well-
developed metameres whose appendages serve for walking or swimming.

i6

OUTLINE OF THE ARACHNID THEORY.

In the insects, the first four metameres fuse with each other, and with the
procephalon, to form the so-called "head," the last three metameres usually
remaining separate. (Figs. 3, A, 5, A.)

In phyllopods (Branchipus) the first two metameres, and possibly an evanes-
cent third, or premandibular, fuse with each other and with the procephalon.
The remaining three metameres, the mandibular and two maxillary, fuse with
each other, forming a group by themselves distinct from the anterior division.

In arachnids, such as the scorpions, spiders, trilobites, and merostomes,
all six thoracic metameres unite with one another and with the procephalon to
form the cephalothorax, leaving on the hsemal side little or no indication of the
larger divisions, or of the more primitive division into metameres.

On the neural side, the metameric structure is
always retained. The reduction in size, and the
modification of the first two or three pairs of appen-
dages for feeding purposes, are usually clearly indica-
ted, the last three or four pairs serving mainly for
locomotion.

In Limulus, the subdivision of the thorax into
an anterior and a posterior division at first sight does
not appear to exist; but I have shown that in abnor-
mal embryos the first two or three thoracic metameres
act as a unit, in that they frequently separate from
the posterior ones opposite the large thoracic sense
organs, or they fuse with each other, or disappear
entirely, or otherwise manifest a distinct independence
in their development. The large thoracic placodes,
the forerunners of the auditory placodes of verte-
brates, mark this latent cleavage line between the
FIG. io. Mesothyra (after Haii and group of oral metameres of the diaccphalon, and those

Clark). Upper Devonian. t_ i ^i /-r*'

belonging to the mesocephalon. (Figs. 141, 142,
184-188.)

The endocranium arose primarily in association with the dicephalic met-
ameres, but in the higher forms takes its origin from the mesocephalic meta-
meres also. With the concentration of all the cranial neuromeres, the endo-
cranium embraces, or underlies all of them except the more posterior ones of
the branchiocephalon.

Oral Arches. The basal joints of the thoracic appendages, especially in

the arachnids, are greatly expanded where they join the body, forming oblong

arches to which the slender, more movable part of the appendage is attached. In

the arachnids, these basal arches may be located some distance from the median

.line, on the lateral wall of the head. At least four or five of these anterior thoracic

THE DICEPHALON AND MICROCEPHALON.

arches persist as the circumoral, visceral arches of vertebrates, that is, as the
pre-maxillary, maxillary, mandibular, and hyoid arches, and possibly the first
gill arch. (Figs. 32-34, 160-172.)

Taste Buds, Slime Buds, ami Cranial Ganglia. In the typical appendicular
arches of arachnids, there is a lobe on the median or neural side that forms the
mandibular or coxal spurs, and in which are located important groups of sense
organs, i.e., gustatory buds and slime buds. They are the forerunners of the

\ /' >

1 1 f

o -y

if

D :ljg

\

FIG. ii. FIG. 12.

FIG. ii. Diagrams of marine arachnids, to illustrate the relations of their organs to those in the ostracoderms.
FIG. 12. C, hypothetical form, intermediate between a merostome and an ostracoderm (Cephalaspis) ; D, is
an accurate restoration of a small cephalaspid (sp. nov. ?) from Scaumenac Bay, P. Q., except the external gill,
r.v. g. which are hypothetical.

" epibranchial organs," "lateral line organs," and "gustatory organs" of verte-
brates. At an early embryonic period, in the wide zone between the nerve cord
and the coxal and gustatory spurs, and in close connection with the latter, immense
oblong ganglia (pedal ganglia) are developed from thickenings of the overlying
ectoderm. (Figs. 36-39, 134-137.) These ganglia arise independently of the
medullary plates. Later, they unite the proximal end of the pedal nerve with the
corresponding neuromere. They are the forerunners of the cranial ganglia of
vertebrates.

Segmented Sense Organs. In the scorpion, each appendicular arch, except
the first, has, on its lateral margin, close to the base of the coxa, two sensory cups,
in form and in minute structure very similar to the conspicuous pits on the outer
surface of the neuromeres. (Figs. 15-16, 74, s.so.) All these pits quickly lose
their sensory character and later apparently disappear or are converted into gan-
glion cells.

i8

OUTLINE OF THE ARACHNID THEORY.

Similar segmental sense organs are seen in Limulus, but farther removed from
the bases of the appendages. The one that develops into the so-called "dorsal
organ" (auditory pit of vertebrates) lies opposite the fourth appendage. (Figs.
140-153, s.o. 4 ) Later it becomes greatly enlarged and is a conspicuous feature
on the haemal margin of the thorax till after the last moult of the trilobite stage.
At the height of its development, it is a disc-shaped thickening, slightly pigmented

and sensory in appearance. (Fig. 131.)
The four remaining pits (Fig. 140) are
very faint and transitory, although in the
corresponding regions of the adult, there
are patches, or knobs of skin that are highly
sensitive and richly supplied with nerves.
The thoracic segmental sense organs of
Limulus and the scorpion lie nearly in line
with the cephalic sense organs, and are
probably serially homologous with them.

The Diencephalon and the Mesencep-
halon. We may recognize two groups of
thoracic neuromeres, the diencephalon and
mesencephalon, approximately correspond-
ing with the external divisions of the thorax.
The diencephalon, or tween-brain,
consists of the first one, or two or three,
neuromeres that surround the oesophagus.
It includes the large, lateral stomodaeal
ganglia that are attached to the median wall
of the cheliceral neuromere, but which arise
as thickenings, or evaginations, from the
side walls of the oesophagus. These neu-
romeres contain the swallowing center and
an important center for all the taste organs
of the more posterior thoracic appendages.
(Fig. 114.)

The enlargement and closer union of the thoracic neuromeres, and the back-
ward overgrowth of the rostrum and the optic ganglia, ultimately lead to the
closure of the mouth. After it closes, the inner end of the stomodaeum persists
in vertebrates as the epithelium of the saccus vasculosus, the passageway between
the circum-oesophageal neuromeres becomes the infundibulum, and the stomo-
daeal ganglia, arising from its deeper side walls, the lobi inferiori. (Figs. 43 and
44.) The last position occupied by the arachnid mouth may be identified in

FIG. 13. Bunodes lunula. Restoration from
numerous specimen in the author's collection
obtained from the island of Oesel, Russia. Photo-
graph of enlarged plaster model by the author.
X about 2.

THE METACEPHALON. 1 9

vertebrates, as the opening behind the cerebellum, now closed by the choroid
plexus of the fourth ventricle. (Figs. 3, 43, 44, 46, 58.)

The mesencephalon consists of the last three or four thoracic neuromeres;
they are usually conspicuous for their distinctness, great breadth and volume,
and for the large size of their ganglia. In the vertebrates, they form the pos-
terior portion of the crura cerebri, and are still further accentuated as one of the
principal divisions of the brain, by the migration of the optic ganglia of the lateral
eyes backward and upward till they come to overlie them as the tectum opticum.
The parietal eye ganglia overlie the diencephalon as the ganglia habenulae. (Figs.

43>44, 57,58.)

The Suprastomodceal Commissure and the Cerebellum. In all arthropods,

the lateral stomodaeal ganglia are united by a large commissure that forms a
prominent arch over the anterior or neural surface of the stomodaeum. This com-
missure is one of the most conspicuous and constant landmarks in the arthropod
brain. (Fig. 3, st.co.) In the insects, it contains a large, median mass of gan-
glion cells, arising as an evagination, or as a thickening, in the anterior, median
wall of the stomodaeum, close to its external opening. (Fig. 3, a.) The projecting
arch of the commissure becomes crowded backward by the backward migration
of the mouth and rostrum, and by the increasing size of the lateral eye ganglia,
forming in vertebrates the rudiment of the cerebellum. (Figs. 3, D, and 46.)

Thus the median stomodaeal ganglion of arthropods and the cerebellum of
vertebrates are the only brain structures that may be said to arise originally in the
median line above the neural surface of the brain; the parietal eyes, the ganglia
habenulae, and the optic lobes being originally paired structures arising from the
lateral margins of the medullary plate.

4. The Metacephalon, or Vagus Region.

The metacephalon, or vagus region, forms a remarkable intermediate zone
between the mesocephalon and the branchiocephalon. It consists of from one
to four metameres that usually atrophy, or fuse with one another at an early
period, leaving little or no external trace of their existence in the adult. Their
feeble development is the principal cause of the sharp constriction which, in many
insects and arachnids, separates the thorax from the abdomen. (Figs. 3, 6, 14,
15, 16,46,47, 57, M. c. or?'g.'~ 4 .)

The vagus appendages rarely serve as locomotor or respiratory organs. They
show a marked tendency to become unpaired; they may dwindle into insignifi-
cance, or they may be retained as highly specialized sense organs (chilaria and
metastoma of merostomes; genital papillae and pectens of scorpions).

The vagus neuromeres, on the other hand, are well-developed, but they fuse
with one another so quickly that it is very difficult to distinguish their boundaries
after the early embryonic stages. Their motor elements are greatly reduced and
the sensory ones correspondingly enlarged, owing to the reduction of the corre-

2O

OUTLINE OF THE ARACHNID THEORY.

spending trunk muscles and the absence of appendages, or their conversion into
sense organs. In them is located an important decussation of the longitudinal
tracts passing from the cord to the brain, and vice versa; and the vagus neuro-
meres are the most anterior ones in which such a crossing takes place. (Figs.
65, 66, 114, v.dec.}

The vaults nerves have special relations with the heart, intestine, and integu-
ment. They are the only segmental nerves that are persistently directed back-
ward into foreign territory, a result that is due in part to the forward concentration
of the vagus neuromeres, and in part to the backward growth of the nerves and
the atrophy of their native metameres. (Figs. 38, 42, 57, 70, 71.)

FIG. 14. Diagram of an insect embryo (Acilius) in
mercator projection.

FIG. 15. Scorpion embryos in mercator
projection.

The interpolation of the vagus neuromeres between the mesencephalon and
the branchiencephalon is a very important and striking feature in the morphology
of the arachnids. They form a compact group, or distinct brain region, which in
its anatomical and physiological characters, and in the distribution of its nerves,
is very similar to the vagus region of vertebrates.

5. The Branchiocephalon.

This group of metameres, four or five in number, is the least specialized of
any so far considered. The appendages may be well-developed (Limulus and
many Crustacea) or they may be rudimentary. In the higher forms, this region
is chiefly notable as the site of the respiratory organs, i.e., the trachea*, gills, lung-
books, and heart. (Fig. 3, br.c.)

The niesodenn is complete, each metamere containing well developed somites

THE ENDOCRANIUM. 21

and lateral plates. (Figs. 16, 142.) The most characteristic organs found in
these metameres, such as the cartilaginous branchial bars and the segments of
the heart, arise from the mesoderm.

The neuromeres usually remain separate, but there is a tendency for the
anterior ones to move forward and join the vagus group. In vertebrates, the
entire group has joined the vagus neuromeres, forming the most posterior part of
the medulla. (Fig. 58.)

Nerves. In the arachnids, the great complex of vagus and branchial nerves
has already made notable progress in that separation of components from the
primary segmental nerves, and in their regrouping into compound nerves whose
constituent parts have a common function, that is so characteristic of vertebrates.

We may recognize, for example, the beginning of the lateral line nerve in
the combined sensory components of the first three vagal appendages of the
scorpion. In Limulus, the primitive condition of the vertebrate cardiac nerves
is seen in the eight pairs of segmental cardiac nerves that arise from the vagal and
branchial neuromeres. (Figs. 59, 78, c 7 I4 .) The visceral arch nerves are repre-
sented by the branchial nerves, and the hypoglossal, by the combined group of
motor components that supply the great, branchio-thoracic muscles. (Fig. 77.)
The intestinal nerves are also indicated; i l ~

It is only necessary to unite the vagal and branchial neuromeres into a
more compact mass, and to complete the union of the sensory, branchial, hypo-
branchial, cardiac, and intestinal components into compound nerves, to realize
the characteristic condition so familiar in vertebrates. (Compare Figs. 5 7 and 58.)

It will be observed that the similarity exists, not only in the union of the
originally separate components into the same physiological groups, but that the
number of neuromeres and components is approximately the same; that their
topographical position is the same; and that the general course and distribution
of the resulting nerves is the same.

The Endocranium.

All the higher arachnids are provided with a cartilaginous endocranium that
is the forerunner of the primordial cranium of vertebrates. It may be traced
back to such primitive arthropods as Branchipus, Apus, and other phyllopods. In
Branchipus, it is a small plate of cartilage, lying on the haemal side of the mesen-
cephalon, and serving for the attachment of the mandibular muscles.

In the higher arachnids, it is more voluminous, serving mainly for the at-
tachment of the leg and jaw muscles, and for the great longitudinal muscles that
move the cephalothorax on the branchial section of the body. Its structure is
similar to that of the primordial cranium of vertebrates, and it has the same topo-
graphical relation to the brain and to the alimentary canal. The rudiments of
the following parts may be recognized: occipital ring, trabeculas, pituitary fora-
men, and palato-pterygoid arch. (Figs. 209-220.)

22

OUTLINE OF THE ARACHNID THEORY.

The Mesoderm.

Origin of the mesoderm. To understand the peculiarities of the cephalic
mesoderm, we must consider its origin as a whole.

In Limulus, the mesoderm arises in part from the telopore, a shallow, terminal
depression overlying a confused mass of proliferating nuclei destined to form meso-
derm, yolk cells, and endoderm. (Figs. 128 and 140.)

As the embryo elongates, the depression maintains its terminal position,
changing to a longitudinal groove, and finally taking the form of a typical primi-

FIG. 16. Scorpion embryos in mercator projection.

tive streak. (Figs. 129, 130, 140, f.p.}. From the primitive streak, a sheet of
mesoderm extends forward and laterally, finally breaking up into somites and
lateral plates.

In the abdominal, or branchiocephalic and vagal regions, the somites are
hollow, contain true coelomic cavities, and are quite distinct from the overlying
ectoderm. The corresponding lateral plates are sharply segmented, and they
are united, for a short period, with the overlying ectoderm. They appear to be
formed from the ectoderm by a local, inward proliferation, that takes place, not
only in the region of the germ wall, but along the lines that mark the anterior
and posterior boundaries of the lateral plates. (Fig. 128, a.)

On the peripheral margins of the expanding mesodermic area, no segmenta-

THE MESODERM. 23

tion of any kind is visible. Ectoderm, mesoderm, and yolk cells form a common,
thickened rim, or germ wall, similar in general appearance to the early stages of
the primitive streak, and extending along the entire lateral margins of the germinal
aiea. (Figs. 140-142.)

The post-oral mesoderm therefore arises from three distinct sources. The
axial portion, consisting of the double line of mesoblastic somites, arises from the
primitive streak; it represents the trail of mesoderm cells left behind as the telo-
blasts of the primitive streak migrate backward. The greater part of the lateral
plate mesoderm is formed from the proliferating cells of the germ wall, as it
spreads over the surface of the yolk in a lateral direction. But on the median
side of the germ wall, the definitive ectoderm continues to proliferate inward for
a considerable distance along the lines that separate the lateral plates. The cells
thus produced form a part of the lateral plates, and the proliferating lines break
the lateral sheet of mesoderm into distinct segments.

The dicephalic and mesocephalic (thoracic) mesoderm of arachnids presents
a most important modification. It forms at first, a well defined band on either
side of the nerve cord. Each band then becomes divided into distinct ccelomic
chambers or somites; but segmented lateral plates are absent, the mesoderm of
that region consisting of scattered cells that are not visible in surface views.
(Figs. 15, 1 6, 19-21.) From the thoracic somites, or head cavities, arise the
muscles of the appendages, the cartilaginous cranium, and the secreting cells of
the coxal gland, or head kidney.

The procephalic mesoderm is scanty and unsegmented, forming a thin
walled, unpaired ccelomic vesicle that breaks down into scattered cells. The
procephalic mesoderm appears to arise from the primitive cumulus before apical
growth begins.

Comparison. With the progress of cephalization in the arthropods, there
has been, therefore, a steady decrease in the volume of mesodermic structures.
In the higher arachnids, mesoderm is almost absent in the procephalon, and the
lateral plates are absent in the dicephalic and mesocephalic regions. The result,
or cause, if you will, is the absence of the thoracic sections of the heart and of the
longitudinal, intersegmental muscles; the shortened thoracic tergites then fuse
with one another and with the procephalon to form a continuous unsegmented
shield, or cephalic buckler.

In the vertebrates, the decrease in volume of the cephalic mesoderm is carried
still further, affecting the anterior head regions, as well as the more posterior
ones, that in the arthropods are usually well equipped with mesodermic structures.
This decrease is due chiefly to the progressive atrophy, or fusion, or condensation
of what were originally freely movable parts, and the consequent reduction in the
number and volume of cranial muscles. For example, practically all the haemal,
longitudinal, intersegmental muscles disappear with the fusion of the branchial
region with the head. The several pairs of originally separate leg-jaws fuse into
unpaired oral arches, only one of which is freely movable. The mesocephalic

24 OUTLINE OF THE ARACHNID THEORY.

locomotor appendages and their voluminous muscles disappear, and also numerous
cndocranial muscles, owing to the union of the endocranium with the dermal
skeleton. Finally the branchial appendages lose a part of their muscles in their
conversion into lung-book-like gill pouches.

This progressive degeneration of the cephalic mesoderm, from before back-
ward, has been, therefore, an ever present factor, exercising a persistent and power-
ful influence over the form of the head and the structure of the brain thoroughout
the whole arthropod- vertebrate phylum.

The Vascular Area and Concrescence.

Vascular Area. The mode of growth of the extra embryonic area, the
concrescence of the germ wall, and the character of the mesoderm in the various
regions is shown in Fig. 138.

in pi. mxl

FIG. i". A-C, Scorpion embryos in side view, semi-diagrammatic. The thoracic appendages are removed in B
and C. D, Diagram indicating relations of the cephalic organs in arachnids to those in vertebrates.

The margin of the germinal area belonging to the thoracic metameres is
greatly thickened, forming large masses of spherical or oval cells containing a
small excentric nucleus, and a brilliantly refractive, colorless thread, usually
coiled with great regularity in the long axis of the cell. (Fig. 131.)

Some of these cells are ultimately converted into muscles, others remain as
free amoeboid cells, and in the adult may be found in great numbers scattered
among the connective tissue lacuna?, in the anterior part of the cephalothorax.

Whether the degenerating muscle cells of the cephalothorax are to be re-
garded as true blood corpuscles or not is doubtful; but it is evident that owing
to the increase in size of the yolk sphere, there is already established, in the higher

THE CEPHALIC NAVEL.

arachnids, an extra embryonic germinal area, and that certain parts of this area
may be regarded as the beginning of an extra embryonic vascular area. The
peripheral ends of the vagus and abdominal lateral plates give rise to the heart,
pericardium, longitudinal haemal muscles, and to blood corpuscles.

Concrescence. As the lateral margins of the germinal area grow faster
than the median portion, concrescence of the germinal wall will ultimately occur
in the precephalic and post caudal regions. In very large yolked eggs, pre-
cephalic concrescence will tend to bring the cardiomeres into conjunction, either
in front of, or underneath the procephalon, that is in their characteristic position
in vertebrates. (Figs. 17-23, 138, 140, 141.)

The post caudal concrescence will tend to unite the posterior margins of
the germ wall behind the real apex of the body, giving rise to the various
phenomena in vertebrates that have been confused with "gastrulation," " con-
crescence of the lips of the blastopore," and with apical growth.

The New Mouth, Cephalic Navel, or Hagmastoma.

In the arachnids, there is a special area on the anterior haemal surface,
just in front of the procephalon, that we shall call the cephalic navel. It prob-
ably occurs in all arthropods, under various modifications, as the so-called dorsal
organ. It is primarily a thickening of the haemal blastoderm, entirely outside, or
beyond the germinal area. In the arach-
nids, the thickened blastoderm gives rise
to an immense mass of proliferating cells
that are ultimately invaginated into the
yolk, where they degenerate and are absor-
bed. This infolded area of degenerating
cells forms the central point toward which
all the surrounding organs converge; the ,>,-
germ wall, with its appropriate structures
advancing toward its sides and posterior
margin, and the procephalon toward the
anterior one. There is thus formed, either

in front Of, Or below, the procephalon, a FIG. iS Anterior end of an embryo scorpion,

i i-i 11 ,i showing forebrain completely covered by the palial

vortex center toward which all the sur- fold

rounding organs move, and into which is

infolded the remnants of the haemal blastoderm (Figs. 23, 127, 138, 139, c.ni\]

The opening between the enteron and the exterior, thus virtually established on

the haemal surface, finally closes in the arthropods, but in the vertebrates a

permanent opening is established at this point, that becomes the new mouth, or

the hoemastoma.

Although the cephalic navel ultimately closes in the arthropods, prophetic
signs of its future function are not lacking, for on the site where it is formed,

rnKSy-'T-j'tiyr^ ^^rB>

-Bf&fti&K^Zysgtt.'.
<*?:- ': r -^a"-4-r.-.v

^m:m&* 1

^*&v^

3^--

s&

26 OUTLINE OF THE ARACHNID THEORY.

there are frequently developed adhesive discs (phyllopods), or root-like out-
growths (cirripeds, copepods) that serve as organs of attachment, or for the ab-
sorption of nutriment.

The cephalic navel of arthropods may be regarded as one of the inevitable
products of apical growth on a spherical yolk surface, just as the belly navel of
vertebrates is a product of the peculiar method of closing up the haemal surface.
The center, around which the converging lips of the cephalic navel are formed,
is the degenerating area of haemal blastoderm, often called the dorsal organ.

The Closure of the Old Mouth or Neostoma.

In the arachnids, there is a progressive enlargement and fusion of the an-
terior cephalic neuromeres, that gradually leads toward the narrowing of the
passageway for the stomodseum, and ultimately to the closing of the mouth. The
backward growth of the rostrum and the transfer of the optic ganglia to the
region overlying the mouth, due apparently to remote, but persistent and cumu-
lative causes, are contributory factors in bringing about this result.

These conditions at first lead to a profound modification of the mode of life,
making a liquid, or finely divided diet a necessity, and ultimately to the utilization
of the cephalic navel as a new entrance to the alimentary canal.

Conclusion.

In the arachnids, the body is built up by successive generations of new groups
of metameres, or tagmata, at definite historic periods in the evolution of the phylum.

The process of cephalizing the anterior regions of the body consists in the
gradual and extensive elimination of motor elements and the establishment of a
definite sequence of functions and organs, according to an inherently necessary
order.

The first five tagmata embrace the first sixteen metameres and lay the founda-
tions for the head in vertebrates. Each tagma is characterized by a special number
of metameres, by peculiarities in the number and structure of its neuromeres,
sense organs, ganglia, nerves, mesoderm and endo-skeleton, and by their sequence
and mode of growth, that are in essential agreement with those in the corre-
sponding divisions of the vertebrate head.

The arachnid body consists of metameres added to the primitive head, which
represents the remnants of the coelenterate body. The greater part of the arachnid
body and its primitive head forms the vertebrate head. Nearly all the vertebrate
body consists of a new generation of metameres, not represented in arachnids.

The conditions created by apical growth, by cephalization, and by the increase
in the volume of the yolk sphere, lead to the closure of the old mouth, and to the
formation of a new one on the haemal surface, the primitive dorsal organ forming
the starting point for the cephalic navel, that ultimately becomes the new mouth.

CHAPTER II.

OUTLINE OF ARACHNID THEORY; CONTINUED.

I. COMPARISON OF ADULT ARTHROPODS WITH ADULT VERTEBRATES.

The preceding analyses have shown, that beneath a heavy disguise of con-
tour and surface detail, the structural plan of an arachnid and of a primitive
vertebrate is after all the same. Let us now consider several types of adult
arthropods and see how they compare with vertebrates.

I. Orientation of Neural and Haemal Surfaces.

It will be seen that although the location of the eyes and the shape of the
body indicate the usual position of the animal during locomotion, they afford no
certain evidence as to which is the neural and which the haemal surface, for the
pattern formed by the sense organs on the neural surface of the cephalothorax
of some arthropods may be very similar to that on the haemal surface in others,
and this fact must be borne in mind when comparing them either with ostraco-
derms or with true vertebrates.

In the phyllopods, and in many other Crustacea that swim neural side up by
means of oar-like cephalic appendages, the center of gravity usually lies below
the attachments of the swimming appendages. In such cases the parietal ocelli
and lateral eyes lie near their original embryonic position, on the upper, or neural
surface, as they do in vertebrates. (Figs. 7, 9, 244, 247 and 260.)

Where locomotion is effected either side up, as in Limulus, the prevalent mode
of life may be indicated by the position of the eyes and legs, and by the shape of
the body. Limulus, for example, uses its sixth pair of legs as pushing poles,
as it moves over soft bottoms, or crawls along partly buried in sand, with
little more than the median and lateral eyes exposed. During the adult stage,
however, it frequently swims, neural side up, for considerable periods, and per-
sistently does so in the larval or trilobite stages, the sloping, anterior margin of
the shield, like a well turned bow of a boat, holding the head up and the body
properly balanced. The same modes of life and dual methods of locomotion
undoubtedly occurred in many trilobites and merostomes, and when the free
swimming life predominates, one or more pairs of appendages are enormously
enlarged to form heavy, oaf-like swimming appendages. The lateral eyes may
then lie well forward on the head, between the neural and the haemal surfaces

(Fig- 5)-

27

28

OUTLINE OF THE ARACHNID THEORY.

In Bothriolepis (Figs. 247 and 248), we have a similarly shaped body, with
similar oar-like cephalic appendages, and from the various positions in which
they are found in the deposits, there can be no doubt that they crawled, partly
buried in soft mud, with the ocular, or neural side up, but swam with the neural
side dow T n, the center of gravity lying below the attachment of the arms toward
the bottom of the boat-shaped head. The same was probably true of Cyathaspis
(Fig. 244), Tremataspis (Fig. 236), Pteraspis, and probably to a less extent of
Cephalaspis (Fig. 232).

" ' T - ;

mp

FIG. 19. Embryos of a spider in side view.

The prevailing position among vertebrates is unquestionably with the neural
side uppermost, although, as we have just seen, the most primitive vertebrates
may move about with either side up. It is by no means true that the prevailing
position of the invertebrates is with the neural side down. In many annelids,
there appears to IK- no fixed position for the neural and haemal surfaces. In
most crawling arthropods (insects and spiders), the neural side is directed down-
ward, but probably in the vast majority of phyllopods, cladocera, copepods,
merostomes, and trilobites, and in the larvae of decapods and cirripeds, the pre-
vailing position, when swimming freely, is with the neural side uppermost, and
that is the approximate position in practically all the adult cirripeds.

BUNODES. 29

It is thus clear that the position of the animal during locomotion has no
morphological value whatever. It is necessary to emphasize this point, because
the ancient superstition, to the effect that it is always the same surface of a verte-
brate or of an invertebrate that points heavenward, or that it is the baptismal name
of a surface that determines its identity, is still deeply rooted in the minds of an
incredible number of zoologists.

II. COMPARISON OF ADULT ARTHROPODS AND VERTEBRATES.

Bunodes. The form that perhaps most nearly realizes the generalized
arachnid type we have tried to portray is Bunodes, a small, silurian merostome
from the island of Oesel, Russia. (Fig. 13.) In my visit to this island in 1901,
a large collection of these forms was obtained from which I have made a large
scale model, showing in detail the essential features of the haemal surface. This
animal is remarkable for the fact that it has no recognizable exoskeleton. The
fossils consist of well denned, but very thin, carbonaceous films, in a fine chalky
matrix. They are found side by side with small eurypterids that are covered
with a delicate chitenous membrane, still retaining apparently its original, chem-
ical structure, and close to fragments of Tremataspis, consisting of perfectly
preserved, calcareous, dermal plates. It is therefore probable that Bunodes had
neither a chitenous nor a calcareous exoskeleton.

The general form of the body is intermediate between that of Limulus and
that of a trilobite, or of a typical merostome. All the five head divisions, except
the diacephalon, are clearly indicated, and they are surprisingly like those in
larval Limuli (Fig. 152). There is a distinct procephalon, six thoracic, two vagal
(chelarial and opercular?), and five branchial metameres. The most remarkable
feature is a pair of short, slender antennas clearly seen in one specimen.

For the sake of exposition we may picture to ourselves the manner in which
an adult arachnid, or other arthropod, might be moulded into a vertebrate, although
it is manifestly impossible for any adult animal to be converted into another.
We may start with a form like Limulus, or Bunodes, or an eurypterid, or with an
adult phyllopod, like Branchipus, or a cladoceran, or cirriped.

In practically all these animals, extensive lateral, or pleural folds develop
on the sides of the cephalothorax, that either extend in a nearly horizontal plane,
to form a broad, shield-shaped cephalothorax, with backwardly directed cornua,
as in the marine arachnids (Fig. 155), or the folds may be directed toward the
neural surface, forming, in extreme cases, the bivalve shield, or mantle, of phyllo-
pods (Fig. 273), ostracoda (Fig. 307), cladocera (Figs. 8 and 9), and cirripeds
(Fig. 275). It may enclose the head, or the entire body, in a large peribranchial,
or atrial chamber, which contains, or into which opens, the nutrient, excretory,
respiratory, and genital organs. Another characteristic feature is the often
enormous labrum, or rostrum, that shows a persistent tendency to migrate back-
ward, forming an overhanging lid to the mouth (Fig. 7). The rostrum and the

OUTLINE OF THE ARACHNID THEORY.

St.

- Prc

*LcUI

^

fe-

p-p' c

FIG. 20. Spider embryos in mercator projection. Camera-outlines

FIG. 21. Spider embryo in meicator projection. Camera-outlines.

COMPARISON OF ADULT ARTHROPODS AND VERTEBRATES. 3!

atrial folds, together with the branchial and oral appendages, thus tend to enclose
the mouth in an ever deepening chamber. When this condition approaches its
extreme development cirripeds, cladocera, etc. (Figs. 273-275) the mouth be-
comes very inaccessible, and food can only reach it in a finely divided condition,
carried there by roundabout ways, in the currents of water produced by the swim-
ming, the oral, or the branchial appendages. Or the mouth may become com-
pletely closed, as in many dwarf, or parasitic cirripeds. (Figs. 280 and 281.)
Under these conditions the form and general appearance of a phyllopod-like
arthropod, with its large branchial, or atrial, chamber, and its oar-like cephalic
appendages, approaches that of some simple ostracoderms, like Cyathaspis, or
Pteraspis. (Comp. Figs. 176 and 244.)

If we compare an adult Limulus viewed from the neural surface, with Cephal-
aspis seen from the same surface (Figs, n and 12), it will be seen that such an
arachnid could be made into an ostracoderm by the union and backward growth

ro.

FIG. 22. Young spider, showing the procephalon, transferred from the neural to the haemal surface, and the loca-
tion of the thoracic appendages, mouth, heart, and respiratory organs. Thoracic appendages removed.

of the anterior margins of the cephalothorax, thus enclosing the mouth and
appendages in a large branchial chamber, like that in some phyllopods (Figs. 9-
10). The eyes and olfactory organs could remain in their original embryonic
position near the center of the head; the olfactory organs in front, the three
parietal ocelli in the center, and the lateral eyes on either side. (Fig. 12.) The
enlarged coxal joints of the anterior thoracic appendages, extending on to the
haemal surface, would form the visceral arches about the mouth, the free append-
ages forming the external gills and the jointed, oar-like arms. A varying number
of infolded branchial appendages, similar to the lung books of arachnids, would
initiate the true gill pouches, and finally the elongated post-abdomen would form
the beginning of the flexible trunk, with its pleural or lateral folds, from which
the post-cephalic appendages later arise.

From the cephalaspids we may easily derive the remaining ostracoderms.
In Bothriolepis, the old cephalo-thoracic portion remains comparatively small,
while the abdominal buckler has become greatly enlarged and closed on its neural

32 OUTLINE OF THE ARACHNID THEORY.

surface to form a true atrial chamber that encloses the gills and cloaca. (Fig. 5.)
The jointed, oar-like appendages, which belong to one of the posterior meso-
cephalic segments, are attached to the angle of the cornua, that are here very small
compared with those of Cephalaspis.

Bothriolepis retains the hinge-like joint in the vagus region, which is such
a prominent feature in trilobites, merostomes, and other arachnids. The same
joint is a conspicuous feature in Dinichthyes, Coccosteus, etc., a group of primitive,
fish-like animals that probably unite the typical ostracoderms with the true
vertebrates. (Fig. 250.)

In Tremataspis (Figs. 236 and 237), there are probably several pairs of
small cephalic appendages, comparable with external gills, that protruded from
the openings on the oral surface; the larger, oar-like pair, at the beginning of the
series, being especially noteworthy. The exhalent branchial currents and the
excretory products, no doubt pass out of the posterior end of the atrial chamber,
as in Bothriolepis.

The assumed changes above described affect, in the main, the external form
of the animal. The internal structure might remain essentially as it now is in
arachnids, and, except for certain organs, it would harmonize with the structural
plan in vertebrates. For example, it would be necessary, in order to complete the
transformation of an arachnid into a vertebrate, to close the old mouth and to
connect the new one and the gill pouches with the enteron. The factors involved
in these changes are described elsewhere. For the present, it is enough to recog-
nize the fact that these events have taken place, in some way and at some time,
whatever the method or cause may have been.

On the other hand it will be observed that many internal organs, that we are
accustomed to consider as characteristic of vertebrates, are already present in
the arachnids, in their proper position and relations, and merely have to be en-
larged or improved, or even left as they are, to agree with those in vertebrates or
ostracoderms.

For example, there is already present in Limulus, in addition to the brain,
sense organs, and other structures that have been considered, a head kidney,
cox. o., heart, //, aortic arches, a. 0., and cardinal sinuses, card. s. foreshadowing
those in vertebrates. (Fig. 2.) There are infolded gill sacs and gut pouches in
arachnids, that are precursors of the gill clefts, thyroids, and other enteric diver-
ticula in vertebrates. (Figs. 179-182.) There is in Limulus and other arachnids
a large cartilagenous endocranium and gill bars, so similar in form, location, and
histological structure to those of vertebrates, that they might readily pass for
those of some primitive, unknown member of that class. (Figs. 210-220.) There
is, in Limulus, an internal, dermal skeleton, made of chiten, it is true, but never-
theless consisting of a network of trabecuke, cancelke, Haversian canals, lacuna. 4 ,
and canaliculae, so much like those of certain ostracoderms (Pteraspis) that it is
doubtful whether fossilized fragments of one skeleton could be distinguished from
those of the other, if their real origin was unknown. (Figs. 196-207.) And

COMPARISON OF ARTHROPOD AND VERTEBRATE EMBRYOS. 33

finally, there is present in all arthropods that have been carefully studied in regard
to this organ, a median, subneural cord agreeing in position, development, and in
some cases in function, with the notochord of vertebrates. (Figs. 221-231.)

It is evident, therefore, that the resemblance in form and general appearance
between the ostracoderms and the marine arachnids is not a fanciful one, to be
classed as a meaningless coincidence, or as due to mimicry, to parallelism, or to
a particular mode of life. The resemblance is real, and pervades the whole
organism, and can be satisfactorily explained only on the assumption that there
is a close genetic relationship between the two classes.

III. COMPARISON OF ARTHROPOD AND OF VERTEBRATE EMBRYOS.

A comparison of adult arachnids with adult vertebrates helps us to see the
morphological relations that exist between the two types, but it cannot tell us how
one arose from the other. That is the function of comparative embryology, for
the rise of one great class from another takes place during the malleable embry-
onic periods, when transitional stages are created by a slow yielding to the impact
of successive readjustments between organs developing under unequal and un-
stable conditions.

Hence the supreme test of any broad theory of phylogeny is its ability to pre-
sent an unbroken series of embryonic stages, naturally or inevitably leading from
one type to the other, and to point out the efficient causes for them. This embryonic
series should include, at the proper period, the characteristic anatomical structures
of both types. The established direction of growth shown by various systems of
organs, and the general conditions that control growth in the lower type, should
persist in the higher, supplying a past cause for the creation of the fundamental
features of the new type, and a present one for those now appearing in it. There
should be no changes demanded that necessitate the sudden destruction of old
organs, or the abrupt creation of new ones; that interrupt the continuity of in-
dividual life, or that break, or entangle, the necessary morphological relations of
one organ to another. This dual embryonic series should run parallel with,
and should supplement and elucidate the series of adult forms left along the trail
made by the two types in their slow process of evolution.

I have made a series of models that show how the embryos of vertebrates and
arachnids conform to these requirements. (Figs. 24-34.) They are intended
to illustrate the principal stages in the development of a primitive vertebrate
supposed to be descended from arachnids. The series begins as an arachnid
embryo and leads, without any greater changes than are found in the develop-
ment of the higher animals, through the typical embryonic stages of forms like
Limulus and scorpion, into a vertebrate embryo of the fish- like or amphibian type.

The series shows us that the early stages of vertebrate embryos, in all essen-
tial respects, run parallel to, or are identical with, those of arachnids; and that the
same morphogenic forces which created the cephalothorax of arachnids find their
full expression in the head of vertebrates. It shows that both embryos begin

34

OUTLINE OF THE ARACHNID THEORY.

pae

their growth from the same surface of the egg and spread over the yolk in the
same directions, enclosing the opposite side in the same manner; that there has
been no transfer of the nerve cord from one surface to the other, as claimed by
Gaskell, and that the medullary plates in both types are homologous, and not, as
claimed by C. L. Herrick, one dorsal, the other ventral.

Form Controlling Factors in the Early Stages. Apical growth and the

volume and composition of the yolk sphere are impor-
tant factors in the development of the embryo, because
the physical and chemical composition of the yolk
sphere controls the rate of radial growth, while the
circumference of the yolk sphere, and the ratio
between the rate of apical and bilateral growth, deter-
mines the relative time and place at which certain
organs arise, and the physical conditions under which
they develop. Owing to the relatively large volume
of the yolk sphere in the arachnids, neither the coelente-
rate nor trochosphere stages can assume the form of
the ancestral, free swimming animal, i.e., a nearly
spherical body growing in each of three dimensions at
about an equal rate, for they are reproduced in the
arachnid egg under totally different conditions. They
appear at a time when cell growth is beginning on the
outer surface of a relatively large sphere of inert ma-
terial, and the various organs must be mapped out in
one plane, like a Mercator projection of the earth's
surface. Moreover at these early stages, the develop-

FIG. 23 . -Diagram illustrating ment of the deeper lying organs is delayed, owing to

a hypothetical, transitional condi- the impenetrability of the yolk and the lack of respira-
tion, between the embryo of a
marine arachnid and that of a
primitive vertebrate. It shows the
convergence of procephalon, appen-

the form of a film in which the rate of growth, in the

This film increases

tory facilities.

Thus all the early stages must be expressed in

dicular arches, and mesodermic

lateral plates around the dorsal , .

organ to form the cephalic navel, three dimensions, IS VCry Unequal.

or the aniage of the hsmostoma. i n length by a process of apical growth, which takes

The uncovered yolk, that is sur-
rounded by the concrescing germ place at one end only; in breadth by bilateral growth,

walls and cardiomeres, constitutes i ,1 i i j- i ^L. T<U 1

the beiiy navel B.N an ^ m thickness by radial growth. These early con-

ditions are identical for all segmented animals, and it

is only necessary to fix the location of the growing apex, and the direction of
bilateral growth, in order to fix, beyond question, the identity of the head and
tail ends and the neural and haemal surfaces.

The Gastrula, Coelenterate, or Trochosphere Stage. The first stage
after cleavage (Fig. 24, A), shows the primitive cumulus, the primary center for the
origin of the germ layers. The central depression, gst, marks an area of inward
proliferation which gives rise to the endoderm, yolk cells, and procephalic meso-

GASTRULATION AND CONCRESCENCE. 35

derm. This stage represents the radiate or ccelenterate phase, and is to be re-
garded as the true gastrula of the arthropod-vertebrate stock. The central de-
pression deepens and later forms the stomodgeum, the outer opening being the
neurostoma. The stomodaeum, at a very early period, is enclosed, or surrounded
by a nerve ring consisting of the stomodaeal ganglia and their commissures, which
probably represents the remnants of the circumoral nerve ring of the ccelenterates.
The stomodasum is, therefore, caught in a trap, the bars of which are continually
growing stronger, and from which it never escapes.

Transition from Radiate to Bilateral Symmetry. The blastodisc, or
cell layers covering the primitive cumulus, gradually spreads out over the surface
of the yolk in all directions. Then, on the posterior side of the disc and independ-
ently of the central depression, a second thickening appears, in which cell growth
and proliferation is especially active. It marks the beginning of apical growth
and of bilateral symmetry, and lays the foundations for the first metameres. The
anterior portion of the primitive cumulus gives rise to the procephalic lobes.
(Figs. 24, 25.)

The Telopore. The rapid cell division at the apex of the developing trunk
may give rise either to an elongated axial groove (insects), or to a terminal in-
folding, or telopore (arachnids), the so-called ''blastopore" of authors. Later
it may be changed to a typical primitive streak (Limulus and scorpion), or there
may be no infolding whatever (Cymothoa). This axial or terminal ingrowth
(Fig. 25), is not to be regarded as a modification, or as an extension of the process
of gastrulation in the procephalic lobes. It is merely a local exaggeration of the
marginal growth of the blastodisc. The infolding is a secondary result of the
rapid tangential proliferation that takes place at the head of the comet-like out-
growth. It may or may not be present.

The Germ Wall. At the close of the primitive cumulus stage, a germ wall,
g. w., is formed on the lateral and posterior margins of the blastodisc. With the
formation of the trunk, it forms the lateral boundaries of the developing metameres.
(Figs. 25, 26). The germ wall, which is merely a thick band of proliferating
cells, similar to the teloblasts at the caudal end, spreads laterally over the yolk
surface (Figs. 31, 32), leaving behind, in addition to the ectoderm and yolk
cells, a sheet of underlying mesoderm that gradually breaks up into somites and
lateral plates.

Differentiation, therefore, takes place along two main axes; from the median
line laterally, and from the head end backward; hence the anterior median part
of the germinal area, which now includes the primitive head and the new body,
is always the oldest and shows the greatest histological differentiation; the marginal
and caudal part is the youngest and is the least differentiated.

Concrescence of the Germ Wall. As the gradually widening germinal
area advances over the yolk, the germ walls of the more posterior segments form
a A , the arms of which gradually unite, forming a double, primitive streak-like
band of nuclei behind the telopore. (Figs. 16, 21, 25, 26, 27, 138.)

OUTLINE OF THE ARACHNID THEORY.

.>;? HI C K

ca.nv

FIGS. 24 to 34. A hypothetical series of arachnid and vertebrate embryos. The purpose of the series is to
show the continuity in the methods of growth and organic differentiation in vertebrates and arachnids. It begins
with the typical arachnid stages and leads up to those characteristic of primitive vertebrates, where without inter-
ruption they are carried on to completion.

FIG. 24. A shows the radially symmetrical germ disc, or primitive cumulus, with its centrally located gas-
trula ingrowth; B, beginning of apical, or teloblastic, growth, and the appearance of bilateral symmetry; C, the
formation of the medullary plate; the unequal expansion of the thickened margin of the germ disc, or germ wall,
g. w., and the infolding of the teloblast to form the telopore, /. p.

FIG. 25. The open medullary plate stage, with its neural crests, the marginal infoldings that mark the beginning of
the forebrain vesicle, and the forebrain sense organs on the outer slope of the neural crest.

FIG. 26. Shows the appearance of the thoracic appendages; the segmentation of the lateral plate mesoderm
in the abdomial region; the beginning of the postanal concrescence of the germ wall; and the infoldings for the
middle cord, or notochord. The telepore is replaced by a primitive streak.

FIG. 27. Shows the appearance of the gustatory lobes, the vagus and abdominal appendages, and the eleva-
tion of the caudal lobe. The olfactory organs have moved forward, in front of the head, and the median eyes
have been transferred to the inner limb of the neural crest. The cerebellum appears as the suprastomodceal
commissure.

FIG. 28. The palial fold has covered nearly the whole of the forebrain, and the optic ganglia are crowded
backward and upward toward the oral region. The pleural folds appear, and the thoracic folds, or the thoracic
shield, extend over the posterior thoracic appendages, forming the beginning of the opercular or branchial fold.

FIG. 29. The optic ganglia have united over the stomodaeum to form the tectum opticum. The neural crests
of the branchial region have closed, except over the mouth which lies just behind the stomodsal commissure, or
cerebellum. The uncovered space marks the location of the choroid plexus of the fourth ventricle.

GASTRULATION AND CONCRESCENCE.

0*7
61

pa.e

my

stco.

st

1K * i 1

.. . ,
1 .Q'-SSSP ,

2

bnv

FIG. 30. The embryo now presents typical vertebrate conditions. The old mouth is practically closed. The
posterior thoracic and vagus appendages appear as the external gills, and the pleural fold as the postcephalic
lateral fold that gives rise to the pectoral and pelvic appendages.

FIGS. 31 and 32. Side views of Figs. 26 and 27, showing the extension of the germ wall over the yolk, and the
elevation of the procephalic lobes above the general level of the yolk.

FIG. 33. Side view of Fig. 29. It shows the projection of the procephalic lobes beyond the anterior surface
of the yolk, thus allowing the oral arches, or the basal arches of the anterior thoracic appendages, to approach
the haemal surface of the forehead. The lateral eye has been carried into the brain vesicles, appearing through the
skin as the kidney-shaped retina.

FIG. 34. Side view of Fig. 30. At least, four pairs of oral arches have united on the haemal suface, giving rise
to the premaxillary, maxillary, mandibular. and hyoid arches. Vestiges of the free appendages persist as the
oral arch papillae, tentacles, or balancers. The large thoracic segmental sense organ opposite the fourth thoracic
appendage and behind the hyoid arch has now become the auditory placode.

^8 OUTLINE OF THE ARACHNID THEORY.

\J

Thus the embryo elongates, apparently, in two different ways; by true apical
growth at the original apex of the embryo, and by the concrescence of the adjacent
parts of the germ wall, behind the apex. Failure to recognize the meaning of
these two processes in vertebrates has led to much confusion.

As the telopore is merely a locally exaggerated marginal growth, its products
are not primarily different from those of the germ walls that concresce behind it.
But it will be observed that in their derivation and in their serial arrangement,
the two sets of products stand in totally different relations to one another, and to
their surroundings. (Figs. 138, 157.)

In vertebrates, as well as in arachnids, neither the telopore, nor the concres-
cing margins of the germ wall have anything to do with a true gastrula, nor is their
mode of growth comparable with the coelenterate method of gut formation. As
indicated elsewhere, post-anal concrescence is the inevitable result when a living
film, extending by apical and marginal growth, spreads over a spherical surface.

The Nervous System follows in the paths first laid down by the expanding
germ layers. The procephalic lobes, developing from the territory of the primi-
tive cumulus, and the lateral nerve cords on either side of the line of apical growth.
A slipper-shaped medullary plate is thus formed that in both arthropod and verte-
brate embryos has essentially the same structure, location, and mode of growth.
In both types we may recognize the marginal sense organs of the procephalic
lobes and the central, stomodaeal infolding. This infolding, with a rostrum-like
elevation on its anterior margin, is frequently a conspicuous marking in the middle
of the cephalic lobes of amphibia (Rana, and Necturus) (Fig. 25.)

In this stage, the olfactory lobes make their appearance as a deep fold across
the anterior border, and the edges of the neural crest begin to grow over the
lateral margins of the medullary plate. A little later, the thoracic appendages
appear as gill arches and external gills. (Figs. 26-28.)

The Primary Sense Organs'. In the following stages (Figs. 27-34), the
primary sense organs on the margins of the cephalic lobes move into, or toward,
their final position, and the gustatory organs make their appearance on the inner
margins of the basal lobes of the thoracic appendages. The olfactory organs,
ol, o, move toward the anterior median line but remain in the surface ectoderm,
outside the brain chamber; the two pairs of ocelli are caught in the palial
fold and carried onto the membranous roof of the brain vesicle to form the parietal
eye; the kidney-shaped lateral eyes lie on the outer edge of the fold, not quite
inside of the brain chamber. (Fig. 27.)

Meantime the cornua, c, of the thoracic shield and the edges of the abdominal
pleurites appear. (Figs. 28, 33.)

The hsemal side of the cephalothorax is unsegmented. The greatly thickened
germ wall in this region concentrates around a point between the anterior end of
the heart and the forebrain, where a great mass of cells, the remnants of the
haemal blastoderm, are engulfed in the yolk and absorbed. (Fig. 33, c.mn'.*)

Vertebrate Stages. Up to this point, our vertebrate-arachnid embryo has

CHARACTERISTIC VERTEBRATE STAGES. 39

been passing through the coelenterate and arthropod stages of its development, the
later ones, as we have represented them, being essentially like those of Limulus and
the scorpion, although not presenting any characters foreign to a vertebrate. In
the following stages, the vertebrate characters appear.

The changes that most affect the general shape and appearance of the brain
are the transfer of the lateral eye placodes to the inside of the cerebral vesicle,
and the union of the optic ganglia over the neural surface of the brain to form the
ganglion habenulae and the tectum opticum, or optic lobes. (Fig. 28.) As the
latter increase in size, they crowd the stomodseal commissure and its ganglion
backward, over the posterior part of the midbrain region, where they form the
rudiment of the cerebellum. (Figs. 29, 30, 107 and 108.)

When the convex, kidney-shaped lateral eye placode is transferred to the
brain wall, it becomes a concave, horseshoe-shaped retina. Still later, it becomes
a circular one, with a median fissure and centrally located nerve, both conditions
being the direct result of its ancestral shape and mode of growth. (Fig. 106.)

The Auditory Pit develops from a prominent, disc-like placode, or segmental
sense organ, which in Limulus lies on the cephalothoracic shield, opposite the
third or fourth thoracic segment. (Figs. 29-30.) With the concrescence of the
anterior oral arches on the haemal side of the head, the disc shifts its position to
that part of the head where it makes its first appearance in vertebrate embryos.

(Figs. 33-34-)

The Heart has been formed in typical arthropod fashion, by the concrescence
of the lateral plates of the vagus, and the anterior abdominal metameres, which
accounts for the fact that, in both arthropods and vertebrates, the anterior heart
nerves arise from the corresponding vagus and branchial neuromeres. (Figs. 32
and 33.)

In the formation of the vertebrate heart, new factors may arise in the greatly
increased volume of the yolk sphere. In this case the branchial metameres, lying
near the equator of the egg, must extend their lateral plates completely round the
yolk before a heart segment can be formed. Hence, in the larger yolked eggs, it
is only the vagal and anterior branchial metameres that are in a position to form
heart segments in time to nourish the growing head structures. If the younger
and shorter caudal metameres produced heart segments, they would necessarily
arise later and would be separated from the head by the barrier of the abdominal
yolk navel. (Fig. 34.) The heart would then be a single tube at either end and
a paired tube in the middle. (Figs. 23, 139, C.)

The thoracic, and the post branchial cardiomeres have been eliminated in
vertebrates, as they have been in arachnids, and the heart develops, as nearly as
one may determine, from about the same group of vagus and branchial metameres
in both cases. But the posterior end of the vertebrate heart still extends partly
round the yolk navel; hence the divided posterior end and divergent vitelline
veins; and it is crowded into an area on the hasmal surface that is growing shorter
while the heart is growing larger, hence the auriculo-ventricular curvature.

40 OUTLINE OF THE ARACHNID THEORY.

The Cornua of the cephalothoracic shield are retained as the opercular
fold, which extends over the posterior appendages, as suggested for Cepha-
laspis, to form a respiratory, or atrial chamber. The projecting margins of the
abdominal segments or pleurites are retained as the lateral fold, from which the
paired, post-branchial appendages arise. (Figs. 29-34.)

The Oral Arches and the Haemostoma. Both the formation of the
haemostoma, or new mouth, and the transfer of the basal joints of the anterior
thoracic appendages to the haemal side of the head to form the oral arches, are
the inevitable results of the processes that have been steadily going on during the
phylogeny of the arachnid cephalothorax. These processes are: the increased
size of the yolk sphere; the increased size of the forebrain neuromeres; and the
progressive degeneration of the cephalic mesoderm. The way in which these
changes affect the location of the jaws and the shape of the head, during the early
stages of development, is shown in Figs. 31-34. It will be seen that as the fore-
brain increases in volume and in precocity, the apex of the head is elevated and
thrust forward off the surface of the egg. As the haemal ends of these anterior
metameres are greatly reduced in volume, or absent, the other head structures,
which were originally neural or lateral in position, such as the anterior meso-
blastic somites and the appendages, are drawn toward the haemal side of the head.
Here they converge around the ingrowing, haemal surface, or cephalic navel
(dorsal organ) that represents the beginning of the buccal infolding, the basal
joints of the appendages forming the beginning of several pairs of oral arches, i.e.,
premaxillas, maxilla?, mandibles, and hyoids.

The more posterior arches are not subject to these conditions; hence they
tend to remain in their original position on the neural or lateral surface. But
in the later stages of the higher vertebrates, even they may be transferred to the
haemal surface. (Figs. 307, 308.)

The mouth parts of our embryo are now in the ostracoderm and cyclostome
stage, one that is seen temporarily in all higher vertebrate embryos (see develop-
ment of the jaws in the frog) (p. 257), and permanently in the adult cyclostomes
and ostracoderms. (Figs. 159-174.) The true vertebrate condition is attained
by the union of two pairs of arches to form a single, fixed, upper jaw, and the
union of a third pair to form a movable, unpaired, under jaw, or mandible.

The older stages of our arachnid-vertebrate embryo, Fig. 34, are character-
ized by an increase of the cranial flexure, bringing the heart close under the
anterior end of the brain, and producing that forward dislocation of the hypo-
branchial muscles, so characteristic of vertebrates; by the opening of the gut pouches
into the lung-books; by the appearance of true vertebrate appendages as post-
cephalic outgrowths of the marginal fold; by the increasing size of the cartilage
cranium and gill bars; by the substitution of a subdermal skeleton for an epidermal
one, and by the conversion of the arthropod lematochord into the notochord.

CHAPTER III.

EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS.

I. MEANING OF THE TERM BRAIN.

In the vertebrates, the term "brain" vaguely signifies the specialized an-
terior end of the neuron. In the invertebrates, the term may be even more vague,
in that it is often used to signify only that part assumed to lie originally in front
of the oesophagus, that is, the supra-oesophageal ganglion. Or the term may
signify that ganglion, plus a varying number of post-oral neuromeres.

The lack of precise definition in both cases is significant, and justifies the use
of the term, as we shall use it here, namely, to signify a varying number of neuro-
meres consolidated in the region of the primitive mouth.

The number of neuromeres thus set apart, their specialization, and the
intimacy of their union, gradually increases throughout the arthropod-vertebrate
series, and furnishes an impressive picture of persistent, progressive specializa-
tion.

In the arthropods, there are many oscillations in the total number, and in
the grouping, of the brain neuromeres. The primary causes of their union arc-
too complex to be analyzed, except in the broadest way; but we may readily recog-
nize a steady progression toward a definitely organized collection of neuromeres
that it is entirely proper to call a brain in the vertebrate sense, for it contains
approximately the same total number of neuromeres as the vertebrate brain; and
it is divided into similar groups of neuromeres, each of w r hich is associated with
nerves, sense organs, and other structures similar to those in vertebrates.

The evolution of the brain cannot be effectively studied apart from the body
regions to which it belongs, for each moulds the other and reflects the other's
changes. The events that created the vertebrate brain, and whose influence is
still effective in moulding its form and function, are to be found in the arthropods.
There, all the initial phases in the successive incorporation of one region of the
trunk after another into a more complex "head," and of one part of the cord after
another into a more and more complex "brain," have taken place, and probably
nearly all the more important steps in the process are there crystallized into
recognizable form.

The five groups of neuromeres included in the first fifteen or twenty that
make up the vertebrate brain may be definitely identified with the corresponding
divisions of the arthropod brain. We cannot hope to identify more than that

41

4 2

EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS.

since those that follow and which make up the greater part of the spinal cord were
acquired after the evolution of vertebrates from arachnids had taken place.

In other words, the ancestors of vertebrates were animals provided with a
comparatively small number of neuromeres, 21 , most of which had already
been consolidated into a complex brain of the vertebrate type. One of the im-
portant events in the early evolution of the new or vertebrate type was the rapid
increase in the number of metameres by the regular process of apical growth.
The new metameres formed a new trunk or body, while nearly the whole of the
old arachnid trunk (head, thorax, and abdomen, 14-16 metameres) was still
further consolidated to form the head of the new type. The whole process thus

FIG. 35. Diagrams to explain the probable relations between the structure of a trochosphere and the early
embryonic stages of a primitive arthropod; A , Trochosphere in mercator projection, seen from the neural, or sub-
umbrella surface; C, same from the side seen as a solid object; B, early stage of an arthropod embryo, seen in
mercator projection; D, same seen as a solid object, from the side. In A and C, the circumoral area, with its
system of radial and circular nerves, forms a part of the sub-umbrella of the trochosphere. In B and D this area
is supposed to be infolded, giving rise to the proximal portion of the stomodceum, from which the system of stomo-
dseal nerves and ganglia arise. The ancestral coelenterate body, according to this interpretation, is represented in
the arthropod embryo by the procephalic lobes and stomodaeum; the arthropod trunk, with its lateral and
median nerve cords, is a new formation, arising as a local outgrowth from the ancestral coelenterate body, or from
the procephalic lobes of the arthropod embryo. On the aboral surface of the trochosphere is the area of yolk de-
posit and the " closing in" point, a pauperitic, degenerative region that is called the cephalic navel.

presents a striking analogy to the way in which the primitive body of segmented
animals was formed as a new outgrowth from the body of its coelenterate ancestor,
which then became the head of its descendant. (Fig. 35.)

*********

II. THE STOMOD^AL NERVES.

We recognize two distinct systems of nerves in segmented animals. One
belongs to the stomodaeum, and probably represents the remnants of the cir-
cular and radial sub-umbrellar nerves of a ccelenterate-like ancestor; the other
consists of longitudinal and transverse nerves that developed in the tentacle-like
out-growth that gave rise to the body of the new animal. (Fig. 35.)

The stomodaeum is looked upon as representing, in part, the infolded sub-
umbrella. When invaginated, it carried with it the primitive system of circum-
oral nerves, which then arise as circular and longitudinal nerves from the walls
of the stomodaeum. The outermost circular nerve (prototroch nerve ( ?)), is repre-

THE BRAIN. 43

sented by the supra-stomodaeal commissure with its anterior median, and two
lateral, ganglia. These nerves and ganglia are without doubt very ancient
structures, and their position and mode of development clearly indicate that they
belong to a different system of nerves from those in the remaining part of the head
or trunk.

There is probably a distinct post-cesophageal ganglion and commissure
belonging to this system, although I have not succeeded in locating it, or in dis-
tinguishing it from the more anterior post-oral commissures. The supra-stomo-
daeal commissure always sends nerves to the labrum, or rostrum, which receives
nerves from this source only. The innervation of this pair of appendages, their
median position in front of the mouth, and between the right and left halves of
the forebrain, distinguish them from all others, and indicate their probable origin
from tentacle-like organs of some very remote ancestor.

Originally the stomodaeal neives appear to have been intimately connected
with the two median longitudinal nerves of the trunk, i.e., with the median cardiac,
on the haemal side, and the median sympathetic on the neural. Both these con-
nections are lost in the adults of the higher arachnids, i.e., in Limulus, although
in the scorpion the connection with the cardiacs seems to be retained.

The dividing line between the ccelenterate nervous system of the primitive
head and that belonging to the bilateral outgrowth from it, cannot be accurately
determined, and indeed there is no reason to suppose the two were ever distinct
systems, the post-oral nerves being merely extensions of the older ones in
the head.

III. THE FRAME-WORK OF THE NERVOUS SYSTEM.

The nervous system of segmented animals may be reduced to a system of
longitudinal and transverse strands or cords.

Longitudinal Cords. In the arachnids, eight longitudinal nerve cords may
be recognized: a median haemal one, from which arises the cardiac ganglion; a
median neural one, or middle-cord (Mittelstrang of Hatschek), from which arises
the so-called median sympathetic nerve; a pair of ventral cords, which give rise
to the main axial nervous system, or neuron (brain and spinal cord) ; the paired
pericardials; and the lateral sympathetics.

The Median Nerve, " Median Sympathetic," or "Middle-Cord," of arthropods
appears to have extended backward, from the posterior part of the oesophageal
region, or of the circumoral nerve ring, the whole length of the body. I have not
been able to determine the peripheral distribution of its fibers. The main nerve
and its sheath undergo many modifications. In the higher arthropods and verte-
brates, the nerve itself atrophies and ceases to form a functional part of the nervous
system. It serves, however, as a center for the development of voluminous,
resistent envelopes from which is evolved the notochord. The history of the middle
chord will therefore be described in the chapter on the evolution of the notochord

44

EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS.

Transverse Cords. Numerous transverse, or circular bands intersect the
longitudinal ones, and lay the foundations for the transverse commissures, and
for the segmental peripheral nerves. The latter usually lead by smaller branches
into a subdermal plexus, from which the nerve ends are distributed to their re-
spective terminals.

The ventral cords and the middle cord are confined to that surface of the
embryo that is the first to develop. Their position during the early stages is the
same in all segmented animals, and their presence definitely locates the primitive
oral, or neural surface of the bodv.

:ta

PIG 3 g Brain and nerve cord of a young Limulus in the second larval stage. A, Haemal surface; B, neural surface
FIG. 37. Sections of same. A, Through the optic ganglia and olkK-torv organs; B, through the middle of the
hemispheres and the posterior part of the forebrain; D, through the cheliceral ganglia; E, through the suprasto-
modeal commissure and the lateral stomodaeal ganglia.

The longitudinal cords serve to conduct nervous impulses in a longitudinal
direction; in them are located the great majority of the nerve cells. The trans-
verse bands serve to conduct nervous impulses in a centripetal or centrifugal
direction. The comparatively few nerve cells that belong in them, as a rule, lie
near their central or peripheral terminals.

THE SPECIALIZATION OF THE NERVE CORDS. 45

The Process of Specialization. The axial or central nervous system under-
goes progressive evolution, or specialization, in a transverse and in a longitudinal
direction.

The first process consists in the segregation of similar nerve fibers and cells
into concentric, overlying longitudinal zones or tracts, the most notable example
of this being the assembling of motor elements toward the haemal surface, and
of sensory ones toward the neural surface of the cords.

The second is the transverse division of the cords into blocks, or neuromeres,
which then, singly or in groups, become the centers of some particular function.
The linear specialization of the neuron is due to the gradual elimination of the
heart, digestive and locomotor organs, from the anterior body metameres, and to
the increased size of the sensory and ingestive organs. These changes lead to a
great reduction in the number and volume of the motor nerve elements in the
anterior metameres, and to the location of functional centers in the neuromeres
according to a definite order, which follows that established in the corresponding
groups of metameres. See page 209. This order, which is initiated at a very
early period in the history of segmented animals, is as follows: olfactory; coordinat-
ing; visual; ingestive (i.e., masticatory, swallowing, and gustatory); auditory;
locomotory; respiratory (cardiac and branchial); digestive, and urogenital.
This process of cephalization progresses in a cephalo-caudal direction, the func-
tional centers becoming more and more sharply localized in the direction and
order named above.

IV. THE DIFFERENTIATION OF PERIPHERAL NERVES.

The primary system of transverse nerves forms the foundation of the peri-
pheral nervous system. The evolution of these nerves consists mainly in the
resolution of the primary network into special nerve bundles composed of fibers
having similar central and peripheral terminals.

The principal stages of the process appear to be as follows : i . Each neuromere
is at first connected with several pairs (four?) of transverse nerves, all of which
may contain both motor and sensory elements. 2. The number of nerves for
each neuromere is ultimately reduced to two main pairs, an anterior and a posterior.
3. The roots of the anterior nerves gradually shift toward the neural surface of
the cord; the posterior ones retain a more haemal position. The two series of
nerves thus formed, are called the neural and the haemal nerves. 4. The neural
nerves develop ganglia on their proximal ends, and in those regions of the body
where appendages are developed, supply only the appendages. The haemal
nerves are without ganglia and supply the remaining parts of the metamere. 5.
In the cephalothoracic, or head region, the neural and haemal nerves remain
separate (vertebrates and arthropods), while in the more posterior regions they
may unite, for a longer or shorter distance, forming single nerves with two sets of
roots, ganglionated neural roots, and non-ganglionated haemal roots. 6. Both
neural arid haemal nerve roots contain motor and sensory elements, but at an

46 EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS.

early period in the evolution of arthropods the sensory elements become more and
more predominant in the neural nerves, and the motor elements in the haemal ones,
this condition being most strongly marked at the anterior end, and diminishing
gradually in a caudal direction.

Factors that Modify the Arrangement of Peripheral Nerves.- -The more
important factors that modify the primitive segmental arrangement of peripheral
nerves are as follows: a. the location, isolation, and size of the peripheral terminals;
b. the elimination of other terminals; c. the organic union of similar terminals
belonging to different metameres; d. the relative age of the metamere in which they
belong.

ol.o.

38

FIG. 38. Brain of a young Limulus about three
inches long; neural surface.

FIG. 39. Same; haemal surface.

a. The segregation of like nerve fibers into peripheral nerves, or into nerve
tracts in the central nervous system, is determined by the time and place of origin
of the peripheral terminals.

Wherever there are highly specialized organs, morphologically isolated, the
associated nerve fibers and nerve cells show a similar isolation or segregation, the
growth of each correlated part keeping pace, in the main, with the growth of
the other. The primary sensory organs are superficial in position and lie in the
ectoderm, close to the lateral margins of the neuron. The motor ones are deeper,
more lateral or haemal in position. The corresponding nerves have, in the main,
similar relative positions, and these factors have controlled from the outset the

THE ARRANGEMENT OF PERIPHERAL NERVES. 47

segregation of motor components in a haemal direction, and the sensory ones in a
neural direction, both as regards their location in the peripheral nerves and in
the central nervous system. With the imagination of the nerve cords, these con-
ditions were still further exaggerated by the union of the neural crests in the median
dorsal line, and by the position of the mesoblastic somites. (Fig. 137.)

The wide separation of the neural and haemal nerves, as for example in the
thorax of Limulus and the scorpion, is due on the one hand to the location and spe-
cialization of the appendages, coxal sense organs and ganglia, and on the other to the
location of the more peripheral trunk muscles and sense organs. It no doubt
had its origin at a very early period in the evolution of metameres.

b. Elimination. In the arthropods there is a progressive elimination from
the anterior metameres of the motor, nutritive, cardiac, and respiratory organs,
leaving little but the leg and jaw muscles, and the primary sense organs, such as
the eyes, olfactory, gustatory, auditory, and tactile organs. The nerve elements
associated with those organs disappear with them. Those that remain increase
in volume and independence with their corresponding peripheral terminals, while
their central terminals tend to completely monopolize their appropriate neuro-
meres. In this way the primitive character of the segmental nerves may be lost or
greatly modified. This is the case in the procephalon, where the only peripheral
nerves that remain belong to the eyes and olfactory organs, all other peripheral
elements having been eliminated, if they ever existed there.

c. Union. Where organs belonging to different metameres perform the same
function their nerves tend to unite, forming a common bundle, or nerve, or tract.
Such compound nerves, consisting of the united branches of separate segmental
nerves, may themselves simulate independent segmental nerves, and greatly dis-
guise the original segmental arrangement. Examples of this mode of segregation
are seen in the segmental cardiacs, the hypobranchial, the intestinal (Figs. 57, 58^,
and to a lesser degree, the gustatory nerves of Limulus.

c. Historic Factor. If we attempt to homologize the nerves in one part of
the head with those in another, or with those in the trunk, we meet with insuper-
able difficulties because, as we have seen, each group of metameres has a history
of its own that is different from that of all the others, and this history is reflected
in the structure of its nerves and neuromeres. The attempt to homologize the
structures in the head with those in the trunk or tail, except in the most general
way, is an illogical and hopeless undertaking, for the caudal metameres belong
to later generations that came into existence under new conditions and were
provided with different organs from those in the old. Except for a small number
of the most anterior ones, the trunk and caudal metameres of vertebrates did not
exist in the arthropods. They arose with the vertebrate stock and never developed
any organs comparable with the cephalic appendages, jaws, gill arches, or visual
organs. Hence it is clear that there can be no exact homology between the head
metameres of an arachnid or a vertebrate and a trunk metamere of the same
animal. For that reason, therefore, we may not consider the cranial nerves, or

4 8

EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS.

cranial neuromeres, or cephalic sense organs, as modifications of those in the
trunk, or vice versa, without conveying an entirely false impression of their real

history and meaning.

In the lower arthropods, the peripheral nerves are generally arranged through-
out the whole body, in typical segmental fashion. In the higher arachnids, due
to the operation of the above described factors, this clear cut metamerism declines,
or is greatly obscured. The broad distinction between cranial and spinal nerves
becomes clearly established, and the extensive elimination of motor elements, as
well as the local segregation of sensory and motor components of different nerves

p.e

... p-e

..p.e

.et

st.c

vg.n.

A B

FIG. 40. Models of the brain of a young scorpion, just hatched. A, Hasmal surface; B, neural surface.

into compound nerves having a similar function and distribution, has given to the
entire system the same structure and general arrangement of parts seen in the
vertebrates.

In Limulus, for example, this process of specialization has produced the
highly characteristic olfactory, pineal eye, and lateral eye nerves, as \vell as the
compound system of gustatory, branchio-thoracic (hypoglossal), cardiac, and
intestinal nerves. These nerves are already so complex and highly modified
that the original segmental arrangement is now exceedingly difficult or impossible
accurately to determine.

The same conditions, but in a still more exaggerated form, are seen in verte-
brates, and in part justifies the revolt of certain American neurologists against the
apparently hopeless task of determining the segmental value of vertebrate cranial
nerves and their relation to the dorsal or ventral roots of spinal nerves. They
have laid great stress on the analysis of nerves into their functional components;
but in perfecting a highly artificial system, they have neglected the deeper mor-
phological problems involved in their more primitive segmental arrangement. It
is clear that both the old and the new method must be retained. But neither

THE EVOLUTION OF NEUROMERES. 49

method alone applied to the vertebrates can ever give us a true picture of their
ancestral condition. That can only be obtained from the arthropods where the
highly specialized condition seen in the vertebrates has its origin.

V. NEUROMERES AND METAMERISM.

Metamerism of the body and the subdivision of nerve cords into blocks or
neuromeres are characters that were probably slowly evolved in bilateral animals;
not inherited, even in a rudimentary form, from ccelenterate ancestors.

The evolution of neuromeres probably began in the trochozoa. They are
well developed in the annelids and in the arthropods, especially in the ab-
dominal regions. In the higher arthropods, the clear-cut distinction between
adjacent neuromeres of the head is greatly obscured by their fusion into larger
groups, and by the segregation of their constituents into new groups, according

FIG. 41. Model of the forebrain region of an embryo scorpion, stage G, Fig. 18.

to their function. In vertebrates, the post-cephalic part of the neuron, which
has been more recently acquired, and which is not represented in arthropods,
is never divided into distinct neuromeres, and probably never was so divided.

Even in the arthropods and annelids, it is doubtful whether there is any such
thing as a neuromere, complete in itself and devoted to a single body joint or
metamere. There are certainly none in Limulus, or in the scorpion, and the
lower down we go, as for example into the phyllopods, the less sharply denned the
neuromeres become; that is, the ganglionic masses are more diffuse, and the pe-
ripheral nerves more numerous, and not so strictly segmental in their origin or dis-
tribution (Branchipus).

In Limulus and scorpion, where there appears to be such an exact and exclu-
sive association of the body segment with its neuromere and nerves, there is no
such exclusive association in fact, because many motor neuromeres and the cen-
tral ends of many sensory fibers are located in some neuromere anterior to the one
where the nerve fibers leave the cord to reach their peripheral terminals. That

4

50 EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS.

is, the central nerve terminals and the centrally located nerve cells, in many cases
lie in different metameres from the peripheral organs with which they are asso-
ciated. (Figs. 59, 60.)

This condition appears to prevail in the most primitive arthropod neuro-
meres, hence that complete functional and morphological correspondence, sup-
posed to occur between a body joint and a nerve cord joint, does not exist. Meta-
merism has developed to a different degree in the two systems and affects them
in quite a different manner. The morphological segmentation of the nerve
axis does not coincide with that of the body, and the functional segmentation of the
nerve cord does not coincide with its morphological segmentation, for both motor
nerve cells and sensory dendrites are frequently located in a neuromere in front
of the metamere in which the corresponding nerve fibers leave the cord, and in
which they have their peripheral terminals.

A partial explanation of the lack of correspondence between functional and
morphological metamerism of the nerve cord is afforded by what takes place in
embryo scorpions. Here each neuromere is composed of two distinct segments,
and as the space between the abdominal ones increases, the anterior segment of
one neuromere unites with the posterior segment of the one in front of it, thus
completely changing the original grouping of the half neuromeres. It is not
clear whether this takes place in Limulus or in the other arthropods I have
studied, but it probably does, otherwise it is hard to understand how the cell
bodies of the motor neurones are located in the neuromeres in front of the one
from which the corresponding motor nerves leave the cord.

It is therefore clear that Loeb's attempt to prove that each abdominal neuro-
mere in Limulus is a complete reflex center for its corresponding gill, is based on a
misconception of the structure of the nerve cord. His interpretations of his ex-
periments are incorrect because, as we shall show later, they are based on a
misunderstanding of the structure of a neuromere and the distribution of its nerves.

VI. THE PRIMITIVE SENSE BUDS.

The main nerve trunks in the arthropods represent bands of metamorphosed
sense organs, and they coincide with the lines along which such sense organs
were distributed in the remote ancestral forms. The transformation of these
primitive sense organs into nerve cells constitutes an important step in the evolu-
tion of the central nervous system. Many details in this process are still retained
in the embryos of arachnids.

In the scorpion, the entire brain and cord is an aggregate of innumerable,
closely packed sense buds which, under a low power, produce a mottled, or pitted
appearance that is very characteristic. (Figs. 15 and 16.) Under a higher
power, and in sections, each bud appears pear-shaped, with a goblet-shaped cavity
opening to the exterior at one end, and leading into a narrow vertical canal at the
other. They consist of typical sensory cells, having the same shape, arrangement,

THE PRIMITIVE SENSE BUDS. 51

and rod-like ends as those in the segmental sense organs on the outer margins of
the coxae. (Fig. 74, E.)

The primitive sense buds appear as soon as the six thoracic appendages are
outlined (stage B), and are at first uniformly distributed over the entire cord and
cephalic lobes, with the sole exception of the olfactory lobes. (Fig. 15.) At
a later stage, E, those on the lateral margin of the cord are distinctly larger than
the rest, forming two dark bands. From the buds on the posterior lateral margin
of each neuromere, arise the ganglion cells at the roots of the post-thoracic nerves
(spinal ganglia). The buds on the smaller, or originally posterior segment of
the neuromere give rise to the cluster of motor nerve cells which are found near
the anterior nerve roots.

1 'X : , ^A

''" II.'H. It 'n h*
"" it, it.

FIG. 42. Brain of adult scorpion, from the side.

As development proceeds, the central cavity of the bud closes, the sensory
cells lose their cylindrical form, and their hair-like, or rod-like outer ends disap-
pear; finally each bud forms a small cluster of ganglion cells.

In the late embryonic stages of the scorpion, the metamorphosed sense buds
form long conical masses of cells with the proliferating apices directed inward.
Their appearance is then much like the cell clusters formed by neuroblasts.
(Figs. 227-228.)

Cell division in the sense buds diminishes after their metamorphosis, the
ganglion cells reaching an approximately fixed number at an early embryonic
period. This, however, does not apply to the minute cells in the hemispheres,
in the olfactory lobes, or in the pedal ganglia of Limulus, for these cells appear
to increase in number steadily, at least as long as the animal continues to grow
in size.

In Limulus the cells descended from a given sense bud, during the late larval
periods, form well defined clusters of pear-shaped ganglion cells, with a special
neuroglia investment. Each cell of the same cluster appears to project its fibers
along the same path, to the same terminals. (Figs. 61-64.)

In Limulus it has not been possible to identify each nerve-cell cluster with

^2 EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS.

the antecedent sense buds, for the latter are best seen in the embryos of scorpions,
while my most detailed work on the cord has been done on Limulus. But the
conditions in the two animals are so similar that there can be no reasonable doubt
that an arthropod neuromere consists of distinct clusters of nerve cells, each sur-
rounded by a special sheath of neuroglia, each projecting its fibers along the same
paths to the same terminals, and each directly descended from one or more
embryonic sense buds.

In the early stages of the neuron in vertebrates, as in the late stages in the
scorpion, the nerve cells are often arranged in parallel vertical rows, which may
be interpreted in the same manner as in arthropods, that is, as the ontogenetic
remnants of ancestral sense buds.

There are many familiar instances where nerve cells arise from the same
points in the ectoderm as the sensory ones. It is highly probable, in such cases,
that the nerve cells are ultimate phases in the specialization of sense cells. For
example, in Acilius, a few cells of large size leave the embryonic retina at a com-
paratively late stage; they finally join the optic ganglion and become giant nerve
cells, having such a peculiar form and location that they may be readily recognized
through life (Patten). Ganglion cells may also arise from the gustatory epithelium
in Limulus, or from the epithelium of lateral line organs in vertebrates. But in
none of these cases has it been clearly shown, to my knowledge, that a functional
and structurally complete sensory cell is bodily metamorphosed into a ganglion
cell. However, just such a metamorphosis as this does take place in Limulus and
Branchipus, where the large rod-bearing visual cells are converted into true gang-
lion cells, which still retain indications of their primitive grouping into ommatidia
and remnants of the visual rods. (p. 162 and Fig. 109, ,4.)

The transformation of well developed sense buds into ganglion cells, as just
described for the neuron of arachnids, is not, therefore, without precedent.

In most arthropods, the primitive sense buds, while undoubtedly present in
some form, are not as well developed as they are in scorpions. Hence certain
authors have failed to recognize their real character, and have interpreted them
as neuroblasts, or even as nutritive folds, or as folds produced by growth pressure.
Such interpretations are untenable. It is true that the sense buds may be repre-
sented by small conical groups of cells, or nuclei, arising from the proliferation of
a single deep-lying cell, or nucleus, or "neuroblast." But the formation of these
iK-un (blasts is to be regarded as an abbreviated method of repeating the sense bud
stage so clearly seen in the scorpion.

Even these neuroblasts may be omitted, or their appearance postponed to a
relatively late embryonic period; the entire cord then has its origin in a few
terminal neuroblasts (or telo-neuroblasts), as in Cvmothoa.

CHAPTER IV.

THE SUBDIVISIONS OF THE BRAIN.

I. THE PROSENCEPHALON, OR FOREBRAIN.

The fore brain of arthropods is that part of the neuron that usually lies in
front of the stomodaeum. In the embryos it is the anterior expansion of the
medullary plate called the procephalic lobes. As nearly all traces of mesoderm
and appendages have disappeared from this region, there is but little evidence
accessible to indicate the presence there of metameres. In many arthropods
the lobes are divided into three main divisions, with no recognizable separation,
at any time, between them and the postoral sections of the nerve cords; hence we

mxl

Kt.

ht

FIG. 43. Sagittal section of a young scorpion.

may, for the present, regard the main divisions as greatly reduced metameres,
and the central portions as neuromeres.

Acilius. The structure of the procephalic lobes is best seen in the embryos
of those insects which lead an active larval existence, as for example in Acilius.
(Fig. 14.) Here they are divided transversely into three similar parts, which
probably represent all there is left of three procephalic metameres. Each meta-
mere is also divided into three parts: a. a median one, representing a forebrain
neuromere, corresponding to the postoral neuromeres; b. a middle part, repre-
senting a segment of the optic ganglion; and c. a lateral one, forming a segment of
the optic plate, each plate containing two ocelli. Between each segment of the
optic ganglion and the optic plate is a deep infolding, iv. l ~ 3 , which later closes,
covering up the optic ganglia, but leaving the ocelli and neuromeres in their
original position.

53

54

THE SUBDIVISIONS OF THE BRAIN.

A pair of small appendages, ro, lie near the first metamere. Later, they fuse
to form the labrum. The second metamere has no appendages. The third one
is closely associated with the antennae.

From the upper or neural surface of the first ( ?) and second forebrain neuro-
meres, are developed dense masses of small cells, with deeply stained nuclei, that
give rise to the characteristic mushroom bodies of insects. They may be recog-
nized in apparently all classes of arthropods, attaining enormous size in the ants,
bees, wasps and spiders, and reaching extraordinary dimensions in Limulus.
In structure and function (Limulus), they are true coordinating centers, and
are to be regarded as the earliest stages in the phyllogenetic development of the
cerebral hemispheres of vertebrates.

m

-* ( 1 J - v * ' )-f

FIG. 44. Sagittal section of a primitive vertebrate embryo, showing the relation of its' principal organs to those in

the arachnids; schematic.

In the arachnids, the cephalic lobes differ from those of Acilius: a. in the in-
distinct segmentation of the optic plate, b. in the relatively late appearance of the
segmental sense organs (median, and lateral, eyes, and olfactory organs), and c.
in the peculiar character of the first metamere. Other differences appear later,
as we shall presently indicate.

In the scorpion (Fig. 15), which may be taken as the type, the first metamere
is never divided into neuromere, optic ganglion, and optic plate, but forms at the
outset a deeply grooved transverse band, ol.o. The walls of the infolding contain
minute, deeply stained nuclei, that make it very conspicuous, both in sections and
surface views. The band marks the primitive anterior end of the brain and is the
anlage of the olfactory lobes. The infolding deepens, at first more rapidly at
either end, and ultimately carries the whole lobe below the surface, and back-
ward, underneath the brain. Here it forms a hollow, bilobed transverse band,
conspicuous in all subsequent stages, when the brain is viewed from the haemal
side, but almost entirely concealed below the hemispheres when seen from the
neural side. (Figs. 40-42, 43, 46, 47.)

THE PROSENCEPHA1.0N.

55

Die.

The cerebral hemispheres arise as mushroom-like expansions of the
second neuromere. In Limulus, they are very conspicuous in the early stages, and
ultimately grow to an enormous size. They consist of dense masses of minute cells,
with deeply stained nuclei, unlike any others in the nervous system. (Figs. 37 and 38.)

As these cells multiply, the hemispheres project above the surface of the
brain and then mushroom, forming large, overhanging lobes. We may dis-
tinguish anterior, lateral, and posterior lobes, the latter being much the largest.
In addition, there is a large lobe on the median face of each hemisphere. (Figs.
47. B, 48 and 49, g.c.) The hemispheres, throughout life, are connected with the
neural surface of the second neuromere by a thick, vertical stalk, or peduncle,
composed of nerve fibers.

As the hemispheres increase in volume, the
posterior lobe completely overlaps the third neu-
romere, and the lateral and anterior lobes partly
envelop the haemal surface. In the adult, the
hemispheres are irregularly convoluted, and
their median faces are flattened against each
other so that they form a large spherical mass
that has a striking resemblance, in external form,
to the hemispheres of vertebrates. (Figs. 38-48.)

In the scorpion, the hemispheres are much
smaller than in Limulus and are crowded farther
forward by the optic ganglia, which have almost
united in the median line behind them. Later
the whole prosencephalon is bent toward the
haemal surface, through an angle of something
more than 90. (Fig. 47, A.) When the fore-
brain flexure is completed, about the time of
hatching, the hemispheres lie on the anterior
haemal surface of the procephalon. (Figs. 42
and 43, c.h. or h.} This flexure is very marked
in all arachnids, so far as known, except in
Limulus.

The third neuromere undergoes very little change. It may be recognized for
a considerable period as a separate neuromere whose neural surface is covered
with tufts of large ganglion cells. It is gradually incorporated into the thick
mass of tissue that constitutes the body of the forebrain commissures, and upon
which the hemispheres rest (basal ganglia). (Fig. 46.)

The history of the procephalic sense organs and their nerves and ganglia
will be considered under their appropriate heads.

II. THE DIENCEPHALON.

The diencephalon in arthropods consists of a variable number of neuromeres
surrounding the mouth. The first neuromere following the procephalic lobes
(antennal neuromere of insects, cheliceral neuromere of arachnids) mav be re-

Ntc

Brc.

br n.wv

FIG. 45. Diagram of the arachnid
brain, showing the number and grouping
of the neuromeres, the ventricles, the vagus
lobes, and the longitudinal gustatory
tracts and their relation to the stomodseal
ganglia.

5.6

THE SUBDIVISIONS OF THE BRAIN.

garded as the initial neuromere of this subdivision of the brain. Its large size
and its special relations, on one side with the hemispheres, and on the other with
the stomodaeum and the gustatory organs, lend to this neuromere and those im-

st.co
oliv

P 1

D

ol.o.

FIG. 46. Semi-diagrammatic sagittal sections, showing the relations of the piincipal nerve centers in the
brains of arachnids and vertebrates. A, Embryo scorpion (stage B, Fig. 15); B, embryo scorpion (stage G, Fig. 18);
C, hypothetical intermediate condition, based on the conditions in both scorpion and Limulus. The embryonic
palium and the anterior neuropore have been carried over into the adult, and the lateral eye ganglia have been pro-
jected onto the neural surface, otherwise the typical arachnid conditions remain essentially unmodified. D shows
the probable position and relation of these parts in a vertebrate. The ventricles are indicated in Roman numerals,
the neuromeres in Arabic numerals.

mediately associated with it in the circumoral region, a special distinction that
justifies their elevation to the rank of a distinct brain region.

% * x ****;)=#

In the scorpion (Figs. 15, 16, 43), by the time the embryo hatches, the fore-
brain is bent through something more than 90, onto the anterior ha?mal surface

THE DIENCEPHALON.

57

of the egg. The angle of this bend lies behind the cheliceral neuromere, which,
therefore, faces forward, connecting the forebrain, now on the haemal surface of the
egg, with the thoracic neuromeres on the neural surface. (Figs. 43-46.)

In practically all adult arachnids, the chelicerse move forward to the very an-
terior end of the head and lie close together in front of the rostrum and stomo-
daeum, instead of behind them, as in the earlier stages. The result is that the
cheliceral nerves, instead of arising from the sides of the brain, like all the other

vg.n.

st.n.

C

FIG. 47. Sagittal sections of brain models. A, young scorpion; B, Limulus; C, a hypothetical brain, combining
the principal characters of the brain of Limulus and scorpion, and with the parts in the position they are supposed
to occupy in a primitive vertebrate.

nerves to the appendages, arise from the median, neural surface, and point
cephalad and neurad. (Fig. 40.)

In Limulus, the cheliceral neuromere is less conspicuous in the older stages
because it is partly covered by the posterior, lobes of the hemispheres, which grow
back over it. (Figs. 37, 38, 47 and 48.) As the cheliceral neuromere moves
forward, it unites so intimately with the third neuromere of the forebrain that it
is difficult to distinguish the boundaries between them. Both neuromeres help

THE SUBDIVISIONS OF THE BRAIN.

n>

form the basal ganglia that lie underneath the lobes of the hemispheres, and
which may be said to form the floor of the prosencoel. (Figs. 57, 58.)

*********

Minute Structure. The minute structure of this region has been worked

out in some detail in Limulus.
Two great masses of neurones that
probably belong to the third pro-
cephalic neuromere are found on
the median, neural surface of the
brain, underneath the posterior
lobes of the cerebral hemispheres
(Fig. 49, ch., H.c.} Their neurites
extend caudad and outward, and
then cephalad, forming a large
part of the posterior cerebral ped-
uncle. On reaching the base of
the hemispheres, they spread out
into great fan-shaped masses that
penetrate into the cortex of every
lobe and convolution, except the
median or gustatory one. (Fig. 48
and 49, G.c.) They run parallel
with similar fibers arising from the
two clusters of association cells,
H.a.s. lying above the gustatory
lobes. They terminate in minute,
spherical masses of neuropile that
form an indistinct, sub-cortical
layer in each lobe, and near which
the neurites of the cortical, granule
cells terminate. Some of the fibers
appear to terminate between the
cortical cells. (Fig. 50.) Numerous
branches from these neurites ramify
in the forebrain commissure, c.o.,
and in the cheliceral lobes, ch.l.

The cheliceral lobes (Fig. 49,
ch.l.) are large spherical masses of
neuropile lying on the anterior
lateral margin of the cheliceral
neuromeres. Their lateral surface is covered with small cells, whose neurites
together with many others, ramify in their interior. The most conspicuous ones
are those belonging to the two sets of cerebral association cells, ch, H.c. and H.as,
and the terminal dendrites of the main gustatory tract, G.tr.

FIG. 48. Median surface of a model of the forebrain of a
young Limulus about four inches long.

THE DIENCEPHALON.

59

The middle portion of the cheliceral neuromere forms the posterior portion of
the great mass of commissural fibers and neuropile upon which the hemispheres
rest. (Fig. 48, b and c.) One may recognize in it: coarse fibers of the lateral cell
clusters, Co 5 ; fibers from the large, central cells of the olfactory lobes (Figs.
48 and 5i,o/.f 1 .); a dark, central mass of neuropile, b. in which innumerable neu-
rites, apparently from all parts of the brain, terminate; and a thin layer of com-
missural fibers extending from one crus to the other, c.

The more anterior portion of the commissural mass (Fig. 48), represents the

cort.

FIG. 49. Diagonal section of the forebrain of a young Limulus (about four inches long), methylene-blue prepara-
tion stained with carmine. Camera outlines.

commissural bundles of the second and third forebrain neuromeres, a and d;
and the several olfactory, ol.c 1 ^ 4 , and optic commissures, op.g. 4 .

The Stomodasal Ganglia and The Suprastomodaeal Commissure. The

cheliceral neuromere is always intimately associated with the system of stomodseal
nerves and ganglia. The lateral stomodaeal ganglia lie on the median side of the
nerve cords, st. g. The stomodaeal commissure, st.c., which always crosses in front
of, or over, the stomodaeum, forms one of the most conspicuous and constant
landmarks of the arthropod brain.

In the scorpion, the anlage to the lateral stomodseal ganglion may be faintly
seen from the surface, on the anterior median face of each half of the cheliceral
neuromere. (Figs. 14 and 15, st.g.) The same anlage may be seen in sections,
in Blatta, Acilius, Buthus and Limulus, as a thickening or evagination of the side

60 THE SUBDIVISIONS OF THE BRAIN.

walls of the stomodaeum. (Fig. 53, c.) The evaginated part separates from the
stomodseum and, uniting with the adjacent neuromere (cheliceral or antennal),
forms the lateral stomodaeal ganglion. In insects, a frontal, or median stomodaeal
ganglion arises in a similar manner from the anterior median wall of the stomo-
dseum. (Fig. 14.)

Nerves extend backward from the stomodaeal commissure into the labrum,
which never receives nerves from any other source. From the deep, or haemal,
end of each lateral ganglion, a nerve extends along the side walls of the oesophagus,
connected by several transverse bands with a median nerve arising from the frontal
ganglion. (Fig. 35.)

In arachnids, the median stomodaeal nerve seen in the insects is absent, and
there are no traces of ganglion cells in the commissure.

The stomodaeal nerves and ganglia represent a distinct system of nerves that
cannot be compared with any others. That it is a very ancient system is shown by
its vigorous growth at an early ontogenetic period. The ganglia, nerves, and
commissure, form a special system controlling the peristaltic actions of the stomo-
daeum in swallowing, grinding, or sucking food. These reflexes may possibly be
directly stimulated through the sensory cells in the inner lining of the stomodaeum
or in the lips; bul , in Limulus at least, an essential condition appears to be an initial
stimulation of the gustatory organs in the jaws, or of the olfactory organ. From
the gustatory organs, important nerve tracts converge toward a common center
in the cheliceral neuromere, and tow r ard the lateral stomodeal ganglion. (Fig.

114.)

Comparison of the Diencephalon of Arachnids and Vertebrates.-

When the mouth of the arachnids was shut off from the exterior by the backward
overgrowth of the rostrum and of the optic lobes, and by the closing up of the cerebral
vesicle, the stomodaeum and the adjacent ectoderm remained in the vertebrates as
the epithelial lining of the third ventricle and adjoining chambers; and the opening
through the floor of the brain, which served as the passageway for the old oesopha-
gus, remained as the infundibulum. The inner end of the stomodaeum, that pro-
trudes through the infundibulum, became the sacci vasculosi; the lateral stomo-
daeal ganglia, the lobi inferiori; and the stomodaeal commissure, the anlage of the
cerebellum. (Figs. 3, 46.)

The median haemal portion of the cheliceral neuromere, which is the princi-
pal center for the olfactory, gustatory, and stomodaeal impulses, corresponds with
the hypothalmic region, while the cheliceral lobes and the cerebral association
cells, ch.H.c, mark the beginning of the thalamus. On the roof of the ventricle,
the median ocellus persists as the parietal eye. (Fig. 47, c.) Let us examine
these comparisons more carefully.

*********

The Stomodaeum. In existing arachnids, the roof of the diencephalic region
consists of the epithelium that was produced by the backward migration of the
rostrum and the mouth. (Figs. 3, 46, and 47.) Owing partly to the manner in

THE DIENCEPHALON OF ARACHNIDS AND VERTEBRATES. 6 1

which the entire brain has slipped forward, underneath the skin, and in part to
this backward growth of the rostrum, the stomodasum becomes divided into two
sections, an inner one passing through a narrow opening between the crura to the
enteron; the other extending backward, over its outer surface, to the mouth.
The outer section is dilated in most arachnids to form a large chamber or sucking
stomach. It is merely a matter of terminology whether we call the mouth the
opening of the original infolding, leading directly through the brain, or the opening
which lies much farther back beneath the overhanging rostrum. In the verte-
brates, both the original infolding and its secondary extension may be recognized.

As we have shown elsewhere, the closing of the primitive oesophagus was due
to several factors, among which were: the crowding together of the cranial neuro-
meres; the increasing size of the palial fold; the backward growth of the rostrum
and optic ganglia along the anterior median line; and the deepening of the median,
neural groove by the precocious thickening of the lateral cords. These condi-
tions lead to the infolding of the entire neuron at an early embryonic period, and
to its complete separation from the overlying ectoderm. Thus, not only were the
eyes enclosed within the brain chamber, but the passage way for the stomodasum
first became greatly constricted, and then the opening into it was covered over by
the neural crests and optic ganglia, thus forever closing the entrance to the enteron
from that direction.

The several processes seen in the arachnids, in vertebrate embryos, are blended
and abbreviated into a simple marginal overgrowth and an axial depression
of the medullary plate. The chamber thus formed over the cheliceral neuromere
then becomes the third ventricle; the epithelium of the extra-neural part of the
stomodaeum merging with, and forming a part of, the epithelial lining of the ad-
jacent cavities. The primitive stomodaeal infolding may still be seen in the
amphibia, as a minute pit in the middle of the procephalic lobes, near their anterior
margin. (Fig. 46.) This pit lies in the position of the future infundibulum and
appears to deepen, giving rise to it. The epithelium forming the floor of the
depression is continuous with the epithelial layer that covers the inner surface of
the adjacent brain cavities, and represents the deeper end of the stomodaeum, now
converted into the membranous saccus vasculosus.

From the posterior lateral walls of the infundibulum, two rounded ganglionic
lobes are evaginated, the lobi inferiori. They correspond with the lateral stomo-
daeal ganglia of the arachnids. Like them, they have direct nervous connections
only with the adjacent epithelial sac (stomodaeum), although the nerve centers
themselves are of considerable size.

According to Johnston, the epithelial wall of the saccus contains large spindle-
shaped, sensory cells, bearing a tuft of cilia which projects into the cavity of the
tube (ventricle). From them arise nerve fibers that help form a nerve plexus
over the outer surface of the sac. The afferent and efferent fibers form two
lateral symmetrical tracts which run through the corpus mammalare to the ventral
part of the thalamus.

*********

62

THE SUBDIVISIONS OF THE BRAIN.

The Optic Ganglia. The palial overgrowth carried the united parietal eyes
over the region of the old stomodaeum, thus helping to form the roof of the third
ventricle, and giving rise to the ganglion habenulae and its commissural strands.

The lateral eye ganglia also united to form a part of the brain roof, but were
crowded still farther backward, beyond the tween-brain neuromeres, carrying w r ith
them the stomoda?al commissure. The latter then became the anlage of the cere-
bellum, and the optic ganglion became the tectum opticum. (Figs. 3 and 46.)

The optic tracts extend diagonally backward and upward from the optic
chiasma to the optic ganglia, and help to form the external lateral walls of the
diencephalon; the primitive cerebellar tracts extend diagonally downward and
forward, over the inner or ventricular surface of the diencephalon to the lobi
inferiores. Thus the location and direction of these important fiber tracts still

tell the history of the parts in which they term-
inate. (Fig. 46, D.}

core.

C.Cor.

*

The Cheliceral Neuromere. The an-
terior neural portion of the cheliceral neuro-
mere is probably in part comparable w r ith the
thalamus division of the diencephalon. It con-
tains the great masses of association cells going
to the hemispheres and cheliceral lobes (Fig.
49, ch.l.} and also the cheliceral nerves and
ganglia. (Fig. 49, ch.g.) The cheliceral nerves
unlike all the other cranial nerves, arise from
the median neural surface of the neuromere.
They are probably crowded against the cere-
bellar commissure by the enlargement and
backward migration of the optic lobes. (Figs. 47, 57 and 58.) They are represented
in vertebrates apparently by the fourth nerves, which seem to arise from the roof
of the' brain, between the optic lobes and the cerebellum, although their roots
have their origin far forward, on the floor of the midbrain region. The excep-
tional location and direction of these nerves in vertebrates, therefore, is in harmony
with their exceptional location and direction in arachnids; and the extraordinary
resemblance between them affords collateral evidence in confirmation of the ex-
planation just given for the origin of the tectum opticum and the cerebellum.
Otherwise these peculiarities of the fourth nerve are inexplicable.

FIG. 50. Portion of the cerebral cortex
of a young Limulus. Camera, Zeiss obj. 16
mm., oc. 18. Golgi preparation.

It will be observed that the whole floor of the vertebrate brain consists of
more or less modified neuromeres. Those structures which now form the roof
of the brain, such as the palium, ganglion habenuke, optic lobes, and cere-
bellum, are not in any way comparable with neuromeres; they have no segmental
value, and they now have no genetic relations with the neuromeres over which

THE DIENCEPHALON OF ARACHNIDS AND VERTEBRATES. 63

they happen to lie. The tectum opticum, for example, really belongs to the third
forebrain neuromere, in front of the diencephalon, and except as a matter of
convenience, cannot be classified as part of the mesencephalic neuromeres.
(See p. 157.)

* * ** * * * * *

Summary. Thus we have in the diencephalon of vertebrates a remarkable
combination of special characters; viz. a sharp cranial flexure; a funnel-like depres-

-ol.m.n.

ey.n.

FIG. 51. Forebrain of a young Limulus, haemal surface. .4. A single optic fibre, showing the arrangement of
its principal branches. Methylene-blue preparation. Camera outlines.

sion in the floor, with voluminous nerve centers (lobi inferior!) on its side wall;
the presence of important nerve tracts arising from the cerebellum, the olfactory
and gustatory organs, and that converge toward a common center in the in-
fundibular region; the presence of a membranous sac that contains the remnants
of sensory cells, and a special set of neuro-muscular reflexes. Each of these
characters is without parallel elsewhere in the brain of vertebrates. They indi-
cate that this particular region either has some very unusual part to play in cere-

04 THE SUBDIVISIONS OF. THE BRAIN.

bral activities now, or did have some such function in the past; but what that
function was, or is, or w r hat is the history and the meaning of these parts, neither
vertebrate anatomy or physiology gives us the slightest clue.

But when we compare these conditions with those in the arachnids, their
meaning is sufficiently clear. The infundibulum is the passageway for the old
stomodaeum, and the latter is the saccus vasculosus. The lobi inferiori are the
lateral stomodasal ganglia; the nerve plexus of the saccus, the stomodaeal nerves;
the tween-brain flexure is the one which occurs in arachnids between the supra-
and infra-stomodaeal ganglia; the remarkable centralization of fiber tracts in the in-
fundibular region is the retention of the ancestral condition seen in arachnids,
where fiber tracts from the olfactory and gustatory organs, and from the stomodaeal
commissure, converge toward the swallowing centers, or the ganglia that control
the neuro-muscular apparatus of the stomodaeum.

In both vertebrates and arachnids, there is: a. but one passage, or opening,

tr

op.tr.-~

FIG. 52. Semi-diagrammatic, longitudinal section of the lateral eye ganglion of a young Limulus.

in the floor of the brain, and in both cases that passage is of similar relative
dimensions \b.\\. lies just behind the hemispheres and the lateral eye nerve roots ; and
r. just in front of the anterior end of the notochord; and <l. in front of the first
pair of somatic motor nerves; e. in both cases, the opening lies approximately
between the right and left halves of the fourth and fifth neuromeres; /. in both
cases evaginations from the median face of the half neuromeres form special
ganglia (stomodaeal ganglia, lobi inferiores), quite unlike any other cranial ganglia;
g. in both cases, these ganglia have similar associations with the stomoda?al com-
missure (cerebellum) , with the epithelial tube lying in the opening through the brain
floor, and with the olfactory and gustatory organs; //. and although of considerable
size, the ganglia have no direct nerve connections with any organs external to
the brain.

The diencephalon, therefore, of arachnids and vertebrates, may be defined
as one, two, or more, neuromeres surrounding the primitive stomodaeum, and
uniting the primitive forebrain (supra 1 eesphageal ganglion) with the midbrain,
or thoracic neuromeres. It lies in the angle of the oldest, and most striking

THE MESENCEPHALON. 65

cranial flexure, and marks the anterior termination of the notochord. The
chamber lying between and above these neuromeres (the third ventricle) was
for a certain period an extension of the stomodasum, or an antechamber to that por-
tion that actually perforates the brain floor. The final closing of the entrance of
this antechamber took place probably at a point just back of the cerebellum
(rhomboidal fissure). (Figs. 3, 46, 58.)

III. THE MESENCEPHALON.

In typical arachnids there are six thoracic neuromeres. The first one (two or
three) (cheliceral neuromere) has already been described as the tween-brain, the
remaining ones constitute the midbrain. They differ from all other neuromeres
in their great breadth, and in the enormous size of their ganglia and gustatory

nerves.

v.n.

St.co. chn.

A

A S ^:.9A?$F W S

c

FIG. 53. Longitudinal, horizontal sections of the head of an embryo scorpion, showing origin of the lateral
stomodaeal ganglion. A, At the level of the suprastomodaeal commissure; B, at a lower level, showing the inner
end of the lateral stomoda?al ganglion; C, still deeper level, showing its origin from the sides of the stomodaeum.
Camera outlines.

In the scorpion, they rapidly increase in size with the growth of the append-
ages, forming a compact group in which the original segmentation is clearly in-
dicated by the arrangement of the nerves and cross commissures. They are
never so completely fused in the adult as to lose their identity, differing in this
respect from those in the forebrain in front of them, and in the vagus region
behind.

In the adult scorpion, the cross commissures are very short, and the thick
cords, or crura, are but slightly separated, leaving a very small opening for the
passage of the oesophagus. (Figs. 40-43.)

In Limulus, this opening is larger, the anterior ends of the crura are widely
separated, and the elongated anterior commissures are bent into wide loops by the
backward movements of the oesophagus. (Figs. 38-39.) These conditions, and
the absence of a tween-brain flexure, give the brain of Limulus a different out-
ward appearance from that of the scorpion, although structurally they are very
much alike.

The great size of the neuromeres and the divergence of the crura make this
the broadest and most voluminous part of the brain, giving it a rhomboidal out-
line, when seen from above.

66

THE SUBDIVISIONS OF THE BRAIN.

Mesencoele. The mesencoele is formed in part by a marginal, epithelial
overgrowth, and in part by a deep median depression or infolding, between the
two cords. The epithelial overgrowth is formed along the whole margin, as a
thin overarching shelf that projects about a third way over each cord. (Figs.
136, 227 and 231.) Later, the cords settle bodily below the surface, and as there
is no delamination of a superficial epithelium, a wide opening is left that is
gradually closed in by the union of the thin edges of the two overgrowths.

As the two cords in this brain region remain horizontal, the potential mesen-
ccele is broad and shallow, except in the middle line where it extends to the bottom
of the deep median fissure.

In the more posterior parts of the brain, in the vagus and branchial regions,

similar overgrowths occur. But the cords are
much narrower here and their margins are brought
together, like the closed covers of a book, by the
deep median infolding, so that the chamber is con-
verted into a deep, narrow fissure. The bottom
of this fissure is converted into a "canalis cen-
tralis," when the neural commissures grow across
the fissure, just above the floor. (Figs. 55, 69.)

Comparison with Vertebrates. The mid-
brain neuromeres of arachnids are represented in
vertebrates by the group of conspicuous neuromeres
forming the floor of the brain, from the infundi-
bulum to the vagus region. The region is charac-
terized by: a. its great width; b. its enormous
FIG. 54. Vagus and branchial cran i a ] ganglia, widely separated from the crura

neuromeres of an embryo scorpion

about ready to hatch. Camera outline, and associated with the oral and hyoid arches; c. by

the segmentally arranged gustatory organs; and d.

by the unusual distinctness of the neuromeres in the early embryonic stages, in
marked contrast with the regions just in front and behind.

In vertebrates confusion has arisen from the failure to distinguish between
the true neuromeres on the floor of the brain and the various structures that have
been forced out of their original positions onto the roof of the brain. The cerebel-
lum and the optic lobes, as we have indicated above, are of very unequal value,
and in no wise comparable with neuromeres. Their position in vertebrates is a
purely secondary one, a long way caudad to their original position and connec-
tions. The optic lobes, that in vertebrates form the roof to the midbrain, clearly
belong to the procephalic neuromeres, while the primitive cerebellar "neuromere"
is a special commissure primarily associated solely with the diencephalon. The
area covered by these structures therefore, varies greatly, and has no constant
relation to the underlying neuromeres.

If we remove the cerebellum and optic lobes from the brain of a vertebrate

THE MESENCEPHALON.

6 7

embryo (frog), it will be seen that the underlying neuromeres form a natural
group, that corresponds approximately with the posterior four or five thoracic
neuromeres of arachnids. It is impossible, at present, to accurately fix either the
anterior or the posterior boundaries of this group, for we do not know how many,
if any, have been added to the cheliceral neuromere to form the diencephalon, or
whether or no the last, or sixth, thoracic neuromere has fused with the first vagus.
There should be, according to our provisional interpretation, five neuromeres,
from the fifth to the ninth inclusive, in this region; and that number corresponds
pretty nearly with the estimated number of neuromeres, and with the number
of head cavities, segmental nerves, and visceral arches that are known to occur
there. (Fig. 58.)

IV. THE METENCEPHALON, OR VAGUS NEUROMERES, AND V, THE BRANCHIEN-

CEPHALON, OR BRANCHIAL NEUROMERES.

The Metencephalon. In many arthropods, the transitional region between
one group of metameres and the next is often marked by an abrupt change
in the size of the pertinent organs, and by their union in the middle line and

, Kco.
I ch.

FIG. 55. Section through the first vagus neuromere of Limulus, showing the neural and haemal commissures, central

canal, and lemmatochord.

ultimate disappearance; at the same time, in the next following segments, the same
kind of organs may be greatly enlarged. These conditions are similar to those
seen at the point of incomplete fissure in annelids.

The most striking cleavage zone of this character in the arthropods is the
vagus region. It lies between the thorax and abdomen, and is one of the most
conspicuous transitional zones in the whole body. It is present in many insects,
Crustacea, trilobites, etc., but its character is best known in the arachnids.

In Limulus, there are two vagus metameres, the chilarial and opercular,
whose neuromeres form an important part of the brain. The external boundaries
of the two metameres are at first similar to those of the other abdominal segments.
(Figs. 141, 142.) But ultimately, the only remaining external traces of the chil-
arial segment are the chilarial appendages and the narrow ridge on the posterior

68

THE SUBDIVISIONS OF THE BRAIN.

margin of the thoracic shield. (Fig. 152, ch.pl.) The tergite of the opercular
metamere is a narrow wing plate, still clearly outlined on the lateral margins, but
in the middle it is completely fused with the abdominal shield op.pl. The hinge
of the cranial buckler comes between the tergites of the chilarial and the opercular
metameres. (Fig. 155.)

The chilarial and opercular neuromeres are completely fused with each other
and with the posterior end of the midbrain; and in the adult only the commis-
sures and the distribution of the corresponding nerves afford a clue to their
identity. (Figs. 65, 66, and 70.)

In the scorpion the first four abdominal metameres belong to the vagus
group. At an early period, there is a pair of rudimentary appendages on each of
these four metameres. (Figs. 15 and 16.) In young scorpions, the first two pairs

"op.

op.tr.

FIG. 56. Cross-section of the mesencephalon of a young Limulus brain. On the right, the section passes
through the fourth pedal ganglion, with its gustatory, entocoxal, and pedal nerves, and the ascending roots of the
fifth and sixth gustatory nerve roots. On the left, the section passes through the root of the fourth haemal nerves,
showing its relation to the commissures and to the great longitudinal tracts. The figure is constructed from
methylene blue and von Rath's preparations. The capital letters indicate the same neurones as in Figs. 65 and 66.

form minute papillae near the unpaired genital opening. The third pair form the
pectines, and the fourth, in part, the first pair of lung books.

The four neuromeres become fused into a dense triangular mass, crowded
forward beneath the midbrain. Its posterior end is elevated, producing a pro-
nounced hind-brain flexure. (Figs. 40, 43, 54.) The vagus neuromeres are rela-
tively narrow, and as the corresponding appendages are crowded toward the
middle line, their pedal nerves arise in a characteristic manner from the neural
surface, not from the sides.

The Branchiencephalon. In the arachnids, the anterior vagus meta-
tneres are characterized by their highly modified tactile or sensory appendages,
the posterior ones only being respiratory. In the transitional forms between
vertebrates and arachnids, respiratory segments were doubtless added to the

THE METENCEPHALON AND THE BARNCHIENCEPHALON.

6 9

head, from time to time, thus leading to the union of the entire group of
branchial neuromeres with the posterior portion of the brain.

We therefore look on the hindbrain of vertebrates as composed of two main
parts successively added to the midbrain, namely: a. the vagus section, consisting
of from two to four fused neuromeres, associated with appendages that have under-

Olfactory

olfactory organ

Coordination

hemispheres

Visual

parietal eye
lateral eye

Swallowing

MI nin t,| i, 'inn

infundibulum

Chewing

and

Gustatory

leg-jaws
oral-arches

Locomotor
Auditory
Equilibrium
Vagus

tactile
lateral line

Respiratory

cardiac
branchial
hypobranchial
sympathetic

Digestive

excretory
genital

FIG. 57. Semi-diagrammatic figure of an arachnid brain and nerve cord. It represents, in the main, the
conditions in Limulus. The parietal eye and the vagus lobes and nerves, however, have the location and arrange-
ment characteristic of the scorpions.

FIG. 58. Here the same parts are shown in the position they are supposed to have in vertebrates. The prin-
cipal changes are in the position of the optic ganglia, which have moved backward and upward to form the
optic lobes; the pedal ganglia have become the cranial ganglia of the fifth, seventh, eighth, ninth, and tenth nerves;
the coxal gustatory organs lay the foundations for the principal lines of canal organs. The branchial neuromeres
have fused with the brain, and the components of the several branchial and vagus nerves going to the same kind of
organ have united, forming compound nerves distributed respectively to the respiratory organs, sense organs,
heart, intestine, and the great branchio-thoracic, or hypo-branchial muscles. The similiarity in the linear distribu-
tion of the principal functions, or the nerve centers controlling them, is shown by the inscriptions between the
figures.

gone extensive reduction and modification, the most important remnants forming
organs of a tactile or gustatory nature; this characteristic group of neuromeres
is largely sensory, and now forms the hindbrain, or metencephalon of arachnids;
and b. a second group of neuromeres belonging to the branchial segments, and

70 THE SUBDIVISIONS OF THE BRAIN.

which were mainly associated with the gills, heart, and viscera; they united with
the brain at a later period.

The combined vagal and branchial neuromeres of arachnids form the hind-
brain, or medulla, of vertebrates. (Figs. 57-58.)

In arachnids, all the branchial neuromeres do not form a part of the brain,
except possibly in certain specialized land forms; but they already show the begin-
ning of that functional segregation of sensory, branchial, cardiac, and motor
nerve fibers that is so characteristic of the hindbrain neuromeres of vertebrates.
(See cardiac and hypobranchial nerves).

In arachnids (scorpion and Limulus) there are in all seven vagus and branchial
neuromeres, which is close to the estimated number in this region in vertebrates.
However, this number probably varies in vertebrates, as it does in arachnids, and
only an approximate numerical agreement and serial location is offered as having
significance. The segregation of the vagal and branchial nerves (in both verte-
brates and arachnids) into the groups mentioned above, is of much greater sig-
nificance than the agreement in the number of neuromeres.

CHAPTER V.

MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS.

METHODS.

In order to better understand the minute structure of the brain, it is essential
to first determine the structure of one of the isolated neuromeres of the spinal
cord. I have used for this purpose one of the branchial neuromeres of Limulus.
But even here the structure is so exceedingly complex that it is possible to work
out or to represent but a very small part of it in detail. Our observations on the
structure of an invertebrate neuromere will be of special interest, because they
are based on the gross anatomy and embryology, upon the minute structure as
obtained by several methods of analysis, and upon the physiology.

For the development of the primitive sense buds and the early embryonic
stages of the cord, I have used the scorpion. For the distribution of the fibers
and cells, I have used the methylene blue "intra vitam" method on Limuli from
2 to 6 inches long; the brain and cord being mounted whole, or sectioned. The
adult nervous system has been tested for its physiological reactions, and the dis-
tribution of the peripheral nerves has been followed with care. Its minute
structure was worked out from sections prepared by several methods, the most
satisfactory ones being the usual Golgi method, and von Rath's picro-osmic-
platinum chloride mixture, followed by methyl alcohol.

I. THE BRANCHIAL NEUROMERES IN LIMULUS.

Development. In Limulus and in the scorpion, during the late embryonic
stages, a thin epithelial overgrowth forms on the lateral margins of the cords, and a
deep infolding appears in the median line between them. The lining to the me-
dian infolding is in part converted into the epithelium of the canalis centralis,
in part into the inner neurilemma, or neuroglia, and into the lemmatochord.

The lateral cords themselves sink bodily below the surface, without separat-
ing off a surface epithelium, and are covered by an ectodermic layer formed from
the marginal overgrowth united to the local (interganglionic) remnants of the
middle cord. (Figs. 227, 231.) As in the arthropods generally, they lie in a
horizontal plane, separated by the median infolding that forms the middle cord.
In the vertebrates, similar conditions prevail, except that the median margins of
the lateral cords are more deeply infolded than the lateral ones, thus bringing the
neurogenic surfaces of the cords face to face. Thus the two lateral cords of the

72 MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS.

arachnids form the lateral walls of the neural tube of vertebrates, the middle cord
canal (canalis centralis) being at the inner surface, the neural crests on the outer
one, and the nerve fiber layers on the lateral wall of the tube. (Figs. 134-137.)

g.Cut.

Commissures. In arachnids, two main sets of transverse commissural
fibers are formed in each neuromere, the neural and the haemal commissures.

The haemal commissures are the first to
arise. They make their appearance on the
haemal side of the epithelium of the median
groove as two separate bundles, an anterior
and a posterior one. (Fig. 64, a.Ji.co. and
p.Ji.co.) The haemal commissures are im-
portant features in all arthropod neuromeres,
and represent in part the remnants of the
transverse fiber tracts that extend round the
body and unite the longitudinal cords. Seen
in sagittal sections of the adult cord, they
appear as two large irregular bundles,
separated by a narrow space through which
the neuroglia of the median canal and me-
dian fissure is continuous with that of the
lemmatochord.

At a much later period a new set of com-
missures appears above the floor of the median
groove, thus converting that part of the groove
into a canal; a large anterior one, an.n.co., a
small middle one m.n.co., and a small but
sharply denned posterior one, p.n.co.

As the longitudinal commissures approach
the neuromere, they divide into distinct neu-
FlG 59 _ The branchial and abdominal ral and haemal tracts //./r. and -n.tr. In the

young Limulus three inches { ^ fi] j each t dther tcrminate
long. The intestinal plexus, i. pi, longitudinal

abdominal, /. ab, general cutaneous, g. cut, seg- j n neuroplle maSSCS situated On the COrr6S-

mental cardiacs, c, and the great branchio- ,..,,., i ,

ponding side of the neuromere, or run straight
through it, the neural tracts passing above the
neural commissures, and the haemal tracts below the haemal commissures.

The peripheral nerves consist of two main pairs. (Figs. 59, 60.) The
anterior pair is the more complex. Owing to the central relation of its fibers,
its position on leaving the cord, and its relation to the great muscle masses, it may
be spoken of as a haemal nerve. It is comparable with the motor or ventral root
of a vertebrate spinal nerve. It is a mixed, non-ganglionated nerve, and contains
general cutaneous, somatic motor, cardiac, and visceral, or intestinal elements.

THE CELL CLUSTERS OF THE BRANCHIAL NEUROMERES.

73

n.tr.

G

nn.n.ca--

B--
i.n.tr. -

The posterior nerve is more voluminous, but less complex than the anteiior
one. It arises from a large conical "ganglion," consisting of neuropile and
ganglion cells, situated on the posterior neural surface of the cord. (Fig. 63, C.}
It supplies the gill muscles and the sense organs of the corresponding branchial
appendage. The majority of the elements are sensory, and are confined in the
main to the neural side of the nerve. It is therefore a neural ganglionated nerve,
comparable with the posterior, or ganglionated
root of a vertebrate cranial, or spinal nerve.

Cell Clusters. The nerve cells in the
neuromere are arranged in clusters that are
remarkably constant in their size, location, and
relation to fiber tracts. Each cluster probably
represents the remnants of one or more primitive
sense buds.

In exceptional cases, the methylene blue fails
to affect the cells and fibers, but stains rather
sharply the neuroglia, thus giving excellent pic-
tures of the nerve cell clusters and their sheaths.
(Fig. 61.) The clusters are mainly confined to
the neural surface and lateral margins of the
neuromere. The more important ones are as
follows :

(/. A cluster of large cells in two or more
groups, on both neural and haemal sides of the
anterior lateral margin. (Fig. 61, ,4.) Their
axones cross to the opposite side, forming part of
the anterior haemal commissure, entering the
haemal nerve as its third root. (Figs. 61 /j.r. 3 and
62, a.) Before crossing, each axone gives off a
large collateral, a', that extends backward into
the longitudinal haemal tracts.

b. A very large cluster of medium size cells
(Fig. 61, B.), whose axones, b. converge into a
large bundle, directed vertically. On reaching
the haemal surface of the cord, each fiber divides,

one branch entering the main longitudinal haemal tracts and extending backward, ?
as a coarse unbranching fiber, to the more posterior neuromeres. (Fig. 62, b.};
the other branch, b', crosses to the opposite side, in the posterior part of the
anterior haemal commissure, behind the fibers forming the third root
of the haemal nerve. They are probably association elements. (Figs. 61, 62
and 64.)

c. Numerous clusters of minute cells, on the neural surface of the pedal
ganglion. Their axones terminate in the large mass of interwoven fiber bundles

n.n

FIG. 60. Two branchial neuromeres,
seen from the neural surfaces, showing the
location of the principal masses of neu-
ropile, fiber bundles, and neurones.

74

MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARCHNIDS.

that constitute the exceedingly complex core of the ganglion. Probably sensory.
(Figs. 62, C. and 63 C.)

d. A cluster of about thirty very large motor cells on the posterior median
part of the neural surface. Their axones form a compact bundle which extends
vertically and then crosses to the opposite side, the two crossing bundles constitut-
ing a large part of the posterior haemal commissure. (Figs. 60, 61, 62 and 64,
D and </.)

hr

a

...I, r 1

FIG. 61. One of the anterior branchial neuromeres of a young Limulus. On the right, the principal cell clusters
of the neural surface are shown; on the left, the principal nerve roots, fiber tracts, and neuropile centers. The
, capital letters indicate the cell clusters, and the small letters, the corresponding fibers; Hr. >- 6 , the five roots of the
hremal nerves. Composite figure, based on methylene blue and von Rath's preparations.

The crossed fibers extend backward along the haemo-lateral side of the cord
to the next posterior neuromere, forming the fifth root to the haemal nerve, //. r 5 .
They are very conspicuous in sections on account of the large size and pro-
nounced coloring of the axis cylinders and their sheaths. On approaching the
next following neuromere, the bundle becomes more compact and gradually
moves toward the outer margin of the cord, where it turns sharply forward and

THE CELL CLUSTERS OF THE BRANCHIAL NEUROMERES.

75

outward into the haemal nerve. It there divides into two bundles; one La b, con-
stitutes the nerve supplying the hasmo-neural and longitudinal abdominal muscles;
the other, extending onward into the main trunk, forms the branch that supplies
the branchio-thoracic muscles (hypoglossal elements).

e. A group of large cells, lying on the posterior haemal side of the pedal
ganglion. (Figs. 60, 61, 63, and 64, E and e.) Their axones are directed diago-

a

h.r.

FIG. 62. Anterior branchial neuromere of a young Limulus seen from the haemal surface. On the right, the
superficial, longitudinal, haemal tracts are shown; on the left, the underlying haemal cross commissures, and their
relation to the principal groups of neurones. Outside the main figure, on the left, is a cross-section of a neural
nerve, A 7 . A 1 "', and a haemal nerve, H. A 7 , On the right, the minute structure of some of the nerve tubes in
the haemal nerve, m:t, and in the neural nerve, s. /, is shown.

nally forward, inward and upward; they then cross over to the opposite side, and
return again to the haemal surface where they extend forward as fine unbranched
fibers on the lateral margins of the longitudinal, haemal tracts.

The crossing bundles constitute apparently the whole of the posterior neural
commissure. (Fig. 64, p.n.co.}. Before and after crossing, the axones give off
numerous dentrites which ramify in the neuropile core of the pedal ganglia (Fig.
63, E.) Probably association fibers.

/. A medium sized cluster on the anterior lateral margin of the pedal ganglion.

76 MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS.

(Figs. 60, 61, 63, 64, F.) Their axones form a rather loose bundle (ill defined in
sections) extending downward, inward, and forward along the neural portion
of the cord, to the next anterior neuromeres. Before and after crossing, the large
irregular fibers give off numerous collaterals which ramify diffusely in the central
fibrous portions of the neuromere. (Fig. 63, F.} Termination unknown; probably
association fibers.

g. On the anterior lateral part of the neural surface, small clusters not
clearly defined, that send axones diagonally inward and forward. (Figs. 61 and
63, G.) The scattering axones appear to cross in both the anterior neural and
the anterior haemal commissures. Some appear to send collaterals backward to
the next posterior neuromere. Termination unknown.

//. Along the lateral haemal margin, on either side of the pedal ganglion,
are several groups of cells that are usually very conspicuous. (Fig. 62, H l ~ 3 .)
They differ from the other neurones in that each cell gives rise to a large number
of dendrites and axones. The dendrites are minute and their innumerable
branches fill the core of the pedal ganglion, often giving it a dark blue, finely
granular appearance. The axones are large, irregularly branching fibers ex-
tending outward, as bundles of parallel fibers, onto the haemal surface of the pedal
nerve. These neurones are the only ones of their kind and are characteristic of
the pedal nerves, both in the thorax and in the abdomen.

The axone bundles from the several clusters, H*~\ converge toward the
haemal side of the pedal nerve where they form a distinct bundle, readily recog-
nizable in sections. (Fig. 68, h.) They are the motor nerves that supply the gill
muscles.

/. Two large groups of neurites on the anterior hamio-lateral margin, sending
great bundles of fibers forward and inward into the anterior portion of the an-
terior haemal commissure. (Fig. 62, / and 64,/.)

_/'. A large group of cells on the posterior haemal margin, projecting their
fibers forward and then across to the opposite side, in the anterior portion of the
posterior haemal commissure. (Figs. 62, and 68, J,von Rath's preparations.) The
cells and fibers of this group have not been identified by the methylene blue
process.

NERVE-ROOTS.

The Neural Roots. The neural or branchial nerve arises from a large
ganglion on the posterior neuro-lateral surface of the neuromere, and extends
upward (neurally) and outward to the gill. In cross sections (von Rath's
preparations) near the neuromere, it consists of two portions, the larger one
formed of a coarse polygonal meshwork of neuroglia, each mesh crowded with
black dots, representing the cut ends of innumerable nerve fibers. (Fig. 62, s.t.}
These sensory fibers constitute about three-quarters of the entire nerve. Most
of them terminate in very fine dendrites, in the large oval mass of neuropile that

BRANCHIAL ]\ 7 ERVE ROOTS.

77

constitutes the posterior lateral portion of the core of the pedal ganglion. (Fig. 63).
A small group of libers extends beyond the main core into the median neural
region of the neuromere. (Figs. 60 and 61, n.p.) This neuropile center is very
dense and stains with great intensity in von Rath's preparations.

The haemal fascicle consists of small nerve tubes with sharply denned axis
cylinders, separated by a wide, clear space from the outer sheath. They are
motor fibers arising from the peculiar huemal neuromeres, H 1 ' 3 . (Fig. 62, m.t.}
They supply the branchial muscles.

hr

h.n

FIG. 63. Anterior branchial neuromere of a young Limulus, showing the course of the principal neurones. Neural

surface; methylene-blue preparation.

The oval mass of neuropile in the ganglion of the branchial nerve consists of
an extraordinarily complex system of interwoven bundles of very fine fibrils. In
this neuropile the following fibers terminate: i. the dendrites of about three-
quarters of all the fibers of the branchial nerve: these fibers are sensory. 2.
the collaterals of the motor neurones, H l ~ 3 . 3. the collaterals of the E neurones
whose axones cross in the posterior neural commissure, and 4. the dendrites of the
minute C neurones that constitute the principal cellular covering of the ganglion.

78 MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS.

The Haemal Nerve Roots. The haemal nerves arise from the anterior haemal
margin of the neuromere and extend upward and outward. In the more pos-
terior segments, their apparent point of attachment shifts forwards to a point
midway between the two neuromeres, but without changing the root terminals.
(Fig. 59.) They contain the following fascicles:

a. A bundle of large motor fibers arising from the D neurones on the neural
surface of the opposite side, in the next anterior neuromere. (Figs. 61,62. Ji.r 5 .)
It runs along the haemo-lateral margin of the cord, becoming more and more dis-
tinct as it approaches the next following neuromere. There it bends outward
onto the anterior haemal surface of the haemal nerve, dividing into two fascicles.
One forms the small, purely motor nerve supplying the haemo-neural and the
longitudinal abdominal muscles, Lab.; the other passes into the main part of the
nerve, and separates farther on, as the branch that supplies the branchio-thoracic
muscles. (Fig. 59, b.th.) (hypoglossal elements.)

b. A large fascicle of pale fibers (Figs. 61 and 62, H.r'), that extends along
the anterior neural margin of the nerve. On entering the cord, it runs diagonally
forward, inward, and upward, terminating in an elongated mass of neuropile,
on the median, neural surface of the next anterior neuromere. Its peripheral
termination is unknown. Probably cardiac.

c. A large central fascicle, H.r 2 , terminates in a conspicuous, isolated mass of
neuropile of the same side, near the anterior median region of the same neuromere.
Some of the fibers pass through the neuropile, neurad, and cephalad, joining the
median, longitudinal, neural tracts. Probably general cutaneous fibers.

d. This fascicle, H.r 3 , is not easily followed in sections, but its fibers are
frequently seen in methylene blue preparations. They spring from the large
neurones A, on the opposite side of the neuromere. Their collaterals are shown
at a'. (Figs. 6r, 62.)

e. This fascicle extends backward toward the neuropile center of the bran-
chial nerve, H.r*. (Fig. 61.)

/. The intestinal fascicle is a small bundle of fine fibers, int. In the anterior
segments, it leaves the cord with the haemal nerve; in the more posterior ones, it
arises separately. (Fig. 59, 7 1 " 14 .) In the first ganglion (Fig. 61, int.), it runs
along the posterior side of the haemal nerve and then turns sharply forward over
the neural surface of the sensory root, H.r 1 , to a small, ill defined group of cells
lying in group A.

We find, therefore, in the haemal nerves, the following roots or fascicles : two
sensory roots terminating in neuropile on the neural surface of the cord, on the
same side, one in the same neuromere, H.r 2 , the other in the one next in front
of it, H.r 1 . (Fig. 60.) Two roots, ending in cell groups on the opposite side of the
cord, one group, D, on the posterior neural side of the next anterior neuromere, the
other, on the anterior neural surface of the same neuromere, .4. A fifth root,
H.r*, extends caudad, disappearing in the neuropile near the base of the neural
nerve. The intestinal branch should perhaps be counted as a separate nerve.

COMMISSURES OF THE BRANCHIAL NEUROMERES.

79

It is only in the second neuromere that it is anatomically a branch of the haemal
nerve; in the more posterior neuromeres it arises from the side of the neuromere
and entirely separate from the haemal nerve roots.

Commissures. The transverse commissures of the cord may be divided
into two sets: a. the primary, or haemal commissures, passing underneath the
epithelium of the embryonic median groove, and representing the primitive nerve
tracts uniting the right and left cords; and b. the secondary, or neural commissures,
crossing the neural fissure above the floor of the median groove. The neural
commissures, phyllogenetically and ontogenetically form much later than the
haemal, and only after the median groove becomes deep enough to bring the
superior median margin of the two cords into contact.

The anterior and posterior haemal commissures are separated by an opening
in the floor of the neural canal through which the neuroglia passes to the underlying
lemmatochord (Figs. 64 and 68.)

The anterior haemal commissure consists of several indistinct bundles. So

an.n.co. m.n.co. p.n.co.

B

B

m.n.co. p.n.co.

I-.,-- D

h.tr.

FIG. 64. Sagittal section of two anterior branchial neuromeres, showing the relation of the principal neurones

and fiber tracts to the cross commissures.

far as could be determined, the anterior portion consists of fibers from neurones
I; the middle portion from neurones A; and the posterior portion from neurones
B (Fig. 64).

The posterior haemal commissure contains fibers from group J (anterior
bundle) and from group D (posterior bundle).

The haemal commissures therefore contain, among fibers of undetermined
character, crossed motor axones and various collaterals.

The neural commissures are three in number; an anterior, a middle and
a posterior one. (Fig. 64.) The sources of the fibers in the anterior neural
commissure could not be certainlv determined. Those of the middle commissure

j

m.n.co., are derived from neurones, F, and those of the posterior commissure from
neurones E. No other fibers could be located in these commissures. The neural
commissures appear to be largely composed of association fibers.

The Neuropile Centers. The neuropile centers are dense masses of inter-
woven terminal dendrites. They appear in von Rath's preparations as dense
black masses of fine fibrils, and in methylene blue, as masses of fine blue dots or
lines, according to the character of the stain.

MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS.

The principal centers are : a. a large oval center at the root of the pedal nerve,
forming the medullary core to the pedal ganglion. (Fig. 60.) A prolongation of
it extends cephalad and mesad, forming a compact, oblong mass, near the center
of the neural surface of the neuromeres; n.p. b. A center for the cephalic root
of the haemal nerve, Hr', extending along the median neural surface of each
ganglion, c. A center for the middle root of the haemal nerve, H;- 2 , on the ante-
rior median face of the neuromere. d. The haemal tracts; large, spindle-shaped
tracts, one on either side of the median line, on the haemal surface. (Figs. 62, 67,
68, /. h.tr.) (e) Four small masses, on the haemal surface, between the anterior
and posterior haemal commissures and the longitudinal haemal tracts. For longi-
tudinal tracts in sections, see Figs. 67 and 68.

II. THE CEPHALIC NEUROMERES.

We are now in a position to describe the arrangement of cells and fibers in
the cephalic neuromeres.

The brain neuromeres, in the main, closely resemble those of the cord.
The principal differences in form are due to their linear union and to the lateral
divergence of the crura. The histological differences are due mainly to the
absence of motor neurones such as the hypobranchial, intestinal, and cardiacs, to
the greater size and isolation of the pedal ganglia, and to the presence of the
large gustatory nerves.

Cell Clusters. The nerve cells are arranged in clusters, of varying sizes,
that have special neurilemma sheaths, as in the cord, but they are so crowded
together that it is difficult to determine the exact arrangement. They cover the
neural surface and lateral margin of the crura, leaving the commissures, part of
the gustatory tracts, and the haemal surface exposed. (Figs. 65 and 66.)

The Commissures. Each neuromere has several bundles of cross commis-
sures that have terminal relations similar to those described for the branchial neu-
romeres. Owing to the divergence of the crura they form long, backwardly directed
loops in which the commissural fascicles are difficult to identify, except where
they approach the crura.

In very young crabs, sagittal sections show that the commissures of each
neuromere are surrounded by distinct membranes. There are two groups of
commissures for each neuromere, corresponding to the neural and haemal commis-
sures of the cord, and no doubt containing similar components. In the adult,
the median portion of the anterior thoracic commissures form compact bundles
with a common neurilemma sheath; near the crus, the several fascicles separate
to their respective terminals. (Fig. 56.) The more posterior thoracic commis-
sures, and those in the hindbrain, are shorter, and the neural and haemal fascicles
are widely separated, leaving a space between them, which represents the begin-
ning of the fourth ventricle. (Figs. 46, 47 and 55.)

The neural commissures. I have not been able to work out the relation

THE CEPHALIC NEUROMERES. 8 1

of all the commissural elements in detail, although some of them stand out very
clearly. For example, in methylene blue, the posterior neural commissures are
often very conspicuous. They extend diagonally forward (anterior ones) and
outward on to the neural surface of the crura, over the great gustatory tracts, to
the cell clusters E, on the posterior hasmo-lateral margin of its neuromere. (Fig.
65, E, p.n.co.)

A bundle of these fibers, on the anterior side of the second thoracic neuro-
mere, indicates that the cheliceral neuromere, in spite of its distinctly pre-oral
position, has its commissure behind the oesophagus. In most cases, one may
recognize two sets of these neural fibers to each neuromere, an anterior one,
arising from the neurones , and a posterior fascicle, ending in a separate mass of
neuropile on the lateral half of each crus.

The haemal commissures contain several sets of fibers. The ones most
clearly seen in methylene blue preparations are rather large fibers which arise from
neurones A (Fig. 66) and after crossing, enter the roots of the haemal nerves.

Between the nerve roots and the commissure, are two sets of longitudinal
fibers extending outward and backward along the haemal surface of each crus,
one on the median side, one on the lateral. (Fig. 66, left.) Most of the longi-
tudinal fibers come from the opposite side through the haemal commissure; some
of the lateral ones probably come from neurites, B, on the neural surface of the
same side, corresponding with group B of the cord.

The Haemal Nerve Roots. It will be recalled that the cranial haemal
nerves supply the integument of the cephalothorax. The branches, which
in the abdominal nerves run to the great longitudinal muscles, to the branchio-
thoracic muscles, to the heart and to the intestines, are absent from the six pairs
of thoracic, haemal nerves. Hence they have but a single root, mainly, if not wholly
sensory, and representing root two, H.r~, of the branchial neuromeres.

In most preparations, the haemal roots appear to extend only part way through
the crus, terminating abruptly in the main longitudinal tracts on the median side
(Fig. 66, right side). They appear to end there in a mass of neuropile, like that
of the second root of the abdominal nerves, H.r 2 . In other preparations, many
fibers are seen to enter the ha?mal commissure and terminate in the .4 neurones
of the opposite side, which no doubt correspond to the A neurones of the abdomen.

Xo trace of any other roots could be found. From this observation we may
infer that roots two and three, H.r 2 , and H.r 3 , of the abdominal haemal nerves
are sensory general cutaneous; that root one, H.r 1 , contains the cardiac elements;
and that the neurones, D, are distributed to the branchio-thoracic muscles.

The Neural Nerve Roots and the Cranial Ganglia. Owing to the greater
size and specialization of their terminal organs, the neural, or pedal nerves of the
head are much larger and more complex than those of the branchial region, but
in the minute structure of their ganglia and nerve roots they are much alike.

Ganglia. In the adult Limulus (Fig. 218), each cranial ganglion forms a
large oval mass of neuropile, projecting a considerable distance from the sides

82

MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS.

FIG. 65. Brain of young Limulus about three inches long, seen from the neural surface. The. cerebral hemi-
sphere, on the left, is shown in optical section, at the level of the giant association neurones, H . as, and on the right,
at a deeper level, showing the principal cerebral lobes and the gustatory tracts, G. ctr 3 . and g. tr.- Compare with
Fig. 48. On the right, behind the hemispheres, the superficial arrangement of the clusters of nerve cells, and
the cranial nerves are shown. On the left are seen the gustatory nerve roots, the great longitudinal gustatory
tracts, the principal neurones and their relation to the neural commissures, and the principal neuropile masses
in the pedal and stomodaeal ganglia; methylene-blue preparation.

THE CEPHALIC NEUROMERES.

op.tr

p.n

FIG. 66. Same from the haemal surface, showing on the left the more superficial neurones, fiber tracts,
nerve roots, and commissural fibers; on the right, the deeper ones. Note the great extension backward of the
fibers arising from the optic nerve, op. l>, and from the giant nerve cells of the optic ganglia, op. g 4 , and olfactory
lobes, ol. c l and ol. c 3 .

84 MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS.

of the brain. In the young (Figs. 35, 36, 38, 39), they are separated from the
brain by long narrow stalks, or tracts, devoid of neurones or neuropile.

Most of the peripheral fibers terminate in the neuropile core, forming many
minute irregular centers (Fig. 65). There is a special neuropile center on the
anterior neuro-median side of each ganglion, formed by the terminals of a small
bundle of rather distinct fibers. This center in some cases appears like a dark
granular blotch, in others like a beautifully distinct, anastomosing network (Fig.

65, it 1 )- Their peripheral relations were not determined.

The nerve cells of the ganglia are very small and numerous. They are con-
fined in the main to the haemal surface, sending short, vertical axones into the
neuropile, where they divide, one branch passing outward into the nerve, the
other forming branches in the neuropile, which extend toward the crus. (Fig.

66, C).

The Motor Neurones. On the haemal surface of the ganglia, there are two
very conspicuous bundles of coarse fibers, coming from large neurones on either
side of the ganglion. A third bundle joins them, coming from cells on the neural
side of the ganglion. They form the small anterior and posterior ento-coxal
nerves supply that the coxal muscles. (Fig. 66, a.cn.c.\ and p.cn.cx.')

These neurones agree with the H neurones of the branchial ganglia, in that
each one sends a considerable number of axones into the nerve trunk.

The Gustatory Nerves and Tracts. The most striking features of the mid-
brain neuromeres are the large nerve roots supplying the taste organs in the coxal
spurs (Fig. 65, g.n.r.-~ r> ). They extend along the neural face of the pedal ganglia,
sweeping diagonally inward and forward, the inner end of each root overlapping
the next posterior one. The united bundles form an immense longitudinal tract
along the median neural margin of each crus, between the neural and haemal
commissures.

The more posterior roots are the largest, that of the sixth appendage largest
of all. The deep, inner ends of the fifth and sixth roots form large oval neuropile
enlargements on the posterior median face of each crus.

Toward the anterior end, one may recognize two subdivisions to the main
tract. Near the stomodaeal ganglion both divisions turn outward and downward.
then upward, forming a sharp semi-circular turn round the crus, giving the latter,
in cross-sections, a very unusual spiral structure. The larger bundle apparently
terminates in the great cheliceral lobe (Figs. 65 and 114, ch.l) ; the smaller one g.tr
forms a slight dilatation, consisting of very dense nodular neuropile, on the lateral
margin of this lobe, and then passes straight forward, along the neural surface
of the cheliceral neuromere to the median cerebral lobe, G.ctr 3 .

The very large fasciculus of the sixth nerve comes, not from the coxal spurs,
which here form crushing mandibles devoid of gustatory spines, but from the
large spatulate organ on the outer margin of the coxa (flabellum), the function of
which is to test the composition of the water passing to the gill chamber.

THE CEPHALIC NEUROMERES. 85

A small, posterior fasciculus, coming from the chilaria, joins the main gusta-
tory tract. (Fig. 65, clicld.n.)

In methylene blue, the gustatory tracts have a very characteristic appearance,
as each fascicle contains a great many parallel bundles of extremely minute fibers
that look like rows of dots.

In sections (von Rath's method), they may be recognized by their dense black
masses of fine parallel fibers (except in the nodes, where they are twisted and inter-
woven) and by the small quantity of neuroglia contained in them. (Fig. 56,
g.n.r*- 6 .)

In the scorpion, a similar tract may be recognized. (Fig. 69.) But here the
most conspicuous portion is the immense neuropile bodies on the roots of the first
three vagus nerves. These nerves supply the genital papillae and the pectines,
and the immense size of these vagal lobes is due to the great development of
sensory (tactile) organs in the pectines.

Similar lobes are seen in Limulus, but lying farther forward, and associated
with the immense, flabellar nerve (gustatory) belonging to the sixth pair of legs.

III. LONGITUDINAL TRACTS.

There are several well defined longitudinal tracts in Limulus that may be
traced the entire length of the brain and cord, but their relations to the various
centers and to the motor and sensory terminals is exceedingly difficult to deter-
mine. In the main, the sensory elements run on the neural surface of the cords,
and the motor ones on the haemal surface.

We may distinguish the following tracts:

The Longitudinal Haemal Tracts of the Brain and Cord. The haemal
tracts are great sheets of longitudinal fibers covering the ha?mal surface of the brain
and cord. They can be seen in von Rath's preparations, along the haemal surface
of the neuromeres, haemal to the transverse commissures (Figs. 55, 56, 67, 68, /.//.
lr}. They leave the anterior and the posterior ends of the neuromere in the nearly
isolated haemal sections of the longitudinal connectives (Fig. 64). Midway
between the ganglia they cannot be distinguished from the other fibers of the cord.

In methylene blue preparations, these fibers of the cord may be followed at least
from one ganglion through the next without branching, and in some cases through
several ganglia. In the brain, individual fibers may be followed the whole length
of the crus. (Fig. 66.)

In the brain many of these fibers terminate in small clusters of dendrites,
scattered over the crura, just below the haemal surface (Fig. 66, left side) ; in the
branchial neuromeres they are seen on the haemo-lateral surface just neurad of the
longitudinal fibers. (Fig. 62, right side.)

The fibers of the haemal tracts are derived from several sources. One impor-
tant source is the large cluster of B neurones on the neural surface of each bran-
chial neuromere. Their fibers, after reaching the haemal surface, divide, one cross-

86 MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS.

ing to the opposite side in the anterior haemal commissures, the other turning
backward into the haemal tracts. (Figs. 61 and 62.)

In the cerebral neuromeres, the haemal tracts contain similar fibers, derived
from similar cells. The latter may be seen in small clusters between the roots
of the ganglia, on the neural surface of the crura. Their fibers pass vertically
through the crus, joining the longitudinal tracts and the cross commissures. (Figs.
56, 65, 66, B, 6, and b.')

The lateral margin of the haemal tracts of the crura receives conspicuous
fibers that cross in the haemal commissures with the roots of the haemal nerve.
Near the lateral margin of the crura they turn backward and join the lateral
margin of the haemal tracts (Fig. 66, left side). A similar, but less conspicuous, set
of fibers forms on the median side of the tract.

The lateral margin of the haemal tracts also receives a considerable number of
large fibers from the third and fourth lobes of the optic ganglion, op.g 3 ' 4 , and
from the crossed and uncrossed fibers of the large lateral neurones of the olfactory
lobes, ol.c'.

The median margin receives fibers from the large central cells of the olfactory
lobes, ol.c?, and from the giant association neurones in the median lobe of the
hemispheres. (Fig. 49, H. as. tr.)

A remarkable band of fine fibers comes from the optic nerve, passing through,
or over, the lateral margin of the first two optic lobes, along the lateral margin of
the optic stalk, through the tween-brain, and along the median haemal surface of
the crura to the beginning of the cord. (Fig. 66, op.tr.) A similar band of fine
fibers extends along the entire lateral margin of each haemal tract of the cord.
The two bands are united by a narrow commissure extending across the anterior
margin of each neuromere. (Fig. 62, op.tr. ?) It is not clear whether these fine
fibered bands of the cord are continuations of the optic bands in the brain or not.

The great majority of the longitudinal fibers of the crura that are directed
forward appear to terminate on the haemal surface of the forebrain commissures.

The Longitudinal Neural Tracts. These tracts lie close to the median
line, on the neural surface of the cord. They receive all the fibers of the first
root of the haemal nerves, many fibers from the nucleus at the root of the pedal
ganglia, and large unbranched fibers whose origin is unknown, that pass through
the cord over several neuromeres.

In the crura the neural roots of the rui-mal nerves appear to be absent, and
the other constituents of the neural tracts could not be certainly identified.
But we may recognize the following tracts which may or may not be modifications
of those already described.

The lateral or pedal ganglion tracts are large and exceedingly complex,
consisting of a confused mass of interlacing fiber bundles which form the lateral
margins of the crura; most of their fibers come in roughly parallel bundles from
the roots of the pedal ganglia (Fig. 56, /,/;-., right side). On reaching the tract,
the fiber bundles take on a longitudinal trend. The tracts are greatly enlarged

THE NERVE FIBER TRACTS. 87

opposite the pedal ganglia, and between them they are reduced to narrow bands.
(Fig. 56. /./r, left side). At the anterior end of the crura, they appear to pass
outside (neural) the gustatory tracts, onto the posterior neural surface of the
tween-brain commissure. (Fig. 49, l.tr.)

The general cutaneous tracts are two large, continuous columns of
neuropile on the median side of each crus. They lie between the gustatory and
haemal tracts, and extend from the tween-brain to the last vagus neuromere,
(Fig. 56 G.c.tr.)

They usually have a slightly different color and appearance from the lateral
ones, from which they are separated by numerous bundles of vertical fibers. The
latter are arranged with considerable regularity, the more important sources being
neurones B, sending axones haemally, and neurones E, sending them neurally.
(Figs. 56 and 65, B and E.) The principal constituents of the tracts are the
crossed and uncrossed terminals of the cutaneous nerves, h.-n. The majority
of the uncrossed fibers and the optic fascicles, op.fr, extend lengthwise of the
tracts.

Comparison of the Fiber Tracts of the Arachnid and Vertebrate Brain. -
A comparison of the fiber tracts in Limulus with those in the vertebrates presents
great difficulties. These difficulties are partly due to our imperfect knowledge
of the brain of arachnids and of the lower vertebrates, and partly to the fact that
the latter is generally studied for the purpose of explaining the structure of the
higher types of brain, not for comparison with an invertebrate brain, that its
own structure might be better understood. When we know more about the
brain of arthropods, and less emphasis is laid on the artificial system of classify-
ing cranial nerves, now in vogue among American neurologists, many points are
likely to be cleared up which are now obscure.

Nevertheless the facts, so far as we understand them, indicate that the arachnid
and vertebrate brain are in essential agreement in the distribution and relations
of their main fiber tracts. The agreements to which we would call attention are :

a. In both classes, important longitudinal tracts containing the principal
motor fibers extend along the haemal surface of the brain and cord.

b. In both classes, conspicuous sensory tracts lie near the median neural
surface, coming from segmentally arranged taste organs and from other sense
organs, i.e., tactile, temperature, or auditory, having a less precisely determined
function.

c. In these sensory columns, there are local enlargements, or lobes, corre-
sponding with special local functions; i.e., in Limulus, the flabellar lobes of the
eighth and ninth neuromeres; in the scorpion, the pectinal lobes of the tenth,
eleventh, and twelfth vagus neuromeres; in vertebrates, the auditory and the
vagal lobes of their respective neuromeres.

d. In both classes, the numerous gustatory fascicles and those from the
vagus neuromeres form a very conspicuous median neural tract, extending the
whole length of the brain floor. It terminates in a special center, in the dienceph-

88

MIUNTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS.

l.n.tr.

int.

FIG. 68.

FIGS. 67 and 68. A series of sections of the first branchial neuromere of an adult Limulus, showing the location
of the principal cell clusters, commissures, fiber tracts, lemmatochord, and central canal. The numbers 66 to 138
indicate the serial numbers of the sections.

THE NERVE FIBER TRACTS.

8 9

alon, which is in turn connected by special tracts with the olfactory lobes, hem-
ispheres, and cerebellum.

e. In both classes, the nerve roots are arranged in two distinct series, neural
and haemal; each series may contain both motor and sensory elements.

In Limulus, the haemal roots enter the brain toward the haemal surface and
extend horizontally, through the crus and the ha'mal commissures, to the main
nucleus or cell cluster on the other side of the median line. But many fibers
end in dendrites on the same side the nerve enters.

In vertebrates, a similar condition prevails in the ventral or haemal nerves,
for according to Johnston "It is a noticeable peculiarity in the origin of the nerve
(i.e., the third nerve of vertebrates) that a large part of the fibers arise from the

:ran.

r m.ch.j

FIG. 69. Four cross-sections of the brain in the vagus region of an adult scorpion, showing the enormous vagal
lobes, the central canal, and the cephalic portion of the middle cord, or lemmatochord.

nucleus of one side, and cross to enter the root of the opposite side. The same
arrangement is found in the roots of other ventral nerves, but to a much less
degree."

On the other hand, the neural nerves are associated with special ganglia, which
arise independently of the brain, and which are attached to its neuro-lateral
margin. In Limulus, these fibers end, or originate, on the same side the nerve
enters the brain; very few, if any, fibers of a neural nerve arise from cells located
on the opposite side of the median line.

/. In both classes, the floor of the brain is divided lengthwise into two main
columns, a median and a lateral one, by an important series of vertical fibers;
arcuate fibers, vertebrates, B and E, fibers, arachnids. The prolongations of
these fibers run lengthwise in the haemal (and neural ?) tracts and crosswise in
the commissures.

g. In Limulus, there is in the vagus region an important decussation of im-
pulses coming from the trunk (See section on Physiology, p. 191) ; and there is also

90 MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS.

there a special condensation of commissural bundles, that have been crowded
together from before backward, in order to leave a large opening for the end of
the oesophagus. (Figs. 57 and 58.) In the vertebrates, there is a similar back-
ward dislocation of commissural bundles, forming a special group just back of the
choroid plexus of the fourth ventricle, "commissurainfirma." Johnston says (p. 287)
that "Behind the choroid plexus the c. infirma contains the visceral, sensory
elements proper to the segments of the VII, IX and X nerves. It is probable that the
course of the root fibers of these nerves within the brain has been influenced by the
crowding backward of their decussation and median nucleus by the choroid
plexus." The choroid plexus could hardly have the power to dislocate the
cerebral framework. The dislocation was probably brought about, as in Limulus,
by the backward migration of the outer end of the old stomodseum, and when
that closed, the choroid plexus grew over it, leaving the permanently distorted
commissures to testify to the event.

//. In both classes, there is a remarkable ganglionated commissure extending
over the neural surface of the brain, the stomoda?al commissure of arthropods
and the cerebellum of vertebrates. Both structures represent very primitive
commissural tracts, the only ones which develop, primarily, from the roof of the
brain chamber. Both commissures may be ganglionated, and they are the only
ganglionated transverse commissures in the primitive brain. Both commissures
develop in connection with the fourth neuromere. Both have special relations
with the gustatory centers on the posterior median face of the stomodaeal opening
(infundibulum).

In arachnids, this commissural arch lies behind the cerebral lobes and the
optic ganglia. In vertebrates, it is crowded backward by the enlarged optic
lobes so that part of its fibers are directed downward and forward, toward the
posterior wall of the diencephalon. (Figs. 3, 46, 57, 58.)

IV. COMMISSURES.

Summary. The facts, bearing on the cross commissures of arachnids,
that have been brought out in the preceding pages, may be summarized as follows:

i. The right and left cords of the primitive neuron were united by a series
of transverse commissures, two for each neuromere. The anterior commissure
is primarily related to the anterior segment of the neuromere and to its peripheral
nerve, the other to the posterior segment, and to its nerve. The commissures
arise, during the early embryonic periods, as fibrous, non-cellular bands, extending
across the floor of the middle groove. They are separated from one another by
deeper infoldings of the groove. In the adult, they are still separate, and each
contains several distinct fiber bundles, differing in origin and in histological
characters.

These commissures always retain their primitive position on the floor of the
neuron, hence they are called the haemal commissures. They are the primary

THE COMMISSURES. QI

communicating paths between the right and left cords, and between the right and
left peripheral nerves.

During the later stages, the cords increase in thickness, the median in-
folding forms a deep fissure, and then three new commissures appear in each
neuromere, extending across the fissure above the old ones. These secondary, or
neural commissures, consist largely of association fibers.

With the formation of the secondary commissures the bottom of the fissure,
at certain points in each neuromere, is converted into a canal, " canalis centralis"
(Figs. 55, 56, 68, 69, r .f.), that is lined in the earlier stages with an epithelium
derived from a part of the original median infolding. (Figs. 222, 224 and 231.)
The floor of the canal is formed in part by the haemal commissures, and the roof
is formed in part by the neural commissures. When the neuromeres are widely
separated, there are of course wide gaps in the roof and floor of the canal between
the commissures in front and those behind. In the vagus region, owing partly
to the increased thickness of the crura, the canal is greatly enlarged, marking the
beginning of a chamber comparable with the fourth ventricle or metenccele.
(Figs. 55 and 56). Here the neuromeres are more closely united than elsew r here,
and their neural commissures, together with some of those belonging to the more
anterior neuromeres that have been crowded backward into this territory by the
oesophagus, form a special group over the posterior part of the region of the
fourth ventricle. (Fig. 47, C.)

In the scorpion also, the immense vagal lobes are united by tw r o special neural
commissures. (Fig. 47, A and 69.)

The combined neural commissures of the vagal neuromeres of the arachnids,
and the more posterior thoracic ones, consist largely of somatic sensory associa-
tion fibers; they probably represent the beginning of the commissura infima of
vertebrates.

In the arachnids there is a wide gap between the forebrain and midbrain,
where there are no primitive commissures. This opening, or infundibulum, is the
passageway for the old oesophagus. In front of it, the character of the commis-
sures changes greatly, in both vertebrates and arachnids. There are no neural
commissures, and the haemal ones form practically a single, but very complex
mass of fibers. (Fig. 48.) In it we may recognize the olfactory commissure,
representing the commissure of the first cerebral neuromere, and lying, morphologi-
cally, at the anterior end of the nervous system. Owing, however, to the in-
ward and backward migration of the lobes, it lies, in the adult Limulus and
scorpion, on the posterior haemal surface of the forebrain. Fig. 47 A and B.
The forebrain commissure also contains the commissures of the second and third
neuromeres, and of the lateral eyes; but these commissures, which arise at an
extremely early period, are not separated into distinct bundles. (Fig. 48.)

In Apus, the ganglia of the median ocelli unite with each other in the middle

92 MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS.

line, over the neural surface of the brain; they rest on the latter by two short
stalks. In many other phyllopods and arachnids, the optic ganglia occupy a
similar position, in that they lie close together, over the neural surface of the
brain, behind the hemispheres. In the vertebrates, they permanently occupy
this position, and have become united by secondary commissures, one in the
habenuke, the other in the optic lobes. (Fig. 308.)

The most conspicuous commissure in the arthropods and one of the rirst to
appear, is that belonging to the system of stomoda?al nerves. Its ganglia, one
median and two lateral, arise from the walls of the stomoda?um. It is the only
commissure originally provided with ganglion cells, and the only one formed
primarily across the neural surface of the brain. It has especial relations to
nutrition, through its association with the olfactory, swallowing, and taste centers.
It represents the primitive cerebellar commissure of vertebrates, where it appears
to have had similar relations.

V. THE NEUROCCELIA.

Summary. The transformation of the paired nerve cords of invertebrates
into the hollow nerve tube of vertebrates is affected by several independent factors.
These factors make their appearance as active forces in the arachnids, and they
have already established there the salient features of the vertebrate neurocoelia.
These factors are as follows :

1. The infolding for the middle cord initiates the canalis centralis and the
more posterior parts of the brain cavities.

2. The increasing depth of the median groove, and the increasing thickness
of the two cords, brings the median edges of the cords together, and leads to the
formation of the neural commissures, which form the rafters over the median
groove, and aid in its conversion into a canal.

3. The formation of the palial overgrowth for the forebrain, and the mar-
ginal overgrowths in the hindbrain region, initiates the development of the broad,
membrane-roofed ventricles of the whole brain.

4. The stomodteal infolding, between the forebrain and midbrain, establishes
the deep and narrow third ventricle of the diencephalon.

5. The deep transverse infolding across the very anterior end of the medul-
lary plate gives rise to the cavity of the olfactory lobes.

6. The broad chamber formed by the palial overgrowth, and into which the
hemispheres project, establishes the prosenco?le, or the first and second ventricles.

7. The median and the lateral eye ganglia unite above the neural surface
of the brain, one forming a partial roof to the diencephalon, and the other, owing
to the shape of the ganglion, forming a broad, dome-like covering for that part of
the brain chamber known as the mesenccele.

8. The stomodaeal commissure, forced backward by the enlarging optic
ganglia, forms the first stage of the narrow arch (cerebellum) over the future
metencoele.

THE NEUROCCELIA AND THE NEUROGLIA. 93

g. In the forebrain, and in the midbrain regions, the medullary cords, in both
vertebrates and arachnids, remain practically horizontal. As they are very
broad (the diencephalon alone showing a marked lateral compression), and as
the overgrowth is almost entirely from the sides, the resulting cavities are broad
and shallow. The roof is epithelial in character, and all the nervous material,
except the optic ganglia and the stomoda^al commissures, forms the floor of the
chamber.

10. In the region of the cord, however, the infolding combines another factor,
especially prominent in the vertebrates, in that, as the median groove deepens, the
two cords close like the covers of a book, bringing their outer, or neurogenic sur-
faces, face to face, converting the neural groove into a deep lying canalis centralis,
and reducing the marginal overgrowths to the narrow strip of epithelium that
roofs over the posterior fissure. (Figs. 134, 137 and 231.)

VI. THE NEUROGLIA.

The neuroglia arises from the epithelial lining of the middle cord groove or
canal. Referring to the early stages of Acilius (Fig. 221), it will be seen that the
medulla is invaded by numerous small, dark nuclei, that spread out laterally from
the middle cord, forming a uniform envelope about the medulla, the so-called inner
neurilemma. Later these cells multiply and invade the medulla and surround the
nerve cells, forming a coarse, nucleated reticulum or neuroglia.

In the adult, this tissue is easily recognized. In sections of the brain and cord
of Limulus stained in haemotoxylin, Lyons blue, and acid fuchsin, it is intense red,
and the nerve fiber masses blue. In preparations treated by von Rath's method,
it is intense black, nerve fibers gray. In sections of the adult cord (Figs. 67 and
68), the neuroglia network may be seen springing in root-like processes from the
thick layer lining the central canal, as well as from the sides of the neural fissures
and the surface of the medulla.

CHAPTER VI.

PERIPHERAL NERVES AND GANGLIA.

I. COMPONENTS OF A NEUROMERE.

In my first paper "On the Origin of Vertebrates," 1889, it was maintained
that the primitive arthropod neuromere was a complex structure, consisting of
two segments, four pairs of nerves, and a segmented middle cord.

While I have seen no reason to change my view as to the composite nature
of primitive neuromeres, I do not now regard the ancestral arthropod as an elon-
gated worm-like animal of many like metameres, but as a small-bodied one of
about three imperfect segments. In the arachnid and crustacean descendants
of this stock, the evolution of neuromeres, as we have explained elsewhere
(Chap. XIII) was a gradual process that advanced with the successive additions
of new groups of unlike metameres.

II. NERVES OF THE DIENCEPHALON AND MESENCEPHALON. l

A. Neural Nerves.

In Limulus, there are six pairs of thoracic neural nerves. (Figs. 36-39.)
The third nerve is typical. (Fig. 79.) It divides, soon after leaving the brain,
into three sets of nerves. The gustatory nerves, three in number, are ganglionated
and terminate in the numerous sensory buds, of the mandibular spines. They are
absent, or very minute, in the sixth pair of legs. The anterior and posterior
entocoxal nerves, a.e.n., and p.e.n. are motor and supply the tergocoxal muscles;
the median entocoxal nerve, m.e.n., supplies the sensory knobs of the coxopodite.
The main pedal nerve, consisting of two principal branches, supplies the muscles
and sense organs of the appendage.

The Flabellum. There are a few minor differences between the six pairs
of pedal nerves; the most important is an enormous enlargement of the
median entocoxal nerve of the sixth leg to form the nerve of the flabellum. (Fig.
80, flab.)

The flabellum is a large spatulate organ attached to the outer side of the
coxal joint of the sixth leg. It is first seen in the embryos as a rounded knob,
lateral to the sixth leg, and quite separate from it. Hence it has the same rela-
tion to the outer side of the appendage that the mandibular placode has to the
inner. (Figs. 141-148.) There are indications of flabellar placodes on the other

'For nerves of the prosencephalon see Chapters VIII to X.

94

CRANIAL GANGLIA. 95

thoracic segments, but they quickly disappear. The flabellar placode finally
unites with the base of the sixth leg and then appears to be a part of it. The flabellum
lies in the mouth of the channel leading to the gill chamber, and practically all
the water going to the gills, either from the front or from the sides, must pass over
its anterior surface. This surface is pigmented and very richly supplied with
sense organs and nerves, and it undoubtedly serves to test the quality of water
going to the gill chamber, (see p. 113.)

The flabellar placodes are probably represented in scorpion by the lateral
coxal sense organs. (Figs. 15-16, s.so.)

The Cranial Ganglia. The base of each pedal nerve, near its origin
from the brain, enlarges to form an immense spindle-shaped ganglion. Similar
ganglia are present in Branchipus, scorpions, and spiders, and they are also present
on the pedal nerves of many other arachnids and phyllopods.

In Limulus they arise, at an early period, from large ectodermic thickenings
between the base of the legs and the corresponding neuromere. How the con-
nection between the ganglion and neuromere is established was not determined, but
it is certain that the ganglion is not an outgrowth of the nerve cord. The body of
the ganglion separates from the thickening, but retains its connection with the
overlying ectoderm by ganglionated nerve strands. The latter become the
gustatory nerves, and the ectodermic remnant of the thickening becomes the
mandibles with their gustatory spines. The thick mass of slime buds on the
inner face of the mandibles also arises from these thickenings.

In young Limuli, the ganglia are relatively large masses of cells, separated
by clear fibrous stalks from the corresponding neuromere. (Figs. 36-39.) In
the adult (Figs. 70 and 218) they are drawn a little closer to the brain, but are
never completely merged with it.

In scorpion embryos, similar ectodermic thickenings appear at the base of the
thoracic appendages, furnishing the anlage for the coxal ganglia. (Fig. 74, D.}
The thickenings on the third and fourth appendages become greatly enlarged to
form the four hypostomeal spurs that lie on either side of the mouth and rostrum.
(Figs. 15-16.) The median face of these spurs is highly sensitive (gustatory?)
and from them are developed enormous ganglia and thick masses of mucous glands
or slime buds.

In the adult, the median face of the anterior pair of spurs is deeply grooved.
The two grooves lie close together and thus form a thick chitenous tube, lined with
sensory hairs. In feeding, the scorpion thrusts the spurs into its prey and sucks
the blood and other fluids through this tube into the mouth. (Fig. 43, mxL)

The independent origin of the flabellar and coxal spur placodes of Limulus
and the scorpion suggests their homology with the inner and outer branches of
an originally triramous appendage.

The coxal placodes represent the supra-branchial placodes of vertebrates.
Their homology is indicated by their similarity in position, in function, in devel-
opment, and, so far as may be determined, in number. In both cases, the pla-

9 6

PERIPHERAL NERVES AND GANGLIA.

FIG. 70. Nervous system, endocranium and endochondrites of an adult Limulus.

THE H.EMAL NERVES OF THE DIENCEPHALON AND MESENCEPHALON. 97

codes split into a special group of gustatory organs, and into a large cranial gang-
lion. (Figs. 27-34.)

The Ganglia of the Cord. Limulus. The roots of the neural nerves
arising from the postcephalic neuromeres are also provided with ganglia, but they
are not as large as those on the cranial nerves. They merge with the cord at an
early period, and, in the adult form the large swellings on the roots of the branchial
nerves. (Figs. 59-64.)

Scorpion. In the scorpion the anterior pairs of nerves are much larger than
the posterior ones, and spring from the outer or neural surface of the cord. In
stage H, just before hatching, the root of the anterior nerve contains a large mass
of cells, evidently arising independently of the nerve cords, just as the pedal
ganglia do in the thorax. (Fig. 73, D.) In the later stages, just after hatching,
the ganglion is drawn toward and partly merges with the cord. In the adult,
ganglion cells are scattered for some distance over the root of the nerve. (Fig.

73, C.)

Meantime the two haemal nerves move forward, and unite to form a single
one with two roots, which in turn unite, a short distance from the cord, with the
ganglionated neural nerve. There is no actual mingling of fibers, but the nerves
run together, for a short distance, as a single nerve. (Fig. 72.)

The Haemal Nerves. Limulus. A single pair of haemal nerves arise from
the anterior haemal surface of each thoracic nueromere. (Fig. 70, h.n.) They
are much smaller than the pedal nerves, without ganglia, contain motor and
sensory fibers and are distributed mainly to the integument and other tissues of
the thoracic shield. The sixth pair alone sends branches to the heart and intes-
tine. Near the outer margin of the entacoxite, the nerves which are elsewhere
round, become broad, flat bands; the parallel bundles of nerve fibers become inter-
woven in a complicated manner, and there is an increased number of neurilemma
nuclei, but no ganglion cells. Beyond this swelling, the nerve divides into two
main branches, n and li; one going to the neural surface of the carapace, and the
other to the haemal. After several subdivisions (see original memoir), the end
branches of all these nerves form a continuous, subdermal plexus, distributed
over the whole inner surface of the neural and haemal integument, supplying the
skin, glands, muscles and sensory hairs.

Lateral Line Nerve of Cheliceral Neuromere. All these thoracic haemal
nerves are essentially alike, except the first one or that of the cheliceral neuro-
mere. This remarkable nerve (Fig. 70, l.c.n.), at first extends forward, and then,
bending backward in a broad curve, extends the whole length of the body. It
runs close to the neural surface, just outside the bases of the appendages, and does
not begin to branch till it reaches a large sclerite behind the base of the sixth leg.
The main nerve continues beyond this point the whole length of the branchial
chamber, sending one small branch toward the base of each of the five gills. This
is a purely sensory nerve and supplies the skin lining the channel along which the
water is carried to the gills. It is very remarkable that this nerve should cross the

o8 PERIPHERAL NERVES AND GANGLIA.

6

territory of so many other nerves of the same nature, in order to innervate a
region so far removed from its origin. It is suggestive of the lateral line nerve of
vertebrates, but its origin from the tween-brain region is strongly against such
an interpretation. It resembles the large nerve in ganoids and teleosts, the
ramus lateralis accessorius, which arises well forward in the head and is distributed
to the taste buds of the head, back, tail, and fins.

The character of the sensory terminals to this nerve in Limulus is unknown.

III. THE NERVES OF THE METENCEPHALON.

We have already shown that a certain number of abdominal metameres in
arthropods move forward and unite with the thorax, and that there is a great
reduction in their size and an obliteration of their external boundaries. The
appendages and muscles show a similar reduction, but the corresponding nerves,
neuromeres and heart segments are but little changed. In fact, the nerves and
neuromeres may be relatively more voluminous or extensive than elsewhere.
These metameres constitute the vagus zone and their neuromeres the meten-
cephalon. Their nerves may be appropriately called vagus nerves, because, as in
the vertebrates, they extend backward into regions to which they did not originally
belong.

LIMULUS.

Neural Nerves. In Limulus, this region contains two metameres, the chi-
larial and the opercular. The tergites of these metameres are still visible in the
adult, the chilarial tergite forming a narrow band on the posterior margin of the
thoracic shield, the opercular tergite, two wing-like segments on the anterior
margin of the branchial shield. The hinge joint between the two shields lies
between these two metameres. (Figs. 150-155.) The first entapophysis is
formed between the chilarial tergites and the true thoracic metameres. (Fig. 193.)

The chilaria are without question true appendages. Their early develop-
ment is like that of the other appendages, and they have separate nerves, muscles,
mesoblastic somites, and gill bars. The chilarial and opercular neuromeres
have all the typical nerve elements. They resemble the branchial neuromeres
more than the thoracic, although in the adult they are intimately fused with the
hindbrain and widely separated from the branchial neuromeres. Their nerves
pass out of the occipital foramen of the endocranium together with the spinal
cord. (Figs. 70-218.)

The chilarial nerves arise close together from the posterior neural surface
of the accessory brain. They pass out of the endocranium just below the roof
of the occipital ring, enter the chilaria and supply their muscles, the adjacent
skin, and the numerous gustatory spines on their median side. (Fig. 81, n.n 7 .)

The opercular nerve follows the same course, and on reaching the operculum

THE NERVES OF THE METENCEPHALON.

99

FIG. 71. Brain and nerve
cord of a new-born scorpion,
seen from the haemal surface.

PIG. 72. First two free bran-
chial neuromeres, with the lemma-
tochord, spinal ganglia, and spinal
nerves for an adult scorpion.

TOO

PERIPHERAL NERVES AND GANGLIA.

divides into three nerves which then subdivide into motor and sensory branches
(for details, see original memoir, 1893).

The Jiff mat nerves (Fig. 70, h.n 1 and /z.w 8 ), pass out of the endocranium
through the occipital ring and are distributed to the sides of the body, between the
sixth pair of legs and the operculum. For the distribution of the intestinal and
cardiac branches, see pp. 103, 200.

Scorpion.

In the scorpion, the vagus region consists of four metameres, two genital,
one pectinal, and the first branchial, as I demonstrated in my first paper on this
subject, 1889.

The tergite of the first metamere fuses with the thorax and cannot be detected

D

FIG. 73. A, Section of an abdominal neuromere of a new-born scorpion, showing the ganglionated "dorsal,"
or neural, nerve root; B; same through the non-ganglionated hasmal nerve root; C, section of the second free
branchial neuromere of an adult scorpion, showing both neural and haemal nerve roots and the neural and haemal
transverse commissures; D, third branchial neuromere of an old embryo of a scorpion, showing the large ganglionic
lobe at the root of the neural nerve.

in the adult. The other three tergites remain separate throughout life, the second
or genital, and the third or pectinal, being much narrower than the fourth (Fig. 17).
The fusion of these metameres is more strongly marked on the neural than on the
haemal surface.

During the early embryonic stages, one may recognize four distinct pairs of
rudimentary, abdominal appendages (Figs. 15, 16). During the later stages,
the first, lung book appears in place of the last appendage. The third pair gives
rise to the pectines; the first pair disappear altogether by the time of hatching;
while the second pair finally unite in the median line in front of the pectines to
form the genital cushion, or tubercles.

Before they unite, about stage G (Fig. 16, B}, the genital openings may be

THE NERVES OF THE METENCEPHALON.

101

recognized on the median margin of each appendage. About the time of hatching,
the genital ducts are carried forward and unite to form an unpaired opening,
between the remnants of the first pair of abdominal appendages and the second.
During stage G, the brain may be dissected out, and the arrangement of neuro-
meres and some of the nerves observed (Fig. 54). The first two neuromeres are
crowded forward and are overlapped by the posterior margin of the hindbrain.
This produces a sharp haemal flexure in the brain, at the dividing line between the

|g^pf&.f:T

FIG. 74. D, Section through the basal lobe of the third thoracic appendage of an embryo scorpion; E, section of one
of the segmental sense organs on the outer margin of the thoracic appendages. See Figs. 15 and 16.

thoracic and vagus region; the first two vagus neuromeres are thus partly concealed,
in surface views, under the overlapping hindbrain. The vagus nerves, during
these early stages, are small and cannot be followed with certainty. In the adult
scorpion, they present an interesting condition. The nerves are now divided
into two groups, one containing all the neural nerves, the other all the haemal.
(Fig. 42.) This is largely due to the constriction which is such a characteristic

i'ent

tnt.

pit. rhs.p.ent.

FIG. 75. Side view of the endocranium, brain, neural arches, and associated muscles of an adult Limulus; semi-
diagrammatic.

feature of the vagus region in arthropods. All the paired organs in this
vicinity, such as genital ducts and appendages, are drawn toward a median
position, hence in the adult the corresponding nerves take their origin from the
neural surface of the cord, near the middle line.

The Neural Nerves. One small nerve, supplying the sexual ducts and papil-
lae, probably represents the nerve of the second pair of rudimentary appendages
(Fig. 42, i' 1 ). The nerve to the pectines has three roots, the first one g.n. forming

102 PERIPHERAL NERVES AND GANGLIA.

an immense bilobed ganglion (ganglion nodosum) composed of ganglion cells and
concentric lamminse of medullary substance (g.nd).

It is united with its mate by two distinct bridges of nerve tissue that lie some
distance above the surface of the neuromeres they thus form an imperfect roof to
a deep, narrow canal between the two ganglia and the median sides of the under-
lying neuromeres (Fig. 40). The anterior ends of the ganglia may be traced in
transverse sections a long distance forward, as two great longitudinal fasciculae,
just below the neural surface of the thoracic neuromeres. (Fig. 69, v.g.l. and
g.t.r.)

The ganglion on the second root (ganglion fusiforme) is smaller, spindle-
shaped, and as near as can be determined, appears to belong to the third neuromere.
The third root is small, fibrous, and without any ganglionic enlargement.

The Haemal Nerves. The two haemal nerves of the first neuromere remain
separate, as in a typical thoracic neuromere. In each of the three following
neuromeres, they unite to form a single nerve, each with a double root. (Fig. 42,
h.n. 1 ' 4 .) A short distance from the brain all five haemal nerves form a compact
bundle that extends backward through the occipital foramen of the cartilaginous
cranium. (Figs. 71 and 217.) The nerves to the third and fourth neuromeres,
h.n. 3 and h.n. 4 , some distance from the brain, fuse to form a single nerve supplying
the first and second lung books and the ventral surface of the body (Fig. 71).
On its way to these organs, it passes over the ventral surface of the liver, to which
it possibly gives branches. The anterior haemal nerve of the first vagus neuromere,
h.n. 1 runs close to the coxal gland, and dividing into numerous branches, is lost
on the surface of a thick, peritoneum-like membrane. The posterior nerve,
h.n. 2 extends along the arthroideal membrane, supplying numerous sense organs
on the lateral and the haemal surface of the abdomen. The fourth vagus nerve,
h. n. 4 supplies the skin and the longitudinal muscles on the ventral surface of the
abdomen.

IV. NERVES OF THE BRANCHIENCEPHALON.

The branchial neuromeres differ from those of the brain in that they remain
separate through life. Their nerves are noteworthy for their association with the
respiratory muscles, the heart, and the intestine.

Limulus.

The Branchial Nerves. In Limulus, the branchial, or neural nerves, contain
both motor and sensory fibers. They arise from large ganglia on the posterior
neural surface of the neuromeres ; on entering the gills they divide into three branches.
(Fig. 82.) The external branch, eb.n. supplies the abductor muscles and the skin
on the anterior lateral surface of the appendage. The median branch g.n.
supplies the adductor muscles and the gill books. The internal branch i.bn.
upplies the skin and muscles in the terminal portion of the appendage.

THE NERVES OF THE BRANCHIENCEPHALON.

I0 3

The Haemal Nerves (Figs. 59 and 70), arise from the anterior margin of
the neuromere and extend outward over the neural surface of the abdominal
muscles. They divide into five principal branches; one goes to the enteron, one
to the longitudinal abdominal muscles, one to the branchio-thoracic muscles,
one to the heart, and one to the integument.

ol.o

rost.

Cran.

ht.

Cran.

78

C

FIG. 76. Side view of the brain, endocranium, alimentary canal, and principal vascular channels in Limulus; semi

diagrammatic.
FIG. 77.- Same, showing the relation of the compound branchio-thoracic, or hypobranchial nerve to the haemal

nerves of the vagus and branchial-neuromeres. Semi-diagrammatic.

FIG. 78. Same, showing the relation of the segmental cardiac nerves, s.c.M.6-i3, to the heart and to the vagus and

branchial neuromeres. Semi-diagrammatic.

The Enteric Nerves. In Limulus, the enteric nerves are intimately associated
with the nerves to the longitudinal abdominal and haemo-neural muscles. The
enteric nerves form a plexus which covers the entire mesenteron and the plexus
is united with the roots of all the haemal nerves, from the sixth to the sixteenth, by
paired rami communicantes. (Fig. 59 z' 7 ~ 14 .) Those from the sixth and seventh
neuromeres pass through the foramina in the posterior lateral wall of the
endocranium. (Figs. 59 7 ' 8 and 218.)

104

PERIPHERAL NERVES AND GANGLIA.

The rami from the sixth thoracic, the chilarial, and opercular neuromeres
mingle with a plexus of nerves distributed over the longitudinal abdominal muscles ;
from there branches pass forward, ramifying over the surface of the mesenteron
as far as the stomodasum.

In the branchial neuromeres, the nerves supplying the longitudinal abdominal
muscles and the intestine are separate. The former arise from the anterior side
of the hsemal nerve root (Fig. 60, Lab.), and the latter from the posterior side.
In the more posterior neuromeres, the intestinal branches gradually shift their
point of origin from the root of the haemal nerve to the median margin of the neu-
romere. In the branchial segments, the intestinal rami send a small branch to
the corresponding hasmo-neural muscle.

The enteric nerves appear to represent the initial stages of the sympathetic

4-

man.

FIG. 79. Muscles and distribution of nerves in the third leg of Limulus, from the anterior side, i-cox., Coxo-
podite, or first joint; 2-bas., basipodite, or second joint; $-isc., ischiopodite, or third joint; 4-car., mer., fused car-
popodite and meropodite, or fourth joint; s-pro., propodite, or fifth joint; 6-dac., dactylopodite, or sixth joint; apo.,
apodeme.

MUSCLES: 30 and &, Plastro-coxal muscles inserted upon anterior side of witocoxite; 3' and 3^, tergo-coxal
muscles inserted upon anterior side of entocoxite; e. 2 ' 6 , extensors of second to sixth joints;/. 2 - 6 , flexors of second
to sixth joints; f.m, flexor of inner manible.

NERVES: a.e.n., Anterior ento-coxal nerve; br., brain; e.p.n., external pedal nerve; h., haemal branch of in
tegumentary nerve; h.n. 3 , haemal nerve; in.n. 3 , integumentary branch; i.p.n., internal pedal nerve; m.e.n., median en-
coxal nerve; m.ti., mandibular nerves; n., neural branch of integumentary nerve; n.n. 3 , neural nerve; p.e.n.,
posterior ento-coxal nerve.

system of vertebrates. It is a noteworthy fact that in Limulus the anterior cranial
nerves are not directly united with the enteric plexus by segmental communicating
branches. The most anteiior connecting branch that is recognizable belongs to
the ninth cranial neuromere, resembling, in this respect, the well known condition
in vertebrates.

The Longitudinal Abdominal Muscles and Nerves. The longitudinal
abdominal muscles arise from the posterior haemal side of the endocranium and
pass backward, giving slips to each pair of the abdominal entapophases and to
the abdominal endochondrites. (Fig. 75.)

The muscles are provided with a rich nerve plexus, extending their whole length.

THE HYPO-BRANCHIAL MUSCLES AND NERVES. 105

(Figs. 57, 59, Lab.) The fibers arise from small branches of the sixth to the sixteenth
haemal nerves. The branches are given off from the anterior side of the haemal
nerve, close to the cord. The neurones lie on the opposite side of the next anterior
neuromere, with those that supplythe branchio-thoracic muscles. (Fig. 60, D, Lab.)
In the seventh and eighth metameres, the plexus appears to be continuous with
that going to the intestine.

The General Cutaneous Nerves are largely, if not wholly, sensory. They
extend over the surface of the branchial plastron, dividing into numerous branches
on the margin. (Figs. 59, 70, g.cut.) The fibers that enter into these branches
probably form the second root, h.r. 2 (Fig. 61.)

Cardiacs. For a description of the segmental cardiacs, see p. 200.
The Branchio-thoracic, or Hypo-branchial Muscles and Nerves.

The hvpo-branchial muscle is a large compound muscle derived from the
eighth to thirteenth metameres inclusive. The neural end of each component is
separate and terminates in a tendinous infolding of the ectoderm at the base of
its corresponding appendage. The haemal ends form a single massive muscle
which shifts its position a long ways forward into the haemal region of the thorax,
where it is attached to the inner surface of the shield, in front of the anterior end
of the heart and the forebrain. (Fig. 78, B.)

The muscle aids in the performance of the complicated respiratory move-
ments, drawing the bases of the gills forward and upward; it also aids in flexing
the thorax on the branchial section of the body.

The hypo-branchial nerve 1 forms a great longitudinal trunk extending over
the neural surface of the muscle. (Figs. 59 and 77, b.th.n.) It receives its fibers
from the eighth to the fourteenth haemal nerves, via short communicating
branches. It is, therefore, to be regarded as a compound nerve formed by the
united branches of at least seven segmental nerves. For the greater part of its
course, it forms a compact longitudinal trunk, giving off at regular intervals
branches to the proximal ends of the muscle slips, near their tendinous at-
tachment to the base of the gills. At its anterior end, it breaks up into many
branches that are distributed through the single muscle into which the six
separate muscles merge. One nerve separates from the anterior end of the main
trunk and supplies the inter-tergal, or arthro-tergal, muscle. (Fig. 77, ///./.)

The nerve fibers arise from clusters of large D neurones lying on the opposite
side of the cord, on the posterior margin of the neuromere, in front of the one
where the nerve enters the cord. (Fig. 60.) The very large axones cross in the
posterior haemal commissure and pass backward, as a conspicuous bundle of
large nerve tubes, h.r. 5 , to the haemal nerve root. Some of the fibers go to the
longitudinal abdominal nerves, but the main bundle passes on with the haemal
nerve, leaving it farther on, to enter the main hypobranchial.

The remarkable condition of the hypobranchial muscles and nerves of
Limulus is, no doubt, one that has its counterpart in other arachnids. At present,

1 Lateral Sympathetic of Patten and Redenbaugh.

io6

PERIPHERAL NERVES AND GANGLIA.

practically nothing is known about the anatomy of these muscles and nerves in
other invertebrates.

In Limulus, the fact that is specially noteworthy is that the six originally
vertical components of the branchio-thoracic muscle have been converted into a
nearly horizontal, or longitudinal, compound muscle, thereby destroying all
correspondence between the metamerism of the neural and haemal surfaces as far
as this muscle is concerned.

The hypo-branchial muscle, by this change in position, gains in effectiveness
as a respiratory and flexor muscle, but it would be a mistake, I believe, to accept

6-d

fcas

man

i.tn:

FIG. 80. Muscles and distribution of the nerves in the sixth leg of Limulus, from the anterior side, i-cox.,
Coxopodite, or first joint; 2-bas., basipodite, or second joint; 3-isc., ischiopodite, or third joint; 4-car., mer., fused
carpopodite and meropodite, or fourth joint; $-pro., propodite, or fifth joint; 6-dac., dactylopodite, or sixth joint;
apo., apodeme; bt., brain.

MUSCLES: 6a and 66, Plastro-coxal muscles inserted upon anterior side of entocoxite; 6c and 6d, tergo-coxal
muscles inserted upon anterior side of entocoxite; e. 2 'l , extensors of second to seventh joints;/. 2 "", flexors of second
to seventh joints; i.m., inter-tergal muscle.

NERVES: a.e.n., Anterior entocoxal nerve; e.p.n., external pedal nerve; /;., hmal branch of integumentary
nerve; k.n.6, haemal nerve; i.n. 6 , intestinal nerve; in.n. 6 , integumentary branch of haemal nerve; i.p.n., internal pedal
nerve; l.c.n., lateral cardiac nerve; m.c.n., median cardiac nerve; m.e.n., median ento-coxal nerve or flabellar nerve;
m.n., mandibular nerve; n., neural branch of integumentary nerve; n.n..&, neural nerve; p., pericardium; p.e.it.,
posterior ento-coxal nerve; s.c.n.6, segmental cardiac nerves.

that as a primary cause of the change in position, or as an explanation for the
existence of that particular function. The real reason lies deeper, and is to be
seen in those changes that have gradually reduced the volume of the haemal organs
in the anterior head region. This atrophy or reduction of the haemal surface of
the head during the early embryonic periods, inevitably draws the haemal structures
of the post-cephalic metameres forward, and is the initial cause of that forward

VAGAL AND HYPO-BRANCHIAL NERVES.

107

displacement and condensation that we have just described in the haemal ends of
the hypo-branchial muscles.

This condition is a very ancient one, for these very muscles, in this position,
no doubt cause that folding of the thorax onto the abdomen which is so common
in trilobites. I have seen the same thing in Bunodes, very much to my astonish-
ment. For sections of specimens that appeared to be headless, showed that the
cephalo-thorax was present, but doubled over so as to lie with its neural surface
flat against the neural surface of the branchial region.

The general trend of the branchio-thoracic nerves no doubt has been deter-
mined by these morphological changes in the muscles; but the union of these
several nerves into a common trunk is to be regarded as an expression of the
tendency to gain simplicity by the merging of several separate agents, performing
the same function, into a single one.

V. RELATION OF THE VAGAL AND HYPO-BRANCHIAL NERVES IN ARACHNIDS TO

THOSE IN VERTEBRATES.

The entire system of nerves belonging to the vagal and branchial regions in
the arachnids, represents the initial stages in the evolution of the vagal and bran-
chial complex in vertebrates. We can already distinguish in the arachnids the
beginning of that remarkable segregation of
similar components into compound nerves,
that in the vertebrates has given rise to the
branchial, hypoglossal, cardiac, visceral, and
lateral line nerves; and the beginning of that
readjustment in the position of the terminals
that has given to each set of components their
characteristic distribution and direction of
growth. (Figs. 57, 58.)

The branchio-thoracic muscles and nerves
of Limulus are clearly comparable with the
hypoglossal nerves and muscles of vertebrates.
In both vertebrates and arachnids, the nerves
arise: a. from a large number of post-vagal
neuromeres (five branchial and one oper-
cular); b. they are either haemal nerves (ven-
tral roots) or branches of haemal nerves; c. they
are united to form a compound longitudinal
trunk, terminating in an extensive plexus; d. the distal ends of both muscles and
nerves migrate a long distance forward onto the anterior hcemal surface of the
head, thus causing the nerves to follow their characteristic U-shaped course, and
disguising the original relation between the metameric arrangement of organs on
the neural and hcemal surfaces; e. the distribution of the neural nerves (branchial
arch nerves) is not affected by these changes.

FIG. 81. Muscles and nerves of the chi-
laria of Limulus, from anterior side. The ap-
pendages are revolved outward about 45.
fo.c.7, Capsuliginous bar, or branchial cartilage
of the chilaria.

MUSCLES: T a ' e , Plastro-coxal; 7 / and g,
tergo-coxal; i.m., inter-tergal; up..m.1 , veno-
pericardiac.

loS

PERIPHERAL NERVES AND GANGLIA.

In the arachnids (scorpion), four abdominal neuromeres have migrated for-
ward to unite with the hindbrain. Of these four, the last one is a true branchial
neuromere.

In vertebrates, all the branchial neuromeres have fused with the hind-
brain, probably in some such manner as that indicated in Fig. 68. The
hypobranchial nerves united to form the hypoglossus, having the peculiar distri-
bution indicated above, although in a more exaggerated form. The neural roots

o!

FIG. 82. Muscles and distribution of nerves in the first gill of Limulus. The appendage is flexed upon the
abdomen, and is seen from the neural side, a.e.9, Abdominal endochondrite; b. c. 8 and be. 9, branchial cartilages
of operculum and first gill; i.l., inner lobe of gill; ni.l. median lobe of gill o.L, outer lobe of gill.

MUSCLES: a.b.m.9. Abductor muscle of gill; b.t.m., branchio-thoracic muscles; e.b.m.9, external branchial
muscle; i. b. m. 9, internal branchial muscle; i.I.m., inner lobe muscles; l.a.m., longitudinal abdominal muscles: o.l.m.,
outer lobe muscles.

NERVES: e.g., First abdominal ganglion; e.b.n., external branch of neural nerve; g.n., branch of neural nerve
supplying gill book; h.n.9, haemal nerves; i.b.n., internal branch of neural nerve; i.n.9, intestinal nerve (two branches
are shown, a posterior and an anterior one); in.n.,9 integumentary branch of haemal nerve; l.s.n., hypo-bran-
chial nerve; ni.b.n., median branch of neural nerve; III.H. 9, neural nerve; s.c.n.,9 segmental cardiac nerve;
v.c., ventral cord.

united with one another, and with the posterior neural roots of the vagus, as they
have to a certain extent in the scorpion, to form the series of nerves supplying the
gill arches. The nerves supplying the important sense organs in the group of
modified vagal appendages, gave rise to the lateral line nerve; and the combined
cardiac and intestinal components, that in the arachnids are confined to this
region, gave rise to the corresponding elements in the vertebrates.

We need not carry this comparison any farther, for the conditions are ex-
tremely complicated, and there are many variations peculiar to each class. But
that this entire region has undergone the same kind of changes in vertebrates that

VAGAL AND HYPO-BRANCHIAL NERVES. 109

we now see taking place in the arachnids is, I believe, beyond question. The
fact that the branchial appendages in arachnids, as nearly as we may deter mine,
belong to the same group of metameres as in vertebrates, and the fact that the
total number of branchial segments in Limulus and the merostomes is very
nearly the same as in vertebrates, i.e., seven appendages, four or five of which are
gill bearing, as against five or seven gill bearing arches for vertebrates is sug-
gestive, but perhaps of less significance than the fact that in both cases, there is a
much greater forward growth and concrescence of the structures on the haemal
surface than of those on the neural, thus producing that apparent lack of harmony
in the serial arrangement of nerves, neuromeres, gill arches, and myotomes, so
disturbing to the student of vertebrate cephalogenesis.

The conditions become still more significant when we recall that they are the
inevitable results of very remote factors that are common to both types, such as
the absence of lateral plates to the mesodermic segments in the anterior part of
the head, the gradually increasing size of the yolk sphere, and the precocious
development of the forebrain.

CHAPTER VII.

GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS.

In the arachnids, we may recognize three main groups of sense organs;
primitive segmental, special cutaneous, and general cutaneous.

a. The primitive segmental sense organs include the median and the lateral
eyes, the olfactory and the auditory organs. With the exception of the last named,
they are so highly developed and have been so long established that they and
their nerve centers constitute the very foundations of the forehead and forebrain.
b. The special cutaneous sense organs include the gustatory buds, slime buds,
and other chemotactic, or tactile organs that have well denned nerves, ganglia,
and central terminals, and that are located in definite fields, or areas, such as
the coxal spurs, chilaria, pectines, nabellum, etc. They attain their highest
development in the midbrain and hindbrain regions, c. The general cutaneous
sense organs may be of the same nature as the special cutaneous, but they are
irregularly distributed, and without separate, well defined nerves or ganglia;
we also include in this category temperature organs and free nerve endings.
They are supplied mainly by the subdermal plexus formed from the terminal
branches of the neural, but especially of the haemal nerves. These nerves may
arise from all the main divisions of the neuron except the forebrain.

I. GENERAL CUTANEOUS SENSE ORGANS.

Temperature Organs. Reaction to changes in temperature in the lower
animals is probably much more delicate and more generally exercised than has
been suspected. The temperature sense is not dependent on the location of its
organs in a particular part of the body, for changes of temperature are diffusely
distributed, and are not likely to affect the animal at one point more than another.
An effective response results in the transfer of the whole body to surroundings of
a different temperature; thus the temperature organs primarily control them ove-
ments of the animal as a whole, or its migrations, or distribution in space.

One would hardly suspect that such a heavily armored animal as Limulus
would be very sensitive to changes in temperature, yet that such is the case may be
easily demonstrated. When placed on its back and allowed to become perfectly
quiet, it may be fanned, or the surface of the carapace, or the gills, or the legs,
may be touched with an object the same temperature as the air, without causing
any reflexes; but the instant any of these parts are touched ever so gently with the
finger, or if water a little warmer or colder than the surrounding air falls on them,

no

TEMPERATURE AND GUSTATORY ORGANS. Ill

or even if one gently breathes on the gills and under surface of the body, the animal
at once becomes greatly agitated.

The most sensitive areas are the margins of the carapace, and especially the
margins of the gill chamber along which the main current of water passes to the
gills, and the anterior surface of the branchial appendages themselves.

We cannot positively identify the temperature organs. They appear to be
short, spike-like projections in which terminates a small tuft of sensory cells.
They are distributed over all parts of the carapace and are supplied by the terminal
plexus formed by the branching of the hasmal nerves.

They are seen to best advantage in the gills of young Limuli, 2-4 in. long.
In those parts of the gills that are most sensitive to heat, i.e., the outer surface of
the terminal joints of the exopodites, one may see, in successful methylene blue
preparation, a loose subdermal nerve plexus continuous with small clusters of
spindle shaped sensory cells. From each cluster a very fine fiber extends outward,
through a chitenous tubule, to a short spike situated on the outer surface of the
gill (Fig. 86, A, t.s.)

Free Nerve-Ends. In the abdominal appendages that have been injected
with methylene blue, large nerve branches may be seen going to the soft integument
around the joints of the endopodites. Each branch ends in a group of bipolar
or multipolar cells; from them arise many branching fibers that form a rich termi-
nal meshwork, lying in or on the ectoderm, but without association with any
specialized cells. (Figs. 865, 87.)

The hyphas of a parasitic fungus sometimes ramify in all directions through
or over the surface of the chiten. They usually take on an intensely blue
stain, and at first sight might be mistaken for nerve fibers.

II. SPECIAL CUTANEOUS SENSE ORGANS.

The Gustatory Organs of Limulus. Gustatory organs are widely dis-
tributed over the neural surface of the head, but they are most highly developed
in the appendages that come in frequent contact with the food. In other words,

the principal aggregations of these organs are located around the mouth in segment-
ally arranged fields. This condition explains their remarkable distribution in verte-
brates. There they are primarily arranged in several radiating series on the top of
the head, an inconveniently long distance from the present vertebrate mouth, but
close to the central areas where the old invertebrate mouth was located. (Fig. 89.)
The gustatory organs have been most carefully studied in Limulus, and they
form the principal basis for my conclusions. They are abundant in the mandibles
of the thoracic appendages, except the first and last pairs, and in the tips of the
thoracic appendages. Their presence in the mandibles is indicated by a most
beautiful series of reflexes, first described by me in 1892. Organs similar in

112 GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS.

structure to the ones we are about to describe occur in other parts of the head
and trunk, but stimulation of them does not produce any recognizable reflex.

(7. Reactions to Stimulation. If an adult Limulus be placed on its back, it
soon becomes quiet, except that after long intervals the gills are raised and lowered
a few times. If, during the quiescent condition, the jaw-like spurs, or mandibles,
at the base of the legs are gently rubbed with some hard object, such as a piece
or wood, glass, or iron; or if water the temperature of the surrounding medium be
gently poured over them; or if the animal be vigorously fanned, or loud noises
be made near it, only slight, aimless movements of the legs or abdomen are pro-
duced; usually none at all. But if a very small piece of clam, not more than two
or three millimeters long, is gently laid on the surface, say of the third mandible
on the left side, care being taken not to touch any other parts, that leg will be
repeatedly raised and the tip bent toward the mouth, while its mandible will
move back and forth, alternating with the leg movement. Meantime all the other
mandibles and appendages are motionless. One may start in this way one append-
age after the other (except the first and last pairs), until all of them, first on one side
and then on the other, are in action.

If all the jaws are stimulated with food at the same time the normal chewing
reaction takes place as follows: The second and fourth pairs of mandibles move
in unison inward toward the median plane, and downward toward the mouth;
then back again in the reverse order. When they are farthest from the mouth
the corresponding legs (except the second pair in both males and females) are
quickly raised, flexed, and the tips carried toward the mouth, where they remain
an instant, and then fall back on to the under side of the carapace; the corre-
sponding jaw movement then begins again. The third and fifth pairs of append-
ages and the corresponding jaws work in unison in the same manner, but they
alternate with those of the second and fourth. At intervals these movements
cease, the abdomen is raised, and the stout crushing mandibles on the sixth pair
of appendages, which have heretofore remained motionless, are slowly closed with
great force, as though to crush some object too large to be swallowed whole, or
to kill some struggling prey. These powerful jaws then slowly relax their con-
vulsive grasp, and the chewing movements are resumed.

All these movements go on with the greatest precision and regularity, so that
the food that was placed on the jaws is forced into the mouth and down the
oesophagus.

A drop of clam water is sufficient to start the whole reaction, which is per-
formed in the same manner as during the actual process of eating.

If wads of blotting paper are used, wet with ammonia or picric acid, the
chewing movements are reversed, and the offensive object may be snapped up by
the chelicerae and rejected.

Strong smelling food held close to the mouth, or to the jaws, produces no
effect, although chewing movements are instantly produced when the jaws are
touched by it.

GUSTATORY ORGANS. FLABELLUM.

If the mandibles on one side are stimulated, the chelicera of that side, al-
though not stimulated itself, extends rigidly backward, or waves aimlessly back
and forth snapping its chelae and thrusting the tip of the appendage into the mouth.
If the jaws on the opposite side are now stimulated, the chelicera on that side
begins to work also.

The chewing reactions can only be produced by stimulating the spines on
the mandibles, or the smooth, under surface of the inner mandibular spur. Stimu-
lating the skin around the mouth, or in it, does not produce the chewing reflexes.
If the mandibles are amputated, no reaction in
the leg so treated occurs. If the spines are shaved
off, the reaction is produced only after strong stimu-
lation, or by stimulating the under surface of the
inner mandible.

It is thus evident that we are dealing with true
taste organs, and that they must be located in the
mandibular spines.

b. Structure of the Gustatory Organs. The
mandibular spines are thickly covered with minute
pores arranged in vertical lines. (Fig. 83, A .) The
pores lead into canals, each of which contains a
long, slender chitenous tubule that terminates flush
with the outer surface. The chitenous tubule
contains an exceedingly fine, hair-like prolongation
of a gustatory cell. Toward its inner end, the
tubule expands into a peculiar spindle, beyond
which lies the nucleated cell body. The gustatory
cells are united into spindle-shaped clusters, each
cluster corresponding to a single line of pores.
The central ends of the cells are continued as nerve
fibers into the main gustatory nerve, which extends
over the surface of the pedal ganglia, through the

FIG. 83. .4, Gustatory coxal spine

gUStatOry tracts, tO the COmmon Centers in the of Limulus, showing linear arrangement
! i. ill i ,T^- of the gustatory pores; B, longitudinal

chehceral lobes and hemispheres. (Figs. 65-114.) section of a gusta tory spine, showing

the gustatory cells, g.s.c., spindles, sp,

^ * * ^ * ^ and chitenous end tubes, 5r/;./. highly

magnified. C.D, Details of surface

The flabellum is a large spatulatc organ, one terminals -

to one and one-half inches long, attached to the outer side of the coxal joint
of the sixth leg. It lies in a channel leading into the respiratory chamber, so
that the water going to the gills passes over its flat anterior surface. The latter
is perforated with innumerable canals that afford an opening for the elements
of the underlying sense organ, the most voluminous one in the whole body.

Each canal contains the outer end of a pear-shaped sense bud composed
of eight to twelve, or more, sense cells. The buds are loosely united into small

114 GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS.

groups that are surrounded by an ill defined sheath and supplied by a single
nerve. (Fig. 84.)

The slender outer ends of the sensory cells unite to form a dense, conical
body, enclosed in a bulb-like enlargement at the base of a chitenous tubule.
Before uniting, the cell ends become especially distinct and each one develops a
minute, bead-like swelling.

cKt.

FIG. 84.

FIG. 85.

FIGS. 84 and 85. A, Section through the anterior surface of the flabellum of an adult Limulus, showing four
flabellar sense organs von Rath's preparation; B, section throngh one of the gill warts of an adult Limulus, show-
ing the peculiar bell- shaped terminal " hairs ' ' and the associated cluster of sensory cells and chitenous tubules von
Rath's preparation; C, terminal hair of a gill wart, more highly magnified; D, diagram of a slime bud, Limulus; E,
taste bud from the pharynx of an embryo Catastomus (after Johnston); F, a taste organ from the skin of an
adult Lampetra (ajter Johnston); G, a neuromast from the skin of Catastomus (after Johnston); H , diagram of an
arachnid sense bud.

The apex of the cone extends outward as an exceedingly minute liber, through
a small chitenous tubule, probably as far as the outer surface of the flabellum.
Between the slender necks of the organs are a few elongated cells, and similar
ones, but smaller, are seen in the canals through which the chitenous tubules
pass to the exterior.

BRANCHIAL WARTS. 1 15

The thick epidermis is heavily pigmented, and pigment is frequently seen
in the body of the sensory cells.

The sense buds are so numerous that their inner ends are crowded together
several rows deep. The inner surface of the sensory field is very vascular, and
the narrow crevices between the organs are often crowded with blood corpuscles.

The posterior wall of the rlabellum, in marked contrast to the anterior, con-
tains few or no sensory perforations, and the epidermis is thin, nearly colorless,
and with few blood-vessels.

The flabellum is supplied by a very large nerve the root of which passes over
the neural surface of the sixth pedal ganglion and joins the main gustatory tracts.
It does not differ from the fascicles coming from the gustatory cells in the man-
dibles, except that it is larger. It appears to form the greater part of the conspic-
uous neuropile enlargements seen on the median face of each crus. (Fig. 65.)

The rlabellum doubtless serves to test the quality of the water that is drawn
into the gill chamber. I have not been able to detect any characteristic reactions
when it is stimulated.

The rlabellum probably represents the exopodite of the sixth pair of append-
ages. Traces of similar organs are seen for a short time at the base of the other
appendages. (Fig. 141.)

The Branchial Warts. The branchial warts are blister-like elevations about
four mm. in diameter, located on the endopodites of the branchial appendages.
They are covered by a soft, bluish chiten, and lie either folded over the margin,
half on each side of the gill, or in pairs, one member on the anterior, the other
opposite to it on the posterior surface of the appendage. (Fig. 82.)

The outer surface is thickly and uniformly covered with goblet, or bell-
shaped hairs, deeply set in conical recesses. There are two distinct sizes, evenly
distributed in about the proportion of five small ones to one large. The large
bell-shaped hairs lie over the outer ends of large canals which contain spirally
coiled, and very distinct chitenous tubules. (Fig. 85, Br. and C.}

The canals are colorless and, except for the tubule, appear to be empty.
They do not contain blackened fibers or nuclei such as occur in the flabellar
canals. The tubule springs from a small, fusiform cluster of sensory cells lying
well below the surface. Thick nerve bundles, remarkable for the large ganglion
cells scattered over them, leave the inner ends of the cell clusters and uniting with
the other bundles form a loose nerve plexus, which is continuous with termin a
branches of the branchial nerve.

The smaller goblet hairs are without visible tubules, and their faint, under-
lying canals do not perforate the outer chitenous layers. They do not appear
to be connected with nerves.

The inner surface of the flexible chiten that covers the branchial warts ex-
tends inward in the form of thin, vertical walls that form a coarse, polygonal

Il6 GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS.

network when seen in surface views. They are covered with a thick epithelium*
giving them in sections a false appearance of sensory infoldings.

Between two opposing organs is a loose, areolar tissue and a conspicuous
venous chamber.

These organs are clearly of a very special kind. In addition to their unusual
naked eye appearance, they are peculiar in the shape of the terminal goblet hairs;
in the absence of cells or fibers in the underlying canals; in the thick walled, spiral,
chitenous tubule; in the ganglionated nerve branches, and in their inflated elastic
walls that lie on opposite sides of the appendage.

Their peculiar structure indicates that they may be provisionally regarded
as a kind of pressure guage which aids in the control of the heart beat and the
respiratory movements.

The Slime Buds. Slime buds are spherical masses of glandular cells, mingled
with nervous or sensory ones, and richly supplied with nerves.

They vary greatly in their grade of development, and in the relative number
and size of their sensory and glandular cells. They are scattered over the whole
surface of the body, but are especially abundant in certain regions, or areas, that
are known to be highly sensitive.

While at first sight they appear to be merely integumentary glands, closer
examination raises many important questions that are difficult to answer, as, for
example, in regard to their function, minute structure, and development.

It is probable that they play an important part in the reactions toward certain
kinds of stimuli, but whether their secretions serve to protect the adjacent
nerve buds against excessive stimulation (which seems to me very improbable), or
as absorbers and intensifiers of certain substances, has not been demonstrated.

A familiar illustration of a similar condition in vertebrates is the association
of slime, or mucous cells with the sense organs of the lateral line. There is also
an intimate association of mucous and sensory cells in many molluscs, i.e., in
Lima, Area Noas, and in the tentacles of Haliotis.

The facts that have special significance for our problems are as follows:

1. Secretion of mucus. When small Limuli are violently stimulated, the
slime buds discharge an abundance of mucus, and, if the surface of the shell
has been previously wiped dry, it may be seen to collect in small drops over each
pore. When allowed to accumulate, it forms a thick slimy covering to the whole
surface.

2. Distribution. The slime buds are very numerous in certain well defined
areas which, from their location and abundant nerve supply, have every indication
of being highly sensory, as, for example, in the olfactory organs, in the mandibular
spurs of Limulus, and the maxillaria of the second and third pairs of thoracic
appendages in the scorpion.

3. Innervation. These groups of slime buds are innervated by special nerves,

SLIME BUDS. 1 I 7

or by the same nerves that supply the adjacent gustatory organs. Those that are
scattered over the general surface of the body are supplied by branches from a sub-
cutaneous plexus, formed by the ramifications of the general cutaneous branches
of the haemal nerves. In the appendages, the subdermal plexus arises from the
general cutaneous branches of the neural nerves.

4. Structure. Slime buds are found in many Crustacea and arachnids, and,
although but little is known about them, they appear to have a similar structure
to those in Limulus.

The slime buds differ in appearance in different regions, and apparently at
different times. They are generally spherical or oval, with a small central space

FIG. 86. Anterior, or outer, surface of the branchial appendage of a young Limulus, two inches long. A, a
portion of the subdermal plexus of nerve fibers, with clusters of bipolar sense cells whose outer ends terminate in
minute chitenous spikes; .4, one of the sense buds, more highly magnified, with its chitenous tubule, ch.t., that
conveys the terminal fibers to the surface; B, two apparently isolated multipolar ganglion cells, lying just below,
or in, the surface ectoderm of the branchial appendage; (_' , four multipolar ganglion cells from the same region.
Methylene blue.

from which a chitenous tubule leads to the exterior. (Fig. 88.) This tubule
may or may not be convoluted near its origin, but it generally terminates in a
straight delicate tubule that cannot be distinguished from those covering the
outer ends of the gustatory and temperature-cells. All the tubules are shed with
the old shell at ecdysis. They may be seen protruding a considerable distance from
the inner surface of the cast off shell that have been cleaned with boiling potash.
The slime buds contain at least two different kinds of cells, namely, true
slime cells, which may constitute the greater part of the organ, and one or more
sensory cells. The slime cells vary greatly in appearance. In the typical ol-
factory and mandibular slime buds, they are irregularly conical or cylindrical,
their walls are sharply defined, and they contain, at their pointed central ends, a
mass of refractive colorless spherules. The enlarged peripheral ends of the cells,

n8

GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS.

in which is located the small nucleus, may be finely granular, staining a dark
gray or bluish-black in von Rath's fluid. (Fig. 88.)

Each slime bud contains a single ganglionic or sensory cell, distinguished
from all the others by its large size, dark, finely granular protoplasm and indis-
tinct outline. This cell appears to be larger and more distinct in the mandibular
slime buds than it is in the olfactory buds.

Between the outer ends of the slime cells there are minute, rod-like bodies
with a dilatation at their inner ends. They have the appearance, under some
conditions, of being minute sensory cells, but I have not been able to fully satisfy
myself that such is the case. In von Rath's fluid, they become very black,
and in some cases hair-like processes appear to project from them into the cavity
of the slime bud, where they unite to form a small star-like body. (Fig. 85, D.)

In some cases, the buds are greatly distended and the cells appear nearly

FIG. 87. A cluster of ganglion cells terminating in a sub-dermal plexus of anastomosing fibers; from the soft skin

between the joints of the endopodites of the branchial appendages of a young Limulus. Methylene blue.
FIG. 88. Two slime buds grom the olfactory region of an adult Limulus von Rath's prepration. a. Groups of cells
of unknown significance; c.c, central coagulum resting on hair-like projections.

colorless and empty, as though after a certain period of activity they were about
to degenerate.

New slime buds, that have arisen de novo from the indifferent ectoderm,
or by the division of the existing buds, appear in the older stages.

5. In the vertebrates, there is a similar association of sensory and mucous
cells in the lateral line organs. For a long time it was assumed that the lateral
line canals were primarily slime producing organs, and nothing more. When
the sense organs in the canals were discovered, the associated mucous cells were
apparently forgotten.

In the lower vertebrates, the typical taste organs form small clusters of sensory
cells, resembling in structure and innervation the taste organs and the flabellar
organs in Limulus. (Fig. 85, E.) The typical lateral line organs, or neuromasts,
consist of short, hair-bearing sense cells, united with longer so-called "supporting,"
or indifferent cells, which may or may not secrete mucous (Maurer). Thisassocia-

SLIME BUDS. IIQ

tion of mucous cells and hair cells in one organ is comparable with the association
of sensory and mucous cells in the slime buds of arachnids and in the olfactory
organs of insects (Necrophorus, Dahlgren and Kepner). If the short, rod-like
bodies in Limulus are true sensory cells, then the morphological resemblance
between an arachnid slime bud and a vertebrate neuromast is very striking.
(Figs. 85, D-H.)

According to Maurer, there are some cases in the vertebrates where the
lateral line organs still remain in a condition approaching that in the arachnids,
for he regards the slime buds in Myxine and Bdellostoma as probably representing
modified lateral line organs of Petromyzon. In other words, in Myxine and
Bdellostoma, the mucous sacs are sense buds, in which all or nearly all the cells
secrete mucous.

In the vertebrates, however, the secreting function is usually relegated to
separate cells in the adjacent ectoderm, the "supporting" cells apparently re-
taining their secreting function only in exceptional cases (Maurer). In reply
to an inquiry on this point, Prof. C. Judson Herrick writes me that "the line
organs of vertebrates are so exceedingly variable that I would not venture to
generalize, with my present knowledge, on the relation between the sensory and
the mucous cells; but certainly in some cases, and I think as a rule, they are closely
associated. The mucous cells are I think generally absent in the non-sensory
parts of the lining of the canals. As to the function of the mucus, I have hitherto
regarded it as like the mucus of the general body surfaces, protective. But
in view of Parker's work on the function of the lateral canal sense organs as re-
ceptors for slow vibrations, it may be that the mucus and the cilia of the hair
cells both enter into the formation of the cupula which overlies the lateral line
organs much as in the case of ampullae of the internal ear and that the whole cupula
assists in the stimulus of the sensory cells."

However, it is clear that we must go farther back than primitive vertebrates
for our explanation. In Limulus, for example, we have the same kind of gland
cells intimately associated with cutaneous sense organs, and it is extremely improb-
able that the abundant mucus there serves either to protect an already practically
impervious covering, or to assist, by slow vibrations, in the stimulation of the
sensory cells.

6. The function of the slime buds in Limulus is not apparent. The presence
of the mucoid secretion is obvious enough in both vertebrates and arachnids, but
a satisfactory explanation of its purpose is not available; and it is difficult to
account for the rich innervation of these organs in Limulus, or for the presence of
sensory or nerve cells in them, or for their association with other sense organs,
on the ground that they are mucous glands and nothing more.

The only conclusion open to us at present is that first suggested by me in 1889,
namely that in the arachnids and primitive vertebrates, the mucous secretion serves
to absorb certain chemical substances held in solution, and to thus intensify
their action on the nerve ends. This explanation would account for the abund-

120 GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS.

ance of mucous in the olfactory and gustatory organs, and for its absence in the
tactile or auditory ones.

The mandibular slime buds are sufficiently numerous to suggest that they
are in the nature of salivary glands. This, however, does not seem probable, since
there is no way to get the secretions into the mouth with the food; and the mem-
branes immediately within, or surrounding the mouth are entirely devoid of these
organs. Moreover, it is certain that the precisely similar slime buds in the ol-
factory fields, and in the integument of the back or branchial chamber, cannot be
regarded as salivary organs.

7. The slime buds of Limulus and other arachnids are found in segmentally
arranged fields, or groups, that are supplied by special nerves, the most conspicuous
groups being those in the olfactory organ, in the mandibles of the second to the
fifth thoracic appendages, and in the rudimentary vagus appendages (scorpion).
These organs appear at an early embryonic period as thickenings of the ectoderm
and in close association with the cranial ganglia.

The Auditory Organ. In my first contribution, 1889, I maintained that
the large segmental sense organ, which in Limulus embryos lies opposite the fourth
pair of legs, was the probable forerunner of the vertebrate ear. I see no reason
to change my opinion on this point. Although the evidence in favor of this con-
clusion is not voluminous, it is sufficiently precise as far as it goes. In Limulus,
the organ in question is a large discoidal placode, of a sensory nature (Figs. 131,
1 40 to 153), strikingly like the auditory placode of vertebrates in its general outward
appearance, in its minute structure, and in the fact that it is located, as nearly as
one may determine, on the same segment of the head, using as a guide either
the history of the oral arches (Figs. 29-34), or the number of the corresponding
brain neuromere. (Fig. 57.)

It is assumed that in the primitive vertebrates this particular placode, which
lies at the head of the posterior division of the thorax, formed a simple, sac-like
infolding, similar to the auditory sac in decapods, and that from this sac developed
the inner ear of vertebrates.

The placode belongs to the same series as the visual and olfactory organs.
(Figs. 140-148, 5.o 4 .) It increases in size up to the time of hatching. During the
early trilobite stage, the cells become slightly pigmented, take on a sensory ap-
pearance, and a lens-like thickening of the overlying chiten is formed over it.
(Fig. 131, B.) The organ disappears completely at the close of the trilobite stage.

That is as far as the evidence goes. There is no evidence that the placode in
Limulus is auditory; or that it is serially homologous with the antennal auditory
organs of decapods, although that is not improbable.

Gaskell regards the flabellum of Limulus, or the pectines of the scorpion, as
the precursor of the vertebrate auditory organ; but they lie much too far back in
the head to be compared with the ear of vertebrates. His description of the
minute structure of the flabellum is very inaccurate, and his intimation that it is

LATERAL LINE ORGANS. I 2 1

an auditory organ, possibly homologous with the pectines of scorpions, is con-
trary to well established facts.

Lateral Line Organs of Vertebrates. Summary and Comparison. -
The lateral line organs of vertebrates consist of several distinct groups that arise
at an early embryonic period from the neural surface of the head. Each line of
organs makes its appearance as an oval thickening of the ectoderm, located be-
tween the dorsal extremity of a gill arch and the lateral margin of the medullary
plate. (Figs. 26-34.) The thickening gradually extends in a peripheral direc-
tion, and as it does so it separates into a superficial linear series of sense buds
and an accompanying underlying nerve and ganglion. Subsequently an in-
folding of the ectoderm may take place along the line of growth, forming first an
open groove and then a canal, in w T hich the organs are located at regular intervals.
Finally the several canals may unite, forming a continuous system, but each part
that w r as originally a distinct canal is innervated by a special cranial nerve.
(Fig. 89.)

It has been suggested that the anlage of each canal represents a very ancient
sense organ (the so-called branchial sense organ), but so far as I know, no ex-
planation has been offered for the extraordinary fact that these ancient organs
must have originated, not around or close to the vertebrate mouth, as one would
naturally suppose, but from the opposite or aboral side of the head; and not from
a single anlage, but from several.

This condition, however, is perfectly intelligible as soon as we recognize that
the whole system of taste buds and lateral line organs of vertebrates represents
the thoracic and vagal coxal sense organs of arachnids, which there lie on the neural
surface of the head around the primitive mouth, the latter having closed up and
disappeared in the vertebrates.

When the sense organs of the arachnids are projected on the neural surface
of the cephalothorax, the principal groups, each one containing many organs,
appear as oval or circular areas arranged around the mouth. (Fig. 89, A.) We
may recognize three sets: the gustatory organs and slime buds located side by side
in the thoracic and vagal appendages, and the chemotactic general cutaneous
organs located on the neural flanks of the thoracic and branchial regions, and
supplied by a great longitudinal nerve arising from one of the anterior thoracic
neuromeres, /.//.

Taste buds predominate in the second, third, fourth, and fifth thoracic coxas,
i.e., those immediately surrounding the mouth. Sense cells of the same general
type are abundant in the flabellum and in the vagal appendages, but there they
may serve as tactile organs, or for some other purpose.

The organs of the gustatory-tactile type and the slime buds may arise side
by side from the same anlagen, and they may be supplied by the same nerve
trunks and ganglia. Their later phylogenetic history appears to follow along the
same lines in both cases, but there is apparently a tendency to separate, more and

122 GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS.

more, the two kinds of organs, so that each kind assembles in particular areas
and is supplied with distinct nerves arising from distinct brain tracts. We shall
here refer to the common anlagen of both sets of organs as coxal and vagal sense
organs.

There is a sharp distinction morphologically between the anlagen of the
thoracic organs and those of the vagal region. The thoracic anlagen are always
directed forward and outward and are located well on the sides of the thorax.
The vagal anlagen are always crowded close to the median line and are directed
backward, approximately parallel with the nerve cord. (Fig. 89.) The location
and direction of growth of these organs is determined by that of the appendages to
which they belong and is prophetic of their condition in vertebrates.

"When the oral appendages were transferred to the haemal surface (see Chapter
XV), it is probable that the anlagen of the coxal organs were drawn forward and
outward into a narrow band, each one giving rise to a row, or linear series
of taste organs, the general course or direction of the organs, and the accompany-
ing nerves and ganglia, indicating the path of migration of the corresponding
appendage.

The vagal appendages of the arachnids are always carried backward, relative
to the other parts of the same segments, as shown by the invariable direction of
their nerves and ganglia. The conditions that controlled their movements have
no doubt continued to direct the line of growth of the vagal group of anlagen in
the embryos of their vertebrate descendants.

There is nothing to indicate what conditions determined the backward growth
of the immense longitudinal cutaneous nerve, which in Limulus arises from the
first post-oral neuromere. (Figs. 70, 89, l.n.}

The embryological history of the lateral line organs in primitive vertebrates
bears out this interpretation. We may recognize there two principal groups of
organs, one lying in front of the auditory organ and belonging to the oral arches,
the other lying behind the auditory organ and belonging to the branchial region
and trunk. The former represent the coxal organs of the arachnid thorax, the
latter, the organs of the vagal appendages. These two groups of organs grow in
the same general direction in the vertebrates that they do in the arachnids, but
they have extended very much farther in the former.

In the vertebrates the several pairs of anlagen tend to run together, and it
is not clear just how many there are in either region, or which ones of those in the
arachnids they represent.

The second, third, fourth, and to a less degree, the fifth pairs of coxal anlagen
en the arachnids are probably in part retained in the vertebrates, forming the
rudiments of lines of canal organs for their corresponding appendages, which
have themselves furnished the basis of the premaxillary, maxillary, mandibular
and hyoid arches. One or more groups of vagal sense organs gave rise to the
lateral lines of the branchial region and the trunk.

LATERAL LINE ORGANS.

123

Let us now consider the several lines of canal organs as they appear in ostra-
coderms and primitive vertebrates.

In the ostracoderms, they undoubtedly occur in the most primitive condition
known in the adult of any vertebrate-like animal.

In Tremataspis (Fig. 236), the organs were apparently located in short, shal-
low surface grooves; in Bothriolepis (Fig. 247), in continuous open grooves.
When expressed in a simple diagrammatic form, the sensory grooves of the ostra-
coderms appear to originate in the occipital region and to radiate from it in the
following lines: There is a main suborbital (Fig. 89, B.}, i.o.L, continued for-
ward as the rostral line, r.L, in front of the olfactory organs. In Bothrio-
lepis, a branch line arises from it and extends hsemally over the surface of the

A. B. C. D.

FIG. 89. Schematic figures showing the location of the lateral and median eye, olfactory, auditory, gusta-
tory, and canal organs, in the arachnids, ostracoderms, arthrodiri, and primitive vertebrates. All figures seen
from the neural surface.

premaxillae. A mandibular line is not recognized in any ostracoderm, prob-
ably owing to the small size of the mandibles. There is no true supra-orbital
line, probably owing to the median location of the lateral eyes, although the
short-post orbital line of Tremataspis and the longer one in Bothriolepis possibly
represent the proximal end of such a line, s.o.l.

The orbital line appears to be continuous with the lateral line of the branchial
region and of the trunk, by means of short glosso-pharyngeal sections, g.p. Judg-
ing from the embryological conditions in vertebrates, this section represents a
separate line, supplied solely by the glosso-pharyngeal nerve. The main lateral
line extends along the branchial region and in Bothriolepis may be traced for a
short distance on to the trunk. There are two accessory dorso-branchial lines
in Tremataspis and one in Bothriolepis.

124 GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS.

In the Arthrodira (Fig. 89, C), the most important advance is in the appear-
ance of a distinct supra-orbital line, s.o.L, extending forward between the now
widely separate lateral eyes; and a distinct hyomandibular line h.mL extending
toward, and probably onto, the greatly enlarged mandibles. In the arthrodira
the neuromasts apparently never form a series of separate dots and dashes, but
lie in continuous grooves of varying depth.

In true vertebrates, no important changes or new conditions arise. The sev-
eral lines may be deeply infolded and joined at their proximal ends to form a
united series of canals, with the sense organs located in them at regular intervals,
suggesting the interrupted surface grooves of the ostracoderms. (Fig. 89,
B and >.)

CHAPTER VIII.

LARVAL OCELLI AND THE PARIETAL EYE.

I. THE DIFFERENT KINDS OF EYES IN ARTHROPODS AND VERTEBRATES.

Since the principal facts in their embryonic development became known,
it has been generally assumed that the vertebrate eyes originated inside the brain
chamber, and that the retina was a highly specialized part of the brain wall.

There are fundamental objections to this interpretation, namely: a. it reverses
the usual order of histological development, for nerve cells are to be regarded as
specialized sensory cells, not vice versa; and b. it fails to establish any connection
or relation between the eyes of vertebrates and those that are almost universally
present, and often highly developed, in the invertebrates. In fact, it neither ex-
plains how the eyes got into the brain chamber from without, nor under what
conditions they developed u de novo" from within.

The arthropod theory is not open to these objections, for we shall show that
the evolution of a cerebral eye has already taken place in the arachnids, and that
the principal steps in the process are recorded there in great detail.

Eyes of Arthropods.

In the arthropods, we may recognize four types of eyes, namely: paired lar-
val ocelli; parietal eyes; frontal ocelli, or stemmata; and the lateral or compound
eyes.

The larval ocelli, of which there may be six pairs, two for each of the fore-
brain segments, are present in the active larvae of most insects, but disappear
during the metamorphosis (coleoptera, lepidoptera, neuroptera, hymenoptera).
They are cup-like infoldings of the ectoderm, with upright or horizontal retinal
cells or rods. In the insects, the retinal cells are never completely inverted,
and the ocelli never form unpaired eyes enclosed in a common chamber or vesicle.

The Parietal Eye. In the Crustacea and arachnids, two pairs of ocelli unite
to form an unpaired ocellar vesicle, or parietal eye. The ocellar placodes remain
more or less distinct and form the side walls of the dilated anterior, or distal end
of the vesicle. The proximal, or posterior end is generally tubular and may open
on the outer surface of the head; or it may merge with the palial folds and open
into the forebrain vesicle. The parietal eye usually persists through life, and it
may be the largest and most important one functionally.

The frontal eyes or stemmata of insects consist of two pairs or placodes

125

126 LARVAL OCELLI AND THE PARIETAL EYE.

that form a median, tri-oculate group. They arise during the metamorphosis, or
at any rate after the embryonic period, and are quite independent of the primitive
ocelli. They are never involved in a palial fold or in a common vesicle, and
the retinal cells are, apparently, always upright. They are functional eyes only
in adult insects, or in the late larval stages.

In the arachnids and Crustacea (phyllopods, entomostraca), the frontal ocelli
are present in a highly modified form, as two sets of frontal organs two paired and
one unpaired. In Limulus, they become the olfactory organs. In spiders and
scorpions, they are apparently absent. Their nerve roots arise from the median
anterior surface of the forebrain, or from the anterior surface of the optic ganglia
and hemispheres (Limulus).

The lateral or compound eyes are found in adult insects, Crustacea, and
arachnids, including the trilobites and merostomes. Like the stemmata, their
relation to the primary head segments cannot be easily determined, because at the
time the cephalic lobes are most clearly segmented, as in the embryonic stages of
Acilius and the scorpion, the lateral eyes are absent, and they do not appear, if at
all, till near the close of larval life. In Limulus they belong to the cheliceral
segment; in insects, they appear to belong to the antennal segment.

The development of the lateral eyes is essentially the same in all arthropods.
They are derived from large crescentic placodes lying near the posterior lateral
margin of the cephalic lobes close to the edge of the infolding for the optic gang-
lion; but they never lie inside the fold, and the visual cells are never inverted.
The entire visual layer is formed from a single layer of primitive ectoderm.

The placodes are frequently divided, or may be entirely separated, into
two distinct parts, which differ in their histological characters, and in function
(hymenoptera, neuroptera, coleoptera). One part may be especially developed
in males (ephemeridae), or one may serve for vision under water, and the other for
vision in air.

Cerebral Eyes of Vertebrates.

In vertebrates we recognize as belonging to the forebrain, the median or
parietal eyes, the lateral eyes, and the olfactory organs. At an early embryonic
period they lie on the outer margins of the open neural plate, in similar positions
to the ones they occupy in arthropods.

The Parietal Eye. There are probably two pairs of ocellar placodes that
for a short time occupy this marginal position. Later, they are caught in the
palial overgrowth and carried on the inner limb of the closing neural crests to
the median line. There they form a group of one, or two, or three placodes lying
in the membranous roof of the brain. During or after the closing of the cerebral
vesicle, the brain roof is evaginated at the place where the ocelli are located, thus
forming a sac or tube in the blind end of which the ocellar placodes lie.

The extraordinary way in which the vertebrate parietal eye develops is,

CEREBRAL EYES OF VERTEBRATES. 127

therefore, essentially like that of the parietal eye in Limulus and the scorpion.
This fact, and many others to be brought out later, demonstrates that the parietal
eye of the Crustacea and arachnids is a true cerebral eye in the vertebrate sense,
and is identical with the parietal eye of vertebrates.

The lateral eyes of vertebrates represent the compound or convex eyes of
arthropods that have been transferred to the interior of the cerebral vesicle. In
the arthropods the lateral eyes lie near the margin of the cephalic lobes, on the
outer edge of a deep ganglionic infolding. In vertebrates, they are first seen in a
very similar position on the lateral margin of the open medullary plate. Later
they are swept into the infolding brain, turning the retinas inside out. They then
grow out laterally on the end of membranous tubes, in much the same manner as
the median eyes. In arthropods, the lateral eyes usually have a crescentic, or
kidney-shaped outline; in vertebrates, this shape is retained, giving the retinas
their characteristic crescentic outline during the early stages. When the two
limbs of the crescent unite, a circular retina is produced, giving rise to the choroid
fissure and the centrally located optic nerve that, together with the inverted
rods and cones, have long been such inexplicable features of the lateral eyes in
vertebrates.

The olfactory organ in vertebrates arises from three placodes situated on the
anterior margin of the cephalic lobes. They are not drawn into the brain cham-
ber, but remain permanently in the surface ectoderm. They move forward
along the median line followed by two pairs of olfactory nerves, that in the lower
vertebrates may remain separate up to the adult stages. Its structure, develop-
ment, and innervation is therefore similar to that of the frontal organs of the
Crustacea, and the olfactory organ of Limulus.

II. THE EYES AS SEGMENTAL SENSE ORGANS.

The larval ocelli, lateral eyes, auditory organs, stemmata and olfactory or-
gans appear to be local modifications of a series of primitive sense organs belong-
ing to the procephalic and first six thoracic metameres.

In insects and arachnids, the larval ocelli of the procephalic lobes present a
clearly defined segmental arrangement. (Fig. 14.) In scorpion and Limulus,
in addition to these ocelli, there is a transient series of segmental sense organs in
the thorax, which appears to be a continuation of that in the forebrain. (Figs. 15,
16, 140-142.)

In Limulus, the first pair of the thoracic series are the lateral eye placodes, I.e.
The fourth pair, s.o 4 , are large, circular placodes, distinctly sensory in character,
that are retained through the first larval or trilobite stage, after which they dis-
appear. This organ is probably the forerunner of the auditory organ of verte-
brates for it has the same shape and general appearance as the auditory placode
in vertebrate embryos, and as nearly as may be determined, lies on the same
segment. The other placodes are less distinct and are visible for a very short
period only.

In scorpions, on the outer margins of each coxal joint (Figs. 15-16), there

128

LARVAL OCELLI AND THE PARIETAL EYE.

are two transitory sense organs which appear to represent the thoracic series of
Limulus. They disappear before hatching, after contributing an important mass
of ganglion cells to the pedal nerves.

The series of procephalic and thoracic sense organs just described should not
be confused with the segmentally arranged gustatory organs, which belong to a
different system, and w r hich are always located on the median side of the base of
the appendages.

After this preliminary survey, we may consider the several organs under dis-
cussion in more detail.

III. THE OCELLI OF INSECTS.

A very primitive and suggestive condition is seen in Acilius, where the early
history of the ocelli is best known. Here the cephalic lobes are clearly divided
into three segments, each one containing a segment of the brain, one of the optic
ganglion, and one of the optic plate. (Fig. 14.) Three deep infoldings, iv,

1 3

FIG. 90. The ocellus of an insect larva, Acilius (eye V). This ocellus looks forward and outward.

form on the median side of the plate, carrying the three-lobed optic ganglion be-
low the surface. The openings soon close, without the formation of a palial fold
like that which covers the whole forebrain in the scorpion.

The ocelli are formed by separate, pit-like infoldings of the optic plates, the
retina forming from the bottom of the pits and the dioptric apparatus from the
lips of the closed vesicles. (Figs. 90-91 and 102.)

At the close of larval life, the ocelli break awav from the surface ectoderm

PARIETAL EYE OF THE SCORPION.

I _><;

and become lodged deep in the head, on the surface of the optic ganglia, where
they degenerate.

The frontal ocelli are new formations, usually appearing at the beginning of
the metamorphosis, and differing from the larval ocelli in their mode of develop-
ment, time of appearance, and relation to the brain.

IV. THE PARIETAL EYE.

Parietal Eye of the Scorpion.

The development of the parietal eye in the scorpion and spiders furnishes
the best picture of the process by which ocelli are carried into the brain
chamber to form a true parietal eye like that in
vertebrates.

The evolution of the brain chamber and
the parietal eye is essentially the same in scor-
pions and spiders. (Figs. 15, 20, 21.) I will
describe the condition in the former.

The cephalic lobes soon divide into three
segments that have a very constant and charac-
teristic form in the arachnids. (Fig. 15.) One
may distinguish the centrally located brain neu-
romeres, br. 1 ' 3 , two prominent optic ganglia, and
a marginal plate, with deep infoldings between it
and the ganglia.

The whole of the first segment forms a dark
infolded band, extending across the anterior
margin of the cephalic lobes. From it is formed
the olfactory lobes (organ stratifie of St. Remy).

The lateral lobe of the second segment forms
the optic ganglion of the median eyes, p. e.g.,
and the one behind forms the ganglion of the
lateral eyes, I. e.g.

Between the two ganglia and the lateral margin of the cephalic lobes are two
infoldings, the floor of which is formed by the lateral portions of the optic gan-
glia, iv~-iv 3 .

The median ocelli will develop from the extreme lateral margins of the
cephalic lobes, opposite the second pair of infoldings, and the lateral ocelli oppo-
site the third pair. The ocelli, however, are not visible till later.

The arachnid cephalic lobes are clearly comparable with those of Acilius, the
principal differences lying in the union of the parts of the first segment to form the
olfactory lobe, and the small size and late appearance of the ocellar placodes.
9

FIG. 91. The ocellus of an insect larva,
Acilius (eye /). This eye looks directly
upward.

1 30 LARVAL OCELLI AND THE PARIETAL EYE.

Another important difference lies in the method of closing the ganglionic
infoldings, which is as follows: in the scorpion, the openings to the two pairs of
marginal infoldings lengthen till they merge with each other and with the one in
the olfactory lobes. A continuous groove, varying in depth, is thus formed
around the sides and anterior margin of the cephalic lobes. The edge of the
optic plates projects over the groove forming a thin-walled fold, which repre-
sents the beginning of the palial fold, its free margin being the neural crest.

The margin of the palial fold now advances inward and backward over the
outer surface of the forebrain. At the same time the olfactory lobes sink below
the surface, and slide backward, underneath the second segment, leaving only
a small, median part visible from above.

As the palial fold advances, the optic plate is rolled inward, transferring the
median eye placodes from the outer limb of the fold to the inner. When the
placodes have been carried about half-way across the surface of the brain, pig-
ment develops in them that may be seen, in surface views, through the overlying
ectoderm. (Fig. 16, A.) As the edge of the palial fold moves still farther back-
ward, the outline of the two eye sacs becomes distinctly visible. (Fig. 16, B.)

Finally both sacs merge into a single bi-lobed sac, with a narrow neck, or
epiphysis, that opens to the exterior through a small pore, which we shall call
the anterior neuropore. (Fig. 18, a.n.p.}

The neck to the eye sac elongates somewhat, its walls thicken and become
lined with chiten. It is still open in young scorpions, and remnants of it may
persist through life. (Fig. 43, e.t.}

The posterior edge of the completed palial fold extends straight across the
posterior boundaries of the forebrain. (Fig. 18.) When the latter is bent back-
ward onto the haemal surface of the egg, the edge of the fold forms the anterior
edge of the cephalo-thoracic shield. (Figs. 17, 43.)

By the time the eye tube and palial fold are completed, the anterior portion
of the palium, that is the part overlying the hemispheres, and the part originally
connected with the anterior wall of the inferior lobes, has thinned out and is no
longer recognizable. The position it would have, if retained up to that period,
is indicated in Fig. 43, pi.

It is clear that the anterior neuropore in the scorpion represents the point
over the forebrain toward which the palial folds converge and finally unite. The
pore leads, not only into the proximal end of the eye stalk, but also into the fore-
brain vesicle and into the olfactory lobes. Furthermore, it is clear that there
is no real difference between this method of forming a parietal, or cerebral eye,
and that in vertebrates. In the latter animals, the eye tube usually appears at a
relatively later stage, as an outgrowth of the completed palium or roof of the brain,
near the place where the anterior neuropore closed. In arthropods, the same
final condition is shown, and in addition, all the preliminary steps by which the
eyes were transferred from their original position to the brain roof.

The lateral ocelli lie for a considerable period on the external surface of the

THE PARIETAL EYE OF LIMULUS. 131

procephalic lobes, close to the margin of the palial fold, but, unlike the median
ocelli, they are not swept into the infolding, and hence onto the brain roof. They
develop into typical external eye-pits, which permanently remain in their original
position as regards the procephalic lobes. But in the adult, after the forebrain
has been folded back onto the haemal surface, they lie on the anterior lateral
margins of the cephalo-thoracic shield, on the haemal surface of the body, instead
of the neural. (Fig. 17.)

The Parietal Eye of Limulus.

In Limulus, the cephalic lobes, at first sight, bear no resemblance to those of
the scorpion, or of Acilius, but a more careful examination will show that the
essential features are the same in all of them.

e.g.

A

FIG. 92. A, Ocellus of Lycosa (middle one of the three lateral ocelli); B, retinal portion of the same, more
highly magnified, showing the retinal cells, each with a large outer nucleus, n, and a smaller inner one, n l ; the
lateral rods, rd, are in parallel rows, fenced off by vertical walls of dense pigment; concave reflecting membranes
underlie each double row of rods.

Development. The cephalic lobes at first form two wing-like expansions of
the neural plate, with the stomodseum on the extreme anterior margin and the
chelicerae on the posterior one. (Fig. 140.) No division into segments is visible
at this stage.

A little later (Fig. 141), one may recognize the various parts that belong to
these segments, viz., two large infoldings in the olfactory lobes representing the
first segment, ol.l.; two pairs of minute pores representing the marginal infoldings
for the median ocelli on the second segment, p.e.; a large olfactory placode on the

1^2 LARVAL OCELLI AND THE PARIETAL EVK.

anterior edge of the lateral eye ganglion, representing the sense organ of the
third segment, ol.o.; and the compound eye placode itself, about opposite the
chelicerae, and belonging to the fourth segment, I.e.

The compound eyes arise just behind the true cephalic lobes; apparently they
are not represented in the scorpion or in spiders, or in the embryonic cephalic
lobes of those insects that undergo a metamorphosis.

During the following stages, the two pairs of ocellar tubes unite in the median
line in front of the olfactory lobes, forming a single median tube or epiphysis,
directed forward, below the skin. (Fig. 142, c.p.) Its distal end is dilated and
contains, as shown by its structure in the later stages, four ocellar placodes, two
paired and two practically unpaired ones; its posterior end opens on the surface
of the head by an oval pore situated just in front of the hemispheres, an. p.

Meantime the two paired olfactory placodes move mesially and a new un-
paired olfactory placode appears just in front of the pore of the eye tube. The
compound eyes migrate in the opposite direction, toward the posterior haemo-
lateral surface of the thorax.

We may harmonize these conditions with those in Acilius by assuming
that the two pairs of ocelli of the second segment are the only larval ocelli of the
acilius type retained in Limulus; and that the three olfactory placodes and the
compound eyes represent respectively the three stemmata and the compound
eyes of insects, which do not appear there till the close of the larval life. In
other words, in Limulus the secondary, or imaginal, set of eyes, and the primary,
or larval set, appear at the same period, the more recent organs being reflected
back into the same embryonic period as the more ancient ones.

While these events are taking place, the palial fold is forming in separate
sections, one being directed backward and inward over each lateral eye ganglion
op.g.\ another over each infolding for the olfactory lobes, ol.I, and a third over the
ocellar plates, an. p.

The margins of all three folds gradually move toward the anterior margin of
the hemispheres where they unite to form a common opening, the anterior neitro-
pore. This pore appears to be merely the opening to the united ocellar tubes,
but in reality it represents more than that. It is obviously comparable with the
anterior neuropore of the scorpion, differing from it only in that it lies farther
forward. In both cases, the main opening represents the point toward which all
the epithelial overgrowths of the forebrain converge and the last point to be
covered by them. This interpretation is no doubt the correct one, for it is clear
that the opening offers access, as it does in the scorpion, not only to the eye tubes,
but also to the cavities of the olfactory lobes, the spaces between the hemispheres
and the palial wall, and the spaces between the under surface of the hemispheres
and the floor of the forebrain. (Fig. 47, B.}

Change of Position. The distal end of the ocellar tube is at first diiected
horizontally forward toward the ectoderm that forms the anterior margin of
theprocephalon. (Fig. 142.)

THE PARIKTA1. F.YK OF THK I.IMll.rS.

As the embryo develops, this layer of ectoderm extends forward, carrying
the ocelli with it and drawing out the ocellar sac into a long epithelial tube or
epiphysis. The procephalic ectoderm then forms a vertical wall covering the
median anterior surface of the egg; still later it is bent backward onto

s

9

10

FIG. 93. Various forms of retinophora, isolated by maceration and showing the position and shape of the retinal
rods. Cross- sections of the rods are shown over each figure, the place where the section is taken being indicated by
the letter 5. i . Upright terminal rod from ocellus I ' of Acilius; 2, horizontal terminal rod from sides of ocellus II of
Acilius; 3, a giant retinal cell with short horizontal rod, from ocellus //; 4, retinal cell, with lateral rod from com-
pound eye of Limulus; 5, retinula cell from the compound eye of Tabanus; 6, retinal cell from the ocellus of Lycosa;
7, retinula cell, with serrated rod, from the compound eye of Pinaeus; 8, inverted retinal cell from the eye of Pecten;
9, rod cell from retina of an amphibian (species of Diemyctylus) , showing two nuclei, n'and n, and indications of
division of rod into two parts with either a canal or fiber running through a part of the rod; 10, cone cell from
same animal, showing double nature of the cell as well as of the cone. The body corresponding to the second
nucleus lies at n'

the haemal surface of the buckler, where it represents the exposed surface
of the anterior end of the primitive head, or procephalon; the posterior end
of the head, coincident with the forebrain, remaining on the neural surface.
(Figs. 152, 153, pc.c.)

134 LARVAL OCELLI AND THE PARIETAL EYE.

While this is taking place the ends of the anterior liver lobes unite in front
of the cephalic lobes, thus apparently isolating that part of the head containing
the ocelli, from the neural portion containing the olfactory organs and cephalic
lobes. (Figs. 149, 151.)

At this stage, the surface contours of the forehead cannot be clearly dis-
tinguished. But during the early trilobite stages, after boiling in caustic potash,
a distinct suture is visible on the cephalo-thoracic shield, marking the boundaries
of the primitive procephalon. (Fig. 152, pr.c.}

This suture quickly disappears, and in all subsequent stages the only part
of the primitive fore-head visible on the haemal surface is a narrow patch bearing
the ocelli. (Fig. 155.)

Appearance of tlie Placodes. We may now confine our attention to the later
stages of the parietal eye.

After the trilobite stage, one pair of ocellar placodes form the lateral walls
of a terminal dilatation, that may be called the ecto-parietal eye. (Fig. 102, D.}
Their cells become invested with black pigment and they take on the character of
typical visual cells. (Fig. 94, l.cc.p.e.)

The other pair form the walls of a second median dilatation that we shall
call the endo- parietal eye, en.p.e. It lies below the surface, and on the proximal
side of the ecto-parietal eye. Its cells are unlike the usual retinal cells in shape,
arrangement, and pigmentation; but they are provided, temporarily, with plate-
like visual rods or rhabdoms.

Nerves. In young Limuli, three to four inches long, four nerve fascicles may
be seen at the distal end of the eye tube, one for each retina of the ecto-parietal
eye, and two for the unpaired endo-parietal eye. (Figs. 94, 101, A.)

In the middle section of the tube, the four nerves unite to form a common
layer of fibers outside the epithelial walls of the tube. Toward its proximal end,
the nerve fibers separate from the epithelial walls of the tube and again divide
into four fascicles, or two pairs of roots, the larger pair ending in two conical
ganglia on the haemal surface of the olfactory lobes, the smaller one in two smaller
ganglia situated a little farther back. (Fig. 51, cy.r 1 , cy.r 2 .)

Thus the evidence afforded by the infoldings on the cephalic lobes, the struc-
ture of the terminal sac, of the eye tube and the four nerve roots, show that the
"unpaired eye" of Limulus is formed by" the partial fusion of two separate pairs
of ocelli.

Structure of the Retinas. From the earliest larval stages, the difference in
structure between the endo- and ecto-parietal eyes is very striking. The ecto-
parietal retinas contain, besides numerous indifferent cells, well defined ommatidia
consisting of from five to seven cells with the visual rods arranged in star-shaped
rhabdoms near their outer ends. (Fig. 94.) The visual cells contain a relatively
small amount of reddish-brown pigment, and little, or none, of the white pigment.
The endo-parietal eye, in young Limuli three to four inches long, is a thick- walled,
pear-shaped vesicle lying well below the surface and almost inaccessible to light.

THE PARIETAL EYE OF THE LIMULUS.

An unpaired tubercle, or a more transparent spot in the chiten, usually marks
its location from the exterior. (Fig. 201.)

The thick inner wall of the vesicle, en.p.e., now consists of irregular elongated
cells with small nuclei. The cells may show an obscure arrangement into large
groups, and are completely filled with minute granules, which are snow-white
by reflected, and greenish-black by transmitted light.

The outer wall consists of a few prominent sensory cells, whose pointed
outer ends terminate in nerve fibers. They are devoid of either white, or colored
pigment, or of visual rods, rt' 1 .

This eye reaches the height of its development in the young animals from four
to six inches long, and from that stage on it appears to undergo a slow histological
degeneration, but without perceptible diminution in size.

C

en.

FIG. 94. The three chambered parietal eye vesicle of Limulus. A, from above; B, in cross-section; C, in long-
itudinal section. Semi-diagrammatic.

In the older animals, the distinction between the inner and outer walls dis-
appears, and the entire eye then consists of a solid mass of large vesicular cells,
with minute nuclei, crowded with "white pigment."

After the early larval stages, all traces of the epithelial walls to the primitive
eye sacs have disappeared; the eyes appear to be separate organs, except in so
far as they are innervated by separate branches of a common nerve.

The development of the eyes has shown us that the epithelium of the eye
tube merely represents the tract of ectoderm that separated the ocellar placodes
from the brain, before they were enclosed in the brain chamber; along the inner
surface of this tract the nerve fibers passed from one to the other. When the pla-
codes were infolded, the connecting paths of ectoderm were infolded also, forming
the \valls of the eye tube or epiphysis. When the fibers of the optic nerve grew
from the eye to the brain, or from the brain to the eye, they were compelled to

136 LARVAL OCELLI AND THE PARIETAL EYE.

follow the old paths, that is, the outer surface of the eye tube. When the nerve
fibers separate from it, the tube is left as functionless epithelium, which may, in
whole or in part, disappear.

The dilated proximal end of the eye tube, from which the nerve roots have
separated, remains for a long time (up to three to four inches long), adhering to
the anterior surface of the hemispheres, beneath the thick neurilemma sheath.
In the adult all traces of it have disappeared. The distal end of the tube like-
wise disappears, so that finally the three ocelli are united to the brain by a single
solid nerve with four terminal branches and two pairs of roots, each of the four
roots ending in a distinct ganglion.

The parietal eye of Limulus differs from that of the scorpion in the great
length of the eye tube, in the presence of the endo-parietal eye, and in the location
of the ganglia on the haemal surface of the brain, instead of the neural. These
differences, although sufficiently striking, are not fundamental, but due merely
to differences in the relative rate of growth of the adjacent organs in the two
animals. One cause of the difference was the closing of the anterior neuropore
in front of the hemispheres in Limulus, and behind them, in the scorpion. More-
over, as the parietal eye in the scorpion lies (morphologically) behind the hemis-
pheres, and over the neural surface, the ocellar ganglia are drawn upward,
toward the median neural side, as near to the eye as possible. In Limulus, the
parietal eye has migrated forward, and then backward on the haemal surface,
drawing the nerves and ganglia forward and haemally. (Fig. 47, .4 and B.~)

The Parietal Eye of Branchipus.

The early stages in the development of the median ocellus of phyllopods
and other Crustacea are imperfectly known. But its structure in the adult in-
dicates very clearly that it is the same kind of an eye as the median one in Limulus
and other arachnids, and probably develops in a similar manner. That is, it
consists of two pairs of ocelli enclosed in a median sac that opens to the exterior
for a time at least by a short, median duct, or epiphysis. (Fig. 102, .4.)

The parietal eye of Branchipus is probably typical of many Crustacea.
Its characteristic features appear in the nauplius at a very early period. In
Branchipus it is a tri-lobed vesicle consisting of two communicating sacs.
(Figs. 95 and 96.) The larger, outer one, or ecto-parietal eye, has thick, lateral
walls representing two ocellar placodes or retinas. The distal ends of the retinal
cells are directed inward and are capped with minute lateral rods, or plates. The
cavity of the sac is coated with a layer of dense black pigment, apparently the
product of two large cells whose nuclei are seen in the posterior lateral part, pg.c.

The inner sac, or endo-parietal eye en.p.c. is conical and with a minute cen-
tral canal or crevice toward which the inner ends of the retinal cells converge from

THE PARIETAL EYE OF BRANCHIPI S.

r 37

all sides. It is colorless, and doubtless represents one completely fused pair of
ocelli, corresponding with the inner, colorless eye sac of Limulus.

In sagittal sections, the pigmented floor of the outer sac, in younger specimens,
appears to be continuous with the outer ectoderm, leaving a narrow pore or crevice

nc.

&

ecpe. .

\i<y -

enp.e. .->HV !^_

ufe

i\ en, e.

FIG. 95. Parietal eye vesicle of a young Branchipus, with the adjacent frontal organs (or lateral olfactory organs).

Composite frontal section. Camera outline.

by which the cavity of the eye sac communicates with the exterior. (Fig. 102, .4.)
This opening is doubtless comparable with the eye tube of scorpion and Limulus.
As the tube itself is very short the opening leads directly into the common eye
chamber containing the three ocellar placodes.

There is no lens for any part of the eye, light having free access to the shallow
retina of the outer sac from either side, and to the inner sac from all sides.

FIG. 96. Sagittal section of the parietal eye of a young Branchipus. Camera outlines.

The ecto-parietal eye has two distinct nerves distributed over the outer
surface of the retinas, n.ec.e. They arise from ganglionic enlargements of the
anterior median portion of the brain. The endo-parietal eye has a single nerve,
n.en.c.

138 LARVAL OCELLI AND THE PARIETAL EYE.

The Parietal Eye of Apus.

In Apus, the conditions are a little more complicated. Here, as in many other
phyllopods, there is a remarkable skin fold directed forward, forming a broad,
shallow chamber over both the compound eyes and the ocelli. (Fig. 102, B.} It
opens to the exterior by a narrow pore plugged w r ith chiten. (Fig. 98, O.) This
opening should not be confused with the epiphyseal pore of scorpion and Limulus.
The parietal eye forms a closed chamber with a retinal placode on each side
wall, and two unpaired placodes, one on its posterior, the other on its inner wall.
(Figs. 97-99.)

Each placode consists of a single row of large, colorless, columnar cells.
Their distal ends are buried in a dense mass of dark brown, or black pigment;
their proximal ends are colorless.

o at

c.e v.

I

a rt

FIG. 97. Sagittal section of the parietal eye vesicle of an adult Apus. m, Fold covering the lateral eyes; O,
opening of the lateral eye vesicles, c.c.v; o.at., remnants of canal leading into parietal eye vesicle; a.t.. cavity of
the same; p.rt., posterior retina; a.rt., anterior ratina.

As in Branchipus, there are two large cells which appear to give rise to the
greater part of the pigment that fills the cavity of the vesicle. (Fig. 98, p.g.c.)

When the pigment is partially dissolved, it is seen that each retinal cell is
capped with a large brush-like mass of fine fibers (retinidium) , apparently the
free ends of nerve fibers passing through the interior of the cells, or over their
outer surfaces. They are comparable with the nervous network described by
me in the visual rods of Pecten, Acilius, Lycosa, etc., except that they are not reg-
ularly arranged, and are not imbedded in a dense, transparent matrix, which
usually forms the most conspicuous part of a visual rod.

The parietal eye sac of Apus probably contains the retinas of four distinct
ocelli, which during development migrated from the sides of the head toward
the median line. There they became enclosed in a common sac, that opened to

PARIETAL EYE OF APUS.

139

the exterior by a short duct or pore. The remnant of this duct is seen in the
adult in the deep recess on the posterior outer margin of the eye sac. (Fig. 97,
o. at., and 102, B.)

The fold of skin that covers both lateral and median eyes was no doubt a
later formation, having nothing to do with the original parietal eye infolding.

The parietal eye of Apus lies entirely below the surface. There are no over-
lying lenses, or thickenings of the adjacent ectoderm, to control the direction of
the light. The latter may enter the paired retinas from the sides, and the un-
paired ones from in front, or from behind.

c e

pg c

\

Q rt

FIG. 98. Parietal eye vesicle of Apus, in cross-section.

FIG. 99. Same as preceding figure, with
pigment removed, showing the coarsely
fibrillated visual rods, r.t.d., on the inner
ends of the retinal cells.

There is no reason to doubt that the tri-oculate median eye of decapods,
copepods, trilobites, and merostomes, in structure and development is essentially
like that of Limulus, scorpions, spiders, Apus, and Branchipus. The evidence
presented clearly indicates that this group of ocelli is very constant throughout
the Crustacea and arachnids, and that it has certain remarkable features which
distinguish it from all other visual organs. There is no parallel to the way in
which these ocelli develop except in the parietal eye of vertebrates, and there is
no explanation available for the condition seen in vertebrates except the one offered
by the arachnids.

The Parietal Eye of Vertebrates.

The parietal eye of vertebrates was long ago demonstrated to be a vestigial
eye, although there are some authors who still refer to it as a mysterious organ of

140 LARVAL OCELLI AND THE PARIETAL EYE.

unknown function. It is difficult to understand how any one familiar with visual
organs, could fail to recognize in the parietal eye of Petromyzon, or Hatteria, or
Lacerta, a visual organ of some kind. The pigment, lens, retinal cells, and nerves.
are unmistakably parts of an organ that served at one time as an eye. whatever
its function may be now.

The conflicting accounts of the parietal eye are due in part to the various
conditions in which it appears in different groups of vertebrates, but mainly
to a fundamental misconception of the ground plan of the organ and how it
happens to get inside the brain.

It has not been clearly recognized that the parietal eye is a paired organ
arising originally outside and beyond the boundaries of the brain; that it contains
several distinct sensory placodes; that there is a fundamental distinction between
the sensory placodes and the non-sensory epithelial tube that connects them with
the brain; and it has been very difficult to eliminate the idea that the paraphysis is
an eye or a part of one, or that it produces some part of the parietal eye.

From our new point of view, the parietal eye of vertebrates is a most signifi-
cant and illuminating organ. The best insight into its meaning may be obtained
by studying its structure and development in the lamprey.

Petromyzon. My observations on the development of the eyes of this
animal in the main, agree with those of Sterzi, especially in regard to the nature
of the early epyphysial outgrowth.

The Parietal Eye Vesicle. In larvae about o mm. long, a single median
parietal eye tube is seen just in front of the superior commissure. The dilated
end of this tube appears to divide into two lobes, the larger one lying outside of,
and a little in front of the other. The inner one lies somewhat to the left of the
median line, the outer one to the right; the displacement, however, is not enough
to indicate that the two sacs are right and left lobes of a single pair.

I have no material representing the stages between 6 mm. and 30 mm.
larvae, and I do not know just what takes place at this critical period, but the next
following stages seem to indicate clearly enough that, meantime, the inner sac has
become separated from the main tube, giving rise to an endo-parietal, or "para-
pineal" eye; while the outer sac remains connected with the primitive eye tube,
giving rise to the "pineal" or ecto-parietal eye. (Fig. 100, ec.p.e.and en.
p.e.) The floor of each sac is now divided by a deep longitudinal groove, consist-
ing of undifferentiated epithelium, into two symmetrically placed, concave discs,
each disc probably representing a retinal placode. (Fig. 101.) Both sacs develop
a small amount of brownish pigment, which is, however, masked by a large
quantity of the characteristic white granules. The entire organ, when seen with
the naked eye, is a glistening white spot that looks precisely like the endo-parietal
eye of Limulus. In both Limulus and Petromyzon, the granules are soluble in
weak acid.

The outer eye sac presents the most characteristic retinal structure. In
six inch ammoccetes, the retinas consist of a layer of sensory cells, each bearing

PARIETAL EVE OF PETROMVZON.

a long, fibrous, colorless rod suggestive of those seen in Apus. (Figs. 99 and 100. )
A layer of nuclei and fibers is seen below the columnar cells. Its outer wall, in
its central portion, consists of similar cells and rods. They have been regarded
as forming an imperfect lens, but their histological structure indicates that they

B

FIG. 100. The parietal eye vesicle of a young lamprey, 6mm. long. .4. Sagittal section; B . cross-section.

FIG. 1 01. Plan of the parietal eye vesicle with its nerves, ganglia, and epiphysis, seen from the neural surface.

A, Young Limulus; B, young lamprey.

represent the remnants of visual cells, although they are not so well developed as
those on the floor of the sac. The two sets of rods meet in the middle of the sac,
their distorted ends forming a distinct cleavage band. On the periphery of the
eye the walls consist of a single layer of short, columnar cells.

142 LARVAL OCELLI AND THE PARIETAL EYE.

The amount of pigment, and its distribution, varies greatly in different
individuals and at different stages. In many cases, the cells of the outer walls
are colorless, and the inner wall, and especially the two layers of rods, are densely
crowded with pigment, a condition similar to that seen in Apus and Branchipus.

The inner sac cn.p.c. resembles the outer one, except that its retinal cells are
less highly specialized, and its outer wall consists of a thin layer of indifferent,
columnar cells.

The groove on the floor of the outer sac is hardly recognizable anteriorly,
but it gradually deepens toward the posterior margin, where it leads into the en-
larged, distal end of the epithelial eye tube or epiphysis. This part of the tube
persists in the adult as the conical "atrium" of Studniaka. The proximal part
of the tube likewise persists as a small solid cord, extending over the outer surface
of the ganglion habenula. A trace of its original opening may be seen as a conical
recess, in front of the superior commissure. (Figs. 100 and 101, 141.)

A similar groove, leading into a short blind tube, is seen in the floor of the
inner sac, d 2 . This tube leads toward the base of the "atrium," but at this stage
does not unite with it. It undoubtedly represents the remnants of the connection,
existing during the early stages, between the inner sac and the main eye tube.

After the metamorphosis the parietal eye loses the clear cut histological
details seen in the early stages, and is then undoubtedly of less importance
functionally.

The Parietal Eye Ganglia, or Ganglia Habenula; , consist of a main right and
left ganglion, each consisting of an anterior and a posterior lobe. We may there-
fore, recognize four lobes, or two pairs of ganglia, for the parietal eye, a condition
in complete harmony with the presence of two pairs of retinal placodes in
the eye.

The left ganglion is smaller than the right and differs from it in minor,
histological details. It gradually moves forward and mesially, till the anterior lobe
lies close to the posterior, inner wall of the inner sac, with which it is connected
by a large bundle of nerve fibers. This nerve divides into two, one passing on
either side of the median groove. (Fig. 100, n 2 .)

The larger, outer eye is said to be connected by nerve fibers with the larger,
or right ganglion. I have not been able to satisfy myself that this was the case.
In fact the nerves to the outer sac are small and very difficult either to identify,
or to follow to their terminals.

The right and left ganglia are connected by at least two commissures that
originate in two large cores of neuropile. (Fig. 101.) From the latter, two
pairs of nerve tracts arise, the anterior pair, a.ti'., passing downward and forward
to the median face of the olfactory lobes; the posterior pair, />.//'., downward and
backward to the floor of the midbrain.

It is a surprising fact that the two anterior bundles are of approximately
equal dimensions, while of the posterior pair, the right is very much larger than the
left.

PARIETAL EYE OF THE PETROMYZON.

143

The inner and outer sacs of Petromyzon and the two similar ones in teleosts
have been regarded as right and left mates of a single pair, on the ground that
they are, for a short time at least, somewhat asymmetrical in position, one being
a little to the left, the other to the right of the median line; furthermore, it is claimed
that in the lamprey the larger outer sac is innervated mainly from the right
ganglion habenulae, and the inner one from the smaller, left ganglion habenulae.

The evidence however, is ; by no means conclusive. My own observations
lead me to the conclusion that the inner and outer parietal eyes are just what they
appear to be, namely, two unpaired organs of slightly unequal value, one of which
has been crowded away from the median line.

Olo

.ec.pe.

er\pe.

ien.pe.
'ec.pe.

FIG. 102. Semi-diagrammatic, sagittal sections of the head, showing the relative position and character
of the parietal eye, the epiphysis, neuropore, etc., in A, Branchipus; B, Apus; C, scorpion; D, Limulus; E, vertebrate.
On the right the eyes are shown on a larger scale.

Evidence for this conclusion is afforded by the parietal eye of the cyclostomes
and less directly by the parietal eye of arachnids.

i. In the first place, in the lampreys, during the earliest stages, one sac
lies directly behind the other, and there is nothing to indicate that one is the right
or left mate of the other. Whatever asymmetry appears in the eye sacs is seen
later, and is comparatively slight. The same condition appears to prevail in
teleosts, according to Hill's observations, although he interprets them differently.

144 LARVAL OCELLI AND THE PARIETAL EYE.

2. The two parietal eye sacs in the cyclostomes not only stand very nearly,
in the median plane, but each sac contains a right sensory placode, or retina,
separated from a left one by a median groove, or by an unspecialized band of
tissue. Thus there are two symmetrical retinal placodes in each parietal eye.

3. In young lampreys about two inches long, each ganglion habenula is
divided into a smaller anterior lobe, united by two nerves with the inner sac,
and a larger posterior one, probably united in a similar manner with the outer
sac. Thus there are apparently four ganglia corresponding to the four placodes,
These facts are incompatible with the assumption that one sac is the right or
left mate to the other.

4. The asymmetry of the ganglia is pronounced. The left anterior lobe
ultimately takes up a central position below the inner sac, and remains com-
paratively small. The other three lobes become very large, especially the two
on the right; but the reason for this unequal development is not apparent, since
the nervous connection with the right sac is insignificant.

5. A comparison with the parietal eye of arachnids (Fig. 101), shows that
the inner sac of petromyzon (parapineal eye) corresponds to the endo-parietal
eye of Limulus, both sacs agreeing in position, in their lower grade of histological
structure, in their innervation, and in their relation to the epiphysis. The outer
sac of the lamprey corresponds with the outer one of Limulus, both sacs agreeing
in relative position, in being symmetrically bi-lobed, and in the presence of the
more highly specialized visual cells and rods.

The Lenses of the Parietal Eye.

It will be recalled that in the simple isolated ocelli of insects, the chitenous
lens and the thick transparent ectoderm that serves as a vitreous body are parts
of the optic cup, or of the lips of the cup. (Figs. 90, 91.)

When there are well defined lenses to the parietal eye, as in many arachnids,
they are formed from isolated thickenings of the ectoderm and of the overlying
chiten, wherever the distal end of the eye tube reaches the surface of the head,
however remote that point may be from the one where the retinal placodes first
appeared.

In the scorpions, the parietal eye has a highly developed vitreous body and
two lenses. (Fig. 105.)

In Limulus, there are two well developed lenses, one for each retina of the
outer sac (Fig. 94). But the inner sac never has over it a true chitenous lens, or
any ectodermic thickening which may represent the remnants of a vitreous body,
although there may be a tubercle like thickening of the chiten, or a semi-transpar-
ent spot. (Fig. 201.)

In the phyllopods, although the parietal eye is often very highly developed,
it lies well below the surface, and there is no thickening whatever of the adjacent
ectoderm, or of the chiten, to form a lens or vitreous body for them. The fre-

LENSE OF THE PARIETAL EYE.

145

F'

FIG. 103. Cross-sections of the procephalic lobes of an embryo scorpion. E, Posterior part of stomodaeal region,
showing the third cephalic neuromere, en 3 , the lateral eye ganglion, I. e.g., and the corresponding imagination, and
the posterior margin or the palial fold; E', same stage farther forward, showing the stomodseal ganglion and its
commissure, the second cerebral neuromere, the parietal eye ganglion and its corresponding infolding, iv~. Com-
pare Fig. 15, B. F and F' are corresponding sections in an older stage, Fig. 16, A, showing the crowding of the
lateral eye ganglion over the cerebral neuromeres and the appearance of the parietal eye, p.a.e., on the inner
limb of the palial fold. Camera outlines.

.- pa.e.

FIG. 104. Selected sections from a continuous series through the procephalic lobes of an embryo scorpion, in stag

G (Fig. 1 6, B) . Camera outlines.

10

146

LARVAL OCELLI AND THE PARIETAL EYE.

quent absence of a lens and vitreous body, in the otherwise well developed parietal
eye of arthropods, is remarkable, since it does not occur in the other types of
arthropod ocelli. The fact is all the more significant when we recall that in
vertebrates true lenses to the parietal eye are never present. In place of them, we
find a transparent spot, or tubercle, or a thin place in the overlying tissues. The
thickening of the outer wall of the eye vesicle, which may possibly serve, in ex-
ceptional cases, as a lens (reptiles), is probably the remnant of a retinal placode.

Location of the Placodes. The location of the retinal placodes in the
parietal eye vesicle varies greatly. In the arthropods, they may lie in the side
walls (Branchipus), or in the outer wall (scorpion and Limulus), or in the inner
wall or floor, as in Apus. The prevailing position in arachnids is in the outer wall,

A B

FIG. 105. A, Section through the posterior margin of the parietal eye in stage H, showing the approaching
union of the two retinas, and the palial folds; B, section through the parietal eye of a newly born scorpion, showing
the parietal eye vesicles, and the ventricle, V, formed by the optic ganglia, the palial folds, and the forebrain
neuromeres. The ventricle extends forward and downward, into the cavity of the olfactory lobes, ol.v.

thus inverting the cells, and turning the rod bearing end toward the cavity of
the vesicle. But the retinal cells have a remarkable method of readjustment,
so that in the later stages, they appear to be standing in an upright position.

In vertebrates, the placodes in general appear to occupy the floor of the
vesicle, but they may develop on both walls, as in Petromyzon.

Minute Structure. In the arthropods, there is nothing constant in the
histological structure of the parietal eye retinas. The principal elements are
columnar, sensory cells arranged either in a continuous layer, with terminal
rods projecting into the eye chamber (Apus, Branchipus), or they may be
arranged in definite groups, or ommatidia, consisting of from two to five or
more cells with plate-like rods attached to the side walls of each cell (Limulus,
scorpion, Phalangium, Lycosa).

Where the eye, to all appearances, has become functionless, i.e.. endo-parietal
eye of adult Limulus, the cells form a confused mass, without any definite arrange-
ment in layers, or in respect to the source of whatever light may reach them.
The black pigment is then absent and the cells are filled with a dense mass of
glistening white granules. Even in this degenerate condition, the visual rods
may be retained as irregular plates, singly or in groups, attached to the side walls
of the retinal cells.

PARIETAL EYE. 147

In the scorpion and other arachnids (Limulus, Galeodes and Phalangium)
a transformation takes place in the arrangement of the retinal cells, shortly after
the eye assumes its definite form. In the scorpion, owing to the method of in-
folding, the retinal cells are inverted, the nerves being distributed over the outer
surface of the sac, and the rods turned toward the lumen of the vesicle. Later,
however, the nerves, entering from the side, appear to penetrate the retina about
midway between the inner and outer surfaces. In the adult, the rods are located
on the sides of the cells, near their outer ends, and the nerves then enter the oppo-
site, or inner end. Just how this apparent, or actual, reversal of the retinal
cells takes place, I have not been able to determine.

In the scorpion, Limulus and Phalangium, the rods lie in isolated groups, on
the sides of the cells, just below the outer surface of the retina. But in the parietal
eye of Galeodes and of spiders, where the same method of development prevails,
the rods form in the adult a continuous layer outside the retinal cells, and there
is no indication as to what was the nature of the of the post-embryonic trans-
formation that brought the rods and nerve ends into that position.

I. Summary.

We may summarize our conclusions in regard to the parietal eye as follows:

1. All vertebrates possess remnants, more or less distinct, of a median or parie-
tal eye which in some forms contains true retinal cells and visual rods, and is
connected by several (4 ?) distinct nerves with as many ganglia.

2. There is but one median or parietal eye consisting, however, of several
parts.

3. The eye proper consists of three or four sensory placodes, each one
representing the retina of a simple ocellus of the arthropod type. The placodes
form the walls of a sac on the end of a membranous tube projecting from the
roof of the tween-brain.

4. The placodes have a paired arrangement and probably represent two pairs
of ocelli, located originally in the ectoderm, just outside the lateral margins of
the open medullary plate.

5. They were ultimately forced into, or carried into, the brain chamber by the
same forces that produced the brain infolding. The placodes are carried on the
crest of the brain infolding toward the median line, meantime shifting from the
outer, to the inner, limb of the fold. When the crests unite, the four placodes
form a compact group on the membranous roof of the brain. At that point a
tubular outgrowth of varying length is formed which has a vesicle or dilatation at
its distal end, in the walls of which the placodes lie. This vesicle with its four
placodes is the parietal eye.

6. The primary vesicle may now be constricted, forming two unpaired lobes,
or the lobes may separate, forming two separate sacs, a larger, anterior and outer
one, the ecto-parietal eye, containing the two most highly developed placodes,

148 LARVAL OCELLI AND THE PARIETAL EYE.

and an inner posterior one, or endo-parietal eye, containing the remaining two
placodes, now completely united into one organ, and with greatly reduced struc-
tural details.

7. The membranous tube, or epiphysis may disappear in whole or in part,
leaving the terminal eye sacs either isolated, or united by distinct nerves with the
parietal eye ganglia, or the ganglia habenulse.

8. The parietal eye of vertebrates is homologous with the parietal eye of such
arthropods as Limulus, scorpion, spiders, phyllopods, copepods, trilobites, and
merostomes, but not with the frontal stemmata or other ocelli of insects.

9. In the arthropods, various stages in the evolution of a cerebral eye are
shown in detail, from functional eyes on the outer margin of the cephalic lobes,
to a median group of ocelli enclosed within a tubular outgrowth of the brain roof.

The most primitive type of a parietal eye is seen in the nauplii of phyllopods
and entomostraca, where the eye is a pear-shaped sac, opening by a median pore
or tube on the outer surface of the head. (Fig. 272, 308.) In the higher arachnids,
the process of forming an embryonic eye vesicle merged with the process of form-
ing a cerebral vesicle, the external opening of the forebrain vesicle and that of the
parietal eye tube, forming a common opening or anterior neuropore.

10. The parietal eye of arthropods is an important visual organ until the
lateral eyes, which represent a later product, are fully developed. It may then
diminish in size and activity, but it rarely, if ever, wholly disappears.

11. During the evolution of vertebrates from arachnids, there was a consider-
able period during which the lateral eyes were adjusting themselves to their new
position inside the brain chamber, and when they were in functional abeyance.
At this period, ancestral vertebrates were mon-oculate, that is they were dependent
solely on the parietal eye, which had come to them from their arachnid ancestors
as an efficient and completely formed organ.

When the lateral eyes again became functional, the parietal eye began to
decrease in size and effectiveness.

The parietal eye is the only one now present in tunicates. In the oldest
ostracoderms, like Pteraspis, Cyathaspis, Palaeaspis, the lateral eyes are absent, or
at least do not reach the surface of the head, the only functional one being the
parietal eye, which is of unusual size.

In the lampreys we see the same conditions, the parietal eye being very well
developed in the larvae, while the lateral eyes are deeply buried in the tissues of
the head, and useless. During the transformation, the lateral eyes again become
functional, and the parietal begins to atrophy, finally losing many of its structural
details and its function, although still retaining very nearly its original form.

CHAPTER IX.

THE COMPOUND EYES OF ARTHROPODS AND THE LATERAL

EYES OF VERTEBRATES.

Froriep (Hert wig's Handbook of Embryology) states, quoting Kessler, 1877,
that K. E. von Baer's discovery that the eye in the chick is a hollow outgrowth of
the forebrain vesicle, is the most interesting fact in the development of the eye
that could have been obtained, and is without a parallel. He also quotes with
approval Gegenbaur's expression of astonishment that in the entire range of
vertebrates there are no lower stages in the development of such a complex organ
as the eye. The vertebrate eye, he says, Athene-like, makes its appearance com-
pletely formed, and comparative anatomy and embryology are powerless against it.

The problem, however, is not as hopeless as this, for we have shown that the
parietal eye of arachnids furnishes a very striking parallel to the development of a
vertebrate cerebral eye. The arachnid theory also provides a satisfactory ex-
planation for the sudden appearance of lateral, cerebral eyes in vertebrates, and for
their most striking peculiarities. It is clear, on the arachnid theory, that the lateral
eye was delivered to the vertebrates in a high stage of perfection. There are no
transitional stages between the external, convex eye of arthropods and the internal,
concave eye of vertebrates, because there can be no half-way stages between an
eye that stays outside the brain chamber, and one that during development is
carried into the chamber. The eye must either get in early, before the brain
closes, or stay out. Either position, at once and definitely, determines its char-
acter and the way it does its work. Whatever moulding influence the new en-
vironment had on the enclosed eye was felt immediately, and the necessary read-
justments, no doubt, at first followed rapidly and then ceased, leaving the eye
more stable than before, because enclosed in less variable surroundings.

*jl 5fC 5JC 5|C 5jC *j* 5Ji *J* *j*

I. COMPOUND EYES OF ARTHROPODS.

A. Serial Location. The lateral eyes of arthropods are such essential and
constant parts of the head that it is important to determine their origin, and to
what metamere they belong. This, however, is a very difficult thing to do.

The view, often expressed, that the compound eyes are compact groups of
larval ocelli is untenable, since the primitive larval ocelli (coleoptera and lepi-
doptera) degenerate and take no part in the formation of the lateral eyes. More
over, in the very early larval stages of phyllopods, copepods, and many other
Crustacea the larval ocelli and compound eyes are present at the same time and
clearly arise independently of each other.

149

150 THE EYES OF ARTHROPODS.

In forms like Acilius, that give us a most detailed picture of ancestral con-
ditions, not a trace of the lateral eyes appears till late in the larval stages, when it is
impossible to certainly determine their relations to the cephalic lobes, or to other
segmental structures. They are first seen on the haemal side of the head of the
oldest larvae, median to the ocelli. The latter, during the metamorphosis, are
torn away from the ectoderm, apparently by the relative shortening of the optic
nerves, and are finally lodged on the surface of the optic ganglion, where they may
be seen in a degenerate condition, long after the lateral eyes have become func-
tional. In the early embryonic stages of insects that do not pass through a
metamorphosis, and in many Crustacea, the lateral eyes are seen on the posterior,
lateral margins of the cephalic lobes, just lateral to an infolding that gives rise to
the optic ganglion. Here also their relation to the metameres has not been
determined.

Limulus is the only form in which the larval ocelli, frontal ocelli (olfactory
organs), and the lateral eyes, are all present at the same time in an early embryonic
stage. Here it is clear that the lateral eyes arise from the cheliceral or first
thoracic segment. (Figs. 141 and 142.)

I see no serious objections to regarding the lateral eyes of insects as also
belonging to the first appendage bearing segment, and if the "'organ of Tomds-
vary," in the myriapods represents the rudiment of the lateral eyes, as I have
suggested, 1892, then that also would have a similar position, since it is situated
at the base of the antennae, and its nerve is attached to the ganglion of the larval
ocelli in the same way the compound eye-nerve is in Acilius. (See optic ganglion
in Acilius.)

These facts indicate, therefore, that the lateral eyes of arthropods stand
serially behind both the primitive cephalic lobes and the larval ocelli, and belong
to the most anterior appendage-bearing segments of the primitive body or thorax.
I can find no evidence in the structure or development of the lateral eyes to indi-
cate that they are modified appendages.

B. Development. Although the lateral eyes are often post-embryonic
structures, they may, in some forms, arise during the embryonic stages.

In such cases, Vespa, Astacus, Limulus and others, the lateral eye placodes
lie on the external margin of a deep infolding which gives rise to the optic ganglia,
in the same manner that the infolding in Acilius gives rise to the ganglia of the
larval ocelli. (Fig. 14.) The lateral eyes, however, are never involved in
this infolding. It soon closes, and the placodes move away from the margin of
the cephalic lobes onto the posterior haemal surface of the cephalothorax (Limulus,
many trilobites and merostomes), or in some cases, onto its anterior margin,
or they may remain in their original position on the neural surface, (Cladocera).
(Fig. 78.) The position of the lateral eyes in the adult, therefore, varies greatly,
and is either determined by the prevailing position of the animal in relation to
the source of light, or the location of the eyes determines the position of the
animal.

LATERAL EYES OF VERTEBRATES. 151

In Vespa, after the ganglionic infolding has closed, the lateral eye placodes
are themselves deeply infolded and partly covered by thin membranous folds,
but the latter soon disappear and take no part in the formation of the eye. 1

In insects, Crustacea, and Limulus, the eye proper, or ommatasum, including the
cyrstalline cone cells and retinulae, is formed from the single layer of columnar,
ectodermic cells that constitutes the lateral eye placode. The infolding described
by Reichenbach and others, in the crayfish, as forming the deeper layers of the
eye, is merely the infolding that produces the lateral eye ganglion.

II. LATERAL EYES OF VERTEBRATES.

In my first contribution to the origin of vertebrates, 1889, I pointed out the
remarkable resemblance between the early position of the eye placodes in verte-
brates and arthropods, and the similar way in which the neural crests enclose the
forebrain vesicle.

Many contributions have been made since that time, especially in regard
to the vertebrates, that confirm my observations and my interpretations of them,
yet no one appears to have clearly understood the facts or their significance.
It is hoped that a fuller description, with numerous additional figures, will make
these important data intelligible and convincing.

While the lateral eyes of arthropods are never caught in the infoldings of
the embryonic forebrain, as the larval ocelli are, they lie very close to the edge
of such folds, so that any marked deepening or extension of them, brought about
by the increasing size and precocity of the brain and its ganglia, would be likely
to include the lateral eye placodes in the infolding, and thus transfer them to the
inner walls of the brain chamber. In this new position, they would be subject
to entirely new conditions, and they would doubtless quickly undergo important
structural changes.

The structure and development of the vertebrate eye indicate that, in some
of the intermediate forms between vertebrates and arthropods, these events have
actually taken place.

Location. It has long been known that the lateral eye placodes of verte-
brates are visible at a very early stage on the outer margin of the open medullary
plate, (selachians, amphibia, birds). (Figs. 34 and 35.)

As the neural crests advance toward the median line, the placodes are trans-
ferred to the inner limb of the fold, and finally come to lie in the walls of the
brain chamber, in precisely the same manner that the parietal eye placodes reach
a similar position.

Origin of the Choroid Fissure and the Blind Spot. After they are thus
enclosed in the brain walls, they assume the shape and position which is so
characteristic of vertebrates, and which becomes so significant when compared
with the same features in the lateral eyes of arthropods.

1 See the peculiar, hood-like fold over the lateral eyes of Apus and other phyllopods.

THE EYES OF ARTHROPODS.

It will be recalled that in arthropods the compound eye is rarely circular in
outline. It is usually crescentic or kidney-shaped, the convex margin being turned
toward the source of light. A characteristic condition is seen in forms like Limulus,
trilobites and merostomes, where the eyes are located on the sloping haemal
surface of the bucklers, the light coming from above, when the animal is in its
normal crawling position. Here the eyes take the form of convex crescents, or
some slight modification of them, because such a form distributes the maximum
number of ommatidia to best advantage in reference to the direction and the in-
tensity of light. For similar reasons of economy, the optic nerve reaches the eye
at its topographical center, that is, near the middle of its neural or concave margin,
and all the fibers are distributed from that point by the shortest paths to their
respective terminals. (Fig. 106, .4.)

N

FIG. 1 06. Diagram to explain the origin of the choroid fissure in the lateral eye of vertebrates. A. The
extra cerebral, kidney-shaped compound eye of a marine arachnid; B, the same eye, as is appears seen through
the walls of the head in vertebrate embryos. In its transfer to the wall of the cerebral vesicle, the eye is turned
upside down and inside out. The arms of the horseshoe-shaped retina then unite, forming a choroid fissure, while
the optic nerve, entering near the middle of the retina, distributes its fibers over what is now its outer surface.

If such a kidney-shaped eye, lying during the early stages near the margins
of the cephalic lobes, were actually involved in the brain folds, as the larval ocelli
are, it would still tend to retain the same shape and to occupy the same posi-
tion that it did while on the outer surface of the body, that is, the eye would
eventually grow out from the brain wall, on the end of a tube directed back-
ward toward the original position of the eye.

But the kidney-shaped eye, owing to its inversion during the infolding, would
now form a kidney-shaped retina, or sensory placode, with its convex surface
directed inward, instead of outward, and its concave margin directed haemally,
instead of neurally. (Compare Figs. 32 to 34 and 106.) In other words,
the inverted compound eye would have the same peculiar shape and position that
the vertebrate retina has at an early embryonic period. But in its new position
and under the new conditions prevailing w r ithin the head, the open crescentic
form of the placode would probably not be retained. It would be likely to
follow its original method of growth unchecked, till a new position of equilibrium
was attained; that is, it would continue to grow more rapidly on one margin than
on the other, till the two limbs of the crescent unite, forming a concave, circular
retina, with a "choroid fissure" directed haemally, and with a centrally located
nerve at the apex of the fissure, distributing its fibers radially over the concave
surface of the retina. (Figs. 107 and 108.)

LATERAL EYES OF VERTEBRATES. 153

The Retinal Cell Pattern. In Limulus, and no doubt similar conditions
prevail in trilobites and merostomes, the lateral eyes consist of numerous chitenous
lenses, under each of which is an "ommatidium," consisting of a circle of fifteen
or twenty rod bearing cells, surrounding a central one that appears to be more
highly specialized and to have a richer nerve supply than the others.

The ommatidia are separated from one another by circles of unspecialized
columnar epithelial cells. The crystalline cone cells and the corneagen cells of
other arthropods are absent.

When such a simple kind of facetted eye was enclosed in the brain walls
of vertebrates, not only was the primitive shape of the whole eye retained, but
the characteristic pattern in the arrangement of the two different kinds of cells
was also retained. That is, the circles of rod-bearing cells surrounding a single
central one, is probably represented in vertebrates by the circles of rod cells sur-
rounding a cone cell. (Fig. 106.)

The histological changes involved in this transformation are comparatively
small, the most important one being a transfer of the retinular rods from a lateral
to a terminal position. Such changes as this frequently occur in the arthropods.
Compare, for example, the striking differences in the structure of the retinal cells
in the parietal eye of Apus, Branchipus, Buthus, Galeodes, and Limulus.

The Retinal Ganglion. In Limulus, there is a loose layer of ganglion cells
lying just beneath the inner surface of the lateral eye; and a similar one is present
in the eyes of many other arthropods, e.g., retinal ganglion of insects and Crustacea.
When the lateral eye of vertebrates was involved in the palial fold, this layer went
with it, forming the nerve cells that lie outside the stratum of rod and cone cells.
(Figs. 107 and 108, r.g.}

The Lens. A striking feature of the lateral eye is the development of a lens
vesicle from the surface ectoderm and its union with a retinal placode which grows
out from the brain walls to meet it.

The origin of the image forming organ at a remote time and place from that
of the sensory receptive surface, has led many writers to the conclusion that they
represent two originally different classes of organs, secondarily united into one.
Thus the lens vesicle has been interpreted as a specialized gill pocket, or as a
segmental sense organ serially homologous with those of the hindbrain region.
These views are untenable because they are not called for by the facts as we now
understand them. In Limulus and scorpion, we have shown that the cuticular
lens and the lentiginous ectoderm of the parietal eye are formed wherever the
vesicle reaches the surface ectoderm, no matter how remote that point may be
from the original position of the retinal placode. (Figs. 101 and 102.) It is
clear enough, in these cases, that the lens cups are an original part of the eye, and
cannot be thought of as existing apart from it. Precisely the same condition, it
seems to me, prevails in the lateral eye of vertebrates. We would therefore

154

THE EYES OF ARTHROPODS.

eliminate the lens vesicle from the category of organs foreign to the eye. It may
be comparable with the thick-walled ectodermic cup that secretes the chitenous
lenses for the parietal eyes of arthropods. (Fig. 108.) When the chitenous
exoskeleton atrophied, the old lens cup probably remained to form the new lens
of the vertebrate eye.

Origin of the ' ; Imperfections 11 in the Vertebrate Eye. Thus the com-
pound eye of arachnids, which lies on the outer surface of the head, is represented
in vertebrates by the retinal placode which there lies in the walls of the brain
chamber. The vertebrate retinal placode still retains essentially the same con-
tour, surface curvature, and arrangement of visual cells and nerve cells as that of
its arachnid prototype, but owing to the inversion of the placode, which took place
when it was transferred to the brain chamber, the concave surface and the con-
cave margin of the retinal placode face in nearly opposite directions in vertebrates
from what they do in the arachnids.

Thus arose those extraordinary imperfections of the vertebrate eye, which
have so often excited the comments of the physicist, anatomist, and philosopher.
The inverted retina, the choroid fissure, and the blind spot caused by the awkward
entrance of the optic nerve, are the inevitable result of a combination of condi-
tions, some of which, originally, had no relation whatever to the eye. These con-
ditions were established in the arthropods long before the vertebrate stock ap-
peared, and it was a purely incidental, or accidental, result of these conditions that
the eye was swept into the brain chamber, where it did not originally belong. In
other words, the fate of the lateral eye was not decided by what was best for the
eye, as an instrument, or by any selective action, in which the eye itself played
a part. The eye was a purely passive victim of its location, and of its more power-
ful neighbor, the brain. But it survived, in spite of its unfortunate location, al-
though it will forever bear the marks of a displaced and made over organ.

III. THE OPTIC GANGLIA.

In reconstructing the history of the vertebrate brain, the structure and posi-
tion of the optic ganglia of arthropods is no less significant than that of the eyes.

Location. We have already shown that in the embryos of Acilius the optic
ganglia consist of three lobes lying on the lateral margins of the neural plate, each
lobe lying opposite a forebrain neuromere. (Fig. 14.)

When the larval ocelli of insects degenerate, the ocellar ganglia, without
noticeable transformation, become the ganglia of the lateral eyes. But in Limulus
and in the arachnids generally, the ganglia of the parietal eye and those of the
lateral eyes are separate. In most insects and Crustacea, the ganglia retain their
lateral position through life. This is also the condition in young Limuli, but later
the great overlapping lobes of the hemispheres crowd the lateral eye ganglia to-
ward the haemal surface. (Figs. 36-39.) Limulus is the only arthropod, to my
knowledge, in which the ganglia occupy this position.

THE OPTIC GANGLIA. 155

In the phyllopods and arachnids, they are generally drawn upward, so that
they lie on, or over, the neural surface of the brain.

Sections and surface views of scorpion embryos show how this is done.
(Figs. 15, 1 6, 18, 41, 103, 104.) It will be seen that as the palial folds advance, the
optic ganglia move upward and inward till they lie on the neural surface of the
forebrain neuromeres, the parietal eye ganglion lying in front of the lateral eye
ganglion. In this position they give us the clue to the interpretation of the optic
centers in vertebrates for, clearly, one represents the ganglion habenula, the other
the tectum opticum, or the roof of the midbrain. (Figs. 43, 44.)

Parietal Eye Ganglia. We have already shown that the two pairs of gan-
glia, from which the roots of the parietal eye nerves arise, are represented in
petromyzon by a four lobed ganglion habenula. (Fig. 104, B.} The latter
occupies the same relative position as in the scorpion, but a very different one from
that in Limulus. However, the difference is more apparent than real, because the
anterior roots of the ganglia habenulae are directed downward and forward
toward the olfactory lobes, showing not only the direction in which the ganglia
have been shifted, but that their original point of union with the brain is the
same as it is in Limulus. (Compare Fig. 47, A, B, and C.)

The difference in position of the parietal eye ganglia, in scorpion and Limulus,
is due to the fact that in Limulus the eye is drawn forward and haemally by the
extraordinary size of the cephalic shield, and by the rapid growth of the hemi-
spheres, which have grown up behind the epiphysis, instead of in front of it as in
all other arthropods. (Compare Figs. 43, 44, 46, and 47.)

Lateral Eye Ganglia. The characteristic shape of the optic ganglia is well
seen in large-eyed insects, as Vespa, where they consist of three principal lobes.
(Fig. 107, A.) i. A proximal one, that is roughly spherical; 2. a median one,
consisting of an immense concave crescentic disc, and 3. a long narrow one,
extending around the distal margin of the middle lobe. Each lobe contains a
mass of felted fibers of the same general shape as the lobe, and is covered, on its
outer surface, by a thick layer of ganglion cells.

The hemispherical middle lobe is the most conspicuous one, and the one
which, by its contour and dimensions, reflects most accurately the variations in
the eye to which it belongs. This significant fact has also been observed in the
ocelli of Acilius, where each one of the six pairs of eyes has a special shape, or some
peculiarity in the arrangement of retinal cells, which is accurately repeated in the
size, form, and structure of the neuropile core of the corresponding ganglion.

Minute Structure in Limulus. In Limulus, the ganglia have a similar con-
figuration to those of insects. (Figs. 37-39, 51,66.) The inner lobes (Figs. 51 and 52)
contain large association neurites, op.g. 3 and op.g.*; the two outer ones, the central
ends of the optic nerve fibers, op.f., and two relays of optic neurones, op.g. 1 and
op.g. 2

Each of the two outer lobes is a disc- shaped mass of fibers, one surface covered
with a thick layer of nerve cells, the other bare. In certain cases, successfully

156 THE EYES OF ARTHROPODS.

impregnated with methylene blue, only the terminals of the optic nerves are stained.
They are then seen as small bundles passing in definite order through the gang-
lion, between the medullary core and the nerve cell layer. (Fig. 51.) Each
bundle of fibers, op.f., probably represents the terminals of a definite ommatidium.
On reaching the proximal edge of the lobe, two delicate fibers are given off, one
on each side, that penetrate the first core and end there in a few straggling
branches. (Fig. 51, .4, op.f.) Just beyond them, a compact tuft of varicose
fibers is formed on the proximal outer surface of the core. The main liber then
passes to the under surface of the second core, forming with other fibers a
characteristic chiasma, x., and then, bending upward, ends in widely distributed
dendrites.

<>p.n

FIGS. 107-108. Diagrams to illustrate the relation between the brain, optic ganglia and lateral eyes of an
arthropod and a vertebrate. In both cases the parts are projected onto the same transverse plane. FIG. 107.
arthropod. FIG. 108. vertebrate, where the same parts, by the infolding of the medullary plate, have been
transferred to the walls of the cerebral vesicles. The optic ganglia are inverted, forming the roof of the mid-
brain; the compound eye, with its visual cells and underlying ganglionic cells, r.l ., forms the inverted retina.

Many other fibers, like the one just described, enter on the opposite side of
the first core and extend along its inner face, giving off the varicose dendrites; they
then pass to the outer face of the second, ending in the double set of dendrites
on its proximal margin. (Fig. 52.) In figure 51, these fibers are seen as dotted
strands running diagonally across the inner face of the first lobe, and appearing
as continuous strands on the outer surface of the second. On passing from one
lobe to the other, the two sets of fibers form the well known chiasma (Fig. 52,.Y.)
When seen from the surface, the whole effect is that of a single lobe that has been
twisted, through about one revolution, into two lobes.

The surface neurones of each lobe send their fibers into the other medullary
core. For example, the fiber from cell a (Fig. 52), extends along the outer surface
of the second core, parallel with the optic nerve fibers, to the under surface of the
first and then upward, ending in the central part of the distal margin of the core.
Neurones b take the reverse course. The remaining ones take intermediate
courses.

COMPARISON WITH VERTEBRATES. 157

The proximal lobe consists of two parts; an anterior one, composed of large
neurones, sending their main fibers into the olfactory lobes, and collaterals into
the second lobe of the optic ganglion. (Figs. 51 and 52, op.g.*); and a posterior
part consisting of a spherical mass of somewhat smaller neurones, sending their
main fibers backward into the longitudinal tract of the brain (op.f. 3 ), and their
collateral branches into the second lobe of the ganglion (op.g. 3 ).

The optic ganglia are united with other parts of the brain by the following
tracts: a, a distinct bundle of fine libers that passes without interruption through
the optic ganglion into the crura, and run the whole length of the brain (Figs. 51 and
66 op.tr.); b. a tract uniting the ocellar ganglia with the second lobe of the lateral
eye ganglia (oc.tr.}] c. a commissural tract through the olfactory lobe, formed by
the neurones of the fourth lobe (op. f.*);d. a longitudinal tract formed by the neu-
rones of the third optic lobe, and which extend backward the whole length of the
crura (op.f. 3 ); e. and finally, an important tract, the source of whose fibers is
unknown extending from the second lobe of the optic ganglion into the cerebral
hemispheres (Fig. 52, op. Ir. H.}.

IV. COMPARISON WITH VERTEBRATES.

We have already shown that the four ganglia of the parietal eye in Limulus are
comparable with the four lobed ganglia habenulae of the cyclostomes. There is
also a striking resemblance between the lateral eye ganglia of arachnids and the
optic lobes of vertebrates, especially when we make due allowance for certain
peculiarities of position and structure.

We may best explain the origin of the optic lobes of vertebrates on the
assumption that the optic ganglia of some form like Limulus have been bent
backward and upward onto the neural surface of the brain, in the manner shown
in Figs. 44, 57, 58, and 108. In this position, which is similar to the one
they occupy in the scorpion and other arachnids, they have the same general
form and the same relation to the rest of the brain that the optic lobes have in
vertebrates; the three lobes of the optic ganglia corresponding respectively to the
colliculus, the tectum, and the torus.

It will be observed that in this new position (Fig. loS,^), the general contour
of each of the ganglia is retained, but the general appearance of the whole series
is greatly disguised, owing to the change of curvature of the second and third
lobes, and to the diagonal movement of the ganglia in a caudad and neurad
direction. (Fig. 46.) Both the distal and proximal ends of the series are fixed at
the optic commissure, the original point of attachment of the ganglia to the basal
lobes of the forebrain. The convergence of the fiber tracts of the optic lobes
toward the optic chiasma shows that the optic lobes belong primarily to the hemi-
sphere neuromeres, and that their position in vertebrates is a secondary one.

158 THE EYES OF ARTHROPODS.

It is clear that, as regards compactness and economy of space, the change
is an advantageous one.

The optic lobes of vertebrates have been forced into their present position
by the general trend of several growth forces which appear at an early embryonic
period in the arthropod head. We have already referred to some of these condi-
tions. Those that are most persistent and which most affect the position of
the optic ganglia are : i. the overgrowth of the neural crests, which tend to carry the
ganglia from the margins of the medullary plate toward the median line; 2. the
central location of the eyes on the neural surface of the head in many free swimming
arthropods and in ostrocoderms; 3. the tendency of the entire brain to move forward
beneath the integument, while the rostrum and other superficial neural structures
move backward. (Compare Fig. 46.)

When the optic ganglia are once established in a median neural position be-
hind the hemispheres, the increasing size of the latter, and of the optic lobes
themselves, exaggerates still more the backward movement of the neural portion
of the ganglia and of the primitive cerebellar commissure.

Thus the roof and sides of the mesencephalon represent the ganglia of the
compound eyes of arthropods that have worked back into the territory behind
the hemispheres by that struggle for space between growing organs which ad-
justs and readjusts, till each part falls into the place of least resistance.

Whether the optic lobes helped in the closure of the old mouth, or the disap-
pearing mouth and rostrum made a place for the lobes, cannot be determined.
Doubtless these events are part of a general movement, where it becomes im-
possible to distinguish cause from effect.

D,r. L. Griggs, working at Dartmouth, has been able to locate the optic
lobes on the margins of the forebrain region, in the open neural plate stage of
Amblystoma, and has followed their course backward and upward till they
reach their permanent position. The movements of the lobes, as he describes
them, afford a striking confirmation of the interpretation given above.

Conclusion.

i. The lateral eyes are homologous throughout the insects, Crustacea,
arachnids, and vertebrates.

2. In the arthropods they develop historically later than the larval ocelli, and
from a more posterior segment, namely the first appendage bearing segment
behind the primitive cephalic lobes.

3. In the arthropods the lateral eye placodes lie for a time on the lateral
margins of the cephalic lobes, close to the deep infoldings that form a part of the
brain. In vertebrates, the eyes at first have a similar position, but a precocious
enlargement of the cephalic lobes and the neural crests leads to the enclosure of
the compound eye placodes in the brain chamber, so that they appear to form
a part of the brain.

CONCLUSION. 159

4. The characteristic shape of the arthropod eye and the arrangement of its
retinal cells is retained in an exaggerated form in the vertebrate retina, and
affords us the only satisfactory explanation of its inversion, its contour and
mode of growth, its choroid fissure, its arrangement of rod and cone cells, and its
centrally located optic nerve.

5. The parietal eyes of vertebrates belong to the second forebrain neuromere,
the lateral eyes to the third or fourth.

6. The optic lobes of primitive vertebrates represent the compound eye
ganglia inverted and transferred to a position overlying the mesencephalic neuro-
meres. Their genetic relations, as well as their most intimate functional and an-
atomical relations, are with the procephalic neuromeres.

7. The ganglia habenulae of vertebrates represent the ganglia of the parietal
eyes of arachnids, united in the middle line over the region of the diencephalon.
They were primarily associated with the olfactory lobes.

CHAPTER X.
THE OLFACTORY ORGANS AND THE OLFACTORY LOBES.

The agreement between the olfactory organ of Limulus and that of verte-
brates may be traced in respect to so many different characters that the existence
of a genetic relationship between the marine arachnids and the vertebrates is
placed beyond a reasonable doubt. Indeed there is a greater difference in
respect to this organ, between Limulus and other invertebrates than there is
between Limulus and vertebrates.

The olfactory organ of Limulus, in certain respects, stands in a class by
itself. Nevertheless it represents a modification of organs very widely distributed
in the arthropods, and known in insects as the frontal ocelli, or stemmata, and in
the phyllopods and other Crustacea, as the frontal sense organs.

The history of these organs is an important lesson in evolution. It affords an
impressive illustration of the essentially unalterable character of the procephalic
sense organs, and it distinctly sharpens our perspective of the long series of inter-
mediate forms that connect the most primitive segmented animals with the modern
ones.

I. THE OLFACTORY ORGAN OF LIMULUS.

Structure in Adult Limulus. Gross Structure. In an adult Limulus, the
olfactory organ (sub frontal schlerite of Lankester) is a bi-lobed, wart-like thicken-
ing of the cuticula, from 5-8 mm. wide, situated in the median line, 30-40 mm.
in front of the mouth. (Figs. 38, 39, 70, ol.o.) It is innervated by three large
nerves, a median and two lateral ones.

The olfactory cuticula is provided w r ith a central cluster of sensory spines and
is perforated by many sensory and glandular openings. (Fig. 109, .4.) The
under-lying ectoderm is pigmented, and just beneath it are many branching nerve
fibers, together with ganglionic or sensory cells, and a large number, about
1500, flask shaped, or spherical, slime buds.

The most conspicuous parts of the olfactory organ are the slime buds,
which are, with few exceptions, sharply confined within the area of the olfactory
schlerite. They have the usual form and structure, as described in the chapter on
the gustatory organs (p. 116), the only noticeable peculiarity being the clusters
of small ganglionic or sensory cells lying near, or on, their outer surface. (Fig.
88, a.)

Minnie Structure. -The minute structure of the olfactory organ has not been
satisfactorily determined, especially the character of the nerve terminals. So

1 60

OLFACTORY ORGANS OF LIMULUS.

161

far as I have been able to discover without a thorough application of either the
methylene blue or the Golgi method, there are three ways in which the nerves
may terminate in the region of the olfactory organ. The median nerve, be-
fore reaching the organ, breaks up into numerous small branches which are
distributed in the central portion of the organ. (Fig. 109, m.ol.n.) The two
lateral nerves terminate in oblong masses of very large ganglion cells, just be-
neath the lateral margin of the organ. From these ganglia numerous branches
arise that are distributed to the olfactory organ and to a considerable area of the
surrounding epidermis.

The finer branches from both sources form a sub-epithelial plexus, from which
still smaller branches are distributed over the surface of the slime buds. Others,

orr

FIG. 109. The olfactory organ of Limulus. .4, Olfactory organ of the adult, seen from the outer surface;

B, cross-section through the olfactory organ of a young Limulus, about seven inches long (Flemming's solution) ;

C, longitudinal section through the root of the lateral olfactory nerve of a young Limulus, showing the ommatidia-
like clusters of large cells with rod-like enclosures, derived from the primitive segmental sense organs.

possibly derived exclusively from the lateral olfactory nerves, end in peculiar
ill defined masses of cells that are either wedged in between the slime buds, or lie
against the epidermis. (Fig. 88.) They may possibly be connected with slender
sensory cells, similar to those in the gustatory organs, that extend into the
hollow spines and into the narrow canals leading to the outer surface.

Finally there appears to be a system of fine nerve strands that penetrate
the soft chitenous exoskeleton surrounding the olfactory organ, where they form
loose meshworks in superimposed layers. These fibers resemble those seen in
the cornea of mammals, and although of very uniform caliber they appear to dif-
fer from the branching hyphse of the parasitic fungus (Macrocystis) that is fre-
quently seen in this region. The hyphas take up the methylene blue in a similar
manner to nerve fibers, and at first sight might be easily mistaken for them.
However, recent preparations in von Rath's fluid show, in addition to the hy-
phas above mentioned, branching fibers that appear to be true nerve-fibers.

ii

1 62 THE OLFACTORY ORGANS AND THE OLFACTORY LOBES.

Although the minute structure and the function of this organ need further
study, there is no question that it is a true sense organ of great morphological
significance.

The Development of the Olfactory Organ and Nerves. The Olfactory
Placodes. The primary olfactory organ of Limulus represents a segmental sense
organ serially homologous with the lateral eyes and the ocelli. It is first seen as a
pair of sensory thickenings on the anterior margin of the lateral eye ganglion,
behind the median eye tubes. (Fig. 141, ol.o.} It is connected with the middle
lobe of this ganglion by the lateral olfactory nerve. (Figs. 36-39.)

Each organ soon separates bodily from the ectoderm. Although there is no
visible infolding, the cells which have the appearance of visual cells, are inverted
in the process and become filled with a dense mass of white pigment (guanin?).
(Fig. 37, A.) At the same time certain cells filled with the same kind of pigment
migrate forward from each placode, forming a gradually widening, sub-epithelial
plexus of branching pigment cells connected with the anterior margin of the
placode by a short thick stalk. (Fig. 36, p.st.) During the early embryonic
stages the placodes move toward the remnants of the anterior neuropore and
there unite in the median line, meantime acquiring a connection with the anterior
surface of the cerebral hemispheres and the olfactory lobes. (Fig. 142.)

Lateral Olfactory Nerve. In the following stages, the united placodes move
forward beneath the integument toward their position in the adult. During
this process, the lateral olfactory nerves become greatly elongated and the cells
of the original placodes are now scattered as ganglion cells along the nerve,
but forming a special enlargement at either end. These terminal masses consist
of irregular clusters of five or six large pear-shaped cells which greatly resemble
the ommatidial cells of the paired ocelli, not only in their shape and arrange-
ment, but in the presence of the clear refractive rods, or rhabdoms, on their side
walls. (Fig. 109, C.)

In young Limuli (2-3 in.), the peripheral end of the lateral olfactories still
terminates in a compact, club-shaped mass of metamorphosed visual cells. (109,
B.l.oLn.} It also sends out several fine nerve branches which ramify widely under
the skin, in the region surrounding the main olfactory organ. (Fig. 70.) At
the same time the terminal group of cells breaks up into irregular clusters scattered
among the branches of the nerve. In the ordinary methods of preparation, each
cluster has the appearance of an isolated ommatidium composed of large pear-
shaped ganglion cells, whose proximal ends form coarse nerve tubes. There is
another group of cells, similar to those just described, scattered along the proxi-
mal end of the main nerve, some of them outside the brain sheath, but the major-
ity within it, on the root of the nerve as it passes over the surface of the hemi-
spheres. (Figs. 39, 48, 51, 66, 109, ol.l.n. and gc l and gc~.)

Both these cell groups, which contain granules that have a glistening white
appearance in reflected light, are the modified descendants of the cells constitut-
ing the original visual placode. Even in the adult, they still show traces of

OLFACTORY ORGAN OF LIMULUS. 163

the white pigment, of the refractive visual rods or rhabdoms, and of their primi-
tive grouping into ommatidia.

In young Limuli, the roots of the lateral olfactories become less compact, and
as they were seldom seen in methylene blue preparations, it was very difficult to
follow them. They appear to shift their point of attachment from the second
optic ganglion, toward the inner face of the olfactory lobes, near the tract uniting
the median and lateral eye centers (Fig. 51, oc.tr.) Whether they passed through
this tract to the olfactory lobes could not be determined. In a few cases (methyl-
ene blue) a small strand of fibers was seen to leave the main root and pass
mesially toward the horns of the olfactory lobes. (Fig. 51, z.)

Median Olfactory Nerve. When the united olfactory placodes move for-
ward away from the brain, a new outgrowth from the hemispheres and ol-
factory lobes appears which follows the placodes forward, or is drawn out by
them, to form the median olfactory nerve. (Figs. 38, 39, 41, 48, 66, ol.m.n.} It
consists of large globular masses of minute ganglion cells, each lobule con-
taining a central core of medullary substance, similar to that in the
hemispheres.

In young Limuli (2 to 3 inches long) there are four distinct roots to the median
nerve, two haemal ones continuous with the horns of the olfactory lobes, and two
neural ones, continuous with the anterior median lobes of the cerebral hemispheres.
(Fig. 48.) Each root contains a medullary core of neuropile surrounded by a
cortex of "granule cells," the cortex and neuropile passing without perceptible
change into the cortex and the neuropile of the cerebral hemispheres and the ol-
factory lobes. (Figs. 48, 51.)

In the adult, the two haemal stalks disappear, while the two neural ones unite
and shift their attachment in a neuro-posterior direction, so that they are ultimately
widely separated from the apices of the olfactory lobes.

In larvae about two inches long the distal ends of the three olfactory nerves
form a rich plexus of nerves terminating in a small patch of ectoderm that may
then be recognized as the definitive olfactory organ.

Summary. The lateral olfactory nerves, then, are characterized as follows:
The " ganglion cells" are large and pear-shaped, and arranged in small ommatidia-
like clusters. Granule cells and neuropile are never present. The fibers are
coarse tubes, with distinct sheaths. The nerves terminate in the lateral portion
of the olfactory organ and in the surrounding integument. The ganglion cells of
the lateral olfactories are the metamorphosed visual cells of the initial olfactory
organ.

The median olfactory nerve represents a later, or secondary, outgrowth of the
hemispheres and of the olfactory lobes. Its ganglion consists of lobular masses of
granule cells and neuropile, and never contains large cells of a sensory nature.
Its end branches are bundles of naked fibers, or at least they have no visible
sheath. They terminate in the central region of the olfactory organ.

164 THK OLFACTORY ORGANS AND THE OLFACTORY LOBES.

II. THE OLFACTORY LOBES OF ARACHNIDS.

Development. The olfactory lobes (organe stratifie, St. Remyj are prob-
ably present in all arthropods. They always form a conspicuous part of the fore-
brain in arachnids, but their functions and their relations to other parts of the
procephalon are unknown, except in Limulus where they are associated with the
olfactory nerves; their function is thus definitely indicated. It is singular that
Limulus is also the only form in which the lobes come into close morphological
relation with the nerve roots to the median ocelli.

In the scorpion and in spiders, the olfactory lobes arise from the walls of a
deep transverse groove extending across the anterior end of the medullary plate.
The groove probably represents the whole of the first neuromere, hence they repre-
sent the very anterior margin of the primitive nerve axis. (Figs. 15, 16, 20, 46.)

In the later stages, the groove closes and its walls form a conspicuous crescentic
band of small, deeply stainable cells, on the anterior haemal aspect of the fore-
brain. Figs. 41, 42 ol.l.

The Olfactory Lobes of Limulus.

Development. In Limulus, the olfactory lobes appear as two separate in-
foldings. (Figs. 141, 142, ol.l.) Later the lobes unite and migrate backward
over the ruumal surface of the brain, gradually changing from a thick, bi-lobed
transverse bar extending across the very anterior end of the brain, to an elongated
U-shaped disc lying on its haemal surface. (Fig. 36.) In the adult, the posterior
margin of the bow extends backward, well below the middle of the cheliceral seg-
ment, farther back than its position in the half grown specimens show r n in
Figs 47, 5,48, 51.

The lobe is formed from the posterior wall of the original infolding, the mem-
branous anterior wall disappearing during the later stages.

The entire margin of the lobes consists of very small, closely packed cells
resembling the granule cells of the cerebral cortex. As they freely absorb all
kinds of nuclear stains, the outlines of the lobes can usually be seen with great
distinctness.

In young Limuli the anterior arms of the bow-shaped lobes are drawn together,
forming two slender horns which up to the late larval stages, are continuous with
the lips of the anterior neuropore. (Fig. 36, A and B.) At about the time the
neuropore closes (after the trilobite stage) there is a vigorous forward outgrowth
at this point, apparently originating in the hemispheres. This forward outgrowth
carries with it the pointed ends of the olfactory lobes and the peculiar tissue of the
hemispheres, giving rise to the median olfactory nerve and its ganglion. Thus
the stalk of the median olfactory is continuous with both hemispheres and with
both horns of the olfactory lobes.

Structure. As the crabs grow older, the cells on the median portions of the
lobes become very large, and divide into several distinct clusters. All the marginal

THE OLFACTORY LOBES IN LIMULUS. 165

cells, however, remain very small, and of uniform size from one end of the lobe
to the other; toward the center of the lobes, the cells gradually increase in size.
(Figs. 51 and 66.)

The small marginal cells send their neurites into two sharply defined bands
of very dense neuropile extending round the lobes. (Fig. 48, ol.np.) In the an-
terior horns, these bands become smaller, and unite to form a single band. The
latter extends to the apex of the horns, and is continued into the neuropile axis of
the median nerve. (Fig. 51.)

In methylene blue, either one or both of the bands often stand out very clearly,
with only a single regular row of nerve cells visible over each band. The dendrites
of these cells are very minute, show a longitudinal trend, and are confined to their
respective bands. See the posterior median part of the olfactory lobes in Fig. 51.

On the inner face of the deeper band (Fig. 48, ol.c 1 '*) are two small bundles of
longitudinal fibers, derived in part from medium sized cells on the inner margin of
the lobes. (Fig. 51, ol.c 2 .} These bundles are continuous with the tracts arising
from the median eye centers. One or two bundles of heavier fibers lie below and
concentric with the ones just described. They arise from the cells of the fourth
optic lobe, op.f 1 . On reaching the opposite side, they turn outward and back-
ward, and join the main longitudinal, haemo-lateral tracts, c.op.f 1 .

Of the larger central cells of the olfactory lobes, we may recognize special
clusters of medium size cells sending neurites into the neuropile terminals of the
two pairs of median eye nerves, and hence to the circular bundles and to the tract
connecting them with the lateral eyes. (Fig. 51, ey.r 3 .}

Farther back is a large cluster of cells, generally very conspicuous, sending
richly branched neurites outward and backward, underneath (on the neural side)-,
the main marginal bands of the olfactory lobes, into the longitudinal haemo-lateral
tracts of the same side, or in wide curves to the same tracts of the opposite side.
(Fig. 51, olc\]

Finally in the posterior bend of the olfactory lobes there are some very large
deep lying median cells that send their enormous branching neurites backward
into the median neuropile mass of the cheliceral ganglion and hence right and left
along the median haemal side of each crus ol. c 3 .

In the older crabs, the horns of the olfactory lobes gradually withdraw in a
posterior haemal direction and finally lose their connection with the median ol-
factory nerve root. Thus the main center of the olfactory lobes becomes relatively
isolated, unless, as it appears probable, the lateral nerves ultimately establish a
connection with them, through the median and lateral eve tracts.

o *

III. THE OLFACTORY ORGANS IN PHYLLOPODS. FRONTAL ORGANS.

The olfactory organ of Limulus undoubtedly represents a highly specialized
condition of the characteristic "frontal sense organs" of the phyllopods. Each
resembles the other in location, innervation, origin, and histological structure.

i66

THE OLFACTORY ORGANS AND THE OLFACTORY LOBES.

We may recognize two sets of organs in the phyllopods, the paired dorsal
ones and the unpaired ventral ones. They probably correspond to the stemmata,
or frontal ocelli of insects.

Branchipus.

In Branchipus the dorsal or paired frontal-organs consist of a compact mass
of small ganglion cells, with one or two large ones situated on either side of the
ocelli. (Figs. 95, no, B.} The terminal cells are in contact with the unthick-

oc

df.o

4- . . V(

/'$&&.. -
.-j-..^s?j3j$&

m.fo.

b r.

w

l^^ffl%
* ? *$n$?

1r-f ^

B

\

PIG. 1 10. The parietal eye and olfactory organs, or frontal organs, of Branchipus. A, The median olfactory
nerve of a young larva, showing at the base of the nerve, the ganglionic enlargement, w, formed on the anterior
surface of the forebrain; B, a more mature specimen, showing the breaking up of the lobes into a nerve plexus
containing ommatidia-like clusters of cells; C, one of the cell clusters more highly magnified.

ened epidermis in the center of a faint rounded elevation. They are connected
with a small compact nerve, that runs parallel with the ocellar nerves, and that
arises from the anterior surface of the brain near the root of the lateral eye ganglion.

The embryonic organ is formed by the separation, from the base of the lateral
eye ganglion, of a small patch of neuro-epithelium, which then migrates under
the epidermis toward the anterior median line of the head.

The history of this organ, therefore, is practically identical with that of the
lateral, or primary olfactory placode of Limulus.

OLFACTORY ORGAN OF BRANCHIPUS AND APUS. 167

The ventral frontal-organ is unpaired and lies just in front of the ocelli.
In larvae about 10 mm. long, the organ is merely a rounded area, without any local
thickening of the chiten or epidermis, in which terminate a great many line nerve
fibers, B, m.f.o. In very young larvae the latter arise from the united anterior
ends of two thick ridges, or lobes, on the anterior surface of the forebrain. (Fig.
no, A, w.) These lobes are solid masses of cells like those in the forebrain and
undoubtedly arise as an outgrowth from it. In the later stages, therefore long
after the ocelli are fully formed, they increase greatly in size, expanding laterally
and forward, thus forming two wing-like plates, which still later break up into
many scattered sensory buds united by a nerve plexus, B, w.

Each sensory bud contains several radiating cells; the latter are clear on the
periphery, and their pointed inner ends are granular and capped by refractive
plates or rods, like those on the retinal cells. (Fig. no, C.) These buds,
therefore, resemble the isolated ommatidia arising from the lateral olfactory
nerves in Limulus.

In the adult Branchipus, the buds are united with the brain by loose nerve
strands containing dark colored bipolar cells, the remnants of the stalk by which
the median olfactory lobes were connected with the brain. A small cluster of
nuclei, g., at the base of the median nerve represents the remnant of the unpaired
portion of the lobes.

Nowikoff, '05, also recognizes the resemblance of these cell clusters (in Lim-
nadia) to groups of retinal cells, as I had previously done for Limulus in 1893.
He regards them as detached retinal cells belonging to the median ocellus. But
the development of these cells in Branchipus, long after the ocelli are formed, and
the development of the lateral olfactory organ in Limulus, show clearly enough
that the isolated ommatidia are formed from the breaking up of independent sense
organs, quite distinct from the median eye.

The median frontal organ of Branchipus clearly corresponds to the median
olfactory organ of Limulus, not only in its position, but in its development as a
ganglionic outgrowth of the forebrain. There is, however, this difference, that in
Branchipus there are no recognizable hemispheres, and the sensory buds are
formed from the median olfactory outgrowth, while in Limulus they are formed
from the lateral one.

Apus.

In Apus (Fig. in), the frontal organ is represented by a thick oval sclerite
behind the eyes. Here the underlying ectoderm is thickened and contains ver-
tical fibers crossed by several layers of horizontal ones. Between these coarse
fibers is a network of large ganglion-like cells, that appear to be connected with
the branches of two large nerves l.f.n. (lateral olfactories), containing numerous
scattered ganglion cells. These nerves arise from the base of the lateral eye gan-
glia and are distributed over a wide area, behind and between the lateral eyes,
including the thickened ectoderm beneath the sclerite.

i68

THE OLFACTORY ORGANS AND THE OLFACTORY LOBES.

Similar conditions to those in Branchipus and Apus prevail in other phyllo-
pods, but we need not consider them here. It is enough to show that the remark-
able olfactory organ of Limulus becomes more intelligible when compared with the
condition of the frontal organs in phyllopods. In both cases we may witness
important steps in the transformation of primitive segmental sense organs into
a very special condition preparatory for, and in part realizing, a new function.

The causes lying back of this transformation are remote and probably in-
accessible. I formerly supposed that the unfavorable position of the organs in

o-.

m

.f.o

If

"n..

FIG. in. Head of Apus, showing the eye chamber c.e.v. and its external opening, o, the median frontal organ,

m.f.o, and the course of the lateral, frontal nerve, l.f.n.

Limulus might have had something to do with their loss of visual functions, but I
now regard this as merely a coincidence, since their position in the free swimming
phyllopods is not unfavorable to their use as eyes, and yet they have suffered a
similar transformation.

III. COMPARISON OF OLFACTORY ORGANS IN VERTEBRATES AND ARTHROPODS.

i. Number of Placodes. In arthropods the olfactory organ arises from two
pairs of sensory placodes that still retain structures characteristic of visual cells.
According to the amount of median fusion that has taken place, the adult organ
may be regarded as a single unpaired one, (Limulus); or as three, a paired and
unpaired one, (Apus); or as two pairs, (Branchipus and other phyllopods.)

In vertebrates the primitive olfactory organ has been regarded by various
authors as single, paired, or multiple. The first view 7 has been widely entertained,
especially by the older anatomists, and was based largely on the condition in
the cyclostomes and in Amphioxus. Of more recent authors, Burckhart, 1908, is
inclined to regard the vertebrate olfactory organ as formed by the fusion of two
pairs of placodes. Kuppfer distinguishes three parts in Petromyzon, an unpaired
one at the point where the neuron last closes and one on either side.

These conflicting views are intelligible on the assumption that the vertebrate
organ is derived from three or four separated anlagen, as it is in Limulus and the

COMPARISON WITH VERTEBRATES. 169

phyllopods, and that in both classes it may undergo varying degrees of fusion, or
of unequal development of its constituent parts.

2. Number of Nerves. In the arthropods, the olfactory organ always
shows traces of two pairs of nerves, even when the organ itself is practically
unpaired. I pointed out in 1893 that the two pairs of olfactory nerves,
then known in but a few vertebrates, were comparable with the two pairs in
Limulus, but not with any other cranial nerves known elsewhere, either in verte-
brates or invertebrates; I stated that: "It is now known that each olfactory
nerve of the higher vertebrates is represented in amphibia by two distinct nerves,
which have been likened to the dorsal and ventral roots of a spinal nerve. But if
this were so they would differ from all other spinal nerves in that both dorsal and
ventral branches supply sense organs. Moreover, on any supposition they are
entirely different from those belonging to the other sense organs of the forebrain,
such as the lateral and parietal eye, and the auditory organ. This condition is
quite inexplicable on any theory founded on vertebrate anatomy. But this very
thing occurs in the olfactory organ of Limulus, although the meaning of it cannot
be explained there any more than in vertebrates."

It is interesting to recall the statements made at that time, since they have
been in some respects so fully confirmed by the subsequent discovery of two pairs
of olfactory nerves by Pinkus 1894, in Protopterus; by Allis 1897, in Amia; by
Locy 1899, in the elasmobranchs; and by Zewertzoff 1902, in the embryos of Cer-
adotus. If the condition in Limulus had received more serious consideration, it is
very possible that the little ''foot note" to the ancestral history of the vertebrate
brain, which according to Locy, is furnished by the development of the nervus
terminalis, might have expanded into a chapter.

3. Structure and Termination of the Nerves. Arthropods. Both pairs
of nerves, while supplying the same organs, are widely different in their histo-
logical characters, and in their central termination. Both pairs are ganglionated.
The lateral nerves contain very coarse nerve fibers with distinct sheaths, and
scattered clusters of gigantic ganglion cells; they terminate in the base of the brain,
near the roots of the optic tracts. The median nerve contains fine sheathless
fibers, dense masses of neuropile and small ganglion cells; it has its roots in
the olfactory lobes and in the hemispheres.

The olfactory nerves are u sui generis" and are only remotely comparable
with any other cranial nerves, such as the optic nerves, the segmental gustatory
nerves, or with the components of less specialized peripheral nerves.

Verebrates. According to Locy, both pairs of olfactory nerves are ganglion-
ated, and although closely associated in their peripheral termination, have sepa-
rate central origins, hence they are considered to be separate nerves, not as
separate parts of one nerve.

What Pinkus says of the nervus terminalis, viz., "Eine kolbige Anschw T ellung
dieses Nerven, welche durch die Einlagerung grosskerniger, von alien anderen
nervosen Zellen des Protopterus anscheinend verschiedenen Zellen bedingt ist,

1 70 THE OLFACTORY ORGANS AND THE OLFACTORY LOBES.

macht es wahrscheinlich dass wir es hier mit einem neuen Organ zu thun haben"
applies equally well to the lateral olfactory nerve of Limulus.

The nervus terminalis in elasmobranchs may have either a neural or a haemal
origin, but it is generally closely connected with the lamina terminalis (Locy);
or according to Pinkus in Protopterus, it "Am vorderende des recessus praeopticus
das Zweischenhirn verlast," thus indicating its probable origin near the root of
the lateral eye ganglion, as opposed to the origin of the main olfactory from the
dorsal anterior surface of the hemispheres.

4. Origin of Olfactory Ganglia. Arthropods. The lateral placode is
a primitive visual organ which becomes bodily converted into the giant
ganglion cells of the lateral nerves. The median placode is retained to form
the epithelial area in or near which all the nerves terminate. Its ganglion
cells are very minute and arise as outgrowths of the hemispheres and of the
olfactory lobes.

Vertebrates. The difference between the development of the median and
general placodes is unknown.

5. Position of Placode Cells. Arthropods. The olfactory placodes
arise from the anterior lateral margin of the open medullary plate, but unlike the
adjacent visual placodes they are not swept into the neurocoele by the overgrowth
of the palial fold; consequently the sensory epithelium is upright, and does not
form the wall of a closed sac.

Vertebrates. The same.

6. Serial Location of the Placodes and their Migration. In arthropods
(Limulus), the lateral olfactory placodes are originally located on the margins of
the medullary plate (procephalic lobes), between the median ocelli and the lateral
eyes; they therefore appear to form the second set of cranial sense organs and
nerves; the median ocelli forming the first set, and the lateral eyes, the third
(Fig. 142). The lateral olfactory placodes first move toward the anterior
median margin of the palial fold (edge of the neuropore) and then forward,
taking up a position in the adult either on the neural surface (Limulus), the apex
(Branchipus), or the haemal surface of the head (many phyllopods), its position in
each case being determined by local variations in the growth of the forebrain and
the external surface of the forehead. The arrangement of ocelli, olfactory organs,
and lateral eyes in the fully formed head, may, or may not, agree with their primi-
tive serial arrangement on the margins of the cephalic lobes. The olfactory
organs may stand alone in the adult (Limulus) or they may unite with the ocelli
and lateral eyes to form a compact median group (Apus, Limnadia etc.). (Figs.
8 and in.)

In vertebrates the same conditions are indicated, but the serial order of the
ocellar, olfactory, and lateral eye placodes cannot be certainly determined in
vertebrates without locating their positions on the margins of the open neural
plate more accurately than has yet been done. Their serial order on the surface
of the head in the later stages is not decisive.

COMPARISON WITH VERTEBRATES. I'JI

During the closing of the medullary plate, the olfactory organs may or may
not unite in the median line; but they invariably move forward either to the
median neural surface (cyclostomes), or still farther forward onto the anterior
haemal side of the forehead. (Fig. 4.)

In most ostracoderms (Bothriolepis, Tremataspis, Cephalaspis), the olfactory
organs and the median and lateral eyes unite to form a very compact group on
the neural surface of the head, very similar to the grouping in Apus and other
phyllopods, where they may be located on either the neural or haemal surface.
(Figs. 5, 8, 12 and in.)

In the cyclostomes, all the pro-cephalic sense organs are on the neural
surface, but they are not so compactly arranged as in the ostracoderms or in the
phyllopods.

7. The Olfactory Lobes. Arthropods. In the arachnids, the olfactory
lobes make their appearance as a deep transverse infolding on the very an-
terior margin of the medullary plate. They soon sink below the surface and
move backward onto the haemal side of the forebrain. The posterior wall of the
infolding gives rise to the olfactory neurones; the anterior wall is membranous,
and later disappears. The cavity of the infolding, as long as it can be recognized,
communicates with the spaces between the hemispheres, and with those under
the palium, i.e., with the potential first and second ventricles. (Figs. 46 and 47.)
The roots of the median olfactory nerve and the parietal eye nerves may be
located in the olfactory lobes.

Vertebrates. The olfactory lobes arise as deep transverse infoldings across
the anterior margin of the open medullary plate (frog), (Figs. 25 and 26),
therefore from precisely the same location and in the same manner as
in the arachnids. The lobes are finally located on the anterior haemal margin of
the forebrain, and their cavities communicate as in Limulus. They are the only
brain lobes that have a conspicuous connection with both the olfactory organ and
with the parietal eyes. (Figs. 43, 44.)

8. Function. The olfactory organ of fishes is recognized to be an olfactory
organ largely on morphological evidence. Whether or no it actually has what
is commonly understood to be an olfactory function, whatever that may be, rests
on surmise. Nevertheless, it would still be proper to speak of it as an olfactory
organ, even if it were experimentally demonstrated that it reacted to sound or to
light, because we know that it is the true homologue of the olfactory organ in the
mammals.

It is well to bear this in mind in comparing the olfactory organ of arthropods
with that of vertebrates. Although our case rests primarily on morphological
evidence, the evidence afforded by function, while meager, is confirmatory.
Stimulation of the olfactory organ of Limulus with various kinds of food, with acids
and with ammonia, does not usually produce any characteristic reflexes. Even
drops of rather strong hydrochloric acid, or ammonia, have no more effect than
when applied to other parts of the body; they cause a slight start, nothing more.

172 THE OLFACTORY ORGANS AND THE OLFACTORY LOBES.

In order to test its glandular nature, the olfactory organ was cut out, its outer
surface wiped dry, and then the attached nerves stimulated with electricity;
no traces of a secretion appeared. But electrical stimulation of the olfactory-
region in uninjured male crabs in some instances at once produced very remark-
able leg movements, rarely seen under any other circumstances.

When the electrodes are applied to the olfactory organs of the male, if the
experiment is successful, rapid chewing movements of the mandibles are produced,
accompanied by vigorous snapping of the chelicerae, which may finally become
rigid and stretched out backward at full length. At the same time the second
pair of legs (the ones used to seize the females) which during all our preceding
experiments on the gustatory organs have remained motionless, are now quickly
and repeatedly flexed, as though trying to hug or grasp some object and force it
toward the mouth ; all the other legs remain motionless. Stimulation of the region
about the olfactory organ, or along the median line between the olfactory organ
and the brain, or above the brain, may produce the same effect.

These experiments indicate that the olfactory organ is a chemotactic organ,
whose activities are associated with the process of eating, although it is difficult to
explain why the chewing movements are not produced by direct stimulation of the
olfactory organ with food. On the other hand, the extraordinary hugging and
grasping movements aroused in the second pair of legs of the males, when the
organ is electrically stimulated, indicate that it is used in finding the females during
the mating season. That an organ for this purpose must be present seems cer-
tain, for the males during the breeding season seek out the females and attach
themselves to them with great precision. In confinement, the males usually
attach themselves to the abdomen of the females, but males whose olfactory
organ had been cut out did not do so. Smearing the olfactory organs of males
with the ova or secretions of oviducts produces no effect.

In primitive vertebrates, the olfactory organ was doubtless of great importance
in mating, as indeed it is through the whole series of vertebrates. It is of special
interest that they were intimately associated with sexual activities in such remote
ancestors of the vertebrates as the arachnids. In this connection, the olfactory
function of the antennae of insects, and its relation to sexual reproduction will be
recalled.

Summary and Conclusion.

The gustatory organs play an important part throughout the entire range of
arthropods, and they have done so ever since the appendages have been used as
aids to nutrition. In Limulus, they form the most voluminous nerve tracts and
nerve centers of any single set of organs, and the great size of the hemispheres is
largely due to important gustatory centers that are located in them.

The olfactory apparatus and the olfactory function arose in the higher
arachnids through the secondary modifications of preexisting organs that had
some other function or meaning.

SUMMARY AND CONCLUSION. 173

The primary sensory functions of the marine arachnids were, therefore,
visual and gustatory, and the main centers for these functions lie respectively in
the optic ganglia and the primitive cerebral hemispheres.

In the higher marine arachnids, and toward the beginning of the ostracoderm,
or primitive vertebrate stage in phylogeny, the olfactory function, as a secondary
aid to nutritive and sexual activities, became definitely localized, and the most
anterior section of the forebrain was successfully preempted as the main olfactory
center.

The auditory function was definitely localized at a much later period than
any of the three preceding ones, and it has probably for that reason never suc-
ceeded in creating for itself a definite, sharply circumscribed, brain region.

CHAPTER XL

FUNCTIONS OF THE BRAIN.

PART I.

Introduction. In the preceding chapters, we have shown that there is a
far reaching resemblance in structure and development between the brains of
vertebrates and arachnids. In this chapter, we shall show that they agree in
function, and in their physiological relations to other parts of the body.

In the arachnids, the location of several important cerebral centers is already
clearly indicated by the peripheral termination of the associated nerves, as for
example, the visual, gustatory, cardiac, and respiratory centers. Nevertheless,
it seemed highly desirable, indeed imperative, that there should be some experi-
mental evidence to demonstrate the course of the principal nerve impulses, and to
locate by experiment the centers that control a group of similar activities, or that
bring them into coordinate relation with other activities.

Although Limulus has the largest forebrain, or hemispheres, of any inverte-
brate known, it does not approach such animals as the hymenoptera, the cephalo-
pods, the crayfish, or the lobsters, in alertness, or in the variety of its responses to
visual, tactile, or other stimuli. When compared with the members of its own
class, such as the spiders and scorpions, it appears stupid and quite unaffected
by the events going on in the world about it. Limulus, no doubt, appears to lead
a sluggish life in the muddy bottoms of deep waters; but we should be greatly in
error if we were to estimate the probable volume and complexity of its coordinating
centers, or of w r hat corresponds to the hemispheres of vertebrates, by its so-called
' 'manifestation of intelligence. ' ' Indeed Limulus would furnish very little material
that could be used for experimentation or observation along these lines. But,
for the study of some of the simpler reflexes, Limulus is not excelled by any other
animal.

The following experiments were made in the summer of 1897, at Woods Hole.
In the summer of 1898, Mr. Raymond Pearl, then a student at Dartmouth, work-
ing under my direction, repeated many of my experiments, and the following year
added others of his own. More than seventy different operations were performed,
mostly on adult animals. They generally involved the transecting, or the remov-
ing, of various parts of the brain, or cord, in order to determine the path of nerve
impulses, or to locate the centers of control.

METHODS. 175

We shall describe a few of the more important experiments, and summarize
the results of the others that bear on the main problems here under discussion.

*********
The principal method of obtaining the normal reflexes was to place healthy
crabs on their backs on some convenient table, allowing the posterior end of the
abdomen to hang over the edge. After a few minutes their muscles relax, and
unless disturbed, they remain perfectly quiet for a long time. Meantime, local
stimuli may be applied which, if a little care is exercised, usually produce very
definite reflexes without arousing the animal from its comatose condition.

The usual stimuli for the chewing reflexes, were drops of clam juice, or pieces
of clam, or the like, of the same temperature as the air; and a breath- of warm air,
or the gentle touch of the finger tips, for the crossed thoracic, the abdomino-thor-
acic, or other temperature reflexes. Various other stimuli were also used from
time to time, as indicated in the description of results. Having familiarized
mvself with the normal reflexes, the brain or cord was sectioned in various ways.

J J

After the recovery from the shock, which lasts from five minutes to an hour or
two, the crab was tested as before and the difference in behavior noted.

The operations were performed in various ways, the principal difficulty being
to avoid the great loss of blood following any puncture of the skin near the brain
or cord.

When the section had to be accurately located, there was no way but to thor-
oughly bleed the animal, expose the parts, and section as desired at leisure. This
was the method followed in transecting one-half of the abdominal cord at a given
point, and in cutting it in halves lengthwise.

In transecting the collar, the animal was tied down and the legs fixed in a
convenient position; a quick cut was then made across the collar, care being used
to keep the opening in the skin as small as possible. To prevent the loss of blood,
that spurts with great force from the opening, the wound was instantly
plugged with a tight fitting wad of absorbent cotton smeared with vaseline. If
the operation is successful, very little blood is lost, the animal quickly recovers,
and may. live for six or eight weeks, or longer.

The principal errors to be guarded against arise from the difficulty of
making the sections in the desired place, and from the degeneration of the wounded
or isolated parts of the brain. In some cases, an isolated segment of the nerve
collar would degenerate and completely disappear in a few days after the operation;
or the degeneration may extend into other parts of the brain and vitiate the results.
In some of the most successful cases, the cut surfaces of the brain, after a lapse
of several weeks, were covered with an incrustation which, if removed, left the
surfaces almost as clean and sharply defined as .when the wounds were
first made.

To check these sources of error, we have made careful post-mortem examina-
tions and have excluded all those experiments in which there is any doubt about
the location of the wound, or the extent of the degeneration.

i 7 6

FUNCTIONS OF THE BRAIN.

Experiment I- A.

August 16. Sectioned anterior end of left crus. (Fig. 113, A.I.) Before the operation,
it was found that if the crab came to rest on its back, and the margin of the abdomen was gently
touched with the fingers, the legs on the opposite side would always be raised a fraction of a second
before the legs of the same side. When the movements are once started, they become general.

After the operation, the left chelicera is very restless, snapping and moving aimlessly
back and forth. The second left leg is also restless, and usually elevated higher than the others.

I. Thoracic Reflexes. Fifteen minutes later.

a. Hand, placed on left side of thorax, causes a slight start of left legs and left gills.
Reverse experiment gives corresponding results.

FIGS. 112-113. Brains of adult Limuli, that have been cut in various ways in order to determine the func-
tion of the principal brain regions and the course of the nerve impulses. All the figures show the brains from
the neural surface, with the right side toward the reader's right hand. The roman numerals indicate different
operations performed at different times on the same brain.

b. A little later the results are somewhat varied. The right legs are less readily induced
to make reflex movements by stimulation of either side than are the left.

c. Hand placed on the left side of thorax caused a slight stirring of the legs of both sides,
or a raising of the right or left legs, but no purposeful movements on either side. But placing
the hand on the right margin causes well marked, thrusting away movements of the right legs.

J. Twenty-four hours later, repeated c. with same results.

II. Abdomino-thoracic Reflexes, a. Fifteen minutes after the operation. Hand placed
on left margin of the abdomen causes sudden depression of the gills of both sides and raising of
the right legs. Reverse experiment gives corresponding results.

The gills on the side stimulated contract a little before those of the opposite side. This
is in marked contrast with the fact that the thoracic appendages of the opposite side always move
first when one side of tne abdomen is stimulated, showing that the abdominal reflexes
are mainly uncrossed, and the abdomino-thoracic are mainly crossed.

EXPERIMENTS. 177

b. Twenty-four hours later. The right legs frequently perform spontaneous movements,
as in a normal crab, but the left are quiet unless stimulated.

c . Hand placed on the right margin of the abdomen causes very vigorous thrusting away move-
ments of left legs, more vigorous than can be produced in any other way. The right legs may
be raised and flexed in a sort of spasm, or may not; but they never thrash about as the left legs
do. The movements of the left legs are governed entirely by the stimulation, and cease when
the fingers are removed from the margin of the abdomen.

d. When the hand is placed on the left margin of the abdomen the right legs immediately
move back and forth in the usual manner, the left legs either remaining quiet, or spasmodically
flexed. If the crossed reflexes of the right legs are violent they may not cease on removing
the stimulus, and the animal may attempt to regain its upright position.

There is a marked difference between the movements of the right legs, in d, and of the left
ones in c. The movements of the right legs are those of a normal crab when stimulated. The
left legs may move more vigorously, in response to a crossed abdominal impulse, than the right,
but their movements are aimless, and cease with the cessation of the stimulus.

These experiments show conclusively the controlling and directing effect of the cerebral
hemispheres.

Experiment I B.

Same animal. Thirty-six hours later. Cut all the free, post-oral cross commissures of the
thorax, leaving the vagus commissures intact. (Fig. 113, A. II.)

I. Thoracic Reflexes. a. Ten minutes after the operation. Hand on either margin of
the thorax causes slight movements of the legs on the same side, but none whatever on the op-
posite one; except that when the right side was stimulated, the second leg on the left made vigor-
ous movements.

These experiments were repeated at intervals of one or two hours, with the same results,
except that the uncrossed reflexes gradually became more pronounced.

The experiment shows that there are crossed and uncrossed thoracic reflexes, and that the
crossed ones pass to the opposite side through the thoracic and the forebrain commissures.

II. Abdomino-thoracic Reflexes. a. Fifteen minutes after the second operation, hand
placed on the margin of the abdomen produced only faint movements of the legs of the same side.
On repeating the experiment, at intervals of an hour, the crossed, abdomino-thoracic reflexes
gradually appeared, and three or four hours later became well marked.

III. Gustatory Reflexes. a. After cutting the left cms, the normal chewing movements
could be readily produced, except that the second left leg was spasmodic and irregular in its
movements.

b. After cutting the cross commissures, the chewing movements that could be induced on
the right were very feeble; none at all could be induced on the left. These negative results were
probably due to the feeble condition of the animal.

IV. Olfactory Reflexes. On stimulating the olfactory organ with the electrodes, move-
ments of all the right thoracic appendages and the first two on the left are produced.

Experiment II A.

August 6 Female. Sectioned thoracic cross commissures and the right cms back of
sixth leg. (Fig. 113, 5, 7.)

August 14. Crab is very restless when taken from the water. The thoracic appendages
are in almost constant motion, waving about in an aimless manner.

I. Thoracic Reflexes. August 14. Uncrossed reflexes well marked; crossed, indistinct or
absent.
12

178 FUNCTIONS OF THE BRAIN.

II. Abdomino-thoracic. August 14. a. Hand placed on right side of abdomen causes
raising of the abdomen and flexing of left legs. Hand placed on left margin of abdomen, no
result; or if the fingers cover considerable area, a slight raising of abdomen may be produced.

b. August 25. Crab is vigorous. Hand, or even the tip of a finger, placed lightly on the
right margin of the abdomen causes raising of the left legs, followed shortly afterward by the
right, and then by general movements of both sides. Hand placed on the left margin of the
abdomen, and on the left gills, produces at first no effect; but if the stimulus is increased, then
the left legs are raised, followed by general movements, including movements of the right
legs. Experiment repeated many times, with same results.

III. Gustatory Reflexes. a. August 7. Stimulation of jaws caused normal chewing
movements on either side, but movements of one side do not harmonize with those of the other.
b. August 25. Same.

IV. Respiratory Reflexes. August 24. a. When at rest, the left abdominal appendages
are more elevated than the right.

b. Stimulation of the gill warts with clam causes twitching of the stimulated endopodites,
then several lateral movements, the members of each pair stimulated alternately crossing and
uncrossing over the median line, and finally a full, rhythmical, up and down, respiratory move-
ment of all the abdominal appendages.

Experiment II B.

August 25. Cut both crura back of the chelicera;. Subsequent examination showed that
the cut was made in front of the second neuromere on the left, and behind it on the right.
(Fig. 113, B.II.} General movements of the legs and respiratory movements of the gills
followed, but they lasted only a short time.

I. Gustatory Reflexes. a. Immediately after operation, washed away the blood and
stimulated the jaws with clam, producing marked leg movement, as in chewing, but very feeble
jaw movement, b. Repeated the experiment after five minutes with same results, c. Again,
two days later, stimulation of jaws with food produces chewing reflexes, consisting of leg move-
ment only on the left; on the right, no reflexes.

II. Respiration. a. On removing the crab from the water, all the left gills pulsate a few
times, the right remain motionless.

b. When returned to the water after long exposure to the air, the respiration becomes nearly
normal. The left legs are very restless and move back and forth in a lateral direction. The
right legs are relaxed and motionless, except the sixth, which is directed backward and moving
slightly.

c. Respiratory movements may now be induced by rubbing the gills with clam. The same
premonitory twitching and lateral movements as in experiment i. Repeated frequently with
same results. When the movements are well under way, the left gills are raised higher than the
right.

III. Purposeful Movements of Sixth Leg. When the fingers were placed on the left
abdominal appendages, the left, sixth leg was thrust repeatedly backward over the median sur
faces of the gills, with the very evident purpose of thrusting away the stimulating object. The
other appendages moved very slightly, but did not in any way make purposeful movements
on stimulation of thorax.

Experiment III A.

July 29 Female. Cut the nerves to the endopodites of all the left abdominal appendages
about half way up the appendages. No other effects were observed than the loss of sensibility
of the left abdominal endopodites.

EXPERIMENTS. 179

This animal was then used for the following successful experiment. At the time the second
operation was made, the crab was in such good condition and its normal action had been so
little altered that it was not felt that confusion might result from this attempt to economize
material.

Experiment III B.

The second operation was performed August 6, 5 P. M. The autopsy, three weeks later,
showed that the left crus was sectioned close to the spinal cord, and all the thoracic cross com-
missures severed. (Fig. 1 13, C. I.) At the anterior end, the left crus was cur so that a small piece
of the left cerebral hemisphere remained attached to it. But only a very few cerebral cells, if
any ; could have been connected with the crus.

I. Gustatory Reflexes. a. Immediately after the operation, the chewing reflexes were
inhibited. Five minutes later, excellent reflexes, including the chelicerse, were obtained on
both sides; but the two sides were not coordinated.

b. August 7, 9 A. M. On stimulating right jaws with clam, obtain prompt and vigorous
chewing movements of the jaws, but with moderate, or normal chewing movements of the legs.

On stimulating left jaws, obtain at first the same results, but the leg movement gradually
grows more energetic till it is absurdly exaggerated in rapidit, and range, and finally becomes
much confused, the legs moving wildly back and forth, and often clashing with one another.
This rapid movement may be followed by a spasmodic bending of the tips of two or three ap-
pendages into the mouth, where they are held in a trembling tetanus or rigor. All the left legs
are involved in this movement, except the left chelicera, whose nerve was cut a short distance
from the brain.

Repeated these experiments several times on August 10, 14, and 26, obtaining in each case
essentially the same results.

II. Thoracic Reflexes. a. August 7. On placing the fingers on the left side of the
thorax, there is no response, and if the left legs happen to be making the chewing movements,
the latter are not in the least disturbed.

b. Later. Fingers placed on the posterior, ventral surface of the left side cause no move-
ment beyond a slight start when the contact is made, and uneasy opening of the chelae. But
on touching the anterior quarter of the ventral surface, rapid movements of the second and
third left legs are produced. These movements at first do not last long, and are inconspicuous
when compared with the movements that may be produced on the opposite side. But on the
following days they became distinctly purposeful, repelling movements.

c. August 7. On placing the hand on the right ventral side of the thorax, all the right legs
move furiously back and forth in unison, while the left continue their chewing movements as
before.

d. August 10. Hand placed on the right side of thorax causes active, purposeful thrusting
away movements of the right legs.

e. August 25 and 27. Repeated a, b, and d, with same results.

III. Abdomino-thoracic Reflexes. a. Placing the fingers on right margin of abdomen
caused back and forward movements of all the right legs (except chelicera) and with very
marked thrusting away movements of the sixth leg. No movements of the left legs.

b. Hand placed on the left margin caused obscure movements of the right legs.

c. Hand placed on either side of the margin of the abdomen caused faint, rhythmical con-
tractions of the gills of both sides, but movements of the right gills are the strongest.

d. August 26. Repeated a, b, and c, with same results.

IV. Temperature Reflexes. a. August 25. On breathing gently on the ventral surface
of the quiescent crab, that had been lying on its back in the air for some time, a general muscular
spasm is instantly produced. All the legs are waved about, but the left legs are thrown into

l8o FUNCTIONS OF THE BRAIN.

prolonged violent movements during which they are convulsively flexed, and the tips thrown
repeatedly toward the mouth. The right legs, meantime, becoming quiet.

b. As soon as the crab had quieted down, the experiment was repeated, but with the ut-
most care not to produce too violent a stimulus. The little puffs of warm air could be so regu-
lated as to cause the left legs to move, while the right remained motionless. The experiment
was repeated many times with the same results, showing that the left side reacted much more
readily than the right.

V. Respiration. a. August 7. When at rest in the air, the gills are twisted toward the
left, the left gills tightly compressed, the right ones slightly elevated. At first there was a tend-
ency for the left gills to move spontaneously in rhythmical respiratory movements. A week or
two later, the right gills frequently performed the normal yawning movements, or the normal
respiratory movements, the left gills remaining motionless.

b. Placed in water, normal respiratory movements begin at once, except that the left gills
are raised higher than the right.

c. August 26. Crab still vigorous. Repeat a and b with same results.

d. August 27. Stimulation of gills with clam, or finger tips, does not induce respiratory
movements.

VI. Equilibrium ; Locomotion. a. August 7. Crab rights itself repeatedly when placed on
its back in the aquarium. When righted, it constantly moves in a circle toward the left with
right side raised high on the legs, the left side depressed. The caudal spine turned to the
right, at an angle of about 20. The crab, when righted, circulates to the left, because the
right legs alone make the motor movements. The circular movement continues for hours at a
time. August 27, condition same as a.

b. August 7. On removing the crab from the water and placing it on its back, the left
legs move restlessly and aimlessly, often bending the tips into or toward the mouth. Movement
continues for more than an hour. Right legs remain quid, but may move vigorously if properly
stimulated.

c. At certain intervals, when in the air, all the right legs swing in unison forward, and then
with a vigorous stroke backward. The forward and backward movements are repeated many
times with great regularity, precisely as in swimming, except that the gills did not join in the move-
ment. The left legs never made these characteristic movements.

</. August 19. In water the crab sometimes fails to right itself. In such cases, the swim-
ming movement of the right legs may continue for hours with great regularity, but without
sufficient force to move the animal about. The left legs are meantime passive.

e. August 25, same conditions described in c and d are retained.

VII. Autopsy. August 27. Three weeks after the hemisection of the brain the crab was
alive and vigorous. On removing the brain, it was found that all the parts about the wound
were thickly incrusted with a granulated matter, which when removed showed the cut surfaces
of the commissures and of the crura to be very little changed. There was no indication of degen-
eration, or regeneration of the surrounding parts. There was a thick deposit of sepia colored
pigment about the wound in the forebrain. The cotton plugs were incrusted with a granular
matter, and they apparently had not interfered by pressure or otherwise with the action of the
nervous system.

Experiment IV A.

August 25. Large female. Cut both crura in front of the second thoracic appendage.
(Fig. 113, D.I.) Immediately after the operation, the left legs moved restlessly, the right
remained quiet.

I. Thoracic Reflexes. a. Immediately after the operation, no definite crossed or un-
crossed thoracic reflexes could be obtained by warming the sides of the thorax in the usual way.

EXPERIMENTS. l8l

b. Several hours later. There was, in no case, any raising of the legs, or purposeful
thrusting away movements of the legs on either side, although faint movements of the opposite
legs followed a stimulation of the sides of the thorax.

c. If the hands were placed on the margin of the thorax while the chewing movements
were going on, the movements on the stimulated side were inhibited.

II. Abdomino-thoracic Reflexes. The following results are in marked contrast with the
above:

a. Hand placed on the margin of the abdomen causes raising of the legs of the opposite
side, where they are held in a sort of tetanus, as long as the stimulus is applied. When the
fingers are removed, they instantly drop back on the carapace. There is no purpose in the
movements and it is difficult to understand their meaning. The legs of both sides may be
affected, but those on the side opposite to the stimulus are flexed first and to the greater extent.

III. Leg-gill Reflexes. If the surface of the gills and operculum is touched, the sixth
legs at once make vigorous and well directed movements to rub the stimulated spot. If the
stimulation is on one side only, the sixth leg of the opposite side moves first and most vigorously
The fifth and fourth legs may be, to some extent, involved in the movements.

The movements of the sixth legs, under these conditions, are remarkable. The ends of
the legs are rubbed against each other, and over the surface of the gills, something like the
"washing" movements of the posterior pair of legs in a common house fly.

IV. Gustatory Reflexes. a. Immediately after the operation, stimulation of the jaws
with food caused chewing movements of both legs and jaws on the left side, but none on the right
Three hours later, however, vigorous chewing movements were produced on both sides. The
coxal chewing movements were vigorous and normal, the leg movements greatly exaggerated.

V. Respiratory Reflexes. Stimulating the gills with clam produced no effect.

Experiment IV B.

Twenty-four hours later. Crab still shows remarkable vitality and spontaneity. Made
a sagittal cut through the cross commissures of the thorax and right crus, D,II. For a few
minutes after the operation, the left legs are very restless and the right are quiet.

I. Thoracic Reflexes. a. Hand placed on the right margin of thorax produces no
reflexes. On left margin, retraction of the left legs.

II. Abdomino-thoracic Reflexes. a. Hand placed on right margin of abdomen causes
slight movements of left legs, none of right.

b. Hand placed on left margin causes stronger movements than before of the left legs,
none of right.

c. Hand placed on the gills causes violent movement of left legs, none of right.

III. Gustatory Reflexes. a. Immediately after the operation, no chewing movements
could be induced by stimulating the jaws with food. Applying the electrodes to the left jaws
caused movements of the corresponding legs, but no movements followed when they were ap-
plied to the right jaws. The right legs would not respond to any change of temperature,
whether applied directly or indirectly.

b. One hour later, obtained a faint chewing movement on the left side, on stimulating the
jaws with clam; none on the right.

IV. Respiratory Reflexes. After the second operation, respiratory movements of the
gills ceased, and for twenty-four hours no rhythmical movements of the gills could be produced,
either by stimulation with clam, or by placing the crab in sea water. But the next morning the
following curious facts were observed. The crab was found on its back in the aquarium,
just as it had been left the previous day. No respiration had apparently taken place during the
night, as the gills were covered with a light sediment.

a. On removing the apparently dead crab from the water and stimulating the gills with

1 82 FUNCTIONS OF THE BRAIN.

the electrodes, peculiar rhythmical contractions followed. The movements stopped in a few
moments.

b. On rubbing very gently a small piece of clam on the gills, the movements began again,
but more vigorous than before. On returning the crab to the water, the movements, which
had meantime ceased, began again and continued with some interruption, for purposes of
experimentation, for several hours. These movements were as follows: i. The abdominal
appendages remained well elevated, pulsating in short strokes, once about every one and one-
half second, for twenty-five seconds. 2. Two full vertical pulsations then follow, each one being
a vigorous flattening of the appendages against the abdomen, followed by an elevation of the
same to their full height. 3. They move repeatedly back and forth across the median line,
rubbing the posterior surface of one appendage over the anterior surface of its mate, as though
rubbing or washing the gills. They may remain crossed and motionless for several seconds,
but finally return to their first position. 4. One full pulsation followed by 5. a long pause of
twenty-five seconds, and then the whole begins again. One whole series of movements takes
place in about seventy seconds, but this period may be gradually prolonged till all the move-
ments cease.

c. When the crab was taken from the water and placed on its back, the movements grad-
ually ceased.

d. Breathing on the gills produced only a slight contraction of the same. A drop of water
produced two or three faint pulsations. Scratching the abdominal appendages with the fingers,
or with a stick produced no result, but on rubbing them with a small bit of clam, the compli-
cated series of rhythmical movements described in b began at once.

Experiment V A.

10.30 A. M. Large female. Sectioned both crura back of chelicerai. All spontaneous
movements cease. (Fig. 113, E.I.}

I. Thoracic Reflexes. a. Hand placed on either margin of the thorax causes contraction
of the gills of the same side, and later the raising of the legs of the opposite side. In both cases,
the legs on the side stimulated start slightly, but are not raised. No purposeful movements of
the legs are made to remove the irritation.

II. Gustatory Reflexes. Three hours after the operation no chewing movements could
be produced by stimulating the jaws.

Experiment V B.

3 P. M. Sectioned the right half of the ventral cord, in front of the first abdominal ganglion,
.77.

I. Abdominal-thoracic Reflexes. a. Hand placed on the left margin of the abdomen
caused raising of the right legs, first to sixth. Hand placed on the right margin, caused move-
ment of the fifth and sixth legs on the opposite side.

These two experiments show that impulses cross both above, and below, the cut, 77.

Experiment V C.

3.30 P. M. Made a sagittal cut through the vagus neuromeres, but without cutting the
free, thoracic cross commissures, .777.

I. Abdomino-thoracic Reflexes. At first all crossed abdomino-thoracic reflexes ceased.

a. A few minutes later, placing hand on right margin of abdomen causes the fourth, fifth,
and sixth legs of left side to be raised.

b. Hand placed on left side of abdomen causes fifth and sixth legs of same side to be raised,
but these movements are not so strong as when the right side of the abdomen is stimulated.

Crab died after about twenty-four hours.

EXPERIMENTS. 183

Experiment VI A.

August n, ii A. M. Exposed the spinal cord, and made a longitudinal median section
through the first abdominal ganglion. F.I. Rhythmical contractions of the gills followed, lasting
a short time.

Experiment VI B.

The right cord was then cut across about midway between the brain and the first abdominal
ganglion. F.I I.

I. Respiratory Reflexes. a. At first no reflexes followed the section, but after one or
two minutes respiratory movements began. The left gills were raised much higher than the
right, the latter being apparently dragged up by the left gills, rather than by their own action.
When respiratory movements ceased, the left gills remained in a higher position than the right.

b. When placed in the water, the respiration was at first about normal, but in a few min-
utes it almost ceased, leaving the left gills moving slowly, the right motionless. After about an
hour, respiration ceased, leaving the left gills raised, and the right closely pressed against the
abdomen.

II. Abdomino-thoracic Reflexes. a. On placing the hand on the right margin of the
abdomen, all the left legs are promptly raised, remaining in that position till the hand is removed,
when they again fall back slowly into the thorax. The right legs are also raised, but after the
left.

b. Hand placed on the left margin of the abdomen causes raising of the right legs (but the
response is not as prompt and vigorous as that of the left legs in the previous experiment). The
left legs are not raised, but their chelae stir uneasily.

c. Repeated a and b two hours later with the same results.

d. Stimulating with a weak electric current at b, causes contractions of the right gills, also
slight movements of the left gills (see below); at c, causes contractions of the left legs and the
gills on both sides; at a, causes contractions of the right legs.

e. 2.30 P. M. Applying electrodes at a, the right gills are slightly raised, and then moved
back and forth faintly as in respiration. Left gills move much less than right.

/. Stimulating at f, obtain movements of left gills and partial ones of the right, and with
vigorous and immediate movements of the left legs.

g. Stimulation at a, causes immediate movements of the right legs only.

We thus see that stimuli applied directly to the spinal cord produce uncrossed reflexes of
the appendages anterior to the cut, and both crossed and uncrossed below the cut.

But temperature impulses, starting on the sides of the abdomen and traveling centripetally,
produce both crossed and uncrossed reflexes in the thorax. It is not clear why direct stimulation
of the spinal cord should produce only uncrossed impulses above the point of stimulation.

Experiment VI C.

3.30 P. M. Made an accurate sagittal cut through the vagus neuromeres, care being taken
not to cut the first three or four post-oral commissures. F.I 1 1

a. Stimulating at a or c, produced leg movements of the corresponding side, as before.

b. But stimulation of the margin of the abdomen and gills, with the hand, produced no
leg reflexes, although breathing on the legs, or dropping tepid water on them, produced prompt
movements of the same.

It would thus appear that all the uncrossed temperature impulses started in the abdomen
cross in the vagus neuromeres.

184 FUNCTIONS OF THE BRAIN.

Experiment VII A.

Adult male. Sectioned right cord (Fig. 113, G.7). Twitching of the right operculum and
first right gill followed the operation.

Experiment VII B.

Cut the whole ventral cord in halves, lengthwise, G.II.

a. The caudal spine is thrown toward the left and remains so permanently.

b. On breathing on the abdomen and gills, the gills are retracted and the right legs alone
are raised.

c. Placing the fingers on the left margin of the abdomen causes the raising of all the right
legs; the legs are strongly flexed and the points thrown medianly and backward.

d. Hand on the right side produces no results, except that in some cases there is a slight
start of the left legs and a faint gaping of the chelae, but no movement at all resembling those seen
on the opposite side of the body. This impulse probably reaches the forebrain through the
longitudinal integumentary nerve. (Fig. 70.)

e. One hour later Repeated several times (/, b, and c, with same results.

Experiment VIII.

July 29. Male. The right and left halves of the cord were separated behind the opercular
neuromere, by a median, longitudinal cut extending through all the abdominal neuromeres.
All the free, post-oral commissures of the brain were also cut, leaving the vagus commissures
intact.

I. Abdominal and Abdomino-thoracic Reflexes. July 30. Stimulation of the left
side of the abdomen caused a drawing down of the left gills and an immediate and strong upward
movement of the right legs, followed by a slight raising of the legs on the left side. Stimulation
of the right side of the abdomen in the same way, produced the same result, and rice vcrxi.
In both cases there was a movement of the tail toward the stimulated side.

The experiment shows unimpaired crossing of impulses in the vagus neuromeres.

II. Respiration. July 29. The right and left halves of the gills were breathing in alter-
nation, the gills on the right side being raised while those on the left were depressed. The
coordination of the respiratory movements was perfect longitudinally, but the two sides were
beating independently.

Experiment IX.

I. Purposeful Reflexes of the Sixth Legs. July 15. Male. Transected the right half
of the cord between the first and second gill neuromeres. (Fig. 113, H.g.)

July 18 and 19. a. Stimulation of the left side of the abdomen. Caused a drawing down
of the gills on the same side, together with a spasmodic, upward, non-purposeful movement of
the legs on the right side, followed by a purposeful movement of the sixth leg on the left side.

b. Stimulation of the right side of the abdomen gave same results vice versa, except that
in this case the sixth left leg (as before) performed the purposeful movement, the sixth on the
right being merely raised.

Abdominal stimulation produced no purposeful movements of the sixth leg on the cut side.

II. Yawning. After the crab had been out of water for about an hour, it would yawn,
the operculum and the first gill, both right and left, moving in the usual way, while the gills
behind the cut behaved as follows: The left gills were raised in lime with the operculum and
first gill, although they were not raised as high as in the normal animal. The right gills did not
move at all, except as they were slightly dragged upward by the left.

EXPERIMENTS. 185

III. Respiration. Eight hours after operation. In air. As the gills behind the cut
began their opening phase, the left gills started first, and apparently dragged the right ones up
with them. When the gills closed, the left started clown first and dragged the right after. The
operculum and the first gill behaved normally.

When the crab was put back in water, the respiratory movements would begin as described
above, but after a while they would become more nearly normal. From this it would appear
that the coordination of respiratory movements is brought about in some part of the nervous
system anterior to the abdominal neuromeres. Same results were obtained on seven succes-
sive days, the abnormality of the respiration in air being greater than in water.

IV. Swimming Movements. Jul\ 23. The right and left operculum, the right and left
first gill and the four posterior left gills performed normal swimming movements in unison.
All the right gills behind the cut were either quiet, or respiring, while the others were swim-
ming. This indicates a center for the swimming movement of the gills in front of the free
abdominal neuromeres.

Experiment X.

July 19, 1898. Male. Both cords were cut between the neuromeres of the first and second
gills, H. 10.

I. Respiration. a. July 29. In water. All the gills were breathing, but out of rhythm
The operculum and first gill moved together, but out of time with the posterior gills. In the
anterior group, the operculum was the first to start each inspiratory movement, and was followed
up by the first gill. The four gills behind the cut beat fairly well together, but the rhythm within
the group is imperfect and all are out of time with the anterior group. In the respiratory move-
ment of these four posterior gills, the most posterior one was the first to start the upward move-
ment. Results indicate that the gills behind the cut have been separated from their center of
coordination.

b. July 23 to 28. The four gills behind the cut frequently performed the "cross rubbing"
or "scraping" movement, the first gill not participating. This "cross rubbing" movement of
the posterior gills was always followed by strong swimming movements of the operculum, first
gill, and thoracic appendages. At times, the operculum and first gill would stop, while the
appendages behind the cut kept on with unbroken rhythm; or the appendages in front of the
cut would be performing the swimming motions, while those behind the cut were breathing as
usual. This indicates a separate nerve mechanism for respiratory and locomotor activities of
the gills.

Experiment XI.

In another experiment, the cord was cut behind the second gill neuromere, H.i2. Results:
i. the three gills behind the cut made the respiratory movements more vigorously and frequently
than the two gills in front of it; and 2. the operculum and the gills in front of the cut "yawned"
frequently, while those behind the cut were motionless.

II. Swimming Movements. At no time after the operation did the gills behind the cut,
in experiments X and XI, perform swimming movements. The abdominal appendages in
front of the cut made the swimming movements often and in a normal manner.

Miscellaneous Experiments.

The following results were obtained at various times by stimulating the cord and the peri-
pheral nerves with an induced electric current.

a. On stimulating any one of the posterior thoracic haemal nerves on the left side, the right
legs and the right halves of the operculum and gills were raised, the legs pointing toward the

1 86 FUNCTIONS OF THE BRAIN.

region stimulated. The left legs make purposeful movements. The effect is not confined to
the appendage corresponding to the nerve stimulated, since after stimulating one nerve all the
legs of the opposite side are raised.

b. Stimulation of the longitudinal integumentary nerve gave no results.

c. The abdominal cord was transected between the neuromeres of the first and second gills.
Stimulating the proximal end of a haemal nerve in the isolated segment of the cord caused a
raising of the abdomen, followed by a very slight rhythmical movement of the gills.

d. Stimulation of a branchial nerve causes a contraction of the legs and gills of the same
side, but no rhythmical movement.

e. Stimulation of the right or left cord in front of the first abdominal ganglion causes con-
traction of legs and gills of the same side, but no rhythmical gill movements.

PART II.
SUMMARY OF EXPERIMENTAL AND ANATOMICAL RESULTS.

I. GUSTATORY REFLEXES.

a. Normal Action. i. Each leg of the second to fifth pairs, when its taste organs are
stimulated, performs the chewing movements alone, without starting the action in the adjacent
legs of the same, or of the opposite side. 2. Stimulating the chilaria of one side may induce
chewing movements in all the legs of that side. 3. The chelicerae have no "taste spines."
They are brought into action by stimulating the taste organs on one or more of the other legs
of the same side. The action may then be transferred to its mate, if the stimulus is strong
enough. 4. When the taste organs of several legs on the same side are stimulated, all the legs
of that side work in a harmonious rhythm. 5. When both sides are stimulated, the rhythmic
movements of the right legs harmonize with that of the left. 6. There are two independent
movements in chewing, the lateral, or in and out movement of the jaw-like coxae, and the thrust-
ing of the tip of the legs in and out of the mouth.

Structure of the Gustatory Apparatus. The principal reflexes described
above can be explained by the structure of the parts involved. The condi-
tions, so far as we have been able to analyze them by the anatomical and exper-
imental methods, are shown in a diagrammatic form in Fig. 114. We have
shown, for example, by the anatomical analysis, that : i. The taste organ nerves
of the jaws, flabellum, and chilaria form distinct fascicles, whose inner ends
(after giving off local dendrites, g.c. 1 ) unite to form an immense, longitudinal
tract, g.fr., terminating in a voluminous mass of neuropile, or secondary taste
center, on the neuro-lateral surface of the cheliceral neuromere (diencephalon)
near the base of the cerebral peduncles, g.c." 2. A tract extends beyond this
center along the cerebral peduncles to a tertiary center that forms a large lobe on the
median face of each hemisphere, g.c. 3 At the base of the hemispheres, underneath
the posterior median lobes and on the anterior neural surface of the cheliceral
neuromere, is a group of large nerve cells, ch.Hc., which by one set of dendrites
bring the secondary taste center into relation with the cerebral cortex of the same
side, and by another set with the opposite side of the collar, through the forebrain
commissure. 4. Finally a cluster of about fifteen large nerve cells, H.as.,
lying a little above the taste lobe, on the median surface of the hemispheres,
sends one set of extensive dendrites to the entire cerebral cortex, another to the

THE GUSTATORY APPARATUS.

l87

secondary taste center, and a third along the surface of the mid- and hindbrain
neuromeres, probably terminating around the motor neurites at the base of each
pedal ganglion.

ch.

Ollactory

Coordination

feec.orter. Cent
0\f. Vis. gust
1 locom. swim
( respir

Visual
Swallowing

FIG. 114. Diagram of the brain of Limulus, showing the course of the principal nerve impulses, and the loca-
tion of the principal centers of control. The figure is constructed from the data obtained by experimental and ana-
tomical methods. Only the first and fourth thoracic, and the first branchial appendage on the right side of the
animal are shown. The most important points illustrated are (a) the primary, g.c 1 ., secondary, g.c. 2 , and tertiary,
g.c. 3 , gustatory centers; (b) the relation of the motor, sensory, and association neurones to the muscles used in the
chewing reflexes; (c) the crossed thoracic temperature impulses; and (d) the crossing of the branchio-thoracic im-
pulses in the vagus neuromeres. The lettering of the neurones is the same as that in Figs. 56 to 66.

Closely associated with the secondary taste center is the "swallowing center,"
or the lateral stomodaeal ganglion, sl.g., lying on the median margins of the cheli-
ceral neuromere, next to the walls of the stomodaeum. It is connected with its
mate by a special pre-oral commissure, st.c .

l88 FUNCTIONS OF THE BRAIN.

The Nerve-muscle Chewing Apparatus. The leg movement in chewing
is produced by two muscles, a flexor and an extensor, both innervated by a branch
of the pedal nerve. (Fig. 114, c\~ and/? 2 .) The motor nerve-cells of these mus-
cles while not certainly located probably lie in, or close to, the pedal ganglion, Jr.

The i'aw movement is controlled bv nine muscles. The "in" movement is

j J

produced by four plastro coxals, pi. ex., two in front, and two behind, going from
the edge of the plastron to the sides of the coxa;; the "out" movement by the five
coxo-tergals, r.v./., two in front and three behind, extending from the base of the
coxa to the dorsal shield. These nine muscles are innervated by the anterior
and posterior ento-coxal nerves, a.en.cx. and p. cn.cx. that spring from the clusters
of motor cells lying on the haemal side of the brain, one on each side of the pedal
ganglion. (Fig. 66, H.) Each of these motor cells gives off numerous fibers to the
ento-coxal muscles; to the crus of the same side, and to the opposite crus, through
the corresponding commissure. (Fig. 114, //.)

Experimental Results. The results obtained by cutting the collar at various
places are naturally not always intelligible, but when they are they appear to be
in harmony with the anatomical relations just described. These results are as
follows:

i. Cutting the posterior end of one crus, or of both crura, behind, or close
to the sixth thoracic neuromere does not materially affect the chewing reflexes.
2. Cutting across one crus close to the hemispheres, produces increased vigor, and
a diminished coordination in the chewing movements of the legs on the cut side.
The jaw movement is either unmodified or slightly diminished. If both crura
are cut, the above results are obtained on both sides. These operations not only
separate the hemispheres, but the main gustatory and swallowing centers from
the thoracic neuromeres. The results point to the presence of separate control-
ling centers on each side of the forebrain. 3. An isolated segment of the collar,
containing one or two neuromeres, lying between the second and sixth neuromeres,
and separated from the opposite side by cutting its cross commissures, may give
feeble, uncoordinated, gustatory reflexes, but such an isolated portion of the collar
generally degenerated and soon failed to give further response. 4. However, in
experiment III B, the left side of the collar was completely isolated and lived with-
out perceptible degeneration for three weeks. But this segment undoubtedly in-
cluded that part of the cheliceral neuromere containing the secondary gusta-
tory center and the swallowing center, and only a small part, if any, of the
hemisphere. This segment readily produced the chewing reflexes; the reflexes
were normal except for the exaggerated and uncoordinated leg movements on
the isolated side, and the lack of harmony between the chewing movements on
the one side with those on the other. The results show: a. that the coordination
and inhibition of the leg movements on the one side lie in the hemisphere of the
same side, probably in the large median lobe, or tertiary gustatory center; b. that
the rhythmic control of the chewing movements of one side is located in the second-
ary gustatory center in the cheliceral neuromere; and c. that the coordination of

COURSE OF NERVE IMPULSES. 189

the movements of the right and left sides is controlled by means of libers in the
thoracic cross commissures, probably by the crossed collaterals of the motor
neurones.

The Swallowing Reflexes. There are no nerves or sense organs visible
near the lips, or in the soft skin about the mouth; and no reflexes could be pro-
duced by touching these parts with food. " Swallowing" apparently depends on
preliminary stimulation of the coxal taste organs. The chewing movements are
often interrupted by a tetanic spasm of the legs and a prolonged "bite" of the
great mandibles on the sixth pair. During this period, a muscular spasm of the
stomodaeum appears to take place, by means of which the materials that have
been tucked into the mouth by the coxal spurs are swallowed. The reflex is
probably initiated in the stomodaeal ganglion by stimuli received from the anterior
end of the gustatory tract.

Course of Nerve Impulses in the Gustatory, Chewing, and Swallowing
Reflexes. We may picture the probable course of the nerve impulses in the
chewing reflexes as follows : Stimulation of the gustatory cell, g.o, may i . discharge
an impulse by the first set of collaterals directly to the motor neurones of its own
segment. 2. To produce the continued rhythmic discharge, it is apparently
necessary for the impulse to be conveyed to the secondary, or cheliceral center,
and then back to the motor neurones of the chewing muscles. 3. Impulses may