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Full text of "The elasmobranch fishes"

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THE ELASMOBRANCH FISHES 



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ELASMOBEANCH FISHP]S 




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nail'ilut ]-, V,!it','!Zr ri'l't,'"',,!,'!-' ';'':;■""";■' ■;":'•" ■•*''•'":''■:■ •""/"";<• tlir tl.r,sl,L.,-. :!. Jihhu,,!,,,, Imcus, the wlialc shark. 4. C.l.nhi,,,,.. (Srluelw) mummiw, tho baskii.g sliaik. 5. Squalu 
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THE ELASMOBRANCH 
FISHES 



BY 



J. FRANK DANIEL 

Professor of Zoology in the University of California 




UNIVERSITY OF CALIFORNIA PRESS 
BERKELEY, CALIFORNIA 

1934 



First Edition Issued June, 19:22 

Second Edition Issued April, 1928 

Third, Eevised, Edition Issued March, 1934 

Copyright, 1922 and 1928 

BY 

J. FEANK DANIEL 

copyright, 1934 

BY the 

REGENTS OP THE UNIVERSITY OF CALIFORNIA 



sales REPRESENTATIVES 

University of California Press 
Berkeley, California 



Cambridge University Press 
London, England 

For orders originating in Great Britain only 



PRINTED IN THE UNITED STATES OF AMERICA 



PREFACE TO FIRST EDITION 

The Elasmobranch Fishes are, I believe, unsurpassed as material on whicli 
to study the fundamental plan of the vertebrate body. The ease with which a 
large, cartilaginous form may be dissected makes a study of its systems of 
organs a relatively simple matter, and the comparative simplicity of most of 
the systems shows that at least some of the present-day Elasmo])ranchs closely 
approximate the early vertebrates. 

The importance of the shark as a type for classroom study has, I think, not 
been generally appreciated in this country. This has been due in part to the 
difficulty of obtaining specimens in centers removed from the seaboard; in 
part, it has been due to the paucity of available literature. This I say in face 
of the fact that an abundant literature in practically all languages exists. I 
have attempted to remedy this latter ditficulty by adding to each chapter a 
working bibliography. 

In my studies of the Elasmobranchs I have been fortunate in having at hand 
perhaps the most generalized of these fishes, Heptanchus maculatus. In addi- 
tion to a study of the systems of organs in this and in other forms, I have at- 
tempted to collect and unify the work done by many workers on the various 
types. These combined studies I present with the hope that they will serve as 
a guide for undergraduate students of college grade, and at the same time be 
sufficiently inclusive to be used as a book of reference on the entire subject. 

I am indebted to the Scripps Institution for Biological Research for liberal 
support during five summers at La Jolla, and to the Research Board of the 
University of California for a grant in the final finishing of the plates. I am 
further indebted to many students who have helped me. Among them I single 
out Duncan Dunning, who, as a sophomore and junior student, made many of 
the most important drawings in the book. 

J. Frank Daniel 
Berkeley, California, 
January 16, 1922. 



[V] 



PREFACE TO SECOND EDITION 

In the second edition of the Elasmohranch Fishes I have followed the same 
plan as in the first, but I have added subject-matter and illustration which 
should make this edition somewhat more complete. Especially is this true for 
the chapter on the blood system to which has been added the work of Keys on 
the liypobranchial arteries of Hexanchus. The findings on Hexanchus supple- 
ment and add to my work on Hepfanchiis and make it more certain that the 
blood supply to the pectoral area in primitive vertebrates was from tlie hypo- 
branchial system, rather than from the dorsal aorta (the subclavians) as is 
true for higher vertebrates. I have also included in this edition Professor 
Van Wijhe's discovery that the thymus gland in the embryo of Heptanchus 
cinereus is not ductless, and I have added experimental evidence to show that 
the cutaneous vessels in Elasmobranchs are true blood-vessels. 

Berkeley, California, J. Frank Daniel 

February 24, 1928. 



PREFACE TO THIRD EDITION 

Among the contributions added to the third edition of Elasmobranch Fishes 
is the work of Marine on the transformation of the endostyle of Ammocoetes 
into the thyroid gland of the adult Cyclostome. Other contributions made 
since the publication of the second edition are on the lymphatic system by 
Hoyer, on the external carotid artery by O'Donoghue, and on the ampullae of 
Lorenzini by Dotterweich. 

O'Donoghue has shown that the external carotid in Elasmobranchs, as in 
higher types, belongs to the lower jaw, and that the artery in the orbit pre- 
viously designated as the external carotid or posterior carotid is, in fact, com- 
parable to the stapedial artery of mammals. Through this work we now have 
a complete history of the externa] carotid artery from sharks to man. 

Dotterweich has shown that the wall of an ampulla of Lorenzini is made up 
of two types of cells. One of these, a large goblet or gland cell, pours its secre- 
tion into the ampulla; the other type is pj^ramidal and has an inside hex- 
agonal face which is sensory in nature. An efferent nerve supplies the gland 
cell, and an afferent nerve leaves each of the sensory cells. 

Berkeley, California, J. Frank Daniel 

January 4, 1934. 



[yi] 



CONTENTS 

PAGE 

Introduction 1 

I. EXTERNAL FORM 

External Form of Heptanchus maculatus 5 

External Form of Elasmohranchs in General 8 

Transitional series i • • 9 

External form in its development 10 

Form and position of adult fins 12 

External form of fin and its bearing on function 12 

Form of fin in its beginning 14 

Bibliography 17 

II. INTEGUMENT 

Integument of Heptanchus maculatus 23 

Modified scales 25 

Integument of Elasmobranchs in General 26 

Gland cells 28 

Glands of claspers 28 

Poison glands of sting ray 28 

Light organs 29 

Placoid scales 30 

Finer anatomy of scale 32 

Modification of scales 33 

Fin spine 33 

Saw tooth 35 

Sting 35 

Gill rakers 37 

Stomodeal denticles 38 

Teeth as modified scales 38 

Bibliography 39 

III. ENDOSKELETON 

Endoskeleton of Heptanchus maculatus 43' 

Axial skeleton 43 

Skull 43 

Visceral skeleton 44 

Spinal column ■ ... 47 

Appendicular skeleton 49 

Skeleton of paired fins 50 

Skeleton of unpaired fins 51 

Dorsal fin 51 

Caudal fin 51 

Anal fin 52 

Endoskeleton of Elasmobranchs in General 53 

Axial skeleton 53 

Skull 53 

Cranium 53 

Visceral skeleton 62 

Extravisceral arches 67 

Spinal column 69 

[vii] 



46052 



Endoskeleton of Elasmobranchs in General — Continued page 

Appendicular skeleton 75 

Skeleton of paired fins and of fin girdles 75 

Pectoral fin skeleton 75 

Pectoral girdle 79 

Pelvic fin skeleton 81 

Pelvic girdle 82 

Skeleton of unpaired fins 82 

Bibliography 84 

IV. MUSCULATURE 

Musculature of Heptanchus maculatus 89 

Muscles of the eye 89 

Buccal and pharyngeal muscles 90 

Dorsal constrictors 90 

Ventral constrictors 91 

Interarcuales 92 

Adductors 93 

Ventral longitudinal muscles 93 

Muscles of the fins 94 

Musculature of Elasmobranchs in General 96 

Muscles of the eye 99 

Muscles of visceral arches 100 

Superficial constrictors of pharyiLx 101 

Dorsal constrictors 102 

Ventral constrictors 104 

Deeper muscles of pharynx 105 

Interarcuales 106 

Adductores arcus 107 

Hypobranchial musculature 108 

Muscles of the fins 109 

Muscles of the claspers 110 

Electric Organ in Elasmobranchs 112 

Electric organ of rays 112 

Finer anatomy of electric organ 114 

Bibliography .■ •. ^^^ 

V. DIGESTIVE TRACT 

Digestive Tract of Heptanchus maculatus 121 

Mesenterial structures 121 

Buccal cavity 121 

Pharynx and associated structures 122 

Oesophagus 123 

Stomach 123 

Spleen 124 

Duodenum or middle intestine 124 

Valvular intestine 124 

Colon and rectum 126 

Rectal gland 126 

Cloaca 126 

Abdominal pores (Pori abdominales) 126 

Digestive Tract of Elasmobranchs in General 127 

Mesenteries 127 

Buccal cavity 128 

Teeth / 128 

Replacement of teeth • 131 

[viii] 



Digestive Tract of Elasmobranchs in General — Continued page 

Pharynx 131 

Oesophagus 134 

Stomach 135 

Duodenum or middle intestine 136 

Liver 137 

Pancreas 138 

Spleen 138 

Valvular intestine 139 

Colon and rectum 141 

Cloaca 141 

Abdominal pores 142 

Bibliography 143 

VI. RESPIRATORY TRACT 

Respiratory Tract of Heptanchus maculatus 147 

Gill pouch or pocket 147 

Spiracle 147 

Gill or holobranch 148 

Gill supports 148 

Gill filaments 149 

Respiratory Tract of Elasmobranchs in General 150 

Gill pouch or pocket • 150 

Production of respiratory current 155 

Direction of respiratory current 156 

Circulation of blood in filaments 157 

Respiration or the exchange of gases 157 

Bibliography 158 

VII. CIRCULATORY SYSTEM 

Circulatory System of Heptanchus maculatus 160 

Arteries 161 

Ventral aorta 161 

Afferent arteries of the adult 161 

Efferent-collectors 161 

Branches of the efferent-collectors 162 

Hypobranchial arteries 162 

Efferent arteries 165 

Dorsal aorta 165 

Arterial supply to digestive tract 166 

Coeliac axis and its brancries 166 

Superior (anterior) mesenteric and its branches 167 

Inferior (posterior) mesenteric artery 167 

Arterial supply to extremities 168 

Arterial supply to deeper structures 169 

Caudal aorta 169 

Circulatory System of Elasmobranchs in General 170 

Heart 170 

Arteries 172 

Ventral or ascending aorta 172 

Afferent arteries of adult 172 

Capillaries 173 

Efferent-collectors 173 

Branches of efferent-collectors 178 

Hypobranchial arteries 178 

Arterial supply to head 180 

Efferent arteries 182 

[ix] 



Circulatory System of Elasmobranchs in General — Continued page 

Arterial supply to trunk 186 

Dorsal aorta 186 

Unpaired arteries 186 

Coeliac axis and its branches 186 

Superior mesenteric artery 188 

Inferior mesenteric artery 190 

Paired branches of aorta 191 

Subclavians and iliacs 191 

Segmentals 192 

Caudal segmentals 193 

Bibliography 195 

VIII. CIRCULATORY SYSTEM {Continued) 

Circulatory System of Heptanchus maculatus 198 

Veins * 198 

Veins of head 198 

Veins of tail 199 

Veins from kidney or mesonephros 200 

Veins from digestive tract . • 200 

Veins of body wall 202 

Veins of skin 202 

Circulatory System of Elasmobranchs in General 204 

Veins - 204 

Anterior cardinal sj'stem 204 

Renal portal system 208 

Posterior cardinal veins 209 

Hepatic portal system 210 

Development of hepatic portal system 212 

Lateral abdominal system of veins 213 

Veins of heart 215 

Cutaneous sj'stem of veins 216 

Nature of cutaneous system of vessels 217 

Lymphatic vessels 218 

Bibliograph}' 219 

IX. NERVOUS SYSTEM 

Nervous System of Heptanchus maculatus 221 

Central nervous system 221 

Brain 221 

Spinal cord 222 

Peripheral nervous system 222 

Cranial nerves 223 

Occipitospinal nerves 227 

Spinal nerves : 227 

Nervous System of Elasmobranchs in General 229 

Development of central nervous system 230 

Gross form of brain 230 

Internal view of brain 235 

Spinal cord 237 

Peripheral nervous system 238 

Cranial nerves 238 

Spinal nerves 246 

SjTnpathetic or autonomic nervous system 247 

Bibliography 249 

[X] 



X. SPECIAL SENSES 

PAGE 

Special Senses of Heptanchus maculatus 258 

Olfactory organ 258 

Eye 258 

Ear 258 

Innervation 261 

Sensory canal system 261 

Special Senses of Elasmohranchs in General 264 

Olfactory organ 264 

Development of olfactory organ 265 

Gustatory or taste organs 265 

Elasmobranch eye 266 

Structure of adult eye 267 

Finer structure of retina 268 

Development of eye 269 

Accommodation apparatus 269 

Auditory organ 271 

Innervation of ear 273 

Development of ear 274 

Sensory canal system and ampullary and pit organs 274 

Sensory canal system 274 

Function of sensory canal system 279 

Ampullary organs 279 

Pit organs 281 

Bibliography 283 



XI. UROGENITAL SYSTEM 

Urogenital System of Heptanchus maculatus 287 

Urinary system 287 

Genital system 290 

Urogenital System of Elasmohranchs in General 292 

Urinary system 292 

Kidney (Mesonephros) 292 

Ducts of kidney 294 

Finer anatomy of kidney 296 

Nephrostome and segmental duct 296 

Development of nephrostome and segmental duct 298 

Bowman's capsules 299 

Genital system 300 

Genital organs of male 300 

Testes 300 

Finer anatomy of testis 301 

Vasa efferentia 301 

Genital organs of female 303 

Ovaries 303 

Oviducts 304 

Shell glands 304 

Types of egg shells 305 

Uterus 306 

Relation of uterus to embryo 307 

Secondary sexual characters 309 

Bibliography 311 

Index 319 



[xi] 



INTRODUCTION 

THERE LIVES today a vast group of fishes, some of which are littoral, keep- 
ing close to shore; others are the nomads of the ocean, roaming vast ex- 
panses of its waters; others there are which are pelagic, living near its 
surface; and still others that are the inhabitants of the profound depths into 
which sunlight never penetrates — these are the sharks, to the man with nets the 
most worthless, to the naturalist among the most interesting of living things. 
But the vast numbers of today are few in comparison with the hordes which 
have lived and passed in succession before them. They, the rulers of the waters 
in bygone ages, have gone down like primitive man, leaving little to tell of 
their presence. This little, however, is of singular interest. Some of these fishes 
are known to us only by a spine. Others are represented by ])its of armament 
which show that many of the ancient sharks were clad in a protective covering 
far more complex than that possessed by any of the living forms. But most of 
these fishes are known to us by their teeth; of these, the heterodont sharks are 
most instructive. 

Before me are the teeth of a form which swam the primitive seas before the 
formation of our western mountains. Beside them I place the teeth of another 
wdiich was taken with hook and line in the Pacific but yesterday. The vast 
stretch of years separating the life of the one from the life of the other is be- 
yond the comprehension of man, yet the close similarity of plan binding the 
one form to the other clearly indicates that this of the present is that of the 
past projected through the ages. 

Not upon fragments alone does our information concerning these ancient 
fishes rest. Within the past few decades our knowledge has been greatly en- 
riched by the discovery of specimens, many parts of which were in an almost 
perfect state of preservation. This preservation is the more wonderful when it 
is recalled that even the harder cartilaginous parts are subject to rapid decay. 
That some of these specimens have escaped the ravages of time makes us hope- 
ful that others of still more archaic forms will yet be unearthed, to complete 
our records of the ancient history of this group. 

Of the extinct types discovered in an excellent state of preservation, Clado- 
selachus (fig. 10), described by Dean, is one of the oldest and in many ways 
one of the most generalized and interesting of fishes. From it we have learned 
much ; for even to detail, soft parts like muscle fibers and, in some specimens, 
visceral organs have been obtained in a remarkable state of preservation. Two 
other forms, Acanthodes and Pleur acanthus, are of special interest. In an 
acanthodian type like Cliniatius{ ?) (fig. 11) spines have been developed to an 
unusual degree so that even in the paired fins the fin is composed essentially 
of web and anterior supporting spine. In this type the spine is essentially a 
dermal structure encasing the exposed margin of the fins. In other regions of 
the body the scales show the same tendency toward hypertrophy. Thus around 
the eye and along the lateral line they are of unusual size. In Pleur acanthus, 

LI] 



2 THE ELASMOBRANCH FISHES 

there are many of the present-day characteristics peculiar to the sharks. In 
addition, bj' the structure of its limbs and tail, it suggests relationship to the 
interesting group of Dipnoans or lungfishes. 

Equallj' as interesting as the extinct forms preserved to us in the rocks are 
others, less ancient though the}' be, which have come down to us in flesh and 
blood. Of these living representatives of the past some are among the most 




Fig. 10. Cladoselachus. (From Dean.) 

interesting and instructive of forms, interesting as antiquities are interesting, 
instructive as all generalizations are instructive. Among these forms known to 
us on the California coast are Heterodontns francisci (fig. 17), whose genus is 
the sole survivor of a group, twenty-five genera of which are buried in the 
rocks; and Hexanchus corinus and Heptanch us maculatus (fig. 13) , the latter 




Fig. 11. Climatius(^). (From Dean.) 

with generalization of bodily plan surpassing that of any other present-day 
Elasmobranch. 

In addition to the ancient types there are many modern forms. Some of 
them, like Acanthias (fig. 5, Squalus acanthias) , have system upon system so 
generalized as to approximate a ground plan on which nature has built up its 
masterpieces of vertebrate life. Other forms, although simple in part, are 
noted for extreme specialization in certain respects. Among these may be men- 
tioned Cephaloscyllium, the California swell shark (fig. 1), Zygaena, the 
hammer-head common to many seas, and Alopias, the thresher, a single genus 
of world-wide distribution (fig. 2). 

Still others of the recent sharks, although exceeding Zygaena and Alopias 
but slightly in size, are singled out as large sharks. Of this group the forms 
known to occur on the Pacific Coast are CarcJiarias (Prionace) , the "man- 



THE ELASMOBRx\NCH FISHES 3 

eater," or greaX l)lue shark (fia:. 16) , and Cetorhinus, the basking shark (fig. 4) , 
wliich not infrequently exceeds twenty-five feet in length. Finally, as an occa- 
sional visitor np this coast may be added the giant of all fishes, Khinodon 
fi/picus (fig. 3), specimens of which have been known to reach fifty feet in 
length. 

To the above recent Elasmobranchs may be added the flattened sharks or 
rays. Some of the most singular of these are Prist is the sawfish (fig. 19) with 
nose prolonged into a two-edged saw, and Myliobatis the batfish (fig. 8) ; 
Torpedo the electric ray (fig. 6) provided with a powerful battery by means 
of which shocks of electricity may be administered to food or enemy alike; and 
lastly, Cephaloptera, the giant of the ray tribe, growing in tropical seas to 
more than a thousand pounds in weight and sometimes having a measurement 
from tip to tip of pectorals exceeding twenty feet. 

Of the whole group of Elasmobranchs possibly none is of more interest than 
the remaining notidanid sharks {Heptanchus and HexancMis) . Because of the 
generalization found in these forms I propose in the following discussion of 
the Elasmobranch fishes to use Heptanchus maculatus* (fig. 13) as a type with 
which to compare in general other Elasmobranchs. 

* This shark is variously known as Notorhynclnis maculatus Ayres (1855), Heptanchus 
maculatus Girard (1858), or Heptranchias maculatus Gill (1861). After making a detailed 
study of this form and noting the marked differences between it and Heptanchus cinereus I 
am almost persuaded that it merits the generic position ascribed to it by Ayers. Because of 
the simple meaning of the word Heptanchus, however, and regardless of whether the etymol- 
ogy of the word is or is not correct, I have retained the name given by Girard. 



THE ELASMOBRANCH FISHES 




Fig. 12 




Fig. 13 




Fig. 14 



Fig. 12. Eeptanchus cinercus. (From Fitzinger.) 

Fig. 13. Heptanchus macu^atus. (C. G. Potter, del.) 

Fig. 14. Heptanchiis indicus. (From Macdonakl and Barron.) 



EXTERNAL FORM 

EXTERNAL FORM OF HEPTANCHUS MACULATUS 

Hepianchus maculatus (fig. 13), in general shape, lacks the grace character- 
istic of many of the more active and predacious sharks. This is due in part to 
a relatively heavy head; in part it results from the unusual dimensions of the 
tail. In these features all heptanchid sharks (figs. 12 to 14) are similar. In 
Hexanchus, a close relative of Heptanchiis, the body is still more ungainly. 
This is readily apparent when a specimen is brought to shore, for on land the 
body and head are so heavy that they flatten out and become distorted. Not- 
withstanding this lack of grace in the heptanchid sharks, I agree with Dean 
(1895) that "Hepianchus, of all living sharks, inherits possibly to the greatest 
extent the features of its remote ancestors." 

The most superficial feature by which Hepfanchus may be recognized is 
the number of its gill clefts (cL, fig. 15) . In fact it was from this characteristic 
that Hepianchus received its name.^ Anteriorly the clefts are of large size, but 
posteriorly they decrease so that the last, which lies just in front of the pec- 
toral fin, is about half the height of the first. Anterior to the first cleft and 
somewhat dorsally is the modified cleft or spiracle (sp., fig. 15) which in the 
adult is relatiA'ely diminutive in size. 

Other superficial characteristics helpful in distinguishing Hepianchus from 
most other sharks are the position of the nasal apertures and the shape of the 
mouth. The nasal apertures are more nearly terminal than is usual in the 
Elasmobranchs. They appear on the broad snout as relatively small crescents, 
each of which is separated into a dorsal and a ventral part by an overlapping 
median flap. This flap in fact forms of the nasal cup a tube which provides a 
passageway for the water current over the olfactory organ. The mouth is of 
unusual size (figs. 15 and 119) . In side view it appears as a deep horizontal slit 
extending back as far as the segment of the spiracle, and hence cleaving the 
anterior region almost into dorsal and ventral halves. From this type of cleav- 
age there results an exceptionally heavy lower jaw which gives to this form 
much of its grotesque appearance. 

At the side of the head is the eye, which has an orbit of relatively long hori- 
zontal axis. The eye is shielded by an upper and a lower membranous lid, but it 
is unprovided with the so-called nictitating membrane or third eyelid charac- 
teristic of some of the Elasmobranchs. 

This shark also possesses, in common with other Elasmobranchs, the small 
apertures of the endolymphatic ducts which lead to the ear (see p. 42, fig. 45, 
e.cl.). These apertures are near the middorsal line and slightly in front of a 
plane taken through the spiracles. 

1 €7rra, Seven; a7xw, with reference to compressed gill clefts. 

[5] 



6 THE ELASMOBRANCH FISHES 

The paired fins of Heptanchus partake slightly of the enlargement charac- 
teristic of the forward part of the body; the pectorals {pt., fig. 15) are large, 
but the pel vies (ventrals, pi.) are much smaller. When considered in relation 
to other parts of the body the pectoral and pelvic fins appear close together 
but the distance between the two is considerable, the pelvic girdle being in the 
region of the fortieth spinal segment. The inner margin of each pelvic fin of 
the male is modified into a clasper, which extends backward (fig. 14). The 
claspers, however, in the immature Heptanchus inaculatus (see p. 288. fig. 
251b, els.) are small and inconspicuous. Between the inner margins of the 
pelvic fins and back of their bases is the cloacal opening through which prod- 
ucts of excretion as well as the sex cells leave the bodv. 




Fig. 15. Lateral view, Heptanchus maculatus. 

ah, anal fin ; cc, supraeranial canal ; cl., branchial clefts ; dl., dorsal fin ; h inc., hyomandib- 
ular canal; ioc, infraorbital canal; U., lateral line; pL, pelvic fin; pt., pectoral fin; soc, 
supraorbital canal; sp., spiracle; v.l., ventral lobe of caudal fin. 



One of the most marked characteristics of Heptanchus is a single, unpaired 
dorsal fin (dl., fig. 15). It is from this shapely fin that we get the term noti- 
danus (vcjtov, the back; Isapos, comely) . The notidanids include both the lieptan- 
chid and hexanchid sharks. So far as this character is concerned it applies 
equally as well to Chlamydoselachus. 

In Heptanchus this dorsal fin lies above and slightly back of the pelvics, 
having a position as far posterior as that occupied by the second dorsal of 
many Elasmobranchs. The immense size of the caudal fin is due to the unusual 
extent of its lobes and to the width of the ventral lobe (v.l., fig. 15). In the 
ventral lobe the anterior dermal fin rays are especially well developed. These 
fin rays are followed by a shorter series back to a notch near the terminus of 
the lobe, behind which the dermal rays are again longer and extend in an 
almost horizontal direction to the tip of the tail. Between the ventral lobe and 
the cloacal opening is the smaller anal fin (al.) . 

The ground plan of coloration in Heptanchus maculatus (fig. 13) is an al- 
most uniform drab above and light below. Scattered over the drab background 
is a motley pattern of spots from which comes the specific name, maculatus, 
spotted. These spots extend over the dorsal side of the paired fins and to the 
unpaired fins, and vary in size from minute dots to clumps of pigment larger 
than the pupil of the eye. Over the dorsal region of the body, where the pig- 
ment is most dense, they are less conspicuous; as growth proceeds many of 
them become hidden in the general color pattern. 



THP] ELAHMOBRANCII FISHES 7 

Overlying the coat of drab and the pattern of dots there is, in an adnlt, an 
armament of denticles, the so-called plaeoid scales, myriads of which go to 
make np the protective shagreen exoskeleton (see p. 24, fig. 27 ) . This armament 
in the adult Heptanchus is made up of a vast number of closely set scales, 
many of which are more or less spade-shaped. In the more exposed parts of 
the body, however, which are subject to great wear, the scales often become 
modified and plate-like. 

Another characteristic made out in external view is the lateral line {II., 
fig. 15). In Hepianchus, this line is an open groove extending from the end of 
the tail along the side of the body to the pharyngeal region. This groove is con- 
nected with certain canals of the head, w^iich, as closed tubes, are located 
deeper in the integument. Branching from the canals are small chimney-like 
tubes which retain connection with the surface by pores. 

In the walls of this system of grooves and canals are groups of sense organs 
which we shall consider more fully in Chapter X. 

In addition to these lines of pores there are other aggregates of pores (for 
example, p. 260, fig. 227a, soa.) in the region of the head, and pits which are 
located along the dorsal and anterior parts of the trunk. Each of these pores 
is the entrance to a tube wiiich leads to an enlargement or ampulla of Loren- 
zini. The tubes are filled with a jelly-like mucus which, if pressure be put on 
the skin, may be made to exude from the pores. It is from this content that the 
pores of the ampullae of Lorenzini and those of the canal system are known 
as mucous pores. 



THE ELASMOBRANCH FISHES 



EXTERNAL FORM OF ELASMOBRANCHS IN GENERAL 

A comparison of the adult in two types like Acanthias, the spiny dogfish, and 
Urolophus, the small sting ray (figs. 5 and 9), shows the two extremes of body 
form to which the Elasmobranch fishes have diverged. In Acanthias, which is 
beautifully adapted for cleaving the water in the running down and capture 




Fig. 16. Carcharias gJaucus. (From Garman.) 

of prey, the head is pointed, and the rounded body tapers gracefully into 
a powerful organ of locomotion, the caudal fin; while in ZJrolophus, which 
spends much of its time on the bottom, the head and body are depressed, carry- 
ing the branchial clefts to a ventral position, and the pectoral fins, extending 




Fig. 17. Keterodonius francisci. 

from the pelvic fins behind to the tip of the nose in front, have become the 
organs of effective locomotion. 

It is by differences like these that the Elasmobranch fishes have been sepa- 
rated into two general groups (suborders) . One of these, the Selachii, contains 




Fig. 18. Squat ma calif ornica. 

the sharks; the other, the Batoidei, includes the raj'S. While this distinction 
between the Selachii and the Batoidei is of special service in separating forms 
like Acanthias from those like TJrolophus, yet between the two extremes there 
are types which link by link bridge the intervening gap. So effectively is this 



THE ELASMOBRANCH FISHES 9 

accomplished that on the border line between the two the characteristics of 
the one often resemble those of the other so closely that it is difficult to say 
this is a shark, that a ray. This will be made the more evident upon a consid- 
eration of a series of these forms. 

Transitional Series 

In such a series, Carcharias, the "man-eater" (fig. 16) , may be taken as a highly 
specialized type. The fusiform body, even more than that of Acanthias, is 
fashioned for cleaving the water ; the caudal and pectoral fins are powerful ; 
and the sharp pointed teeth are adapted for seizing and holding prey. In fact 
every line of its structure is an index of predacious perfection. To a less extent 
the same is true of Galeus. In Mustelus, although the body is highly special- 
ized in this respect, the teeth, varying from the type, have generally become 
broad and flattened for crushing. 

Heavier of body and clearly less graceful in form is the lamnoid type, 
Lamna cornubica, in which, although the teeth are long and fang-like, the 
body is relatively cumbersome and the pectorals are placed farther back. The 
lamnoids, however, retain their restless nature and are classed among the 
more predacious of the pelagic fishes. 

In Heterodontus (fig. 17) we see a still less graceful type. In it the pectorals 
are expanded, suggestive of a less active nature, more given to foraging over 
the ocean floor in search of a shellfish diet. Furthermore, in Heterodontus the 
posterior teeth are of the crushing type adapted to such food (see p. 130, 
fig. 128) . Yet it is worthy of notice that the most anterior of the teeth are pre- 
hensile showing that the grasping of food is possible. 

Next in the series is Squotina, the angel fish (fig. 18), in which the body is 
greatly flattened. Furthermore, the pectorals extend forward and the dorsal 
fins have shifted to a posterior position. In all these respects Squatina is 
ray-like. But the gill clefts, although covered with a flap, open laterally, and 
other significant internal structures of fin and skull cling tenaciously to the 
shark type. 

In Pristis, the sawfish (fig. 19), the anterior part of the body is still more 
flattened; but the caudal region is like that of a shark. The gill clefts of Pristis, 
however, are entirely ventral in position; and more important still its pec- 
torals have fused to the sides of the head so that in essential respects it has 
passed over the batoid line and is clearly among the rays. 

Slightly more ray-like is Rhiiiohatis product us, the guitar fish (fig. 7) . But 
even here, although the head and body are depressed, the pectorals are rela- 
tively small and the tail is still the organ of active propulsion. From this'shark- 
like ray to others singularly flattened in form are rays in great variety. 
""^In the skate, Eaia erinacea (fig. 20), the pectorals extend forward to the 
region of the nose, and the pelvics are large. Correlated with the greater devel- 
opment of the paired fins the caudal region, including the dorsal fins, is poorly 
developed, the dorsal fins liaving migrated still farther back on the tail. In this 



10 



THE ELASMOBRANCH FISHES 



fish we see sufficient modification of form to insure a new mode of locomotion. 
Here the pectorals become the elfective organs in propulsion, and the tail 
remains at a stage of development insufficient to propel the body. The skate is 
also an organism not only singularly adapted to its method of getting food, 
but equally effective in crouching on the ocean floor so as to evade its enemies. 
Last in the series we may place the sting ray, Disceus thayeri (fig. 21), 
which even more than TJrolophus undergoes profound depression of body. In 




Fig. 19 




Fig. 21 



Fig. 20 

Fig. 19. Pristis cuspidatum. (From Garman.) 
Fig. 20. Baia erinacea. (From Garman.) 
Fig. 21. Disceus thayeri. (From Garman.) 

it the pectoral fins meet in front and behind so that tlie body is essentially 
disc-shaped. Like other rays of this type its tail is provided with a serrate 
sting, but unlike TJrolophus the tail is whip-like and has but slight indication 
of fins or folds. 

External Form ix Its Development 



The transition in external form which we have just considered in the series of 
sharks and rays may be similarly followed in a series of stages in the life- 
history of an individual ray. In other words, while a ray and a shark are very 
dissimilar in the adult, they are much alike in their earlier development. As 
growth proceeds, the ray gradually diverges from the shark-plan so that 
finally a fixed gulf separates the two. To illustrate this divergence we may 
again compare Acmithias and Urolophus. 



THE ELASMOBRANCH FISHES 



11 



At an early stage there appears above the germinal disc a horseshoe-shaped 
mass of tissue, the closed end of which represents the head end, and the open 
end, the tail end. This mass of tissue then becomes spatulate (see p. 230, 
fig. 209). In a further stage, in which the body takes on definitive form, the 
two types are characteristically similar. In both, the optic vesicles stand out 
as prominent structures and the gill clefts, in breaking through, occupy about 
the same lateral position. 

We may figure two stages (fig. 22a-d) which represent the parting of the 
ways. The first (a) and third (c) of these are of Acanthias; the second (b) 
and fourth (d) are of Urolophns. While in Urolophus (fig. 22b) the clefts still 



( 






'^^ 



^ J 





-^•v ' 




1 

ri 



'i ■■'■ 

ABC D 

Fig. 22. Development of body form in Acanthias and Urolophus. 

A. Stage in development of SquaJus acanthias. (Length 20.6 mm.) (From Scammon.) 

B. Stage in development of Urolophus halleri. (Length 22 mm.) (C. G. Potter, del.) 

C. Older stage in development of Squalus acanthias. (Length 28 mm.) (From Scammon.) 

D. Older stage in development of Urolophus halleri. (Length 38 mm.) (C. G. Potter, del.) 

open on the sides, other changes are taking place which immediately charac- 
terize it as a ray. The most notable of these changes is the extension of the 
pectoral fin. 

In the figures of Acanthias (c) and of Urolophus (d) both assume essen- 
tially the features of the adult. While Acanthias retains its slender form, 
Urolophus becomes greatly flattened, the disc-shape being the result largely 
of the growth of the pectoral fins. Each pectoral now takes the shape of a 
battle-axe, the blade of which extends outward. The posterior point of the 
blade projects toward the pelvic fin, while the anterior point extends over the 
branchial region. As the fin broadens, its anterior tip meets the growing antor- 



12 THE ELASMOBKANCH FISHES 

bital process and both anterior and posterior projections of the pectoral come 
in contact with, and fuse to the sides of the head and body. The fusion of the 
forward part thus profoundly modifies the branchial area, and the clefts take 
up a ventral position. As growth continues, the forward extensions of right 
and left pectorals meet in front, completing the disc of the adult body. 

Form and Position of Adult Fins 

It was observed above that a determining factor in the external shape as a 
whole is the form and position of the fins. The pectorals are of first importance 
in such a determination. In fact sharks and rays could be separated with cer- 
tainty by the character of the pectoral fin alone. In general the pectorals of 
the sharks are relatively small, while those in the rays are large. But a point 
of greater importance is the secondary fusion which, as we have just seen, the 
pectorals of the rays make with the head during developmental stages. Border 
types, like Squatina and Rhinohatis, which could not be separated by relative 
size of fins alone, could be distinguished with certainty by the presence or lack 
of such fusion. 

The caudal fin, although of less value than the pectoral, may also be a deter- 
mining factor in external form. The axis of the caudal in the sharks, although 
compressed, is more or less fleshy and the lobes of the fin both dorsally and 
ventrally are well developed. In adult and specialized rays, on the contrary, 
the entire tail may be in a more or less complete state of atrophy. In the skate 
(fig. 20), as an example, it is a long fleshy rod; in Myliohatis, although it may 
reach an extreme length, it is slender and whip-like (fig. 8) ; and in Pfero- 
platea it never develops beyond the rudimentary stage. In transitional tj^pes 
of rays, however, especially among the Pristidae and Rhinobatidae, the axis of 
the tail is muscular, and the lobes of the caudal fin are pronounced structures, 
although they are less well developed than are those, say, of Squatina (fig. 18) . 

Among the other fins the dorsals, which are usually- correlated with the size 
of the tail, are more poorly developed in the rays than in the sharks. Further- 
more, these fins in the rays, if present, generally take up a position relatively 
far posterior to the anal segment. Myliohatis as a type is exceptional in that 
the single dorsal fin takes a position just anterior to the sting. In the skates the 
dorsals are far out toward the tip of the tail. In fact, in some of the rays they 
may not inaptly be said to have migrated practically off of the end of the tail. 
An anal fin is always lacking in the rays; hence this would be of definitive 
value were it not wanting in a few of the sharks such as the Spinaeidae and 
the Rhinidae. 

External Form of Fin and Its Bearing on Function 

If the pectoral fin be of sufficient extent it may perform the function of pro- 
pulsion. But in the sharks propulsion is brought about largely by the caudal 
fin. Two general types of locomotion may be described for the Elasmobranchs. 
2 Excepting in the second dorsal of Lamna, Alopias, etc. 



THE ELASMOBKANCH FISHES 



13 




In one, forward movement is produced principally by the pectorals; in the 
other this function is performed chiefly by the caudal fin or tail. One of these 
types of locomotion we may therefore designate as pectoral, the other as 

caudal. Pectoral locomotion among the Elasmo- 
branclis is confined to the rays while the caudal 
type, so far as I know, is universal among the 
sharks. It is, however, by no means peculiar to 
them, since, as we have indicated, some transi- 
tional rays retain this method of swimming. 

To illustrate pectoral locomotion Urolophus 
may first be considered. In Urolophus progres- 
sion is brought about by a synchronous wave 
movement in both pectorals. This wave begins 
at the anterior margin of the fin and passes 
backward, terminating at the posterior margin; 
thereupon another wave sets in and repeats this 
action. In the skate pectoral locomotion may be 
seen to excellent advantage. In it (fig. 23a), as 
in Urolophus, there is a wave motion which in- 
volves not only the margin, but also the greater 
width of the fin, throwing the pectoral into an 
inverted U with the sharper convexity directed 
forward. As this wave passes backward ( 1-9 ) it 
gains in size and evident momentum, serving as 
an effective pushing surface against the water. 
From this type of wave movement a beautiful 
gliding motion results. A further modification of the type is present in Cephal- 
optera, in which the stroke of right and left pectorals takes place alternately. 

Transitional rays are instructive 
in that they have not yet attained 
the method of pectoral locomotion 
although the pectorals are well de- 
veloped. In Rhinohatis these fins 
may be put to considerable use other 
than in directing the course, as may 
be seen upon grasping the tail and 
attempting to pull the fish out of the 
water. 

In the sharks, in which caudal locomotion is employed, the body itself is 
thrown into undulations which also provide concave surfaces with which to 
push against the water (fig. 23b) . The wave here begins in the body just back 
of the pectoral fins (1) and passes posteriorly and off at the tip of the tail (9), 
and another undulation thereupon begins and repeats the course. At the same 
time and with the greatest effect the dorsal and ventral lobes of the caudal fin 
are directed by the strong muscles as a potent sculling organ. 



Fig. 23a. Diagram to illustrate 

the different phases (1-9) of 

pectoral locomotion in the skate. 

(From Marey.) 




Fig. 23b. Diagram to illustrate the different 
phases (1-9) of caudal locomotion in the 
shark, ScylUum. (From Marey.) 



14 THE ELASMOBRANCH FISHES 

We have thus far spoken only of the fins wliich are propelling. In a type like 
Myliohatis (fig. 8), direction of the course is effected through the paired fins. 
In TJrolophus (fig. 9) the horizontal course is also directed by the pectorals, 
but the vertical direction is controlled in large part by the caudal which is 
used effectively as a rudder. 

In the sharks other fins are of service in directing the course. The use of the 
directive fins may made out by a series of experiments. If a rubber band is 
put over the pectorals of a young shark with the caudal fin free, there residts 
a downward swimming of the fish, the pectorals functioning as organs for 
directing the horizontal course in the water. In function the pelvics are acces- 




—'Ai--' ;.j_ui£i-i. 



Fig. 24. Embryo of ScyUium canicula showing early epidermal fin-folds. (From Mayer.) 

al., anal fin; df., dorsal fin-fold; dl., second dorsal fin; pL, pelvic fin; pt., pectoral fin; 
V.I., ventral fin-fold. 

sory to the pectorals. Both the dorsals and the anal may be bound down with- 
out interfering seriously with propulsion. These fins are of service, however, 
in keeping the fish in a vertical plane. 

FOEM OF FIN IN ITS BEGINNING 

It is evident that the form of the fin in present-day Elasmobranchs differs 
from that of the ancestral type. And the question arises, What was the form of 
the ancestral fin ? To this inquiry many answers have been given, few of which 
have gained a hearing. We may mention briefly two of the best known theories 
for the origin of the paired fins. One is the lateral fin-fold theory of Balfour 
and Thacher; the other is the gill-arch theory of Gegenbaur. 

According to the fin-fold theory the ancestors of the present-day fishes are 
supposed to have possessed a median dorsal fold {df., fig. 24) which was con- 
tinued over the tail as the dorsal lobe of the caudal fin and then forward to the 
anal region as the ventral lobe, {v.l.) ; anterior to the anal region the ventral 
lobe separated into right and left lateral folds not greatly unlike the meta- 
pleural folds of Amphioxus. In certain regions of the dorsal and the lateral 
folds greater development ensued than at the interspaces. These parts of the 
fin-fold hence increased in size and became the present unpaired and paired 
fins, while the intermediate parts finally dropped out. 

The gill-arch theor}^ of Gegenbaur holds that the framework of the girdles 
for the paired fins and of the fins themselves have been derived from the gill 
arches (g.a., fig. 25) and their attached branchial rays (h.r.). The arch itself 
represents the girdle and the formation of the skeleton of the fin proper was 



THE ELASMOBRANCII FISHES 15 

understood to have resulted from a fusion of the branchial rays at their bases 
into a main axis which by extension drew out the adjoining rays so that they 
arose from this main axis (a-c) . 

When we consider that the pectoral girdle in form and position is much like 
a gill arch (see p. 49, fig. 54) , and that the fin rays may be arranged both pre- 
and postaxially, Gegenbaur's theory appeals to us with much force. The appli- 
cation of the theory to tlie pelvic fin is more difficult, however, for the fin is 
assumed to have reached its present position by migrating from the branchial 
region. This assumption strikes one as far-fetched. Indeed, Dean (19026) has 
studied the problem of migration of the fins and concluded for Heterodontus, 
at least, that there is no evidence of migration. 




^/y//.. 






D 

Fig. 2.5. Diagram A-D illustrating the gill-arch theory of Gegenbaur for the origin of paired 
fins, h.r., branchial ray; g.a., gill arch. 

Further objections have been offered against the gill-arch theory. If the 
skeleton of the pectoral fin arose from the branchial rays of a single arch 
doubtless it at first occupied a dorsoventral position, a position disadvanta- 
geous to the fin as a directing organ. Moreover, if the pectoral arose as a modi- 
fied gill arch, why are there so many segments involved in the fin? Again, 
paired fins both in their development and in their structure resemble in a 
remarkable degree unpaired fins. But unpaired fins certainly have not arisen 
as modified branchial arches. Still another objection which has been urged 
against the theory is that the branchial arches lie in the walls of the digestive 
tract (pharynx) and hence within the aortic arches of the blood system. The 
pectoral girdle, on the contrary, is superficial to both the arteries and nerves 
as is shown by the fact that it is perforated hj these as they pass out to the fin 
(p. 78, fig. 84a). It is therefore difficult to see how the pectoral girdle could 
have reached its present position outside of blood vessels and nerves. 

Evidence supporting the lateral fin-fold theory has been obtained from sev- 
eral sources. It has been shown by Dean (1909) , for example, that in Cladose- 
lachiis (see p. 2, fig. 10), one of the most ancient sharks yet discovered, the 
paired fins are essentially lateral folds unconstricted at the base and with 
radial supports nearly parallel. From development also comes evidence for 
the lateral fin-fold theory. In a type like Scyllmm (fig. 24) or Prist iur us or 
Torpedo there may be in the embryo an epiblastic fold along the back and 
over the tail, and in Torpedo embryonic lateral folds may extend from the 
pelvic region behind to the pectoral region in front. Furthermore, Dohrn has 
shown that the skeleton of the fin in Pristiurus arises as a series of parallel 
radials and that the girdle at the same time arises from a fusion of the bases 



16 THE ELASMOBRANCH FISHES 

of the anterior radials. Similarly Balfour has demonstrated for Scyllium that 
the pectoral girdle arises secondarily. It may also be added that the blood sys- 
tem of the paired fins in a generalized type like Heptanchus and Hexanchus 
can most readily be interpreted in terms of a lateral fin-fold theory (see Bib- 
liography p. 195, Daniel, 1926). Other characters of musculature and nerve 
may be siid to bear on this subject. To them we shall direct further attention 
when we consider these systems. 




THE ELASMOBRANCH FISHES 17 



BIBLIOGRAPHY* 

Chapter I 

GENEEAL 

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1879. Brehm, a. C, Die Fische. Leipzig. Thierleben, Bd. 8, pp. i-xvi + 1-426, 11 Taf., 145 
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90-324. 8". 

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14-424, pis. 1-8. 
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344 text figs. 
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Selachii, pp. 17-49. 



* To each chapter is appended a working bibliography. This may be greatly extended by 
the bibliography published by Dean and Eastman, of the American Museum of Natural 
History (1916-17 and 1923, see above). 



18 THE ELASMOBRANCH FISHES 



EXTERNAL FORM 

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THE ELASMOBRANCII PISHES 19 

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Japan. Proc. U. S. Nat. Mus., Vol. 26, pp. 593-674, pis. 26-27, 10 text figs. 
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1879. Miklucho-Maclay, N. de, and Macleay, Wm., Plagiostomata of the Pacific. Pt. I. 

Fam. Heterodontidae. Proc. Linn. Soc. N. S. Wales, Vol. 3, pp. 306-334, 5 pis. 
1883. Miklucho-Maclay, N. de, and Macleay, Wm., Plagiostomata of the Pacific. Pt. II 

(Heterodontus japonicus). Proc. Linn. Soc. N. S. Wales, Vol. 8, pp. 426-431, pi. 20. 
1885. Miklucho-Maclay, N. de, and Macleay, Wm., Plagiostomata of the Pacific. Pt. III. 

Heterodontus zebra (Grey). Proc. Linn. Soc. N. S. Wales, Vol. 10, pp. 673-678, 

pis. 45-46. 
1839. Muller, J., uber den glatten Hal des Aristoteles, und fiber die Verschiedenheiten 

unter den Haifischen und Eochen in der Entwickelung des Eies. Abh. Akad. Wiss. 

Berlin, 1840, pp. 187-258. 
1838-41. MtJLLEB, .J., and Henle, J., Systematische Beschreibung der Plagiostomen. 60 Taf . 

Berlin. 4°. 
1898. Neal, H. v., The Problem of the Vertebrate Head. Jour. Comp. Neurol., Vol. 8, pp. 

153-161, 2 text figs. 
1898. Nishikaava, T., Notes on some Embryos of Chlamydoselachus anguineus Garm. 

Anno. Zool. Japonens. Vol. 2, Pt. IV, pp. 95-102, pi. 4, 7 text figs. Tokyo. 

1906. Eegan, C. T., a Classification of Selachian Fishes. Proc. Zool. Soc. Lond., 1906, Pt. 
II, pp. 722-758, text figs. nos. 115-124. 

1908. Began, C. T., A Synopsis of the Sharks of the Family Squalidae. Ann. Mag. Nat. 

Hist., Ser. 8, Vol. 2, pp. 39-57. 
1908. Eegan, C. T., A Synopsis of the Sharks of the Family Cestraciontidae. Ann. Mag. 

Nat. Hist., Ser. 8, Vol. 1, pp. 493-497. 
1911. Scammon, E. p., Normal Plates of the Development of Squalus acanthias. Normen- 

tafeln z. Entwick. der Wirbeltiere, Heft 12, 4 plates, 26 text figs. (Excellent bibli- 
ography.) 
1898. Sewertzoff, A. N., Die Metamerie des Kopfes von Torpedo. Anat. Anz., Bd. 14, 

pp. 278-282. 
1829. Smith, Andrew, Contributions to the Natural History of South Africa. Zoological 

Journal, Vol. 4, Art. LIV, pp. 443-444. 
1849. Smith, Andrew, Pisces, in Illustrations of the Zoology of South Africa. Plate XXVI 

and description. London. 
1911. Smith, H. M., Note on the Occurrence of the Whale Shark, Ehineodon typicus, in 

the Philippine Islands. Proc. Biol. Soc. Washington, Vol. 24, p. 97. 

1907. Starks, E. C, and Morris, E. L., The Marine Fishes of Southern California. Univ. 
Calif. Publ. Zool., Vol. 3, pp. 159-251, pi. 21. 

1917. Starks, E. C, The Sharks of California. Calif. Fish and Game, A^ol. 3, pp. 145-153, 
13 text figs. 



20 THE ELASMOBRANCH FISHES 

1918. Starks, E. C, The Skates and Bays of California, with an account of the Eat Fish. 

Calif. Fish and Game, Vol. 4, pp. 1-15, 18 text figs. 
1921. Stensio, E. a.: son., Triassic Fishes from Spitzbergen (Upsala). 
1867. Stoker, D. H., A History of the Fishes of Massachusetts. Mem. Amer. Acad. Art. and 

Sci. (n. s.). Vol. 9, pp. 217-256, pis. 36-39. 
1902. Waite, E. E., Studies in Australian Sharks (No. 2). Eec. Australian Mus., Vol. 4, 

pp. 175-178, 1 text fig. 
1906. Waite, E. E., Notes on Fishes from Western Australia (No. 3). Eec. Australian 

Mus., Vol. 6, pp. 55-82, 10 pis., 1 text fig. 
1882. WiJHE, J. W. VAN (see p. 118). 
1889-(95). Woodward, A. Smith, Catalogue of Fossil Fishes in the British Museum. Vols. 

I, II (and III). London. 
1908. ZiEGLER, H. E., Die phylogenetische Entstehung des Kopfes der Wirbeltiere. Jena. 

Zeitschr. Naturwiss., Bd. 43, pp. 653-684, 23 pis., 11 text figs. 



DETAILED STUDY OF TYPES 

1811. BLAi>rviLLE, H. D., Memoire sur le squale pelerin. Ann. du Mus. Paris, T. 18, pp. 88- 

135, 6 text figs. 
1899. Cunningham, E. O., On a Few Points in the Structure of Laborde's Shark (Euproto- 

micrus labordii). Proc. Zool. Soc. London, 1899, pp. 732-734. 
1907. GuDGER, E. W., A Note on the Hammerhead Shark (Sphyrna zygaena) and its Food. 

Science (n. s.). Vol. 25, pp. 1005-1006. 
1933. GuDGER, E. W., and Smith, B. G., The Natural History of the Frilled Shark, 

Chlamydoselachus anguineus. Bashford Dean Memorial Volume. Art. V, pp. 245- 

319, pis. 1-5, 31 text figs. 
1902. Helbing, H., Beitrage zur Anatomic und Systematik der Lamargiden. Anat. Anz., 

Bd. 21, pp. 658-668. 
1904. Helbing, H., Beitrage zur Anatomic und Systematik der Lamargiden. Nova Acta 

Acad. Halle, Bd. 82, pp. 335-524, 2 Taf. Also: Inaug. Diss., Halle, pp. 9-135, pi. 1, 

30 text figs. 
1876. Huxley, T. H., Contributions to Morphology. Ichthyopsida, No. 1. On Ceratodus 

forsteri, with Observations on the CHassification of Fishes. Proc. Zool. Soc. Lond., 

1876, pp. 24-59, 11 text figs. 
1898. Jordan, D. S., Description of a Species of Fish (Mitsukurina Owstoni) from Japan, 

the Type of a Distinct Family of Lamnoid Sharks. Proc. Calif. Acad. Sci., Ser. 3, 

Vol. 1, pp. 199-202, pis. 11-12. 
1909. Kellicott, W. E., The Growth of Parts in the Dogfish. Science (n. s.), Vol. 29, p. 

437. 
1847. Kneeland, S., Dissection of Seymnus brevipinna. Boston Jour. Nat. Hist., Vol. 5, 

pp. 479-485. 
1785. Monro, Alexander, The Structure and Physiology of Fishes explained and com- 
pared with those of man and other animals. Edinburgh. Many illustrations. 
1866. Owen, Eichard, On the Anatomy of Vertebrates. Vol. I. Fishes and Eeptiles. Lon- 
don, Longmans, Green & Co. 
1882. Parker, T. J., Notes on the Anatomy and Embryology of Seymnus lichia. Trans. 

and Proc. N. Z. Inst., Vol. 15, Art. XXIX, pp. 222-234, pis. 31-32. 
1884. Parker, T. J., A Course of Instruction in Zootomy. Macmillan. 
1887. Parker, T. J., Notes on Carcharodon rondeletii. Proc. Zool. Soc. Lond., 1887, pp. 27- 

40, pis. 4^8. 
1874. Pavesi, p., Contribuzione alia storia naturale del gcnere Selache. Ann. Mus. Civ. 

Storia nat. Genova, Vol. 6, pp. 5-72, tav. 1-3. Also: Extr.-Arch. sci. phys. et nat. 

Geneve, T. 51, pp. 307-324. 



THE ELASMOBRANCII FISHES 21 

1891. Platt, Julia B., A Contribution to the Morphology of the Vertebrate Head based 
on a study of Acanthias vulgaris. Jour. Morpli., Vol. 5, pp. 79-106. 

1904. PuNNETT, E. C, Merism and Sex in Spinax niger. Biometrika. Cambridge. Vol. .S, 
No. 4, pp. 313-362, pi. 1. 

1905. Rand, H. W., The Skate as a Subject for Classes in Comparative Anatomy. Amer. 
Nat., Vol. 39, pp. 365-379. 

1874. Turner, W., Additional Observations on the Anatomy of the Greenland Shark 
(Lacmargus borealis). Jour. Anat. Physiol., Vol. 8, pp. 285-290. 

1906. Woodland, W., On the Anatomy of Centrophorus calceus. Proc. Zool. Soc. London, 
1906 (2), pp. 865-886, pis. 57-62, 12 text figs. 



FINS AND THEIR ORIGIN 

1878. Balfour (see p. 17). 

1887. Baur, G., Morphology and Origin of the Ichthyopterigia. Amer. Nat., Vol. 21, pp. 

837-840. 
1906. Braus, H., Die Entwickelung der Form der Extremitaten und des Extremitaten- 

skelets. Hert^vag's Handb. vergl. u. expt. Entwick. Bd. 3, Teil 2, pp. 167-235. 

1879. Davidoff, M., BeitrJige zur vergleichenden Anatomie der hinteren Gliedmassen der 
Fische. Morph. Jahrb., Bd. 5, pp. 450-520, pis. 28-31, 1 text tig. 

1896. Dean, B., Sharks as Ancestral Fishes. Nat. Sci., Vol. 8, pp. 245-253, 4 text figs. 
1896. Dean, B., The Fin-Fold Origin of the Paired Limbs, in the light of the Ptychop- 

terygia of Palaeozoic Sharks. Anat. Anz., Vol. 11, pp. 673-679, 8 text figs. 
1902. Dean, B., Historical Evidence as to the Origin of the Paired Limbs of Vertebrates. 

Amer. Nat., Vol. 36, pp. 767-776, 1 text fig. 
1902&. Dean, B., Biometric Evidence in the Problem of the Paired Limbs of the Vertebrates. 

Anier. Nat., Vol. 36, pp. 837-847, 1 text fig. 
1884. DoaRN, A., Studien zur Urgeschichte des Wirbelthierkorpers. IV. Die Entwicklung 

und Differenzierung der Kiemenbogen der Selachier. Mitt. Zool. Stat. Neapel, Bd. 

5, pp. 102-195, pis. 5-11. 
1865. Gegenbaur, C, Die Brust-Flossen der Fische. Untersuch. z. verg. Anat. d. Wirbelth., 

Heft 11. 
1906. Goodrich, E. S., Notes on the Development, Structure, and Origin of the Median and 

Paired Fins of Fish. Quart. Jour. Micr. Sci., Vol. 50 (n. s.), pp. 333-376, pis. 10-14, 

text figs. 1-3. London. 
1911. Gudger, E. W., The Finned-Tailed Larva of the Butterfly Ray, Pteroplatea maclura. 

Jour. Elisha Mitchell Scient. Soc, Chapel Hill, N. C, Vol. 27, p. 84. 
1889. Hatschek, Die paarigen Extremitaten der Wirl^eltiere. Anat. Anz., Bd. 4 (Verh. 

Berlin), pp. 82-90, 4 text figs. 
1860. Kner, Rudolph, trber den Flossenbau der Fische. Sitzber. der math.-naturwiss. 

Klasse, Vol. 41, pp. 807-824. 

1893. Marey, M., Des mouvements de natation de la raie. C. R. Acad. Sci. Paris, Vol. 116, 
p. 77-81, 2 text figs. 

1886. Mater, Paul, Die unpaaren Flossen der Selachier. Mitt. Zool. Stat. Neapel, Bd. 6, 

pp. 217-285, pis. 15-19. 
1879. MrvART (see p. 86). 
1892. MoLLiER, S., Zur Entwickelung der Selachier Extremitaten. Anat. Anz., Bd. 7, pp. 

351-365. 

1894. I. MoLLiER, S., Das Ichthyopterygium. Anat. Hefte, Bd. 3, pp. 1-160, pis. 1-8, 12 
text figs. 

1895. IL MoLLiER, S., Das Cheiropterygium. Anat. Hefte, Bd. 5, pp. 433-530, pis. 31-38, 
7 text figs. 

1909. MuLLER, Erik, Die Brustflosse der Selachier. Ein Beitrag zu den Extremitaten- 
theorien. Anat. Hefte, Bd. 39 (Heft 118), pp. 471-601, pis. 27-46, 62 text figs. 



22 THE ELASMOBRANCH FISHES 

1907. OSBURN, E. C, New Evidence from Primitive Sharks on the Origin of the Limbs of 

Vertebrates. Science (n. s.), Vol. 25, pp. 729-730. 
1907. OsBURN, R. C, Observations on the Origin of the Paired Limbs of Vertebrates. 

Amer. Jour. Anat., Vol. 7, pp. 171-194, pis. 1-5. 
1907. OsBURN, R. C, The Origin of Vertebrate Limbs. Recent Evidence upon this Problem 

from Studies on Primitive Sharks. Ann. N. Y. Acad. Sci., Vol. 17, Pt. 2, pp. 415-436. 
1906. Rennie, John, Accessory Fins in Raia batis. Anat. Anz., Bd. 28, pp. 428-431, 2 

text figs. 
1877. Thacher (see p. 87). 
1892. Woodward, A. S., The Evolution of Fins. Nat. Sci., Vol. 1, pp. 28-35, 8 text figs. 



II 



INTEGUMENT 

INTEGUMENT OF HEPTANCHUS MACULATUS 

One of the most characteristic structures observed in a study of the external 
form is the integument, in which is located the pigment pattern and from 
which project the shagreen denticles. The two layers of the integument of a 
young Heptanchus are shown in figure 26. The epidermis (e.) is relatively 
thin and the dermis or corium {cr.) is considerably thicker. 

The epidermis when examined in detail shows superimposed strata of cells, 
the innermost layer of which, the basal or ger- 
minative layer {gr.), is of a columnar nature; 
while the outlying strata in which the cells are 
horizontal may be designated as the superficial 
layers of the epidermis. Between these two ex- 
tremes are numerous intermediate cells which 
are irregularly stratified. These cells arise in 
development from the basal layer and pass out- 
ward toward the surface, becoming more flat- 
tened as they move outward. 

At practically any level of the epidermis 
from the deep basal layer outward to the sur- 
face may be found large beaker or goblet cells 
{fj.c. ) of a glandular nature. Each of these cells 
arises as a modified cell divided off from a basal 
layer cell and comes to be essentially a large sac 
with the nucleus located near its base. 

The dermis or corium (cr.) is composed of 
two more or less well defined layers in Heptan- 
chus. The first or more superficial of these is 
made up of a dense mass of irregularly ar- 
ranged cells; the second or deeper layer is com- 
posed of sparser cells which have protruding from them longer or shorter 
processes, which interlace into a network of supporting tissue. 

In the upper layer of the corium and between it and the epidermis are lo- 
cated the pigment masses which are the causes of the color pattern. In the 
adult the pigment cells are black or greyish and are distributed more or less 
evenly over the back and sides, giving to the upper areas their drab hue. Over 
the drab background are scattered the irregular blotches of black which are 
produced by more deeply lying pigment. In the patches the cells are packed 
so closely together that it is impossible to pick them out individually. Pig- 
ment is absent from the integument on the ventral side, which is of a whitish, 
metallic color. 




Fig. 26. Section through the 
buccal integument, Heptanchus 
maeulatus. 

h.m., basal membrane; cr., 
corium or dermis; e., epidermis ; 
g.c, goblet or gland cell; gr., 
basal or germinative layer. 



[23] 



24 



THE ELASMOBRANCH FISHES 



The scales present in the integument of a shark like Heptanchiis provide a 
relatively compact exoskeleton which, in a general way, serves to protect the 
organism against external injury. In form the scales vary considerably de- 
pending upon their position. The first of these, 
represented by figure 27a, were taken from the 
side of the body just above the lateral line. They 
are therefore from a region which is not unduly 
exposed and which may be characterized as 
typical. By looking from the lower left- to the 
upper right-hand corner it will be observed 
that the scales are arranged more or less in 
oblique rows. The same is true, although less 
evident, if they be observed from the lower 
right- to the upper left-hand corner of the 
figure. If a larger piece of the integument be 
drawn and lines be ruled through the rows of 
scales, the lines will form a series of rhomboids, 
producing the diamond-shaped pattern. In such 
a pattern each scale normally points backward 
and is attached anteriorly and centrally. 

The details of a single scale have been figured 
by Steinhard (1903) for Heptanchus cinereus 
(fig. 28). Here an anterior arm {a.a.) is well 
im1)edded in the integument and a posterior 
spine {sp}) projects more or less sharply up- 
ward and backward. Two lateral arms appear 
(La.) which, as a rule, curve more or less up- 
ward and terminate in lateral spines (sp.-). 
Passing along the middorsal line from the an- 
terior arm to the tip of the posterior spine is the 
primary or axial crest (cr.^). Located on the 
lateral arms and branching from the primary 
crests are the lateral crests (cr.-). Other terti- 
ary crests (cr.^) connect primary and secondary 
crests, and a fourth {cr^) arises from the sec- 
ondary crests to pass laterally around the sides 
of the scale. Lying deeply buried in the integu- 
ment is the enlarged supporting base (ba.) 
which, in general, is rhomboidal in shape and is 
attached by a narrow neck or pedicel to the 
main body of the scale. Into the lower substance 
of the base enter strong fibers of connective 
tissue which anchor the scale to the integument. 
Where the base may he broad, as in the scales over any of the exposed areas, 
many such supporting fibers may be present. 




Fig. 27. Scale patterns, Hep- 
tanchus macidatus. A. From 
side of body. B. Modified 
scales from margin of fin. 
C. Stomodeal denticles from 
roof of mouth. 



THE ELASMOBRANCH FISHES 



25 



Modified Scales 



Scales located in a region that is more exposed may differ greatly from the 
typical ones which we have just described. The several areas in which the 
scales may become modified in Heptanchus are along the dorsal margin of the 
back and especially in front of the dorsal fin, 
around and in front of the eye, over the tip of the 
nose and the tip of the lower jaw; in fact, over 
any of the surfaces which are particularly ex- 
posed as the fish swims forward. An extreme ex- 
ample of modification is shown in figure 27b, the 
pattern of which was taken from the anterior mar- 
gin of the pectoral fin. The crests and even the 
spines of the scales previously studied have here 
disappeared. In fact the entire form has become 
so greatly modified as to be hardly recognizable. 
Instead of having the regular spade-shape they 
have become enlarged nodules, the anterior and 
posterior parts of which can be distinguished only 
with difficulty. Not infrequently such a scale in 
this exposed area reaches a size many times that 
of a protected scale. 

A third type may be described briefly. Covering 
the lining of the buccal cavity and the pharyngeal 
box, excepting the posterior part of the roof, there 
are numerous modified scales which are designated 
as stomodeal denticles (fig. 27c). These denticles 
resemble much more closely the normal scales previously described than they 
do those secondarily modified. Indeed, the stomodeal denticles, occupying a 
position which in the young is more protected than the normal type of scale, 
are sharply differentiated into parts. This is especially noticeable in their 
prominent lateral spines; in some even a second pair of laterals is present, 
making five spines in all. It will be observed that the stomodeal denticles 
closely resemble the lower teeth (see p. 122, fig. 121) . 




Fig. 28. Detail of plaeoid 
scale, Hepfanclms cinereus. 
(From Steinhard.) 

a.a., anterior arm ; ha., base 
of scale; cr.^, primary crest; 
cr.^, lateral crest; cr.', tertiary 
crest ; cr.*, fourth crest ; La., 
lateral arm; sp.\ posterior 
spine; sp."^, lateral spine. 



26 



THE ELASMOBRANCH FISHES 



INTEGUMENT OF ELASMOBRANCHS IN GENERAL 




^^"^^^^^J^^a;,^" elf 



The description given for the layers of the integument in Heptanchus may 
serve to portray conditions which are more or less general. There is, however, 
in the different Elasmobranchs considerable variation in the thickness of the 
integument, affecting one or both of the layers. In a type like Scymnus, for 
example, the upper or epidermal layer is thinner than that of Heptanchus, 
yet the corium of Heptanchus is relatively thin, so that its integument as a 

whole is of medium thickness. The 
thickening of the skin may be due 
to the addition of many cells to the 
corium as, for example, in Het- 
erodontus, or it may be brought 
about as in Triakis ( fig. 29 ) , where 
the dermal cells, particularly in 
the superficial layer, are much 
less compact. 

Color in the Elasmobranchs 
while varying considerably usu- 
ally assumes a quiet hue, the bril- 
liant reds or blues characteristic 
of the bony fishes rarely being 
present. The color may be a light 
drab as in Mustelus calif ornicus, 
a deep blue as in the lamnoids, 
brown as in Torpedo {Tetronarce 
calif ornicus), or black as in Tor- 
pedo occidentalis. It may be scat- 
tered over dorsum and venter 
alike as in the deep-sea forms, 
Spinax of the Mediterranean and Etmopterus of Formosan waters; or it may 
be confined largely to the dorsum as in Oaleus, Acanthias, and a host of other 
types; again, it may be collected in a remarkable pattern over a drab back- 
ground as is characteristic of Cheiloscyllium and the Leopard shark, of Tigri- 
num and Rhinodon typicus. Whether the color be light or dark or variegated 
it is produced by chromatophores, several types of which are present. 

The melanophore containing black or brown melanin is usually considered 
to be a connective tissue type of celP rhizopod in shape, and sometimes capable 
of amoeboid movement. Some of these cells in Heterodontus (pg., fig. 30) 
though typically rhizopod are capable of but slight change of form. Other 
melanophores, smaller in size, may have numerous processes which in extreme 

1 There is evidence for the belief, however, that this cell is derived from a smooth 
muscle cell. 




U"^ "^ 



Fig. 29. Section of the integument of the 
Leopard shark, Triakis semifasciatus. 

b.m., basal membrane ; d.p., dermal papilla ; 
€.0., enamel organ; g.c, goblet or gland cell; 
gr., basal or germinative layer of epidermis; 
ll.c, lateral line canal; n., ramulus of vagus 
nerve; pg., pigment; s.c, sensory column. 



THE ELASMOBRANCH FISHES 



27 



occurrences exceed in lengtli many times the diameter of the body of the cell. 
From the possession of great numbers of these processes expanded into a com- 
plex web, the extreme black of a form like Torpedo occidentalis results. 

The golden cells or lipophores (Ip., fig. 30) (Xanthophores) containing the 
fatty pigment, lipochrome, are beautifully shown in the young of Heterodon- 
tiis. Here, with the brown melanophores, they give patches of a warm glossy 
yellow of remarkable ])eauty. In Cephaloscyllium (fig. 1) also, multitudes of 
lipophores produce the sulphur spots so characteristic of this form. In Rhino- 
don (fig. 3) the lipophores associated with reddish brown melanophores form 
the great orange spots or color patches. 

In most of the Elasmobranchs, ex- 
cepting the deep-sea types, pigment 
cells are absent from the venter. The 
metallic white here results from the 
presence of guanin, a waste product of 
metabolism, which impregnates the 
cells (leucophores) ventrally as do the 
pigment granules dorsally. The guanin 
granules although present are not visi- 
ble dorsally, for in this location they 
are obscured by the melanin granules. 
Ventrally they are very numerous, and 
have much to do with the production 
of the light color. Contributing also to 
the formation of a light-colored venter 
is a certain concentration of tissues 
known as argentium. In this concentra- 
tion the underlying tissues, through the deposition of calcic prisms, become 
so compact as to form a highly reflecting surface to which the silvery sheen 
characteristic of a fresh specimen is partly due. 

The function of pigment has often been thought to be the protection of the 
more delicate underlying tissues against the rays of the sun. While this per- 
haps holds in general, such protection from the rays of the sun would not be 
necessary for those forms like Spinax or Etmopteriis which inhabit the pro- 
found depths into which the light of the sun never penetrates. And it is the 
more singular that in such deep-sea Elasmobranchs pigmentation is not con- 
fined to the dorsum but is distributed over the ventral part of the body as well. 
It has been suggested that at depths at which the rays of the sun are unknown 
it may still be that pigmentation is in some way correlated with light; for per- 
vading even the greatest depths there is present phosphorescent light, the 
source of which being diffuse would not result in pigment on the back alone 
but on the sides and venter as well. But the cause of pigmentation in deep-sea 
sharks is as yet not understood. It seems not improbable that the pigmentation 
is correlated not so much with light as with the lower temperature. 




Fig. 30. Chroniatophores of young Hetero- 
donfiis francisci. 

Ip., golden cells containing granules ; 
pg., melanophore filled with brown pig- 
ment. 



28 



THE ELASMOBRANCH FISHES 



Gland Cells 



The integumentary beaker, or gland cell, as such (g.c, fig. 29) is produced 
entirely from the epidermis. In sections taken through the body of the em- 
bryo at different levels it is observed that these cells are distributed over the 
entire surface, with but few exceptions, like the cornea of the eye. They may 
likewise be found in the integument lining the buccal cavity (g.c, fig. 26) and 
the cloaca, these cavities being formed as invaginations from the surface. The 
first indication of the origin of such a gland cell is seen in the enlargement of 

a cell derivative of the basal layer. 
This cell migrates through the inter- 
mediate layers to the surface as do 
the superficial cells, but instead of 
l)econiing flattened, as do the super- 
ficial cells, it becomes vesicular and 
reaches an enormous size. When it 
gains the surface a lumen forms, 
through which the gland pours out 
its product of excretion. 

Glaxds of Claspers 

In addition to the gland cells of 
the type just described there are in 
males of the Elasmobranchs numer- 
ous gland cells which are derived 
from the epidermis and which sink 
in at the base of the claspers. Such a 
gland in Sqiiatina if examined under 
the microscope is seen to be essentially a series of enlarged goblet cells. In 
other forms as has been shown by Leigh-Sharpe (1920-21) the gland may fill 
up the siphon (Lamna) or it may be in only a part of its wall. In Raia circu- 
laris (fig. 31) the gland has been compared to a date stone, bilobed in appear- 
ance with a longitudinal groove running its entire length. Into the groove the 
papillae open to drain the different components of the gland. 




Fig. 31. Transverse section through siphon 
sac (s.s.), Haia circularis, to show clasper 
gland. (From Leigh-Sharpe.) 

e.g., clasper gland; cm., circular muscle; 
l.m., longitudinal muscle ; p., papilla from 
gland ; s.w., siphon wall. 



Poison Glands of Sting Ray 



Goblet or mucous cells which in the sting ray, Urolophus, are present in great 
numbers at the root of and just under the sting, form what some believe to be 
a poison gland (p.gl., fig. 32). It is evident that these would secrete an abun- 
dant supply of mucus which might pass along the ventral groove into the 
wound made by the sting. It is doubtful, however, that this mucus is more 
toxic than is the acrid mucus of other glands. 



THE ELASMOBRANCH FISHES 



29 



Light Organs 




Fig. 32. Transverse section of sting, TJrolophns 
halleri. (A. M. Paden, orig.) 

e., enamel of sting; e.o., enamel organ; d., den- 
tine of sting; j).gl., so-called poison gland. 



In deep-sea fishes, in general, gland cells have contributed to a most remark- 
able specialization : that is, the}- have become converted into light organs or 
photophores. Such organs have been found in various Selachians, principal 
among which are Spinax niger, Laemargus, and Etmopierus. In Etmopterus, 
Oshima (1911) has made out two types of light organs: (1) punctate, cup- 
shaped organs which in the living specimen have a pearly luster; and (2) 
linear, semicylindrical organs which apparently result from a fusion of two 
or more punctate organs. In Et- 
mopterus these light organs are 
located in definite patterns, prin- 
cipally along the sides and ven- 
trally, but they are jiresent also 
on the dorsum. 

Johann (1899) gives a section 
through a luminous organ of 
Spinax (fig. 33a) which shows 
that it is formed as a modification 
of cells in the germinative or 
basal laj^er of the epidermis (gr. ) . 
These cells enlarge and, as a cup, 
sink slightly into the corium. Two 
types of cells are present, one, the lens cell (I.e.), the other, the light or photo- 
genic cell (It.c.) . 

The lens cells are few in number and are located toward the surface where 
they appear as enlarged mucous cells. In their beginning they arise from the 
basal layer and migrate outward, becoming large and granular. Upon coming 
in contact with the surface they pour out some or all of their contents and 
become the lens cells of the adult type. The light or photogenic cells {It.c. ) con- 
sist of a few cells (Sphiax) or a number of them {Etmopterus) which occupy 
an irregular position at the base of the cup. 

Under the basal membrane which supports the cup are blood sinuses {h.s.). 
Usually indenting the walls of these sinuses are masses of enclosing pigment. 
Figure 33b of Etmopterus shows the arrangement of pigment not given in 
figure 33a. This heavy band of pigment lines the bowl of the cup and exten- 
sions pass inward and practically cover the cup as the so-called iris {ir.). 

There is doubt as to how the organ thus described functions. It is possible, 
however, as in some of the bony fishes, that these basal cells, which are essen- 
tially mucous cells, form a luminous secretion, the oxidation of which produces 
the light. That the organ is efi'ective in the production of light has been ob- 
served through the study of living specimens. Thus in Spinax, Dr. Theodor 
Beer has observed a strong phosphorescent light given off along the side and 
ventral region. The light was of variable intensity, glowing for a time and then 



30 



THE ELASMOBRANCH FISHES 



decreasing in brilliancy. Oshima has also studied the luminosity, and has 
noted that in Etmopterus the light was never produced spontaneously but was 
emitted regularly upon the application of mechanical stimulation. Whether or 
not the pigmented iris through contraction and expansion regulates the light 
thus produced has not been sufficiently studied. 

Placoid Scales 

In figure 29, showing the layers of epidermis and corium, is also shown a devel- 
oping placoid scale. The first indication of such a scale is the collection of a 
group of cells in the upper layer of the corium to form a dermal papilla (d.i).). 




..I.e. 




Fig. 33. A. Section through a light organ or photophore, Spinax niger. (From Johann.) 
B. Pigmentation of a photophore, Etmopterus. (From Oshima.) 
i.s., blood sinus; gr., basal or germinative layer of epidermis; ir., iris; I.e., lens cell; 
U.C., light cell. 

As the papilla grows upward it raises the basal layer of the epidermis, making 
of it a cap, the enamel organ (e.o.). By continuous growth the cells of the 
enamel organ assume the high cubical type with their nuclei located toward 
the outer margin. 

The cells of the enamel organ form a layer of enamel over the tip of the 
papilla; while the odontoblasts of the dermal papilla which lie most superfi- 
ciall}^ are the first to lay down dentine. The first layer covers the tip and sides 
of the papilla lying immediately under the thin layer of enamel. Then the 
odontoblasts which are located deeper send out processes around which den- 
tine is deposited. The canals thus formed for the processes themselves are the 
beginning of the dentinal canals and into them the protoplasmic processes of 
those odontoblasts lying still deeper will later enter as the formation of the 
dentine continues. Thus it is that the dentine produced from without inward 
becomes thickened, finally crowding the core of the papilla into narrow 
compass. 

When a scale like the one described above reaches the surface the epidermal 
layers are rubbed off from the tip, and the body of the scale then erupts. In a 
more mature embryo than the one here described many such scales erupt at 
about the same time and come to take up a definite arrangement in patterns in 



THE ELASMOBRANCII FISHES 



31 









Fig. 34. Scale patterns of Elasmobranchs. K. Mustelus calif amicus. (H. M. Gilkey, del.) 
B. Squaliolus. (From Smith.) C. Anchor scale. D. Stomodeal denticles. E. Flattened transi- 
tional scales. F. Nodules from fin margin. (C-F, Heterodontus francisci. Duncan Dunning, 
del.) 



32 



THE ELASMOBRANCH FISHES 



general like that of Heptanchus. In some, however, a pattern exists only over 
limited areas. In others the scales may be confined to rows along the back, or, 
in addition to this arrangement, they may be scattered more or less pro- 
miscuously over the body, as they are in some of the rays. In other rays integu- 
mentary scales may be entirely lacking (Myliohatis, Trygon). 

The individual placoid scale may bear but a single spine as in Must el us 
calif ornicus (fig. 34a), CarcJiarias, Pristiophorus. Or it may be tricuspate, 
multitudes of scales covering the surface as in Heptafichus (fig. 27a), Scyl- 
lium, Zygaena, Pristiurus, and Pentanchus. The scales may be found in geo- 
metrical exactness and beauty as in Sqnaliolus (fig. 34b) , or they may present 
various designs from a simple spade-shape to the anchor scale (fig. 34c) or to 
the complex Greek cross of Heterodontns. The gross anatomy of the type 
described for HeptancJiiis may be taken as an example of more or less gen- 
eralization. 

FINEE ANATOMY OF SCALE 

A section through a placoid scale of Scymnus (fig. 35) illustrates the finer 
structure characteristic of the Elasmobranch scale. In such a section the crest 
surmounting the main body continues backward to the spine. The base is rela- 

atively large and a pedicel or 
neck connecting it with the 
body of the scale is practically 
lacking. The base is fixed to the 
corium by connective tissue 
and is perforated by a central 
canal (c.c.) which leads into a 
large median pulp cavity 
ip.c.) An anterior and a pos- 
terior canal lead off from the 
pulp cavity; and from the an- 
terior and posterior canals, as 
well as from the pulp cavity 
itself, smaller dentinal canals 
(d.c.) extend into the dentine. The enamel (e.) surrounding the exposed part 
and sinking slightly into the integument is much better developed anteriorly 
than it is posteriorly. 

Considerable doubt has been raised as to whether or not the so-called enamel 
of the placoid scale and of the tooth of the Elasmobranchs is in fact compara- 
ble to the enamel of the teeth of higher forms. The studies thus far made show 
that the enamel formed in the Elasmobranch fishes presents a variety of types. 
In the rays it appears to be in all essential respects true enamel ; in some of the 
sharks the evidence is not so clear. That the dentinal tubules in a type like 
Galeus may be traced far into the outer layer shows that there is here no clear 
demarcation between dentine and enamel. For the present, then, we may 
think of the harder outer layer of the scale in some of the Elasmobranchs as 
composed of a substance just beginning to differentiate into true enamel. 




Fig. 35. Sagittal section showing finer anatomy of 
placoid scale, Scymnus lichia. (From O. Hertwig.) 

c.c, central canal; f/.c, dentinal canals; e., enamel; 
p.c, pulp cavity. 



THE ELASMOBRANCII FISHES 



:33 



MODIFICATION OF SCALES 




By a peculiar liypertropliy modifications may arise wliich, altliough essen- 
tially like the primitive shagreen denticles in strnctnre, greatly differ from 
them in form. Such hypertrophy may result in the production of a fin spine 
like that in Heterodontus (fig. 88) and the Spinacidae; a tooth like that in the 
sawfishes, Pristis (fig. 38) and Pristiophorus; a sting like that in the sting rays 
(fig. 42) ; or it may result in other variously modified structures, such for ex- 
ample as the branchial rakers in Cetorhmus (fig. 44) and 
Ellin don. 

Fin Spine 

In some of the types in which fin spines are present they 
are so rudimentary as to be but little larger than enlarged 
scales, as exemplified in Cenfroscymnus. In others, as in 
Heterodontus (fig. 88) and Acanthias, they are pronounced 
structures. In general, they are located just anterior to the 
dorsal fins, the posterior one (fig. 36) being longer than the 
anterior. For almost half its length the spine is buried in 
the integument. The buried part is designated as the root 
or base and the exposed portion the crown or spine proper. 

If such a structure be removed and more closely studied, 
its deeply imbedded base is seen to be triangular in shape. 
The spine contains a large central cavity which when in 
place fits over a cartilage of the fin skeleton. The walls of 
the spine are made of dentine which in the crown consists 
of a double layer. The more superficial layer is bounded 
anteriorly and laterally by a layer of enamel, but enamel 
does not extend over the posterior groove which fits close 
up against the basal cartilage of the fin skeleton. A more or less compact laj^er 
of pigment {pg., fig. 40) separates the enamel (e.) in front from the layer of 
dentine (d.). 

The development of such a fin spine is of considerable interest. Figure 37a 
represents a sagittal section of an early stage of Acanthias in which a mass of 
the epidermis (ep.) has sunk into the dermis, just in front of the dorsal fin. 
The bounding layer of this section becomes the enamel organ (e.o.). The for- 
mation of the enamel of the spine, and of a part of the dentine, is singularly 
modified by the peculiar position of this organ. It will be observed that the 
enamel organ here covers only the anterior upper part and sides of the de- 
veloping spine, instead of forming a cap over the entire structure as it does in 
a placoid scale. As a result enamel is present only on the front and sides of the 
crown, little being produced posteriorly {e., fig. 40). The odontoblasts {od., 
fig. 37a) just under the enamel organ lay down dentine so that these two layers 
so far as they go are like the enamel and dentine of a common placoid scale, 
but tlie greater mass of dentine is formed back of tliis dentine. 



Fig. 36. Second fin 

spine, Squahts 

sucMii. 



34 



THE ELASMOBRANCH FISHES 



Marker! (1896) says that in the formation of this secondary dentine certain 
long fibers {fs., fig. 37a) grow down in a sheath posterior to the core of carti- 
lage (ct.). These fibers penetrate deeply and near the base spread out, curve 
forward, and fuse anteriorly into a closed ring. Now this posterior sheath of 
fibers runs through the odontoblasts in such a way as to leave some of them in 







^s 











A. Young stage 



B. Older stage 
Fig. 37. Development of fin spine, Acanthias. (From Markert.) 

ct., core of cartilage; d., primary dentine; d.', secondary dentine; e., enamel; e.o., 
enamel organ; ep., epidermis; fs., connective tissue fibers; od., odontoblast; pg., pigment. 

front of it and others behind it. Both of these sets of odontoblasts lay down 
dentine so that the fibers (fs.) come to lie between two layers of dentine. 
Finally the fibers themselves also give place to dentine. Hence a transverse 
section of this secondary dentine, taken at the base of the spine where the 
sheath of fibers is closed into a ring, shows a heavy circle of secondary dentine 
like the broad band around the core of cartilage {d\ fig. 40) ; a similar trans- 
verse section taken toward the tip would indicate the layer of dentine as a 
crescent just back of the central core. 



THE ELASMOBRANCII FISHES 



35 



Saw Tooth 



A secondary form of hypertrophy is seen in the saw tooth of the sawfishes. 
Here the teeth are arranged along both edges of the saw (rostrum) as greatly 
modified scales. In the specimen from which figure 38 was taken the teeth 
were asymmetrical, 26 teeth being i)resent on the left edge of the saw and 27 
on the right. In the larger saws, some of which may reach a foot in width and 
six feet in length, the crown of the tooth may reach four inches in length and 
the teeth become most formidable organs of offense. 

A sagittal section through an adult saw tooth according to 
Engel (1910) shows that the core of the tooth, unlike that of the 
spine, becomes converted into long columns of vasodentine, and 
a transverse section through this dentine near the tip also dem- 
onstrates numerous canals around which the dentine is formed 
and through which l^lood vessels pass (see section through tooth, 
p. 131, fig. 129). 

A saw tooth erupting from the side of the rostrum (fig. 41) 
presents something of the appearance of a developing fin spine 
of Acanihias, but with one difference. The saw tooth arises di- 
rectl}^ through the mass of invaginated epidermis {ep.) and 
hence the posterior part is more completely surrounded by an 
enamel organ {e.o.). It follows that there is a layer of enamel 
over that part of the saw tooth just as there is over the placoid 
scale. 

A transverse section through the saw tooth near its tip (fig. 
39) shows that, like the fin spine of Acanthias, it is more or less 
flattened posteriorly. The central part of the immature tooth is 
occupied by the enlarged pulp cavity containing blood vessels 
and numerous odontoblasts. Just outside of the pulp cavity is 
the layer of dentine {d.'), superficial to which is the thinner 
band of enamel or vitrodentine (e.). The tissues outside of the enamel organ 
{e.o., fig. 41) do not take part in the production of the saw tooth. They are to 
be looked upon as the intermediate and superficial layers of the epidermis. 



Fig. 38. 

Saw tooth 

of Pristis 

antiquorum. 



Sting 



A third form of hypertrophy occurs as the variously formed defensive organs 
of the sting rays (fig. 42) . Here a spine arising from the dorsal part of the tail 
grows backward, varying greatly in size and in complexity. In the small sting 
ray Urolophus this spine is but two inches in length; while in the larger ty]ies. 
as in Myliohatis, it may reach a length of four or five inches. Usually the sting 
or spine is simple, but it may be compound, numerous spines arising one be- 
hind the other. Simple or compound, each spine is provided with a sharp 
point, and its sides have smaller and recurved hooks arising from them. 






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THE ELASMOBRAN(!II FISHES 



37 



A transverse section tliroiig'h the sting of Vrolophus (p. 29, fig. 32) sliows 

tliat it is convex dorsally. The ventral side consists of right and left plane 

surfaces separated by a median ventral ridge. Both the plane surface and the 

convex dorsal sides are covered with a thin layer of enamel (e.) under which 

is the thick dentine {d.). But the dentine here is arranged differ- 

A ently from that of the placoid scale. 

I I Figure 43 is drawn from the dorsal side of a sting and represents 

it as a transparent object. In it may be observed numerous longi- 
tudinal and anastomosing canals (r.) surrounded by heavy dentine 
(f/.). This arrangement of canals is much like that which we have 
observed in the saw tooth of Prist is. 

The sting is clearly a protective structure. In those rays in which 
it is attached nearer the body, as in Myliobatis, it is more effective 
than in types like Dasyafis, in which it is located farther out on the 
tail. In either type, the sting is brought into action by thrusting the 
tail upward and forward over the back. In the thrust the weapon is 
driven forward with precision and force, and is removed with great 
difficulty from the wound, the recurved hooks (sp.) forming a 
most painful tearing surface unless the sting can be pushed en- 
tirely through. 

Gill Rakers 



lii 



Fig. 42. 
Sting of 
sting rav. 



In some forms, structures 
internal branchial arches 
rakers, such, for ex- 
ample, as are seen in 
Squalus sucklii (p. 154, 
fig. 147, gr.). These gill 
rakers evidently serve 
as .strainers to prevent food from 
passing out with the respiratory 
current. Structures somewhat like 
these in function but very unlike 
them in form have become remark- 
ably specialized in Cf^or/rnnrs »(fl,r('- 
nius (fig. 44 ) and in Rhinodon typi- 
cus, in which they form a highly 
complex straining apparatus. Each 
raker in Ceiovliinus arises from a 
semilunar base and extends as a 
long slender filament across the in- 
ternal branchial aperture (figs. 44 
and 148). 

It has long been known that one of 
structurally like a placoid scale. In a 



located on the pharyngeal walls of the 
have undergone modification into gill 




Fig. 43. Segment of sting, Vrolophus halleri, 
seen as transparent object. Dorsal view. (A. 
M. Paden, orig.) 

c, canals of sting; d., dentine; sp., recurved 
spine on sting. 

these filaments with its adjoined base is 
section through the base numerous den- 



38 



THE ELASMOBRANCH FISHES 



tinal canals appear which are essentially like those of the scale. Superficially 
the filament is surrounded by a more compact layer, but structurally it is 
similar to the base. The central canal of the filament is surrounded by a "non- 
vascular dentine" in which there is a network of dentinal canals. 

Stomodeal Denticles 

The stomodeal denticles are also modified scales, although it may 
be more correct to speak of them as atrophied rather than hyper- 
trophied. These denticles may be abundant over the larger part 
of the buccal and pharyngeal walls as well as over the branchial 
arches {Heterodontus, fig. 34d), or they may be present in the 
buccal cavity and restricted to the hyoid and first branchial 
arches (Squatina), or they may be confined to the pharyngeal 
margins of the branchial arches (Alopias vulpes). In certain 
types they are rudimentary (Squalus sucklii) and in others they 
have ceased to be developed altogether {Scyllium canicula). 

It has been suggested that the denticles may serve to hold and 
to a slight extent to grind food, but it seems more probable that 
they are structures which, l)ecause of their location and origin, 
are without pronounced function, and hence are usually ves- 
tigial (see Chapter V). This is indicated by the fact, as Imms 
(1905) has suggested, that whether the food be hard or soft they 
develop equally well (as in Galeus, and Mustelus). Further- 
more, like vestigial structures they often appear much later 
than do the scales of the body {Carcharias). 

Teeth as ^Modified Scales 

The most important structures, however, with which placoid 
scales are associated are the teeth. In a type like Accndhias, in 
which these are sharp pointed, superficial resemblance between 
the two is more or less pronounced. In a continuous section cut- 
ting through both the scales and the teeth it has been observed 
that the scales outside of the buccal region have their spines pro- 
jecting backward, while those within the mouth may have their 
spines in a reverse direction, and still be pointing backward. 
Some of the transitionals lietween the two regions, however, 
possess both an anterior and a posterior spine, and hence are so 
generalized that the retention of the latter projection would re- 
sult in a scale, its loss would result in the formation of a tooth. 

In other Elasmobranchs, little outward similarity between 
tooth and scale is seen. Such is in part true of the immense tooth 
of Carcharodon (see p. 129, fig. 127); especially is it true of 
manj' of the plate-like crushing or pavement teeth like those of Myliobatis 
(p. 128, fig. 126b). Invariably, however, the two are essentially identical in 
nature. For a further consideration of the teeth, see Chapter V. 




.Fig. 44. 

Gill raker of 

Cetorhinus 

maximus. 

(From 

Hendricks.) 



THE ELASMOBRANCH FISHES 39 

BIBLIOGRAPHY 
Chapter II 

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1868. Hannover, A., Recherches sur la structure et le developpement des ecailles et des 
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1874. Hertwig, Oscar, Ueber Ban und Entwickelung der Placoidschuppen und der Zahne 
der Selachier. Jena. Zeitschr. Naturwiss., Bd. 8 (N. F. 1), pp. 331-404, Taf. 12-13. 

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40 THE ELASMOBRANCH FISHES 

1897. Jentsch, B., Beitrag zur Entwicklung unci Struktur der Selaeliierzalme. Diss. Leip- 
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1899. JOHANN, L., uber eigeiithiimliche epitheliale Gebilde (Leuchtorgane) bei Spinax ni- 
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der Eochen und Haie (Leipzig), pp. 1-127, pis. 1-4. 
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297-301. 
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Zool. Anz., Bd. 8, pp. 50-51. 
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morata. Zool. Anz., Vol. 8, pp. 388-389. 
1874. LuTKEN, Chr., Sur les differences dans la dentition que presentent, selon le sexe, les 

Eaies (Eaja) qui habitent les cotes du Danemark. Jour. Zool. (Gervais), T. 3, pp. 318- 

321. Eevue: Arch. d. Zool. exper. et gen., Ser. 1, T. 3, pp. xxi-xxiii. 
1896. Markebt, F., Die Flossenstacheln von Acanthias. Ein Beitrag zur Kenntnis der 

Hartsubstanzgebilde der Elasmobranchier. Zool. Jahrb. (Abt. Anat. u. Ontog.), Bd. 9, 

pp. 665-722, pis. 46-49, 10 text figs. 

1895. Maubeb, F., Die Epidermis und ihre Abkommlinge. Leipzig. 355 pp., pis. 1-9, 28 
text figs. W. Engelmann. 

1839. Owen, E., Eesearches concerning the Structure and Formation of the Teeth of the 
Squall, with a New Theory of the Development of the Teeth. London. 4°. Transl. in 
C. E. Acad. Sci. Paris, T. 9, pp. 784-788; Ann. Sci. Nat. Zool., Ser. 2, pp. 209-229. 

1840-45. Owen, E., Odontography. Vol. 1. London. 

1911. OsHiMA, H., Some Observations on the Luminous Organs of Fishes. Jour. Coll. Sci. 
Tokyo, Vol. 27, Art. 15, pp. 1-25, pi. 1, 4 text figs. 

1896. EiDEWOOD, W. G., The Teeth of Fishes. Nat. Sci., Vol. 8, pp. 380-391, 22 text figs. 
1900. EiTTEB, P., Beitrage zur Kenntnis der Stacheln von Trygon und Acanthias. Diss. 

Eostock. Inaug. Diss., Berlin. I-VI, pp. 1-56, pis. 1-6. 
1894. EosE, C, Ueber die Zalmentwicklung der Fische. Anat. Anz., Bd. 9, pp. 653-662, 8 

text figs. 
1894. EosE, C, Ueber die Zalmentwicklung von Chlamydosclachus anguineus Garm. Morph. 

Arb., Bd. 4, pp. 193-206. 

1898. EosE, C, Ueber die verschiedenen Abanderungen der Hartgewebe bei neideren Wir- 
beltieren. Anat. Anz., Bd. 14, pp. 33-69, text figs. 4-28. 

1908. Eynbebk, G. van, Sur une disposition particuliere dans le squelette cutane de quel- 
ques Selaciens. Arch. Ital. Biol., T. 49, pp. 203-212, 12 text figs. 



THE ELASMOBRANCII FISHES 41 

1908. Kynberk, G. van, Di una disposizioue particolarc nello scheletro cutaneo di alcuni 
Selacei. Eend. Accad. Lincei Roma, Ser. 5, Vol. 17, pp. 137-146, 12 text figs. 

1906. Spex'GEL, J. W., In Bezichung aiif Mund- und Schlundziiline dor Elasmobranchier. 
Zool. Anz., Bd. 29, pp. 332-333. 

1861. Steenstrup, M., Sur la difference entre les poissons osseux et les poissons cartilagi- 

neux au point de A-ue de la formation des ecailles. Ann. Sci. Nat. Zool., Ser. 4, T. 15, 

p. 368. 
1903. Steikhard, Otto, tJber Placoidschuppen in der Mund- und Rachen-Hohle der Plagios- 

tomeu. Arch, f . Natur., Jahr 69, Bd. 1, pp. 1-46. Also : Inaug. Diss. Bern, 1902, pp. 

1-50, Taf . 1-2. 

1906. StudniJka, F. K., Ueber kollagene Bindegewebsfibrillen in der Grundsubstanz des 
Hyalinknorpels, im Dentin und im Knocliengewebe. Anat. Anz., Bd. 29, pp. 334-344, 

10 text figs. 

1909. Studnic'KA, F. K., Zur Losung der Dentiufrage. Anat. Anz., Bd. 34, pp. 481-502, 2 
text figs. 

1876. Tomes, G. S., On the Development of the Teeth of Fishes. (Elasmobranehii and 
Teleostei.) Phil. Trans. Roy. Soc. Lond., Vol. 166 (1), pp. 257-267, pi. 31. 

1898. ToiXES, C. S., Upon the Structure and Development of the Enamel of Elasmobranch 
Fishes. Phil. Trans. Roy. Soc. Lond., Vol. 190B, pp. 443-464, pis. 17-18. 

1898. Tomes, C. S., Upon Rose's Proposed Classification of the Forms of Dentine. Anat. 
Anz., Bd. 14, pp. 343-348. 

1896. Treuexfels, Paul, Die Zahne von Myliobatis aquila. Inaug. Diss., Univ. Basel, pp. 
1-34, pis. 1-2. 

1849. Williamson, W. C., On the Microscopic Structure of the Scales and Dermal Teeth of 
Some Ganoid and Placoid Fish. Phil. Trans. Roy. Soc. Lond., ] 849, Pt. 1, pp. 435- 
475, pis. 40-43. 

1851. Williamson, W. 0., Investigations into the Structure and Development of the Scales 
and Bones of Fishes. Phil. Trans. Roy. Soc. Lond., 1851, Pt. II, pp. 643-702, pis. 
28-31. 

1892. Woodward, A. S., The Evolution of Sharks' Teeth. Nat. Sci., Vol. 1, pp. 671-675, 12 
text figs. 

1893. Woodward, A. S., On the Dentition of a Gigantic Extinct Species of Myliobatis from 
the Lower Tertiary Formation of Egypt. Proc. Geol. Soc. Lond., pp. 558-559, pi. 48. 



42 



THE ELASMOBRANCH FISHES 





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ENDOSKELETON 

ENDOSKELETON OF HEPTANCHUS MACULATUS 

The endoskeleton in Hepianchus maculatus consists of a framework of rela- 
tively simple cartilage which serves as a protection for the various internal 
organs and at the same time acts as a support for the attachment of the mus- 
culature of the body. The cartilage of the axial part, the skull and the spinal 
column, is essentially like that of the appendages in that it has but little cal- 
cium deposited in it. 

Axial Skeleton 

SKULL 

The skull of Heptanchus maculatus is composed of (1) a relatively thin-walled 
cranium or brain case to which in the adult are fused the nasal and auditory 
capsules, and (2) a series of cartilaginous visceral arches which support the 
mouth and the pharyngeal region. 

The cranium in dorsal view (fig. 45) is like a violin box, broadly pointed in 
front and almost square at the posterior margin. At the sides it is constricted, 
and back of these constrictions are the heavy postorbital processes {po.p.) . In 
the middorsal line at the posterior margin of the cranium is a ridge, the occipi- 
tal crest {o.cr.) which lies posterior to a pit, the parietal fossa (p./.) ; from 
the bottom of the fossa the foramina for the endolymphatic ducts {ed.) and 
the fenestrae {fn.) lead to the ear. Inclosing the parietal fossa and pointed 
backward is faintly indicated a V produced by the two anterior oblique semi- 
circular canals. This V, together with a similar V produced by the posterior 
oblique semicircular canals (see p.o.s.) and inclosing the occipital crest poste- 
riorly, roughl}^ forms an hourglass. In front of the parietal fossa the roof is 
slightly convex and extends to a large dorsally placed opening, the anterior 
fontanelle (F.). 

On each side of the anterior fontanelle is a small foramen for the ophthal- 
micus profundus nerve {f.o.p}), and at the sides of the posterior part of the 
fontanelle are two (or one) large openings (f.o.VIP) for the ophthalmic 
branch of the seventh cranial nerve. Running on a line posterior from this 
large opening and parallel to the margins of the indenture is a row of smaller 
foramina on each side, through which twigs of the same nerve pass. 

It will be observed that the cranium of Chlamydoselachus in dorsal view 
(fig. 46) is much like that of Heptanchus. 

In side view (figs. 47 and 48) projecting forward there are the elongated 
cartilaginous supports for the rostrum, at the sides of Avhich is the olfactory 
capsule (ol.c) for the nasal apparatus. The cartilages of the capsules are 

[43] 



44 THE ELASMOBRANCH FISHES 

exceedingly thin-walled and open to the exterior by the nasal apertures. Sur- 
rounding the aperture is the arch-like nasal cartilage ( n.c.) . The optic capsule 
does not fuse with the cranium. 

On each side at the posterior third of the cranium is the auditory capsule 
(a.c, fig. 47) in which the semicircular canals and the organs of hearing are 
located. Between the auditory and the nasal capsule is the large orbit for the 
eye. Overhanging this is the supraorbital crest {s.o., fig. 48) , the anterior pro- 
jection of which is the preorbital (pr.o.), and the posterior one the postorbital 
process {po.o.). Ventral to the posterior part of the orbit the cranium bends 
sharply downward forming the basal angle {h.a., fig. 47) . On the anterior face 
of the basal angle is a flattened articular surface against which the orbital 
process of the upper jaw plays. Anterior to the basal angle and extending 
from the margin of the cranium is an antorbital process {a.pr., fig. 47). This 
process is called by AUis (1923) and by others the ectethmoidal process. 

Numerous foramina through which nerves and blood vessels course perfo- 
rate the walls of the cranium. The first of these between the orbit and the nasal 
capsule is the anterior opening of the orbitonasal canal (o-n.), its posterior 
opening {o-7i}) lying in the anterior angle of the orbit. Above this opening is 
a smaller foramen for the anterior cerebral vein (f.a.c). Ventrally and at the 
middle of the orbit is the large optic foramen (f.II) through which the second 
cranial nerve reaches the brain. Directly above it is the ophthalmic fora- 
men through which the superficial ophthalmic branch of the seventh nerve 
(f.o.VII) leaves the orbit. A short distance ventral and anterior to this is a 
small opening through which the deep branch of the fifth nerve leaves the 
orbit. Posterior to the ophthalmic is the small trochlear foramen (f.IV) for 
the fourth cranial nerve which passes to the superior oblique muscle of the 
eye. Behind the optic and below and slightly back of the tip of the postorbital 
process is the large orbital fissure (o.f.) ; through this the fifth, and a part of 
the seventh cranial nerves pass. The sixth nerve enters the orbit through its 
own foramen in the anteroventral margin of the orbital fissure. Below and 
slightly posterior to the orbital fissure is the facial foramen (f.VIP) for the 
hyomandibular branch of the seventh nerve. On a line between the facial and 
the optic foramina are two perforations, the larger and posterior of which is 
for the interorbital canal (i.o.) by means of which the blood sinuses of the two 
orbits communicate. The other of these perforations (f.r.a.) is for the entrance 
of the ramus anastomoticus arter^^ Above this is the small opening {f.III) for 
the exit of the third cranial nerve to muscles of the eye. 

VISCERAL SKELETON 

The visceral skeleton is composed of a series of cartilaginous arches which 
more or less completely surround the buccal cavity and the pharynx. The num- 
ber of these in Heptanchus (nine) exceeds that of any other present-day Elas- 
mobranch. The arches may be divided into two groups. The first group com- 
prises the mandibular and the hyoid arches, each of which is made up of two 
segments. The second group consists of seven branchial arches which support 




Fig. 47. Lateral view of cranium. Heptanchus maculatus. 




ex.h 



Fig. 48. Lateral view of skull, with branchial arches removed, Reptanchus maculatus. 
(Duncan Dunning, del.) 

a.c, auditory capsule; a.pr., antorbital process; h.a., basal angle; h.r., branchial rays; 
ex.h., extrahyoid cartilage; /.//, optic foramen; f.IIl, foramen for oculomotor nerve; f.IV, 
trochlear foramen; f.VIP, foramen for hyomandibular branch of facial nerve; f.IX, fora- 
men of glossopharyngeal nerve; f.a.c, foramen of anterior cerebral vein; f.o.VII, foramen 
for ophthalmicus superficialis nerve (leaving orbit) ; f.r.a., foramen for ramus anastomoti- 
cus artery; i.o., interorbital canal; I., labial cartilage; md., mandible; n.c, nasal cartilage; 
o.f., orbital fissure ; o-n., anterior opening of the orbitonasal canal ; o-n.^, posterior opening 
of the orbitonasal canal ; ol.c, olfactory capsule ; p-q., palatoquadrate cartilage ; po.o., 
postorbital process; pr.o., preorbital process; qd.p., quadrate process; s.o., supraorbital 
crest. 




''■•^iby Ailis ^923) audi- : . ••- the ectethuR .. , 

■amina ^^hroii^h whirl and blood vessels course perff 




(.; rii Li anv. ' 



n? (f.III) for 
e to muscles of tlie eye. 

\ ISCET? AL SKELETON ^'^^^ ^sniaiiua rtBonuQ) 
:8'^£-i IfiirionBid ,.*v.(J ;3l§nB lB8Bcf ,.».(S ;88930iq IjslicfTotnB ,.iq.r> jafoeqeo '(;i<)ixbuB ,.o.d 
j'^l.^ I'sYT^n lotoraofi/oo loi nsxcBioi ,111.\ ;n9«tBioi aijtqo ,11.\ ^egsniipo bLOYdsiixs ,.A.3ia , 
-eioi ." ■ ' 'l(Vrta»BWT'£llid^«nliioYri ibi jtionrB'ioi jT.l'T'.X • nemnioi lasldaoii 

nonii. ido^i-io luneXv^ >%o haitiSToi (.f^.o.^ ; ovian iBOgfl^iBilqoaBoIg ^o nam 

-■■+•■. 1 . .:,..■■ i nviUjjtoi ,.D.t.\ ; (Jidio grriTsei) avion fti(Bi')a"iaqi;2 ^oiiTiliifhifqo loi 

..,v ! , I . ;,>..fr ,.«>ik ^ sFdifctrBrff j-biw ; .gg^IiJtJio l^idal ,J jIgjcibd lB:ticf"i6io}rri (.o.s [ji-jiiB guo 
ym.ijqo luiiy.;.;' " '«-" ■ 'fSfico iBeBJiQiiJio Qilt io gniireqo 101:19*08 [<.«-qi joinsail Lectidio ,.\*' 
..o.oc^ ;9gBli}i(. ')i£rBq ,.p-c^ lefuaqi^o ;^io;j3BiIo ^.'-J.SOijlBflBO lB8£xio,tidio adi, io 













Tie. 47 







FiE. 48 



THE ELASMOBRANCIl FISHES 



45 



the walls of the pharynx. In plan the branchial arches are essentially similar 
to the first two, but in the branchial arches there are typically four segments 
to every arch; these segments differ among themselves in minor details. 

The mandibular or first arch upon which the teeth are borne has become 
the most highly specialized of all the visceral arches. The massive upper pala- 
toquadrate (pterygoquadrate) segment {p-q., fig. 48) of this arch is as large 
as the lower mandil)ular segment or Meckel's cartilage (md.). This arch in 
Hepianchus, unlike that in most sharks, is closely bound to the cranium in two 
places, giving the amphistylic 
type of attachment (amphi, 
both; sfi'/ar, pillar) . Anteriorly 
this union with the cranium is 
effected by the or])ital process 
of the quadrate coming in con- 
tact with the sides of the basal 
angle ( ft. o., fig. 47). Posteriorly 
the union is produced by a 
strong quadrate process (qd.p., 
fig. 48) joining the postorbital 
process of the cranium (po.o.) . 
The upper and lower segments 
of the left side are connected in 
front with similar segments of 
the right side, but this articu- 
lation in Heptanchus is loosely 
made. Posteriorly the upper 
and lower segments are joined 
to each other by a simple double 
joint, and made fast medially 
by pronounced ligaments. 

The hyoid or second visceral arch is, as we have said, composed of two seg- 
ments. In Heptanch us these segments are slender and are entirely hidden in 
side view l)y the mandibular arch. The upper segment, unlike that in more 
highly specialized Elasmobranchs, is not a suspensorium for the mandibular 
arch; the lower segment is long and slender. Connecting the two ceratohyoids 
of opposite sides in the midventral line is an additional unpaired piece, the 
basihyal cartilage {bh., fig. 50a). 

Both segments of the hyoid are provided with numerous cartilaginous rays 
(h.r., fig. 48) which support the main respiratory structures. These rays on 
the hyoid are considerably more complex than are similar cartilaginous rays 
found on the branchial arches. 

The first branchial arch (fig. 49) may be taken as a type. Its segments, from 
the dorsal to the ventral side, are : (1) the pharyngobranchial (pb.), (2) the 
epibranchial (eb.), (3) the ceratobranchial (cb.), and (4) the small hypo- 
branchial (hb.), respectively. These segments slant obliquely forward and 




Fig. 49. First brancliial arch, Heptanchus maculatus. 

li.r., cartilaginous branchial ray; cb., ceratobran- 
chial; eh., epibranchial segment; ex.h., extrabranch- 
ial cartilage; hb., hypobranchial ; pb., pharyngo- 
branchial segment. 



46 



THE ELASMOBRANCH FISHES 



downward, the pharyngobranchial and ceratobranehial lying almost parallel 
to each other. All the branchial arches, except the last, are similar to the first. 
They diifer from it, however, in that their hypobranchial segments are better 
developed. In the last arch a functional hypobranchial is lacking and the 
pharyngobranchial has fused dorsall}^ with that of the sixth arch. 




A B 

Fig. 50a. The median branchial cartilages, Heptanchus maculatus, dorsal view. (H. M. 
Gilkej, del.) 

Fig. 50b. Area of rudimentary arches, Heptanchus maculatus, ventral view. 

ctr.", ninth arch; hb.', second basibranchial ; hh., basihyoid; ch., ceratobranehial; ch., 
ceratohyoid; hh., hypobranchial; mp., median piece; r., rudimentary rays; x., part of 
eighth arch. 

In the midventral line of the branchial basket is a series of unpaired pieces 
which unite the arches of opposite sides. The first of these is the basihyoid pre- 
viously mentioned (hh., fig. 50a). A first basibranchial piece is lacking, 
but basibranchials are present on the second to the fourth arches following 
(hh-''^) . A large median piece posteriorly {mp.) serves for the attachment of 
the fifth and sixth hypobranchials and the seventh ceratobranehial (cbJ). 

All these arches, except the last, have cartilaginous branchial raj's on their 
epi- and ceratobranehial segments. These branchial rays, though simpler than 
those on the hyoid arch {h.r., figs. 48 and 49), serve the same function of sup- 
porting the gill septa. 

The visceral arches are further provided with superficially placed pieces, 
the extravisceral cartilages. Those accompanying the first or mandibular arch 



THE ELASMOBRANCH FISHES 



47 



are tlie so-called labial cartilages (see fig. 48, 1.), a single one of which is found 
in Heptanchus ijiaculatns. This is an irregular cartilage, interesting particu- 
larly because of its unusual development in this species. An extravisceral is 
present dorsally on the hyoid arch (ex.h., fig. 48), but none is present ven- 
trally. Similar cartilages are found both dorsally and ventrally on all bran- 
chial arches except the last. The extrabranchial cartilages (ex.h., fig. 49) 
curve around the tips of the branchial rays as a protection and a support for 
the outer margins of the gill septa. 

In addition to the arches above described there are, especially in some of the 
young specimens of Heptanchus, supernumerary rudiments of still other 



hd. id sbd. 





Fig. 51 



Fig. 52 



Fig. 51. Fifth to eighth segments of the spinal column, Heptanchus maculatus. 

Fig. 52. Sagittal section through sixth to eighth segments of the column. (H.M.Gilkey, del.) 

bd., dorsal basal (basidorsal) plate; bv., ventral basal (basiventral) plate; c, central 
column; chd., unconstrieted part of notochord; f.d., foramen for dorsal root nerve; 
f.v., foramen for ventral root nerve; h.a., haemal arch; id., dorsal intercalary (interdorsal) 
plate; iv., ventral intercalary piece; is., layer immediately around the gelatinous notochord; 
m.s., middle layer in sheath of notochord ; n.c, neural canal ; oz., cartilage ; r., rib ; s., sep- 
tum constricting notochord ; s.bd., so-called neural spine. 

branchial arches. An eighth appears directly back of the last functional arch 
(ch.^, fig. 50a) and even a ninth arch may be indicated nearer the middle line 
(ar.^, fig. 50b), (Daniel, 1916). 

Spinal Column 



The spinal column in Heptanchus, because of its simplicity, is especially inter- 
esting. Unlike that of the higher Elasmobranchs, it consists of a long central 
column {c, fig. 51) which is essentially the enlarged sheath of the notochord. 
Anteriorly the column is more or less continuous with the occipital region of 
the cranium (see fig. 47) and posteriorly it extends to the tip of the tail. Above 
this central cohunn there is a series of neural arches formed for the protection 
of the spinal cord; below it, in the region of the tail, is a similar series of 
haemal arches {h.a., fig. 53) for the protection of the caudal artery and vein. 
A segment of the column in the so-called neck region shows the central part 
well developed. Above this central part are the neural plates making up the 
neural arch. Each arch is composed of a dorsal plate (hd., fig. 51) and a dorsal 



48 



THE ELASMOBRANCH FISHES 



intercalary piece (id.). Both of these cartilages are more or less triangular in 
shape, the former having its base on the centrum, the latter with apex pointing 
toward the centrum. Above the dorsal plate there may be pieces segmented off 
(s.hd.) to form the so-called neural spines, and in the most anterior part of 
the column two such pieces may be present one above the other (fig. 47) . Each 
dorsal plate in the anterior region is further perforated by a ventral root 
(f.v.) of the spinal nerve, and each dorsal intercalary by the dorsal root (f.d.) 
of the same nerve. 

In this region and ventral to the central column are also ventral plates 
(bv., fig. 51). On the third and succeeding vertebrae back to the forty-fifth, 
ribs (r.) are present. The eighth to twenty-fourth ribs in Hepfanclius, like 
some of those in Laemargus (Helbing, 1904, cited on p. 72), are divided into 



f.v.f.d. 




Fig. 53. Lateral view of spinal column in transitional area. Drawai as transparent object. 
(Katharine Rogers, orig.) (For explanation see fig. 52.) 

an anterior and a posterior part, the former of which is a curious plate-like 
process projecting forward and downward. Between two ventral plates there 
is interpolated a small ventral intercalary piece (iv.). 

A sagittal section through this region (fig. 52) shows the central column 
composed of three concentric layers in the notochordal sheath. These layers 
surround the notochord (chd.) and constrict it at intervals into a bead-like 
chain. The outermost of these layers is relatively thin and consists of carti- 
lage; within this cartilage is a second and lighter broad area (m.z., fig. 52) 
which appears to be made up of transverse fibers. Within this second layer 
and bounding the notochord is a third layer (is.) of a white tissue. At regular 
intervals the third layer forms septa (s.) which produce the regular constric- 
tions in the central part of the notochord. It will be observed that the septa are 
more pronounced ventrally than dorsally and that they pass intraeentrally. 

In the midbody the central column assumes its simplest form. Here it con- 
sists essentially of a heavy and but slightly constricted sheath which if it be 
allowed to dry slightly gives greater evidence of constrictions. At about the 
fiftieth segment of this region (fig. 53) some of the dorsal plates extend en- 
tirely to the top of the arch so that no neural spines are present. Following 
these plates, and beginning at about the fifty-sixth segment two types of dorsal 
plates obtain, one of which is high, the other much lower. The higher is per- 
forated by the ventral root of the nerve; the lower is imperforate. The higher 
plate is followed by a dorsal intercalary plate, perforated by the foramen for 
the dorsal root nerve. In the region beyond the fifty-fifth segment it will be 



THE ELASMOBRANCH FISHES 



49 



observed that to each segment two neural arches are present, and that a similar 
condition obtains in tlie haemal arches (h.a.) even farther forward. Such a 
condition is incomplete diplospondyly. 

Appendicular Skeleton 

The appendicular skeleton is the framework for the fins and the girdles to 
which these, if paired, are attached. 

The pectoral girdle (fig. 54) in Heptanchus is a slender arch open dorsally, 
to which the framework of the pectoral fin is attached. It is composed of right 



ms.p. 



•t.pf 





Fig. 54. Lateral view of the skeleton of the pectoral fin and girdle, Heptanchus maculaius. 
(Duncan Dunning, del.) 

a.pl., process for articulation of pectoral fin; co., coracoid; f-pi., foramen for nerves and 
blood vessel; ms.p., niesopterygium ; mt.p., metapterygium ; pr.p., propterygium ; ra., ra- 
dials; sc, scapula. 

and left cartilaginous halves which are united in the middle line below by 
means of an unpaired median piece. The part of the girdle extending the more 
dorsalward is the scapula (sc), tipped by the suprascapula; that part which, 
by means of the median piece, joins a similar part from the opposite side 
below is the coracoid portion (co.). At the middle and posterolateral part of 
each half of the girdle there is an irregular surface for articulation with the 
pectoral fin (a.pl.). In front of and below this projection is a broad surface 
for the attachment of the ventral muscles of the fin. Perforating the girdles in 
this surface is a large foramen (f.pt.) through which the lirachial artery and 
nerves pass to supply the fin. 



50 



THE ELASMOBRANCH FISHES 



Skeleton of Paired Fins 

The skeleton of the pectoral fin (fig. 54) is fan-shaped; the proximal part con- 
sists of three basal cartilages, propterygium, mesopterygium, and metaptery- 
gium ; from the last two, nnmerous rows of radials radiate. 

The propterygium {pr.p.) in Heptanchus maculotus is a small nodule of 
cartilage located upon the mesopterygium. It is followed by four or five large 





A. Female 



B. Male 



Fig. 55. The skeleton of the pelvic fin and girdle, Heptanchus maculatus. (Ruth Jeanette 
Powell, del.) 

j3, beta cartilage ; h}'-, first and second connecting segments ; ha., basal or axial cartilage ; 
ba.p., basipterygium ; pi., pelvic girdle ; ra., radials. 



radials, the first of which may fuse with the mesopterygium. The mesoptery- 
gium (ms.p.) is a stout cartilage, from the enlarged distal end of which 
extend ten or twelve rows of radials (ra.), depending upon the amount of 
fusion which has taken place proximally. The most anterior row is composed 
of large and irregular plates, but the remaining rows are broken up into small 
segments. The metapterygium (mt.p.) is a large triangular cartilage, the base 
of which points ventrally. It is segmented both j^roximally and distally and is 
then continued into the most distal radial. From the metapterygium diverge 
numerous rows of preaxial radials, in addition to which there are clearly 
marked postaxial radials. 



THE ELASMOBRANCII FISHES 51 

The pelvic girdle (pi., fig. 55) is a flattened band of cartilage, slightly con- 
cave dorsally and enlarged at the ends. Perforating the terminal parts of the 
girdle are from one to three foramina (see p. 95, fig. 96) through which nerves 
pass to the pelvic fin. At the termini of the girdle are the articular processes, 
each consisting of two protuberances which fit into depressions (fossae) of the 
pelvic fin skeleton. These are not well shown in figure 55. 




Fig. 56 Fig. 57 

Fig. 56. Dorsal fin, Heptanchus cinereus. (From Mivart.) 
Fig. 57. Anal fin, Heptanchus cinereus. (From Mivart.) 
Z)c., basal cartilage ; ra., radial. 

The framework of the pelvic fin proper consists of a long posteriorly pro- 
jecting basal cartilage, the basipterygium {ha. p.) which bears one or two 
small terminal segments. From this cartilage in the female proceed 21 or 22 
radials {ra., fig. 55a), all of which, except the last five, are segmented. Ante- 
riorly, a much enlarged plate meets the basal piece, forming an obtuse angle. 
From this run three rows of radials. At the proximal ends of this enlarged 
plate and the basal piece, are the two fossae with which the protuberances on 
the pelvic girdle above described articulate. 

In the male the long basipterygium {ha.p.) is continued bj^ the basal or axial 
cartilage of the claspers {ba.). Where the two join there are two segments 
(&.^"-), and dorsal to them is the so-called beta cartilage (^) . 

Skeleton of Unpaired Fins 
DOESAL FIN 

Extending over the forty-ninth to the fifty-fifth segment of the vertebral col- 
umn is the thin basal cartilage of the dorsal fin (see Heptanchus cinereus, 
fig. 56, be). From this plate in Heptanchus maculatus arise seventeen or 
eighteen radial cartilages {ra. ) , one of which, the anterior, is unsegmented and 
a few of the posterior radials may or may not be fused into a single piece. 

CAUDAL FIN 

The segments of the tail show a characteristic diplospondyly in the arches 
both above and below the central column, although the pseudosegments of the 
central column itself are not doubly constricted. The ventral rays of the caudal 



52 THE ELASMOBRANCH FISHES 

fin (see fig. 53) are an integral part of the axial skeleton, being the prolonga- 
tions of haemal spines under the haemal arches (h.a.). These consist of a series 
of rays, two of which correspond to a segment. The dorsal lobe of the fin is also 
supported by rays which begin back of the segment shown in figure 53. These 
dorsal rays are more than twice as numerous as the segments present. 

ANAL FIN 

The base of the anal fin {Ic, fig. 57) ends anteriorly at about the fifty-third 
centrum of the spinal column. The basal piece, barring the fact that it is seg- 
mented in front, is remarkably similar to that of the dorsal. From it, however, 
the radials {ra.) proceed in a less definite fashion. 



THE ELASMOBRANCH FISHES 



53 



ENDOSKELETON OF ELASMOBKANCHS IN GENERAL 



The endoskeleton in the Elasmobranchs in general varies a great deal in its 
composition. While in all it is formed of cartilage and never of bone, yet this 
cartilage differs greatly as to its rigidity. In a shark like Heptanchus macu- 
latus the cartilage is usually of a clear hyalin type and relatively soft. In H. 
cinereus, on the contrary, it is strengthened by a deposit of calcium. In some of 
the more specialized forms calcification is so abundant and so arranged that 
the cartilage is almost as strong as bone. 
Cartilage as such in Elasmobranchs is 
composed of cartilage cells and ground 
substance. The ground substance consists 
of a mucin-like substance in which are co- 
logen fibers. In certain forms elastic fibers 
may also be included in the cartilage, and 
calcification is present in many Elasmo- 
branchs (see Roth, 1911 ) . 

Axial Skeleton 
SKULL 

The skull in the Elasmobranchs varies 
considerably in shape. In the rays it is 
depressed dorsoventrally, while in the 
sharks in general it is similar in plan to 
the skuUas described for Heptanchus. In 
all Elasmobranchs the skull includes the 
cranium, the capsules for the organs of 
s])ecial sense, and the visceral arches. 

CRANIUM 




Fig. 58. Development of cranium, 
Acanthias (Modified from Sewert- 
zoff.) 

a.c, auditory capsule; asp., alisphe- 
noidal cartilage; pr., parachordal 
plate ; tr., trabecular cartilage. 



The cartilaginous cranium or brain case 
in the Elasmobranchs is unlike the bony 
cranium of higher forms in that its sides, roof, and floor are welded into a 
single piece of cartilage. Joined to it also are the auditory and olfactory cap- 
sules for organs of special sense. The cranium is perforated by nerves and 
blood vessels which enter or leave the brain. Below the orbit the cranium may 
project outward as an in£raorl)ital plate (most sharks) or such a plate may 
be absent (rays). 

The rudiments for the brain case are laid down in the embryo of Acanthias 
according to Sewertzoff (1897) as three pairs of cartilages (see figs. 58 and 
70). These are (1) the parachordal plates (j^r.) which lie at the sides of the 
notochord and posterior to the internal carotid foramina; (2) a pair of ali- 
sphenoidal cartilages (asp.); and (3) a pair of trabecular cartilages (tr.) 
anterior to the internal carotid foramina. The parachordal and trabecular 



54 



THE ELASMOBRANCH FISHES 







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THE ELASMOBRANCH FISHES 55 

rudiments by growth extend medially, forming the floor of the cranium; the 
tral)eeulae bend to form the basal angle, and then project forward in the re- 
gion of the rostrum, giving rise to the rostral plate. The alisphenoidals pro- 
duce a large part of the sides of the cranium and extend dorsally, to form the 
roof of the cranium. To these three pairs of cartilages the sense capsules fuse. 
The nasal capsule joins the trabecula (rostral) ; the otic or auditory capsule 
joins the parachordal; while the optic capsules, when present, remain free. 

In the adult sharks alid rays, as we shall see, the extent to wdiich the plates 
increase in size and the modifications to which they are subject vary greatly. 
Indeed, in various species of the same group these differences often produce 
entirely unlike crania. An example of this variation may be seen in Zygaena, 
the hammerhead (fig. 65), in which the cranium in the region of the eye is so 
modified as to be unlike that of the nearlj^ related Gale us. The modification, or, 
better, the progressive develo])ment of the cranium is best understood by con- 
sidering first the simpler and more generalized, and then the more highly 
specialized Elasmobranchs. 

In dorsal view the adult Elasmobranch cranium may be said to take the 
shape of an hourglass (figs. 59, 61, 62). In this hourglass the basal segment is 
formed by the enlarged otic or auditory capsules; the middle part of the in- 
dentation contains the orbits for the eyes; and the top segment is produced by 
the olfactory capsules. Upon this as an apex may arise longer or shorter rostral 
cartilages {rs.). 

In more generalized forms, as, for example, Hcptanchus or Chlamydosel- 
achus, the positions of the semicircular canals of the ear are evident as super- 
ficial ridges on the surface of the auditory capsule. 

The ridge for the posterior canal runs from the parietal fossa or pit outward 
and backward to the foramen for the ninth cranial nerve, while the ridge for 
the anterior canal rises at the parietal fossa and runs forward at right angles 
to the posterior canal. In more highly specialized types, w^here the parietal 
fossa is shallower, external evidence of the canals is less distinct (fig. 62). 

From the bottom of the parietal fossa certain apertures lead to the ear. Two 
of these are for the endolymphatic ducts {e.d., fig. 59) and two other accessory 
apertures are the fenestrae (/».), for the perilymphatic spaces. In some forms 
the parietal pit may be deep and the endolymphatic foramina may be sepa- 
rated only by a short space, as in Heptanchus. Again it may be shallow, where- 
upon these foramina are farther apart (8cylliu)}i). In still other forms, the 
pit is only a slight depression and the endolymphatic foramina are more 
widely removed from each other {Rhinobafis, fig. 62; Raja clavata; Trygon; 
Myliohatis) . The fenestrae are apertures of much larger size than are the 
foramina for the endolymphatic ducts, and have at times been confused with 
them. In a type like Heterodontus (fig. 61) the parietal pit is deep and hence 
the fenestrae are difficult to see. They are evident, however, in Pseudotriacis 
(fn., fig. 59) and in Khinohatis (fig. 62). In a specimen of Galeorhmus, six 
feet in length, and in a large Myliohatis calif ornicus, they were relatively of 
immense size. 



56 



THE ELASMOBRANCH FISHES 






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THE ELASMOBRANCH FISHES 57 

The middle region varies greatly in dorsal view. While it is typically con- 
stricted, secondarily this constriction may be obscured by the expansion of the 
supraorbital crests (Hetei^odontus, fig. 61), or the constriction present in the 
young may be entirely lost in the adult, as in Carcharodon rondeletii (T. J. 
Parker, 1887). In the rays a snpracranial fontanelle is usually present in the 
middorsal line. This fontanelle is small in Trygonorhina, but large in Rhino- 
hatis {F}, fig. 62) and Baja clavata. 

The olfactory capsules {ol.c, fig. 60) which represent the uppermost seg- 
ment of the hourglass are in most Elasmobranchs more or less well developed. 
In some of the sharks they may be relatively of immense size, as for example 
in Zygaena where they are drawn far out with the preorbital processes of the 
cranium (fig. 65). But the capsules are normally smaller as in Pseudotriacis 
(fig. 59) . In Rhinobatis (fig. 64) and many other rays they are large ^ang-like 
expansions to which heavy antorbital pieces are attached. In the middorsal 
line of this median segment there is present in all sharks and rays the anterior 
fontanelle (F.), which in some is confluent with the snpracranial fontanelle 
(Myh'ohatis) . 

The rostral pieces which may form an apex to the hourglass may be flat and 
divergent, as in Heterodontus (fig. 61) and Crossorhinus, or blunt and con- 
vergent as in Scyninus and Laemargus; or there may be two long dorsolateral 
bars which unite forward with a ventral bar as in Pseudotriacis microdon (vs., 
fig. 59) , Mustelus, Galeus, and many others. In Acanthias the ventral cartilage 
is broad and spoon-shaped and the dorsolateral bar is rudimentary, each bar 
being joined to the nasal capsule by a double cord of connective tissue (Wells, 
1917). In some of the rays (Myliohatis) a rostrum is lacking, but this is the 
exception. In the majority of this group it is a long single piece (Rhmobatis, 
fig. 62) , and in some it is of great length, as for example in Pristis the saw ray. 

In a ventral view of the cranium (figs. 60, 68, and 64) a similar hourglass 
shape is apparent. The auditory region at the base is a broad expansion of the 
parachordal cartilage of the embryo to which the auditory capsules have 
fused. In the most posterior region of the midventral line is the notch separat- 
ing the two occipital condyles (ocd., fig. 60) ; by means of the condyles the 
spinal column articulates with the cranium. In primitive sharks like Hep- 
tancluis the column is more or less completely fused with the cranium so that 
no true articulation exists. In the rays the condyles are well developed, but 
here a secondary fusion takes place which afi^ects the occipitovertebral articu- 
lation and the anterior part of the column (see p. 70, fig. 76b) . Posteriorly and 
between the two condyles is the foramen magnum {f.m., fig. 61) through 
which the spinal cord joins the brain. 

The middle segment in ventral view is characteristically different in the 
sharks and rays. In the adult of most of the sharks right and left infraorbital 
plates broaden out into wing-like processes which form a floor for the orbits, 
and consequently ol)scure the constriction. In the rays the form of the hour- 
glass is very evident, for in them, as in Heptanchus, an extension of these 
plates is characteristically absent. 



58 



THE ELASMOBRANCH FISHES 



111 the olfactory region the capsules are seen in their relation to the elliptical 
nasal cartilages (w.c, figs. 60. 63, and 64) surrounding the nasal aperture. 
Projections from the anterior and posterior margins of the ellipse meet and 
cross, usually forming of it a figure 8. A second anterior accessory process may 
be added as in Heterodontus {n.c.~, fig. 60), or may be more completely de- 
veloped as in Scyllium. In the rays the ellipse may be broken into segments. In 
Rhinohatis (fig. 64) the anterior and posterior projections forming the bridge 
are relativelj^ slender cartilages. In Myliohatis numerous accessor}^ projec- 
tions are present. 




Fig. 65. Dorsal view of cranium, Zygaena, left side. (Modified from Gegenbaur.) 
or., orbit; po.o., postorbital process; pr.o., preorbital process. 



In a side view (figs. 66 and 68) the auditory, optic, and olfactory regions of 
the cranium are seen to advantage. With the exception of ChJamydosclachiis 
and the notidanids the auditory region is greatly modified superficially for 
the attachment of the hyoid arch. In most of the recent sharks the articulation 
is made by means of a deep pit (as is present in Heterodontus, fig. 66) . In some 
of the rays a special part from the hyoid arch makes an extended articulation 
with this part of the cranium. 

Anterior to the auditory region is the enlarged orbit in which the eyeball 
rests. Its roof is usually formed by a supraorbital crest, modified posteriorly 
into a postorbital process {po.o.), and anteriorly into a preorbital process 
{pr.o.). 

In the sharks the postorbital process is rarely extended. Exception must be 
made, however, of Zygaena, the hammerhead {po.o., fig. 65) and its near ally, 
the bonnet shark, where it may be prolonged outward to meet the posterior 
part of the preorbital process. Exception should be made also of Chlamydosel- 
achus and the notidanids in which the postorbital process serves in suspend- 
ing the upper jaw. In the rays, except those bearing stings, this process is 
characteristically small or absent. 

The preorbital process in Zygaena {pr.o., fig. 65) arises far out on the cra- 
nium and is divided into anterior and posterior jiarts. The anterior part 



THE ELASMOBRANCII FISHES 



59 




-t.< 



Fig. 66. CraniuiJi of Ileterodontus francisci, lateral view. (Diniean Donning, del.) 




,5p.C. 



Fig. 67. Skull of Mcterodontus francisci (articulate). (Duncan Dunning, del.) 

a.c, auditory capsule; cli., ceratohyoid; ?., labial cartilage; f.op., foramen for profundus 
nerve; f.r-a., foramen for ramus anastomoticus artery; /.//, optic foramen; f.IV, troch- 
learis foramen; f.VII, foramen for hyomandilmlaris branch of facial nerve; f.X., foramen 
for tenth nerve ; lim., hyomandibula ; md., mandible ; o.f., orbital fissure ; o.p., optic pedicel ; 
po.o., postorbital process; p-il-, palatoquadrate cartilage: pr.o., preorbital process; sp.c, 
spiracular cartilage. 



60 THE ELASMOBRANCH FISHES 

joins the nasal capsule and the posterior part extends back to the postorbital 
process (po.o.). Between anterior and posterior parts is located the secondary 
orbit (or.). 

In the notidanid sharks, in Chlamydoselachus, and in the rays there is a 
posterior projection from the preorbital region, the antorbital process (figs. 
69 and -47, a.pr.) . In the notidanids and Acanthias this serves for the attach- 
ment of one of the superior labial muscles. In the rays it unites the pectoral fin 
skeleton to the cranium. 

The eyeball is held out from the cranial wall by a rod, the optic pedicel 
(Heterodontus, fig. 67, o.p.) ; this pedicel in some of the sharks has a terminal 
expansion into which the eyeball fits. In Chlamydoselachus (fig. 46) it further 
serves as a place of origin for the rectus muscles. In the rays the pedicel is 
plate-like and may be fixed to the eyeball (Torpedo). 

The apertures which perforate the orbital region for the cranial nerves and 
blood vessels vary considerably in size and position from those given for Hep- 
tanchus. Ordinarily the optic foramen (/.//, fig. 66) is relatively large and 
occupies a central position in the orbit, but in types like Rhinohatis (fig. 68) 
it is well forward. In the embrj-o (Acanthias) this foramen separates the 
alisphenoid from the trabecular cartilage. The oculomotor and the troch- 
learis foramina take positions respectively behind and above the optic; but the 
trochlearis is variable. In the rays (Rhinohatis, fig. 68, f.IV; Myliohatis) it is 
above but posterior to the aperture for the optic nerve. The orbital fissure 
(o.f.) usually gives exit to the fifth and a considerable part of the seventh 
cranial nerves, and in a type like Rhinohatis is of unusual size. In this type it 
is not unlike the large fissure which early forms a separation of the alisphen- 
oids from the parachordal cartilages in the embryo of Acanthias. In Mustelus 
henlei, where there are special foramina for the superficial division of the 
seventh nerve and the profundus division of the fifth nerve, the fissure is re- 
duced in size. The foramen for the sixth nerve usually opens separately at 
the base of the orbital fissure. The facial foramen for the hyomandibular 
branch of the seventh nerve may be located posterior to the orbit, as in Hep- 
tanchus (f.VII', fig. 47), in Scymnus, and in other forms; or it may be in the 
posteroventral angle of the orbit (Heterodontus, fig. Q>Q, f.VII; and others). 
In the upper anterior angle of the orbit is the ophthalmic foramen (or fora- 
mina) which gives exit to the ophthalmicus superficialis of the seventh nerve. 
In Heterodontus, as is general for the Elasmobranchs, the profundus nerve 
(f.op.) leaves the orbit by an extra foramen ventral to that for the ophthal- 
micus superficialis. 

Anterior to the orbit is the ethmoidal region in which is situated the nasal 
or olfactory capsule. The capsules in the simpler forms are more or less ter- 
minal in position, while in the highly specialized rays, as for example Mylio- 
hatis, the region is bent sharply downward so that the apertures are entirely 
ventral in position. The olfactory cups are usually more or less surrounded by 
cartilage, leaving their apertures as relatively small openings. These openings 
are visible in side view in the sharks only. The nasal cartilages surrounding 
the openings have been described. 



THE ELASMOBRANCH PISHES 



61 




Fig. 68. Crauiiim of lUiinohatis productus, lateral view. (Chester vStoek, orig.) 




Fig. 69. Skull of liaja clavata (articulate). (Modified from W. K. Parker.) 

a.pr., antorbital process; f-II, optic foramen; f.IV, trochlear foramen; f.VII, foramen 
for hyomandibular l^ranch of facial nerve; Ji.md., hyomandibula; 7)id., mandible; o.f., orbi- 
tal fissure; po.o., postorbital process; p-q., palatoquadrate ; pr.o., preorbital process; s.o., 
supraorbital crest; sp.c, spiracular cartilage. 



62 THE ELASMOBRANCH FISHES 



VISCEEAL SKELETON 

The cartilaginous visceral skeleton in the Elasmohranchs usually consists of 
seven visceral arches : the mandibular, the hyoid, and five branchial arches. In 
Chlamydoselachus and the notidanids, however, there are additional branchial 
arches making a total of eight visceral arches in Hexanchus and Chlamydosel- 
achus and, as we have seen, nine in Heptanch us. 



Fig. 70. Development of embryonic visceral arches in Acanfhias. (From Sewertzoff.) 
md.a., mandibular or first arch ; or.2>., orbital process. 

The visceral arches of the adult are more easily understood if we first study 
their early arrangement in the embryo. In a reconstruction made by Sewert- 
zoff of the embryo of Acanthias (fig. 70), all the arches appear as bent carti- 
laginous bars which as yet have not divided into segments. The first or mandi- 
bular arch (md.a.) takes the form of an inverted U. On the upper (anterior) 
limb of the arch is a process which abuts against the trabecular cartilage, and, 
in the adult, forms the orbital process (or. p.) for articulation with the cra- 
nium. The second or hyoid arch at this stage is sigmoid in shape, and even here 
joins the cranium in the region of the auditory capsule. The remaining five 
branchial arches, third to seventh viscerals, are crescentic in shape and like 
the preceding arches show no signs of segmentation at this stage. 

The mandibular arch in all adult Elasmobranchs, as in Hepfanchus, is di- 
vided into (1) an upper palatoquadrate (pterygoquadrate) segment (p-q.) 
and (2) the mandibular segment or Meckel's cartilage (nid., figs. 67 and 69). 
In simple types like the notidanids the palatoquadrate may have two fairly 
well defined regions, an anterior palatine (pterygoid) and a posterior quad- 
rate. In more specialized forms, however, it is impossible to distinguish these 
segments other than by position. On the anterior part of the quadrate segment 
may be present an orbital (palatobasal) process w^iich articulates with the me- 
dian wall of the orbit {Chlamydoselachus, Scymnus, Acanthias) ; or it may be 
wanting, as for example in Heterodontus (fig. 67) and Raja (fig. 69) where the 
mandibular arch is shoved forward. The palatoquadrate cartilage unites in 
front with a similar segment from the opposite side, as does also the mandible. 



THE ELASMOBRANCII FISHES 63 

The union of tlie right and k^ft cartilages may be loosely made (notidanids) or 
it may be firmer as in most forms; that of the mandibular cartilage, while firm 
in the rays, is loose in most sharks except Squatina and Heterodontus. 

The articulation of the upper and lower jaws may be by a joint in all re- 
spects similar to that of Heptanchus; such is the articulation in Chlai)ijjdosel- 
achiis and Hderodontus. Or the joint may be slightly more specialized as 
in Scyllimn. A highly specialized type found in the Elasmobranclis is that in 
Raja (fig. 69), where a single ball and socket joint prevails. The joint may ))e 
bound simply by short ligaments, or it may be firmly held by a most complex 
ligamentous arrangement, as in Heterodontiis francisci (figs. 67 and 128) . 

Following the mandibular arch, and above the quadrate segment, is the 
spiracular cartilage (sp.c, figs. 67 and 69) which supports the filaments in the 
anterior wall of the spiracle, where such exist. This cartilage may be composed 
of several segments (three in Cenirophorus, two in Acanthias) , or it may con- 
sist simply of a single piece. In other sharks where the spiracle is minute the 
cartilaginous support may be absent {Lainna). In the rays (Torpedo, fig. 63; 
Raja, fig. 69; Myliohatis) it is well developed, and serves as a framework for 
the support of the spiracular valve. It follows a true prespiracular ligament. 

The liyoid or second visceral arch, which is simple in primitive sharks, is 
subject to great modification when the Elasmobranchs in general are con- 
sidered. This arch in its generalized condition (Heptanchus) is composed of 
a dorsal segment which suspends a ventral segment. Under such conditions 
the hyoid does not function, or functions but slightly, in suspending the first 
or mandibular arch. The proximal end of the dorsal segment consequently is 
attached loosely by ligaments and hence indents the auditory capsule but 
slightly as in Heptanchus cinereus. In other types it may indent the capsule 
by only a part of its surface (Hexanchus, Chlamydoselachus) . When this oc- 
curs the distal end of this segment suspends the ventral segment of the hyoid 
and the latter is bound by ligaments to the mandible. 

In general, an attachment of the second arch to the mandibular may be 
made by a ligament at the joint and elsewhere. In Chlamydoselachus the form 
of attachment is very simple. While the lower segment depends slightly upon 
the mandible, the latter begins also to depend upon the upper segment for 
support. In other words, the dorsal segment is on its way to become a hyoman- 
dibula or suspensorium. Where it is a true hyomandibula, as in most sharks, 
this upper segment of the hyoid arch is of service primarily to the first arch. As 
a suspensorium it becomes stronger and its articulation with the cranium l)e- 
comes deeper; furthermore, its ligamentous attachment to the mandibular 
arch may be most complex (Heterodontus francisci, fig. 67; see Daniel, 1915) . 

When the upper segment of the hyoid has assumed the secondary function 
of suspending the mandibular arch, that is, where it becomes hyomandibular, 
it may still continue to suspend the lower part of its owai arch also. This is 
indeed characteristic of the sharks. But the lower segment may lose connec- 
tion more and more with the upper (hyomandibular) segment. Instead of 
being suspended from the distal end of the hyomandibula, it may be joined 



64 THE ELASMOBRANCH FISHES 

posteriorly to the middle portion of the hyomandibula {Torpedo, fig. 63). 
Such a union results from the method of growth of the hyomandibula. A proc- 
ess extends from the anterior angle of the upper segment which, in the adult 
Torpedo, forms the suspensorium or hyomandibula. This part suspends the 
mandibular arch while the lower segment is attached to the posterior part of 
the hyomandibula (fig. 69). In some of the other rays the lower segment may 
not be attached at all to the hyomandibula, but may be united with the poste- 




c.b. 




Fig. T. 



Fig'. 71 

Fig. 71. First branchial arch, Heterodontus francisci. (Duncan Dunning, del.) 

Fig. 72. Fourth and fifth branchial arches, Heterodontus francisci. (Duncan Dunning, del.) 

h.r., branchial ray; cl>., ceratobranchial ; eJ}., epibranchial ; ex.l}., extrabranchial carti- 
lage; ph., pharyngobranchial. 

rior part of the cranium (Urolophus) . In a still more specialized form it may 
have no union either with the hyomandibula or with the cranium, but may be 
bound to the tip of the first branchial (third visceral) arch, as in Rhinohatis 
and Myliohatis. In some such occurrences the lower arch may be further 
segmented. 

The branchial arches in general are typically made up of four segments (fig. 
71, Heterodontus) which from dorsal to ventral, as was given for Heptanchiis, 
are: (1) the pharyngobranchial {j)h.), (2) the epibranchial (e.h.), (3) the 
ceratobranchial (ch.), and (4) the hypobranchial {hh., fig. 73) segments. 

The pharyngobranchials are usually flattened cartilages which lie dorsal to 
the pharynx. In .sharks they are usually attached bj^ strong connective tissues 
(ligaments) to the roof of the pharynx or to the sides of the spinal column 
but not to the pharyngobranchials of the opposite side, as is the first in Hep- 
tanchus and Scyllium. In the rays, the pharyngobranchial segment of the first 
branchial arch, as we have said, may join the cranium {Rhinohatis, Trygon). 



THE ELASMOBRANCH FISHES 65 

The most posterior pliaryiigobrancliial, as a usual tiling', in l)oth sharks and 
rays is fused with the one preceding, hence it may lose much of its character- 
istic shape (Heterodontus, fig. 72). 

The epi- and ceratobranchials are the ray-bearing segments of the arches. 
In all, except the most posterior arches, these segments are similar. In the last 
arch, both of these segments are more or less modified. Generally this modifica- 
tion takes the form of a thickening of the ceratobranchial (sharks) and an 
atrophy of the epibranchial because of its fusion with its pharyngobranchial 
cartilage. 

The hypobranchials, although perhaps more regular in a type like Chlamy- 
doselachus (fig. 73a) than in Heptanchus, are generally more variable than 
are any of the other segments. In the sharks the first, if present, is usually 
small and is located between the distal end of the first ceratobranchial and the 
hyoid cartilage (Heterodontus, fig. 73b, hb.^; Laemargus) . The second may 
fuse with a similar one from the opposite side across the midventral line 
{Scymnus, Laemargus) ; or the two may join a median unpaired cartilage, 
the second basibranchial, as in Acanthias, Trygon, Heterodontus (hh., fig. 
73b) ; or they may arch backward to join the enlarged median piece (mp.) as 
in Torpedo (fig. 63) and Rhinohatis. Generally the third and fourth hypo- 
branchials of pentanchid forms, except in some of the rays, are well developed 
and are attached to the large median unpaired piece. In general, except Hep- 
tanchus (fig. 50) , hypobranchials on the most posterior arch are lacking or are 
fused with the unpaired median piece to which the third and fourth hypo- 
branchials are attached. In the rays these segments may be present as plate- 
like cartilages attached to the unpaired median piece (Raia erinacea, fig. 74b) . 

In the midventral line unpaired basal elements join the arches of the right 
and left sides. The element connecting the two halves of the hyoid arch is 
the basihyoid (bh., fig. 73) and those connecting the branchials are the basi- 
l)ranchial cartilages (bb.). The basihyal cartilage may be a broad plate, per- 
forated by the thyroid foramen {Chlamydoselachus, fig. 73a, bh.) ; or it may 
bear an anterior glossal projection as a support for the tongue (Heterodontus, 
fig. 73b, g.p.; ScylUum) . Again it may be a narrow band as in Acanthias or as 
in Raia erinacea (fig. 74b) ; or it may be incomplete as in Torpedo (fig. 63). 
Basibranchials are present as distinct irregular j)ieces of cartilage anteriorly, 
but posteriorly they may fuse into a single mass. Generalized forms are char- 
acterized by numerous basal elements. The first of these may be a peg-like 
structure attached to the basihyoid (Chlamydoselachus) , or it may lie free be- 
hind the basihyoid (Laemargus) . The second basibranchial is usually free and 
the third, when present, is often attached to the larger posterior median piece 
which may or may not be segmented. In the rays only the posterior median 
piece is present (Rhinobatis, and Torpedo, fig. 63) . 

In the embryo of some forms well marked rudiments of still other branchial 
arches persist, as we have seen in Heptanchus. Such rudiments are also pres- 
ent in Chlamydoselachus, where a seventh arch has been described, and in 
Heterodontus and in some of the rays, where a sixth arch may occasionally be 



66 



THE ELASMOBRANCH FISHES 



found. The rudimentary arch in the embryo of Heterodontus consists of at 
least two segments, which in the adult may still be seen, welded more or less 
closely to the fifth branchial arch (fig. 72). Rudiments of such structures are 
of interest in forms like Heptanchus and CJilamydoselachus in which an un- 
usual number of arches becomes functional in the adult. That still other rudi- 
ments are present in the embryo indicates that ancestral forms possessed a 
number of arches exceeding that of present-day types. 





Fig. 73. Median ventral basibranchial cartilages. A. Chlamydoselachus. (From Goodey.) 
B. Heterodontus francisci. 

1)1)., basibranchial; hh., basihyoid ; ch., ceratobranchial ; g.p., glossal process; hh., liypo- 
branchial; mi)., median piece. 



Figure 50b represents the area of rudimentary arches in Heptanchus macu- 
latus. It will be observed that a slight asymmetry is shown which gives a some- 
what greater development of the midventral region on the right than on the 
left side. Through this asymmetry the rudimentary arch and adjoining area 
on the right side are more highly developed than on the left. 

Upon examination of the ventral side of the rudimentary arch on the right 
side I found certain round and pointed rays (/•., fig. 50b) arising from the 



THE ELASMOBRANCH FISHES 67 

seventh eeratobranchial practically at right angles to its long axis. These rays 
extend posteriorly between the plate x and the median piece as clear pieces of 
hyalin cartilage. Whetlier they represent rudimentary branchial rays on the 
seventh arch like those described by Gegenbaur (1872, pi. 12, fig. 5) on the 
anterior margin of the fifth arch for Scjjllium, or have to do with the riidi- 
mentar}^ arch following, is not certain. 

On the middle piece a similar arrangement is found. Here there are three 
pieces which are successively longer toward the middle line. They are essen- 
tially identical in appearance with the rays above described on the seventh 
eeratobranchial segment, but they are attached along their whole dorsal 
length as flattened lamellae. Terminally the median two look very much like 
the rays above described and they are much like them also in that they are of 
clear hyalin cartilage. Farther toward the median line on the middle piece 
there is clear evidence of still another similar group, except that it is no4eepa- 
rated into rays or lamellae. This group is also of clear hyalin cartilage aiff er- 
ing distinctly from the median piece, which, in the specimen, is of a dark 
color. I have interpreted this group as a remnant of a ninth arch (ar.^), al- 
though I am not certain what part it represents. 

The condition found in this specimen is suggestive as to the method of for- 
mation of the enlarged median piece so characteristic of the Elasmobranchs. It 
would appear that in this region the rudimentary arches are forced more and 
more to take a longitudinal direction nearer the middle line, and that the 
median piece represents in its most posterior part the fusion of these arch^^ 
from side to side. 

Extending from all the visceral arches, except the first (mandibular) and 
the last, is a series of cartilaginous branchial rays for the support of gills. 
These supporting rays are confined to the epi- and cerato-segments. The bran- 
chial rays may be complex and branched, the termini fusing into arches on 
the liyoid (sharks) , or they may be comparatively simple and straight (rays) . 
In Torpedo a curious modification of the branchial rays occurs in the form of 
terminal discs (&.r., fig. 63), each of which is almost circular in shape. These 
cartilaginous rays, although fewer the more posterior the arch, may be ex- 
ceedingly numerous anteriorly as in Lamna, or relatively sparse as in Lae- 
margiis. The central ray, the one between the epi- and the eeratobranchial, 
may exceed all others in length. This ray may be postulated as the main axis 
of the fin skeleton according to the gill-arch theory for the origin of paired fins. 

EXTEAVISCEEAL AECHES 

Outside of the deep internal visceral arches are the extravisceral cartilages. 
These may be divided into the labials^ of the mandibular arch, the extrahyals 
of the hyoid, and the extrabranchials of the branchial arches. Normally each 

1 The labial segments are often interpreted as representing arches formerly present be- 
tween the mandibular and hyoid arches. 



68 



THE ELASMOBRANCH FISHES 



of these arches is made up of two segments, one dorsal, the other ventral in 
position. 

Three labial cartilages are usually present on a side in sharks, two dorsal, 
the so-called premaxillary and the maxillary cartilages on the palatoquadrate, 
and one ventral on the mandible. Of the dorsal segments the anterior is the 
shorter. When well developed the labials serve to reduce the gape of the mouth. 





Fig. 74. Extrabranchial cartilages of Baia erinacea. (From Foote.) A. Dorsal view. 
B. Ventral view. 

ex.l)., extrabranchial cartilage; ex.h., extrahyoid cartilage; md., mandible; mp., median 
piece; p-o., palatoquadrate (pterygoquadrate) cartilage. 



An interesting series may be followed in the specialization of these carti- 
lages from the simple condition of a single cartilage to the tripartite condition 
just mentioned. Heptanchus cinereus has a single labial located dorsally, 
which because of its small size long escaped observation (Fiirbringer, 1903). 
In Heptanchus maculatus (1., fig. 48) there is a single cartilage, shaped like a 
tuning fork, which extends across the gape of the mouth and ends dorsally in 
two horns. It gives no evidence, however, of segmentation. In Hexanchus, the 
labial extends across the gape of the mouth and according to Gegenbaur 
(1872) is more or less divided into an anterior and a posterior division. In 
the notidanids, then, we have practically a complete transition from the single 
rudiment to the condition found in more specialized forms. 

It may be said of transitional rays that the labial cartilages are poorly 
developed (Rhinohatis) . 




THE ELASMOBRANCH FISHES 69 

Since the extraliyoids and extrabranchials botli serve the same purpose the 
two types may be described together. These structures support the free mar- 
gins of the gill septa and hence run parallel with the deeper visceral arches to 
which the septa are attached. They may be present on the hyoid and on all the 
branchial arches except the last, as in Acanthias and in Raia erinacea {ex.b., 
fig. 74). In others, while the extrahyoid is lost dorsally it may persist ven- 
trally, making five inferior and only four superior cartilages (Heterodontus 
francisci) . In still others, both segments of the extrahyoid arch may be absent, 
and yet a full complement of extrabranchial arches on the first four branchials 
may be present (Trygon) . In a reduction of the number of extrabranchials the 
posterior cartilages are the 
first to be absent. A fourth ex- ^m^- 
trabranehial may be lacking 
ventrally, leaving three below 
and four above {Scyllium). 

While the extrahyal seg- 
ments present are normally 
small, the extrabranchials over 
the branchial arches may be rig. 75. Sagittal section through a developing verte- 
well developed. Occasionally ^-'^^' S^^U^^^um canimla. (From Schauinsland.) 
the tins of the dorsal and ven- chd., notochord; e.e., elastica externa; e.i., elastica 

interna ; ep., chordal epithelium ; iz., inner zone ; ms., 
tral segments of the anterior middle zone ; oz., outer zone. 

arches overlap as in Hetero- 
dontus (fig. 71) . In most forms, however, the dorsal and ventral segments fail 
to touch (Acanthias) , and in many they are relatively insignificant structures 
(Raia erinacea, fig. 74) . 

We have said above that the extravisceral arch is normally composed of a 
superior and an inferior segment. In a number of species an interesting con- 
dition is found in which lateral pieces, extraseptalia, are also added. These 
may be present as flattened bands of cartilage between the external clefts 
(Torpedo; Raja clavata) or they may be flattened plates lying underneath the 
forward projection of the propterygial segment of the fin skeleton as this 
passes over the region of the gills (Myliohatis) . In Cephaloptera, the devil 
ray, they are relatively large plates bound under the propterygium. 

Spinal Column 

The spinal column in Elasmobranch fishes shows great variation, from a 
simple cartilaginous tube around the notochord, as in Heptanchus maculatus, 
to the highly segmented and calcified column common to many forms. In gen- 
eral, it consists of a central axis made up of centra upon which is a series of 
neural arches, which extend throughout the body. In the region of the tail, 
haemal arches, under the centra, furnish protection for the haemal or blood 
vessels. A vertebra includes a centrum and its neural and haemal arches. 
The vertebrae vary greatly in numbers. In a type like Heterodontus there are 
only a few more than a hundred in the whole column, while in Alopias there 
are more than twice that number in the tail alone. 



70 



THE ELASMOBRANCH FISHES 



b.a." V 



'■•ip^ ■ 



A. Heterodontus francisci. 



In Elasmobranchs the column is particularly instructive because of its rela- 
tion to the still more primitive notochord found in the prechordata and in the 
embryo of all vertebrates. This new cartilaginous column arises around and in 
the sheath of the notochord as a secondary and more effective support. Its 
mode of development may be noticed briefly. 

In the embryo, cells proliferate from the sclerotome or inner part of the 
somite (see p. 96, fig. 97, scl.) and migrate inward to a position around, above, 
and below the notochord. Those which collect at the upper and lower levels 
form four cogs with the notochordal sheath as the center. These cells lay down 
cartilage for the neural and haemal arches and around the notochord. 

Many of the cells around the notochord, however, may perforate its external 
frd UA2 li.^ ,Lj wall (elastica externa) and deposit car- 

tilage within the notochordal sheath. 
A sagittal section through a vertebra of 
Scylliuni canicula is shown in figure 75. 
The sheath between the outer (e.e.) and 
inner (e.i.) layers in which cartilage is 
deposited may be divided into outer 
(oz.), median (mz.), and inner (iz.) 
zones. The median zone is the one in 
which calcium is usually deposited. 

The central column, in 
a simple type like Hep- 
tanchiis maculatus, is es- 
sentially a thin tube of 
cartilage {oz., fig. 52) 
deposited in the sheath of 
the notochord. The mid- 
dle zone here is composed 
of transverse fibers in the 
sheath, which in some re- 
spects appear to be like 
the "chordafaserscheide" 
described for Amphioxits by Von Ebner ( 1895) . As to the layer which is desig- 
nated "iz." in figure 52 I am not sure whether this represents the inner zone of 
more specialized Elasmobranchs or is a part of the elastica interna. 

In types only a little more specialized than Heptanchus maculatus (Chlam- 
ydoselachus and Heptanchus cinerens), a slightly greater amount of calci- 
fication may be present in the column; and in still more specialized forms, 
where the column becomes a stronger support than that just described, calci- 
fication is a pronounced feature. But different regions vary greatly in the 
amount of calcification. For convenience of description the column may be 
divided into three regions. The first of these, the anterior section, joins the 
head; the second is in the area of the trunk; and the third we may designate 
simply as the caudal segment of the column. 




B. Ehinobatis productus. (C'liester tStock, orig.) 

Fig. 7(5. Cervical vertebrae. 

hd., dorsal basal plate (basidorsal) ; hv., ventral basal 

plate; c, centrum; f.d., foramen for dorsal nerve; id., 

dorsal interealarv (interdorsal plate) ; r., rib; shd., neural 

si^ine. 



tup: elasmobranch ftsiies 



71 



The anterior section of the cohunn differs in the various Elasniohranchs in 
its relation to the eraniinn. In the notidanids, as we have seen, the two are 
more or less continuous. This coiitiunity is further emphasized in all general- 
ized forms by the fact that this fused region is perforated by a number of 
oceipitospinal nerves which arise l)etween the vagus and the first spinal nerve. 
In a type like Acanthias in which there are a dorsal and two lateral ridges 
contiiuiing directly from the column to the cranium, the relation of the two 




A. BJiinohatis producius. (Chester Stock, orig.) 



iW^ 



ho. 



B. Keierodontus francisci. (Duncan Dunning, del.) 

Fig. 77. Transitional vertebrae. 
hd., dorsal basal (basidorsal) plate; hv., ventral basal plate; f.d., foramen for dorsal root 
nerve; f.v., foramen for ventral i-oot nerve; iv., ventral intercalary; n.s. (and) sid., neural 
spine ; r., ril). 

regions is not so clear; while in still more specialized forms tlie column is more 
or less clearly separated from the cranium, attachment of the two being made 
by processes of the cranium and the column (rays) . 

This segment of the column (fig. 76a, Heterodontus), unlike that of Hep- 
ianchus, is nsnally clearly divided into centra (c. ) . In practically all forms the 
relation of the dorsal and ventral arches to the centra is equally clear. In a 
type like Pseudotriacis, how^ever, the segmentation is irregular, both in the 
centra and in the neural arches of the column. This irregularity is especially 
marked in the most anterior vertebra which has fused into a solid ring. In the 
rays (Rhinohatis, fig. 76b) the larger part of this anterior region has second- 
arily fused into a solid vertebral plate, the segmentation of which is made out 
only through a study of the foramina and certain lateral processes. 



72 THE ELASMOBRANCPI FISHES 

The trunk vertebrae lie between the pectoral and pelvic regions and their 
dorsal and ventral plates are usually distinct and regular. In some types, 
however, as for example Laemargus horealis, the dorsal intercalary plates 
represent the maximum of change in that each plate is subdivided into two or 
more parts. A similar segmentation in the ventral intercalary plates may be 
present in this area. 

The anterior vertebrae of this region are rib-bearing but there is great 
variation in the number of ribs present. In Laemargus horealis ribs are present 
on only a few vertebrae, while in other forms ribs may extend almost to the 







_i ^ ..' J J , ■< / ' ' 



.■■■ ^ ^ .r -* -■ 



»^"««?«ii;*<ss^-3*3^^^;^^ 



\ 



B 

Fig. 78. Caudal vertebrae. 

A. Heterodontus francisci. (Duncan Dunning, del.) 
'B. Bldnohatis productus. (Chester Stock, orig.) 
h.s., haemal spine; id., dorsal intercalary plate. 

posterior limits of the trunk area. The posterior part of the trunk segment 
offers great variation in the different elements of the column and is of particu- 
lar interest because of diplospondyly or doubling of the segments, which is 
present here. 

Diplospondyly may begin immediately after the last rib-bearing centrum 
(Heterodontus, fig. 77b), or a series of vertebrae may intervene before the 
diplospondjdous vertebrae are reached. In Scyllium, according to Ridewood 
(1899), a brief area of transition follows the rib-bearing segments in which 
the stages from monospondyly to diplospondyly may be traced. The first indi- 
cation of the change is seen in the slight shifting backward of the dorsal 
intercalary plate of the first vertebra behind the last one bearing a rib, so that 
one of the neurals rests directly upon the dorsal basal plate. In the vertebra 
following, this condition is further accentuated and results in diplospondyly, 
which is also seen in succeeding vertebrae. In a type like Rhinohatis (fig. 77a) 
the rib-bearing segments (r.) extend far posteriorly and are separated from 
the true diplospondylous segments by only a few vertebrae. In this type, also, 
the spines are of large size and rest between two or more of the irregular 
intercalary pieces. 



THE ELASMOBRANCPI FISHES 



73 








M 



be 


'S 


c 


^ 






'? 


^ 


c 


« 






"35 


rt 



(^■. 





U' 



p^ 



di m 


tj 


M c3 




o a; 


o 


^ t~ 


'« 


OJ ctf 


o 


-tJ 


i^ 


CD r-> 


'ij 


<^ 


!li 


. 2 


t5 


OJ as 




t^ t> 


< 


bi'^ 





^ ro 



74 



THE ELASMOBRANCH FISHES 



The doubling probably is to be interpreted as meaning that there is need 
for greater freedom of movement in the active area preceding the caudal fin. 
In this area where the most severe strain is imposed upon the column it is 
important, as Ridewood has suggested, that greater strength and at the same 
time greater freedom of movement be obtained. Strength is given by the in- 
creased calcification, and freedom of movement is brought about by increasing 
the nimiber or decreasing the size of the segments; that is, by diplospondyly. 
Diplospondyly may extend practically to the tip of the tail (Acanthias) or 
the terminal segments may be irregularly segmented (heterospondylic). 

As a usual thing the vertebrae in the caudal segment of the column present 
marked regularity in their centra and arches. In Heterodontvs (fig. 78a) the 
radials are more numerous than are the vertebrae. In Lamna the dorsal 
radials are inconspicuous, while those having a ventral position are usually 
large. In this type we see the extreme of heterocercy, in which the axis of the 
body turns sharply upward into the dorsal lobe of 
the caudal fin. In Rliinohatis (fig. 78b) the caudal 
segment is unlike that of the sharks especially in 
that the vertebral column is here practically straight, 
and dorsal and ventral radials of the caudal fin skele- 
ton are of practically equal length. Such a type is 
more nearly diphycercal than heterocercal. 

Helbing (1904) has shown for Laemargus that 
there is a tongue of cartilaginous segments in the 
area between the ventral lobe of the caudal fin and 
the pelvic fin. This tongue is attached to the haemal 
process of one of the vertebrae posteriorly and ex- 
tends anteriorly. While the basal plates of the 
neural arches are uniform in size, the intercalary 
pieces are variable. In the anterior part of this area 
(fig. 78b) they may segment irregularly, while more posteriorly they are con- 
tinuous with the dorsal radials. At the most posterior tip the intercalaries and 
radials represent an irregular mass. 

In various regions of the column the calcification takes up difi^erent lo- 
calities, forming diverse and curious designs. Probably the simplest design 
is that in which a single ring of calcium is produced in the middle zone of the 
notochord sheath (fig. 75). A cross-section through a centrum thus calcified 
shows the ring, of large or small diameter depending upon whether it is cut 
near the end or at the middle of the centrum. A sagittal section would show it 
as a broad V above and as an inverted broad V below within the centrum, 
the two V's being separated at their apices by the notochord. Such a type of 
vertebra has been designated by Hasse as cydospondijlous (fig. 80, Squalus 
sucklii). 

Another design formed by the calcification is a further addition to the 
eyclospondylous type. In this, two or more concentric rings of calcium are 
formed in the sheath of the notochord. The inner ring is thick like that of the 




Fig. 80. Cyclospondylous 
vertebra, Sq^ialus sucTclii. 
ca., calcifieation; chd., 
notochord; n.c, neural 
canal. 



THE ELASMOBRANCH FISHES 



75 



cyclospondylous type, l)ut the outer circle is usually a thin sheet. Such a type 
of calcification is known as tecfospofidyly. A modification of the tectospondyl- 
ous type may add still other outer circles, as, for example, in Squatina. A 
sagittal section through this type would show the heavy inner circles as V's 
above and below, which are concentrically surrounded by sections of other 
circles, appearing as more or less straight lines. 

A most interesting and varied type of calcification is arranged around the 
inner zone of the notochordal sheath as a central hub from which spokes or 
rays diverge in a star-like fashion through the outer zone (asterospondyly) . 





A B 

Fig. 81. A aud B. Stages in development of the pectoral fin of Scylliuyn canicula. (From 
Balfour.) 

ms.p., mesopterjgium ; vit.p., metapterygium; pr.p., propterygium; ra., radial. 

Few calcified rays may be present as in Gal ens, or they may be more numerous 
as in Heterodontus (fig. 79a) and Rhinohatis (fig. 79b) . In Alopias more than 
twenty rays are present. 

So characteristic are the above types of calcification that Hasse has used 
them as a basis for classification. Under such a classification exceptions must 
be made, however, for many variations are to be found. 



Appendicular Skeleton 

SKELETON OF PAIEED FINS AND OF FIN GIEDLES 

PECTOEAL FIN SKELETON 



The skeleton of the pectoral fin, as noted in Heptanchus, consists of a more or 
less horizontal framework of cartilage attached to a vertical girdle. The car- 
tilages making up the framework of the pectoral fin itself are: (1) a set of 
basal cartilages from which projects (2) a series of median cartilaginous 



76 



THE ELASMOBRANCH FISHES 




Fig. 82. The adult pectoral fin, Hcterodontus francisci. (Duncan Dunning, del.) 
7ns. p., mesopterygiuni ; mt.p., nietapterygium ; pr.p., propterygium; ra., radials. 



THE ELASMOBRANCII FISHES 



77 



radials {ra., fig. 82). In some forms there may also be found (3) a series of 
distal plate-like radials {Heterodontus) between upper and lower dermal rays. 

A knowledge of the development of such a skeleton is helpful to an under- 
standing of the adult frame- 
work. In Scyllium canicula 
(fig. 81) Balfour (1881) has 
shown that the pectoral skele- 
ton arises as a horizontal bar or 
jilate of cartilage from which 
radials (ra., fig. 81) extend. 
These radials by fusion at their 
outer tips form a rim from 
which plate-like distal radials 
]iass well out into the fin. As 
growth progresses the original 
bar of cartilage becomes the 
main axis of the fin skeleton, 
the metapterygium {mt.p., fig. 
81a) to which numerous ra- 
dials are attached. Anterior to 
the metapterygium, the so- 
called median piece or mesop- 
terygium (nis.p.) arises sec- 
ondarily; fewer radials pro- 
ject from it. There is next seg- 
mented off from the anterior 
part of the mesopterj-gium a 
piece, the propterygium (pr.p., 
fig. 81b), which bears a single 
plate-like radial. As for the 
pectoral girdle, it is formed 
secondarily from the anterior 
tip of the horizontal bar. 

In the adult shark the sim- 
plicity of plan characteristic 
of this embryonic fin is rarely 
retained (Chlamj/doselachus) , 
yet the fundamental plan here 
laid down is the same, even in 
the most specialized of pec- 
torals. Propterygium, meso-, 
and metapterygium are usu- 
ally present. The propteryg- Fig. 83. The adult pectoral fin, Ehinohatis produc- 
• ' \. n ^ -ii li tus. (Mildred Bennett, del.) 

mm may be fused with the J^_^ mesopterygium ; mt.p., metapterygium; 

meS0])teryginm. as in the adult ne.p., neopterygium ; pr.p., propterygium. 




78 



THE ELASMOBRANCH FISHES 



Heterodontus pMlippi, although this does not occur in Heterodontus francisci 
(fig. 82). The mesopterygium is usually an independent piece, but it may be 
masked by fusion as in Pristiophonis japonicus. A considerable change from 
the embryonic plan of the basals is found in Scymniis lichia, in which only a 
single basal cartilage is present. It is supposed that the missing basal carti- 
lages have secondarily fused in the adult. 

The form of the fin in the rays differs greatly from that of a shark like Hep- 
tanchus in that the basal cartilages are modified in keeping with, the dorso- 
ventral depression and the great extent, anteroposteriorly, to which the fins 
are expanded. The pectoral of Squatina, although shark-like in its articulation, 





-f.pt. 



A B 

rig. 84. Pectoral girdles. A. Heterodontus francisci. B. Torpedo. (Modified from Gegen- 
baur.) 

CO., coracoid; f.pt., aperture for nerve and blood vessels to pectoral fin; sc, scapula. 



is like that of the rays in extent; it has, however, a much heavier mesopter- 
ygium than have the rays. Usually the propterygium of the rays is divided 
into a number of segments which extend forward to join the antorbital process 
{Bhinohatis, fig. 83, pr.p.; Raja, Myliohatis) , or even to the tip of the ethmoid 
region where the two from the opposite sides unite (Urolophus) . The mesop- 
terygia of the rays are very variable. In some forms a mesopterygium is absent 
whereupon the radials extend to the girdle. In other types there is a consider- 
able mesopterygial plate, as in Rhinohatis {ms.p., fig. 83). In others still, a 
second plate back of the mesopterj'^gium may be formed, as in Pteroplatia. The 
metapterygium of the rays, like the propterygium, is greatly developed, pass- 
ing backward to the region of the pelvic fin. 

The propterygial radials of the sharks are usually few in number. They may 
form a single line of segments which may be of more or less regular plates, as 
in Heptanchus or Heterodontus francisci (fig. 82) . In others the rows may be 
more numerous. In the rays there are many rows of such propterygial radials, 
some of which are made up of great numbers of segments (Rhinobatis, fig. 83) . 

The radials attached to the mesopterygium in the sharks are more numerous 



THE ELASMOBRANCII FISHES 



79 



than those of the propterygium. In the rays these are of unusual interest. In 
addition to those extending from the mesopterygial cartilage there are certain 
other radials posterior to this cartilage, as we have said, which extend to the 
girdle. Five such occur in Raja clavata, two or three in Squatina, a shark, and 
a larger number in some other forms. The most interesting thing about these 
extraradials is that in some of the rays they produce, as Howes (1890) has 
shown, a fourth basal, the neopterygium {ne.p., fig. 83), indicated in Rhmo- 
batis and well formed in Pteroplatea. 

In the rays the metapterygial radials are similar to those of the propteryg- 





Fig. 85. Pelvic fin and girdle, CMamydoselachus (A c?, B 5)- (From Goodey.) 
For explanation see fig. 86. 

ium. In the sharks these normally are found in the adult on the anterior side 
of the main metapterygial axis. Not infrequently, however, postaxial radials 
are well developed in the embryo {Acanthias, Carcharias, and many others). 
The significance of postaxial radials has been pointed out by investigators 
seeking a solution of the early form of the paired limb. Those who hold that 
the early type of limb was like that of the present-day lungfish, Ceratodus, 
with a central axis and anterior and posterior rays, think that the postaxial 
rays of sharks are remnants of a past condition. 

PECTORAL GIRDLE 



The right and left limbs of the girdle are incomplete dorsally except in the 
rays, in which the upper tips may be firmly joined to the spinal column or to 
each other. Each half of the girdle is composed of two pieces, one dorsal, the 
scapula {sc, fig. 84), another ventral, the coracoid (co.). The scapula varies 



80 



THE ELASMOBKANCH FISHES 



a great deal in the direction which it takes. In Heptanchiis it slopes very 
obliquely backward, while in Heterodontus and especially in the rays it stands 
more nearly vertical. In general it is capped by a snprascaj)ular cartilage. 





Fig. 86. Pelvic fin and girdle of male. A. Heterodontus francisci. B. Bhinobatis prodvctus. 
(Chester Stock, orig.) 

j3, beta cartilage; 6.^"", intermediate segments; ia., basal or axial cartilage; ba.p., basip- 
terygium; d.mg. and mg. (fig. 85), dorsal marginal; d.tr}-", first and second dorsal termi- 
nal cartilages; pi., pelvic cartilage; ra. (fig. 85), radials; spn., spine; tr.^, accessory termi- 
nal; v.tr., ventral terminal. 

Near the union of the coracoid and scapular pieces, but on the coracoid, is 
the articular process. This area of articulation in the sharks is directed verti- 
cally or obliquely and consequently is usually composed of two convex sur- 
faces {Squatina) . The surface in the rays differs from that in the sharks in 
that it is longitudinal in position and much greater in extent. Torpedo (fig. 
84b) has a type of articulation intermediate between sharks and rays. 



THE ELASMOBRANCH FISHES 



81 



There is a foraiiieii enterino- flie median side of the girdle for the brachial 
artery and for nerves going to the pectoral fin. The canal leading through the 
girdle from tliis foramen sei)arates into a dorsal and a ventral part so that 
laterally there are two foramina leaving the girdle. 

The coracoids from the opposite sides may be separated by a special un- 
paired sternal piece (HepfdncJuis maculotus). Usually, however, they are 
joined ventrally in the sharks; in Heterodontus and Squatina they are firmly 
welded together. In the rays this region is firm excepting in Torpedo. 

PELVIC FIN SKELETON 

The skeleton of the pelvic (ventral) fin is made up of at least two basal car- 
tilages, the basipterygium (ha. p., figs. 85-86) and the anterior basal. From 




Fig. 87. Diagram to illustrate the probable origin of the pelvic girdle. (From Mivart 
after Thacher.) 

ha.p., basipterygium; pL, pelvic girdle; ra., radial. 



these two basal pieces the radials proceed. The basals are supported by a girdle 
w^iich in the Elasmobranchs is not in contact with the axial skeleton. The 
structure of the adult pelvic fin is much simpler than that of the pectoral, but 
in the embryo the two are built on the same fundamental plan. The basipteryg- 
ium (ha.p.) is comparable to the metapterygium of the pectoral. This is 
normally a single piece, but posteriorly it may be broken into three or four 
segments (Centrophorus, Heterodontus, fig. 86a ; Rhinohatis, fig. 86b). The 
anterior basal is somewhat like the propterygium. It apparently represents a 
fusion of the basal parts of the anterior radials, from the distal part of which 
the radials extend freely. In Heptanchus (fig. 55) a segment may join the 
girdle between the anterior basal and the basipterygium which in position is 
like the mesopterygium. The radials belonging to the basipterygium proper 
are more or less segmented anteriorly, but posteriorly, in both male and 
female, they are usually unsegmented. 

The skeleton of the clasper of the male is a continuation in the median axis 
of the basipterygium of the fin, the two being connected by short segments 
{h.^ and h.^, fig. 86a) as in Heptanch us. The terminal part of the basal or axial 
cartilage assumes dififerent degrees of complexity in the various Elasmo- 
branchs. In some, the basal or axial cartilage (ha.) is provided with a single 
accessory cartilage. In others there are present distally an outer marginal 
and a ventral and a dorsal terminal accessory cartilage {Chlamydoselachus, 



82 



THE ELASMOBRANCH FISHES 



fig. 85a.) a still more complex type (fig. 86a) has one or two dorsal terminals 
(d.tr.'^'^) and a ventral terminal (v.tr.), and along the furrow leading to the 
terminal groove there is an accessor}^ terminal {tr.^), and a dorsal marginal 
{mg. and d.mg.) {Heterodontus, Mustelus, Scyllium, CarcJiarias, Raja). 



PELVIC GIEDLE 



The pelvic girdle is probably built on a generalized plan in Chlamijdoselachus 
(fig. 85). Here it is an unusually wide cartilage which is perforated by no 
fewer than six to eight nerves. In most other types the adult girdle is a narrov/ 
band which points backward in the middle line. 





Fig. 88. Dorsal fins of Heterodontus francisci, showing fin spines. (Duncan Dunning, del.) 
A. Second dorsal. B. First dorsal. 

h.c, basal cartilage; ra., radial. 

Figure 87 is a diagram showing the origin of the pelvic girdle as postulated 
by Thacher (1877). This indicates that the anterior fin radials fuse and the 
fusion joins a similar fusion from the opposite side to form a bar which be- 
comes the girdle (pi.) ; the fusion of the tips of the rays back of this becomes 
the basipterygium (ha.p.). 



SKELETON OF UNPAIRED FINS 

The first indication of the unpaired fins in the embryo of Prisfiurus, Dohrn 
(1884) found to be the development of a median longitudinal ridge in which 
cartilage is laid down as a series of parallel rods. In adult Elasmobranchs 
these cartilaginous rods are usually more or less completely segmented into : 
(1) basal (h.c, fig. 89), (2) median (h.c.^), and (3) distal segments {h.c.~). 
The first are proximal or nearest the body axis, and the third often run a con- 
siderable distance into the fins between the two rows of dermal fin rays. 

In the dorsal fins all these types may be present as single segments of the 
radials [Mustelus antarcticus, fig. 89a, or Zygaena) . In some forms the distals 



THE ELASMOBRANCH FISHES 



83 



may be further segmented {Gin(jliimostoma) or they may be absent (Squatina, 
fig. 89b) ; or, finally, all the basal segments may fuse into a single basal plate 
(Heptanchus, Heterodontus, fig. 88). In certain forms the basals may come in 
contact with the column. Such a condition occurs in Squatina (fig. 89b), 
Rhinobatis, and Pristis, in which numerous segments in front of the fin may 
be present joining the neural spines. 





Fig. 89. Dorsal fin skeleton of Elasmobranchs. (From Mivart.) A. Mustehis antarcticus. 
B. Squatina. C. Baja. 

h.c, basal cartilages; h.c.^, median segments; h.c.", distal segments. 



The skeleton of the anal fin, like that of the other unpaired fins, is made up 
of two or three different types of segments, which, in general, show modifica- 
tions similar to those in the dorsals. 



84 THE ELASMOBRANCH FISHES 

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86 THE ELASMOBRANCH FISHES 

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TJIE ELASMOBRANCII FISHES 87 

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1897. RiDEwooD, W. G., Note on the Extrabranchial Cartilages of Elasmobranch Fishes. 
Anat. Anz., Bd. 13, pp. 499-501. 

1899. RiDEWOOD, W. G., Note on the Basibranchial Skeleton of Eehinorhinus spinosus. Anat. 

Anz., Bd. 15, pp. 346-348, 1 text fig. 
1899. RiDEWOOD, W. G., Some Observations on the Caudal Diplospondyly of Sharks. Jour. 

Linn. Soc. Zool. Lend., Vol. 27, pp. 46-59, 2 text figs. 
1884. Rosenberg, Emil, Untersuchungen iiber die Occipitalregion des Cranium und den 

Proximalen Teil der Wirbelsaiile einiger Selachier. Festschr. Dorpat, Vol. 4, pp. 1-26, 

pis. 1-2. 

1886. Rosenberg, Wm., Ueber das Kopfskelet einiger Selachier. Sitzber. Naturf . Ges. 
Dorpat, Bd. 8, pp. 31-34. 

1911. Roth, Wilhelm, Beitrage zur Kenntnis der Strukturverhaltnisse des Selachier-Knor- 
pels. Morph. Jahrb., Bd. 42, pp. 485-555, pis. 5-8, 18 text figs. 

1902. Ruge, Ernst, Die Entwicklungsgeschichte des Skelets der vorderen Extremitaten vou 
Spinax niger. Morph. Jahrb., Bd. 30, pp. 1-27, 1 Taf., 13 text figs. 

1903. Sabatier, Armand, Sur les mains scapulaires et pelviennes chez les poissons chon- 
dropterygiens. C. R. Acad. Sci. Paris, T. 137, pp. 1216-1219. 

1906. Schauinsland, H., Die Entwicklung der Wirbelsaiile nel)st Rippen und Brustbein. 
Hertwig's Handb. vergl. u. expt. Entwlck., Bd. 3, Teil 2, pp. 339-572, text figs. 166-322. 

1911. Secerov, Slavko, tiber die Entstehung der Diplospondylie der Selachier. Arb. zool. 
Inst. Wien, T. 19, pp. 1-28, pis. 2, 5 text figs. 

1925. Senna, Angelo, Contributo alia conoscenza del cranio della Selache (Cetorhinus maxi- 
mus Gunn.). Arch. Ital. anat. e emb., Vol. 32, pp. 84—122, pis. 9-10, 4 figs, in text. 

1895. Sewertzoff, A. N., Die Entwicklung der Occipitalregion der niederen Vertebraten 
im Zusammenhang mit der Frage iiber die Metamerie des Kopfes. Bull. Soc. Imper. 
des Nat. Moscou, pp. 186-284, pis. 4-5, 4 text figs. 

1897. Sewertzoff, A. N., Beitrag zur Entwicklungsgeschichte des Wirbeltierschadels. Anat. 

Anz., Vol. 13, pp. 409-425, 4 text figs. 
1899. Sewertzoff, A. N., Die Entwicklung des Selachierschadels. Ein Beitrag zur Theorie 

der korrelativen Entwicklung. Festschr. z. Carl v. Kupffer, Jena, pp. 281-320, Taf. 

29-31, 4 text figs. (Text and atlas.) 
1911. Sunier, a. L. J., Les premiers stages de la differentiation interne du Myotome et la 

formation des elements sclerotomatiques chez les Acraniens, les Selaciens et les 

Teleosteens. Onderz. Zool. Lab. Groningen, 1911, pp. 1-102, pis. 1-6. 
1877. Thacher, J. K., Median and Paired Fins. Trans. Connecticut Acad., Vol. Ill, pp. 281- 

310, pis. 49-60, 4 text figs. 

1917. Wells, Grace H., The Skull of Acanthias vulgaris. Jour. Morph., Vol. 28, pp. 417- 
444, pis. 1-3. 

1892. White, Ph. J., The Skull and Visceral Skeleton of the Greenland Shark, Laemargus 
microcephalus. Trans. Roy. Soc. Edinburgh, Vol. 37, Pt. 2, pp. 287-306, pis. 1-2. 

1896. White, Ph. J., A Sternum in Hexanchus griseus. Anat. Anz., Bd. 11, pp. 222-224, 
2 text figs. 

1S96. White, Ph. J., Note on the Extra-branchial Cartilages of Scyllium canicula. Anat. 
Anz., Bd. 12, p. 158. 



88 THE ELASMOBRANCH FISHES 

1896. White, Ph. J., The Existence of the Skeletal Elements between the Mandibular and 
Hyoid Arches in Hexauchus and Laemargus. Anat. Anz., Bd. 11, pp. 57-60, 3 text figs. 

1892. WiEDERSHEiM, R., Das Gliedmassenskelet der Wirbelthiere. Jena, pp. 1-266, pis. 1-17, 
40 text figs. 

1904. WiJHE, J. W. VAN, Ueber die Entwicklung des Kopf skelets bei Selachiern. C. R. 6me 
Congr. internat. zool., Berne, 1904, pp. 319-322. 

1922. WiJHE, J. W. VAN, Friihe Entwicklungsstudien des Kopf- und Rumpfskelets von 
Acanthias vulgaris. Dijd Dierk., Afl. 22, pp. 271-298, pi. 11, 4 text figs. 

1886. WoODAVARD, A. S., On the Relations of the Mandibular and Hyoid Arches in a Creta- 
ceous Shark (Hybodus dubrisiensis, Mackie). Proc. Zool. Soc. Lond., pp. 218-224, 
pi. 20. 



IV 

MUSCULATURE 

MUSCULATURE OF HEPTANCHUS I\rACULATUS 

The muscles in Hepianchus may be divided into two groups. In one the fibers 
are short, reaching from one connective tissue septum (myoseptum) to an- 
other; in the other thej' are more 
or less extended and are not con- 
fined within myosepta. The mus- 
cles of the body are of the short 
type; while those of the eyeball, 
of the hypobranchial region, and 
of the claspers in the male are of 
the long type. 

In a side A^ew the muscles of the 
body of Hepfanchus maculatus 
are divided at the lateral line {II., 
fig. 90) into dorsal bundles (f?.6.) 
which attach to the cranium, and 
ventrolateral bundles which at- 
tach to the i)ectoral girdle. Both 
the dorsal and the ventrolateral 
muscles extend to the tip of the 
tail. In these bundles the myo- 
septa {)ns.) are bent into zigzag 
shape. Above the lateral line one 
of the columns has the apices of 
its myosepta directed forward, 
the other backward. Below the 
line there appears to be a single 
column with apex pointed pos- 
teriorly. Some of the anterior 

fibers of the ventral bundle are specialized as the pectoral muscles of the 
pectoral fin. Of the long muscles we may consider those of the eyeball first. 

Muscles of the Eye 

Two groups of muscles are present in the orbit. The first group is placed an- 
teriorly and consists of the superior oblique {s.o., fig. 91) and inferior oblique 
[i.o.) muscles. These muscles extend from the anterior part of the orbit out- 
ward and backward to be inserted on the eyeball. The second group consists 




Fig. 90. Lateral view of body musculature in 
pectoral region, Heptanchtts maculatus. (Evelyn 
Forsytlie, orig.) 

cl., gill cleft; d.l)., dorsal bundle; d.f., dermal 
fin-rays; d.r.m., dorsal radial muscles of pectoral 
fin ; /.&., lateral bundle ; II., lateral line ; ms., myo- 
septum; tr., trapezius muscle; v.h., ventral me- 
dian muscle. 



[89] 



90 



THE ELASMOBRANCH FISHES 



of the four rectus muscles, all of which arise from the posterior surface of the 
orbit around the base of the optic pedicel (o.p.) . The most dorsal of the rectus 
muscles is the superior rectus (s.r.), the most ventral the inferior rectus {i.r.), 
the most posterior the external or posterior rectus (p.r.), and the most ante- 
rior the internal or anterior rectus (a.r.). They pass outward and forward, 
also to be inserted on the eyeball. 

Buccal and Pharyngeal Muscles 



The muscles in the region of the mouth and pharynx may be separated into 

four groups. The first group includes the superficial constrictor muscles; the 

second comprises the interarcuales ; the third 
the adductors; and the fourth the hypobran- 
chial muscles. 

The superficial constrictor muscles are vis- 
ible upon removal of the skin from the re- 
gion around the gill clefts. The levator maxil- 
lae il.mx., fig. 92) apparently is the forward 
continuation of these muscles. It lies just 
back of the eye and has its origin on the cra- 
nium under the postorbital process {po.o.) ; 
it extends from its origin downward and for- 
ward to the palatoquadrate cartilage (p-q.). 
The constrictors appear as eight dorsoven- 
tral bands (csd.'^'^) separating the gill clefts 
and functioning in their closure. Each con- 
strictor, except the first, may be considered 
to be made up of dorsal and ventral com- 
ponents, although the two in Hepfanch us are 
more or less continuous. The constrictors 
l)ound tlie clefts so that the first and second 
are separated by the spiracular cleft (sp.) 

above, and the 2-3, 3-4, etc., are separated by the cleft following. The last of 

these superficial muscles lies in front of the last cleft. 




Fig. 91. Muscles of the eye, Hcp- 
lanchus maculatus, dorsal view. 

a.r., anterior or internal rectus ; 
i.o., inferior oblique muscle ; i.r., in- 
ferior rectus ; n.II, optic nerve ; o.p., 
optic pedicel; p.r., external or pos- 
terior rectus ; s.o., superior oblique ; 
s.r., superior rectus. 



DORSAL CONSTEICTORS 



The first dorsal constrictor (c.sd.'^) arises from the occipital part of the cra- 
nium and from heavy connective tissue (fascia) surrounding the dorsal longi- 
tudinal bundles. As a thin slip of muscle it passes downward and slightly 
backward around the anterior border of the spiracle to be inserted on the in- 
ner and upper margin of the palatoquadrate cartilage (p-q.). Dorsally and 
anteriorly the first dorsal constrictor is difficult to separate from the levator 
maxillae muscle; furthermore the two have a common line of origin and of 
insertion. 



THE ELASMOBRANCII FISHES 



91 



The second dorsal constrictor (csd.-) is a very broad l)and lying between the 
spiracle and the first branchial cleft. It arises also along the fascia of the 
dorsal bundle, its anterior margin in Heptanchus maculatns being continu- 
ous with the first dorsal constrictor. In Heptanchus cinereus it is overlapped 
by the first constrictor. From its place of origin it extends downward and its 
anterior and more superficial fibers are inserted on the quadrate; the fibers 
lying underneath the superficial fibers join the upper segment of the hyoid; 
while those fibers lying back of them pass over to join the fibers of the ventral 
constrictor. In Heptanchus maculatns, where the dorsal constrictor meets the 
ventral there is superficially a heavy connective tissue septum. 




CSV. 



CSV. 



Fig. 92. Lateral view of the constrictor muscles, Heptanchus mactdatu.s. (From Davidson.) 

a.rnd., adductor mandibulae ; a.pr., antorbital process; csd.^-^, first to eighth dorsal con- 
strictors; csv.^'^, second to eighth ventral constrictors; Us., medial levator labialis; lls.^, 
lateral levator labialis; l.mx., levator maxillae; md., mandible; po.o., postorbital process; 
p-q., palatoquadrate (pterygoquadrate) ; sc, scapular cartilage; sp., spiracular cleft; tr., 
trapezius muscle. 

The third to the eighth {csd.^) dorsal constrictors are more slender and have 
their origin largely from the dorsal fascia and by tendons through the tra- 
pezius muscle {tr.). Tlie superficial fibers pass over into those of the ventral 
constrictors, while deeper fibers, acting as interbranchial muscles (see p. 149, 
fig. 143, ib.cl.), lie just anterior to the cartilaginous branchial rays. In a section 
cutting through the dorsal constrictor between two gill clefts, the dorsal con- 
strictors (csd.) are the thicker bundles lying over the margin of the septum; 
and the deeper fibers (ib.d.), comparable to the interbranchial muscle of the 
more specialized Elasmobranehs, extend inward as thinner bands. 



VENTRAL CONSTEICTORS 



The ventral constrictors of Heptanchus cinereus are seen in figure 93. They 
have their origin from a seam of connective tissue in the midventral line. The 
superficial, posterior fibers pass over into the dorsal constrictors, but the 
deeper ones are inserted on the ceratobranchial segments of the visceral arches. 



92 



THE ELASMOBRANCH FISHES 



In Heptanch us maculatus a first ventral constrictor is closely united with 
the second, which as an immense sheath passes from the midventral line to be 
inserted on the mandibular and hyoid arches. By removing a V-shaped seg- 
ment from the anterior third of the mandible and hyoid, it will be seen that 
this bundle is separated into superficial fibers to the 
mandible and deeper fibers to the hyoid cartilage. 

The third to the eighth ventral constrictors arise simi- 
larly from the midventral, triangular seam. Their su- 
perficial fibers are continuous with those of the third to 
the eighth dorsal constrictors (fig. 92). The deeper slips 
or interbranchial parts of these muscles pass through 
the eoracobranchial muscles and are inserted on the 
neck of the ceratobranchial cartilages {ihv., fig. 95). 

The trapezius muscle {tr., fig. 92) arises from the side 
of the dorsal longitudinal liundle and passes backward 
and downward between this bundle and the dorsal con- 
strictors. A ^^11 slip of it is attached to the upper part 
of the last arch, but most of its fibers are inserted on the 
scapula which it draws forward. 



INTERARCUALES 



The interarcuales (fig. 94) are deep muscles which bind 
the branchial arches together. Two systems of these are 
present, one of which is dorsal (ia.d.'^~^) and the other 
lateral {ia.l}'^) in position. The dorsal interarcuales 
consist of bands which bind the pharyngobranehial car- 
tilages together. In Heptanchus maculatus the first or 
anterior interarcualis is large and extends from the first 
to the second pharyngobranehial cartilage. The follow- 
ing dorsal interarcuales decrease in size and bridge suc- 
ceeding pharyngobranchials. 

There is present in Heptanchus the so-called sub- 
spinalis {s.sp., fig. 94) which, unlike the dorsal inter- 
arcuales, takes its origin from the posterior part of the cranium, from the 
spinal column, and from the ventral fascia of the longitudinal bundle. Like 
the dorsal interarcuales it is inserted on pharj^ngobranchial cartilages, but, 
unlike them, it is inserted bj^ a double tendon on the tips of the first and second 
pharyngobranchials. 

The lateral interarcuales {ia.L, fig. 94), except the sixth {ia.lS'), are double 
bands, the anterior of which in eacli pair bridges the angle between pharyngo- 
branehial and epibranchial cartilage of the single arch; while the posterior one 
extends from the following pharyngobranehial to a common insertion with 
the anterior muscle on the upper and posterior surface of the epibranchial. 




rig. 93. The ventral 
constrictors, Heptan- 
cJius cinereiis. (From 
Vetter. ) 

csr.^"^"*, first, second, 
and eighth ventral con- 
strictor muscles. 



THE ELASMOBRANCII P^TSHES 



93 



Tliere are in Heptanclius inaculatus two levator labiales muscles. One of 
these {lls.^, fig. 92) arises from the median side of the antorbital process, 
passes backward over the angle of the jaw, and as a fibrous band divides the 
adductor mandibulae into dorsal and ventral parts. The other labialis (Us.) 
arises from the cranium in front of the antorbital process and nearer the mid- 
ventral line as a wide and loose band of connective tissue ; it passes under the 
orbit, and over the adductor mandibulae to be inserted at the angle of the jaw. 

ADDUCTORS 

The adductor mandibulae (a.md., fig. 92) is an immense and complex muscle 
which closes the jaws. Superficially it is divided into a dorsal and a ventral 
part by the insertion of the first labialis muscle. The fibers of the adductor 
mandibulae arise from the 
quadrate and are inserted in 
two groups : first, a smaller 
deep posterior group is in- 
serted directly on the man- 
dible; second, a major group 
joins the tendon of the la- 
bialis, and fibers are then 
continued from the lal)ialis 
tendon ventrally to insert on 
the mandible. The insertion 
in general is somewhat ob- 
scured bj- a fibrous capsule 
over the ventral part of the 
muscle. 

An adductor is absent 
from the hyoid, but adductors similar to the deep posterior part of the adduc- 
tor mandibulae are present on all the branchial arches. These muscles have 
their origin in a groove on the inner side of the epibranchial (see p. 149, fig. 
143, ad.) and join the ceratobranchial cartilage. They act in closing the bran- 
chial arch and hence in spreading the cartilaginous branchial rays to enlarge 
the gill pocket. 

Ventral Longitudinal Muscles 




Fig. 94. Interareuales muscles, Heptanchii-'i maculatus. 
(From Davidson.) 

eh}''', first to seventh epibrancliial cartilages ; ia.d}'^, 
first to fifth dorsal interareuales ; ia.l}''^, first to sixth 
lateral interareuales; ph}~^, first to sixth pharyngo- 
branchial cartilages ; s.sp., subspinalis muscle. 



The last group of muscles to be considered in the region of the pharynx is com- 
posed of the hy]:)ol)ranchial or ventral longitudinal muscles. These are forward 
continuations of the ventral body musculature, the segmental nature of which 
is seen in a series of myosepta in the coracoarcuales {car., fig. 95). The 
arcuales communes take origin from the coracoid cartilage and are inserted 
on the heavy connective tissue which forms the floor of the pericardial cavity. 
The coracomandibularis (c.md.) arises from fascia above and between the 
anterior projection of the arcuales and passes forward as a large band to be 



94 



THE ELASMOBRANCH FISHES 



inserted at the symphysis of the mandible. The paired coracohyoideus muscles 
{c.hy.) continue forward from the coraeoarcuales as a layer just dorsal to the 
sides of the coracomandibularis. They are relatively broad muscles in Hep- 
tanchus and are inserted on the basi- and ceratohyoid cartilages {ch., fig. 50a) , 
only a few of the fibers reaching the base of the ceratohyoid cartilage. Dorsal 

to the region of the coraco- 
hyoideus muscle is the third 
group, the coracobranchi- 
ales muscles (c.6r.^""). The 
first six of these arise from 
the heavy connective tissue 
dorsal to the coraeoarcuales 
and pass forward, upward, 
and outward as narrow slips 
to be inserted on the hypo- 
branchial cartilages. The 
last or seventh (c.hr.~) arises 
in two parts; one part is 
continuous with the sixth, 
and the other originates di- 
rectly from the coracoid 
cartilage. This muscle in- 
serts as a wide band along 
the whole length of the last 
ceratobranchial cartilage 
and a part of the median 
piece. Slips of the ventral 
constrictor ( interbranchial, 
ihv.^'''\ fig. 95) muscles pass 
through the ceratobranchial 
muscles. 

Muscles op the Fins 




Fig. 95. Ventral longitudinal or hypobranehial muscles, 
Heptanchus maculatus. (From Davidson.) 

hh., basihyal cartilage ; car., coracoarcualis muscle ; 
c&.\ first ceratobranchial cartilage ; c.hr}'', first to 
seventh coracobranchial muscles ; ch., ceratohyal carti- 
lage ; c.hy., coracohyoideus; c.md., coracomandibularis 
muscle; co., coracoid cartilage; rbv.^"", first to sixth in- 
terbranchial slips; md., mandibular cartilage. 



The muscles on the dorsal 
side of the pectoral fin 
(d.r.m., fig. 90) take origin 
from the posterolateral 
margin of the scapula, and from the basal and, in part, from the radial car- 
tilages of the fin skeleton. The radial muscles are distinct in the central part of 
the fin, but on the posterior margin the separation into definite bundles is not 
so evident. The muscles run toward and are attached to the heavy sheet of 
connective tissue continuous with the dermal fin-rays (d.f.). The radials 
while appearing to be long are, according to Davidson, made up of short fibers. 
The ventral radial muscles arise from the posterior side of the coracoid and 



THE ELASMOBRANCH FISHES 



95 



from tlie basal and radial cartilages, and extend outward to be attached to the 
connective sbeath of the ventral side of the fin. 

The muscles governing the clas])ers of the fins in the male are in bundles 
specialized from dorsal and ventral radial mus- 
cles. We may first examine them on the dorsal side 
of the pelvic of the male (fig. 96). Here they are 
continued from the myomeres to the fin, parallel 
with the radials, and are firmly attached to the pe- 
ripheral part of the fin skeleton. The first of these 
muscles is the adductor (ad.), which arises from 
the posterior border of the pelvic girdle and is 
inserted on the distal end of the basipterygium 
(see p. 50, fig. 55b, ha.p.). The external flexor 
muscle if.e., fig. 96), which has its origin along 
the inner margin of the basipterygium {ha.p.), is 
inserted on the "beta" cartilage (B, fig. 55b). In 
Heptanchus the internal flexor (f.i.) arises in 
common with, but deeper than the external flexor 
on the basipteryginm and on the "beta" cartilage 
and is inserted on the segment 6- (fig. 55) and on 
the proximal part of the basal cartilage (ba.) . On 
the dorsal side are also to be seen the dilator (dl.) 
and the compressor (cp.) muscles. The dilator 
arises on the proximal end of the basal cartilage 
(ha.) and is inserted distally with the connective 
tissue covering the tip of the basal cartilage. The 
compressor muscle (cp.) has its origin from the 
"beta" cartilage (/8, fig. 55b) , and passes backward 
and outward to be inserted on the last radial car- 
tilage. One of the muscles associated with the 
clasper, which is not seen in dorsal view, is the 
constrictor of the sac. Its fibers arise from the seg- 
ments &.^"-, and from the proximal end of the basal 
cartilage. Its dorsal fibers are inserted on the curved radial and its ventral 
fibers form the wall of the pterygopodial sac. 




Fig. 96. Muscles of the pelvic 
fin of male, Heptanchus mac- 
ulatiLS, dorsal view. (From 
Davidson.) 

ad., adductor muscle of 
clasper; cp., compressor of 
the sac ; dl., dilator muscle ; 
f.e., external flexor muscle; 
f.i., internal flexor; p?., pelvic 
girdle; ra., radial muscles; 
s.m., muscle of sac. 



96 



THE ELASMOBRANCH FISHES 



MUSCULATURE OF ELASMOBRANCHS IN GENERAL 




In a consideration of the musculature of Elasmobranelis in general we may 
first notice the primitive segments or somites which in the eml)ryo are ar- 
ranged in series from the region back of the ear to the tip of the tail. A trans- 
verse section through the trunk (tig. 97) shows the somite to be made up of 

an outer and an inner layer between which is a 
central cavity or myocoele (mc). The outer layer 
produces the dermatome or cuticle plate (dt.) and 
the inner layer is divided into an upper myotome 
i^uy.) and a lower sclerotome (scl.). These two 
layers extend ventrally as the lateral plate and 
inclose between them the body cavity or coelom 
(c). For a time in the Elasmobranchs these two 
cavities are continuous, but later they are sepa- 
rated by the fusion of the two layers giving a 
somite dorsally independent of the lateral plate 
(seep. 298, fig. 257). 

The sclerotome of the somite, as we have seen in 
Chapter III, produces the elements of the verte- 
bral column, while the dermatome gives rise to 
those connective tissue fibers characteristic of the 
corium and sometimes may also give rise to a 
part of the muscle tissue. The myotome, however, 
produces the mass of skeletal muscle. The lateral 
plates which enclose the coelom are divided into 
an inner splanchnic layer (spl.) which produces 
the muscular layer of the digestive tract; and a 
somatic layer (so.) which thins out ventrally, 
forms the peritoneal lining of the body cavity, and gives rise to connective 
tissue cells. 

The cells of the myotome, the myoblasts, elongate and attach themselves 
both anteriorly and posteriorly to the connective tissue septa (myosepta) sep- 
arating somites (fig. 98). Such a muscle cell or fiber furthermore becomes 
differentiated into longitudinal fibrils and is crossed by a series of transverse 
stripes or bands. In the body of the adult Elasmobranch the fibers generally 
retain the simi)le plan of attachment to myosepta. But the myosepta in the 
adult have secondarily changed their course, always, or at least generally, so 
as to take a zigzag direction. 

The myotome next extends its boundaries dorsally to the middorsal line, and 
ventrally it grows toward the midventral line. The fibers formed dorsally 
between the dorsal septum and the lateral line septum compose the dorsal 
longitudinal bundles (d.b., figs. 100 and 112) which extend from the occipital 



Fig. 97. Transverse section 
through developing somite, 
Pristiurus. (FromEabl.) 

c, coelom; chd., notochord; 
dt., dermatome ; hch., hypo- 
chorda ; mc, myocoele ; my., 
myotome or muscle plate; 
7i.t., neural tube; scl., sclero- 
tome ; so., somatic layer mes- 
oderm ; spl., splanchnic layer 
mesoderm. 




THE ELASMOBRANCH FISHES 97 

region of the skull to the tip of the tail. These fillers universally take a horizon- 
tal direction. Those fibers between the lateral line septum and the midventral 
line are divided into a lateral (l.h.) and a ventral (v.d.) region. The lateral 
muscle (l.h.) is readily recognized by the fact that it lies just ventral to the 
lateral line and is of a dark color. Between pelvic and pectoral regions this 
bundle is folded in in such a way (figs. 99 and 100) as to be overlapped by the 
ventral bundle (v.h.). Anterior to the pelvis the fibers in the lateral bundle 
may take an almost horizontal direction (l.h., fig. 112) but they usually take 
an oblique direction of anterior and 

upward (fig. 100) ; while posterior to .^''-'r'^''^f^i^'. 

the pelvic area the fibers of this bundle 
run more or less horizontally. 

The ventral bundle {v.h.,^gs. 100 and 
112) is highly specialized and its fibers 
take a characteristic anteroventral di- 
rection. In some of the Elasmobranchs, 
as in Zygaena, the ventral bundle is 
further specialized into a rectus ab- 
dominis at the midventral line (r.a., 
fig. 100). 

Figure 99 is a model for Squalns 
siickUi of all muscle fibers between two 

myosepta in the region of the first dor- Tig. 98. Showing the finer anatomy of de- 
, „ T^ m ., Ml 1 ,T , ,1 veloping muscle. (Modified from Erik 

sal nn. Dorsally it will be seen that the Miiiier.) 

septa run posteriorly close together 

and almost parallel with the middorsal line. They then turn anteriorly and 
run forward. Next they bend on an acute angle and curve backward; and then 
they turn forw^ard and downward to the lateral line, where they fold inward 
and are carried forward. When the septa emerge from the fold they are dis- 
placed backward so that as they pass through the lateral bundle they are fully 
a half -segment posterior to the same myosepta above the lateral line. In the 
lateral bundles the septa curve downward and backward and then turn for- 
ward where they fold inward and backward to emerge in the ventral bundle 
(v.h.). In the ventral bundle they run sharply downward and slightly forward. 
They then turn backward, and finally run forward to the midventral line. 

In a transverse section of the tail of Lamna the various muscle cones appear 
in their relation to one another (fig. 101). The rows here are so arranged that 
a dorsomedian {dm.), a dorsolateral {dl.), and a lateral row {l.h.) lie above 
the lateral septum which extends from the spinal column to the skin in the 
region of the lateral line (not drawn in the figure). The ventrolateral {vl.) 
and ventromedian {vm.) rows are below the septum. The bundles are made up 
of concentric lamellae, the concentricity of which is due to the projection of 
the apex of one cone into the angle of the V's in front of or back of it. The 
dorsolateral and ventrolateral bundles are small, the dorsomedian and ventro- 
median bundles are large, and the lateral bundle, composed of nine concentric 



98 



THE ELASMOBRANCH FISHES 




Fig. 99. Model of muscle fibers between two myosepta, Squalus suchlii. (From Coles.) 

en., extension of myosepta forming cone; d.a., dorsal aorta; d.vm., dorsal vertebromuscu- 
lar artery; i., intercostal artery ; i.b., intercostal branches to preceding segments; L., lateral 
bundle; U., lateral line fold; U.', fold between lateral and ventral musculature; ms., myo- 
septa ; rn., renal artery ; V^, F.,, V^, bends in myosepta. 



THE ELASMOBRANCH FISHES 



99 



lamellae, is of immense size. In side view the Vs would appear very long and 
acute, many of them being cut in transverse section. 

We may next consider the specialized or long muscles of the adult which 
are present in tlie hccul ami in the ])liaryngeal region. 

Muscles of the Eye 

The muscles of the eye originate from head somites (fig. 102) which in a way 
are like the body segments or somites previously described. The first or pre- 
mandibular somite, the second or mandibular, and the third or hyoidean 
somite, all take part in tlie formation of these nuiscles. It will be observed 





Fig. 100 Fig. 101 

Fig. 100. Trunk musculature, Zygaena side view. (From Maurer.) 
Fig. 101. Transverse section, muscle bundles of tail, Lamna. (From Ewart.) 

d.b., dorsal bundles; dl., dorsolateral bundle; dm., dorsomedian bundles; I.b., lateral 
bundle; II., lateral line; r.a., rectus abdominis; v.b., ventral bundle; vL, ventrolateral 
bundle; vm., ventromedian bundle; x, part of ventral bundle removed to show overlapping 
of lateral bundle. 

from the figure that somites four, five, and six, which are in the region near 
the enlarging ear capsules, degenerate and consequently take no part in the 
formation of eye muscles. 

The most anterior of these head somites, the premandibular,^ gives rise to 
four of the six muscles of the eye. The first muscle to bud off from this somite, 
arising ventrally as shown by the ruled lines in figure 103 (Lamb, 1902) is 
the inferior oblique (i.o.). Following this the inferior rectus (i.r.) arises also 
from the ventral part of the somite. From the dorsal side the internal or an- 
terior rectus (a.r.) and the superior rectus (s.r.) are budded off. The second 
or mandibular somite divides into two parts, a small upper part and a larger 
lower part. From the upper part the superior oblique muscle (s.o.) arises. 
The cells which form this muscle first take a longitudinal direction; later, as 
they become muscle fibers, they lie more in a dorsoventral direction. From the 
ventral part of the second somite a part of the rectus externus arises (Neal). 

1 In front of the premandibular has been described (Piatt, 1891) still another somite 
(Acanthias) . 



100 



THE ELASMOBRANCH FISHES 



The third or hyoidean somite is simpler than the rest, and from its lower part 
the remainder of the external or posterior rectus muscle (p.r.) arises. The 
relations of the muscles to the somites and nerve supply are tabulated by Neal 
as follows : 

Myotome Muscle Innervated by 

( Rectus superior Oculomotor nerve III 

2q . J 

) Rectus internus Oculomotor nerve III 

(Rectus inferior Oculomotor nerve III 

Iv J 

)Obliquus inferior Oculomotor nerve III 

2d Obliquus superior Trochlearis nerve IV 

2v Rectus externus) ,_ ^ 

„ -r. X X ^ Abducens nerve v I 

3v Eectus externusl 

The position occupied by the inferior oblique muscle of the adult varies 
somewhat in different forms. In Heptanchns the inferior oblique is attached to 
the orbit practically against the superior oblique, while in Acanthias the two 
maj^ be slightly apart. In the saw shark, Pristiophorus, an interesting condi- 




Fig. 102. Diagram showing the head somites in relation to the body somites in Squahis 
acanthias. (From Neal.) The eye muscles are derived from the first three somites. 

hyp., hypobranchial (hypoglossus) musculature; m., mouth; ot., otic capsule; III and VI, 
third and sixth head somites. 

tion obtains in which a second strip of the inferior oblique is attached along 
the infraorbital plate. This is evidently a forerunner of the condition present 
in the rays, in which the whole of the muscle has its attachment as a broad 
band ventral to the orbit {Ehinohatis, Raja). In this respect, then. Prist io- 
phorus is a transitional form between the sharks and the rays. 

Muscles op Visceral Arches 



The levator lahii siiperioris muscle in a type like Acanthias {Us., fig. 104) 
consists of a single muscle on each side, which has its origin on the interorbital 
space ventral to the cranium. The muscle passes l)ackward and outward along 
the upper jaw and over the dorsal labial cartilages. At the angles of the mouth 
it becomes tendinous and is inserted on the mandible. In all probability, this 
muscle is comparable to Us. of Heptanchns. In Torpedo (fig. 105a) (see 
Tiesing, 1896), the muscle is divided into two parts, a median (Us?) and a 



THE ELASMOBRANCH FISHES 



101 



lateral levator (llsr). The lateral levator labii has its origin behind the angle 
of the ethmoidal region and is inserted on the process of the quadrate. The 
median head arises as a broader muscle near the preorbital process and passes 
as a tendon around the angle of the mouth to join the mandil)ular muscle as in 
Acanthias. 

A more complex condition is reached in Rhinobatis and in Baja. In the 
former there are four (or even five) parts to the muscle, and in the latter five 
parts are cliaracteristicall}- present. In Raja (fig. 107b) the first division of 
the muscle Us} represents the me- 



dian division of the muscle in Tor- 
pedo. There is a second muscle (fig. 
107b, Us.-) M'hich is large in Raja 
and which holds a peculiar position 
in Rhinobatis, being surrounded at 
its base by the mandibular muscle 
soon to be described. A third slip 
(lls.^) is isolated, and a fourth and 
a fifth ( ?) join the mandibular 
muscle. 

The levator maxillae superioris 
muscle in Acanthias {l.mx., fig. 104) 
has its origin from the postorbital 
processes and supraotic crest, and 
passes downward in common with 
the first dorsal constrictor, the fibers 
of the latter lying directly against 
the spiracle and the fillers of the 
former (levator maxillae) being 
those of the anterior group. The 
levator maxillae is inserted on the 




093 



Fig. 103. Development of the muscles to the 
eye, AcantJiias, right, median view. (From 
I^amb.) 

a.r.. anterior rectus; cl.g., ciliary ganglion 
of fifth nerve; g.g., gasserian ganglion; i.o., 
inferior oblique; i.r., inferior rectus; o.p., 
optic pedicel; op.V, ophthalmicus profundus; 
p.r., external or posterior rectus; s.o., supe- 
rior oblique; os.V and VII, ophthalmicus su- 
perficialis branches of tlie fifth and seventh 
nerves; s.r., superior rectus; /// and VI, ocu- 
lomotor and aljducens nerves respectively. 



palatoquadrate. In Carcharias, how- 
ever, in addition to its postorbital relations, it extends its origin anteriorly 
along almost the whole of the supraorbital crest. In the rays, this muscle and 
the first dorsal constrictor are distinct and separate. In Torpedo {l.mx., fig. 
105a) the levator maxillae is well defined and runs far forward to be inserted 
on the median palatal part of the palatoquadrate. In Rhinobatis it is much 
broader than in Torpedo and is inserted still farther forward. In Raja {l.)iix., 
fig. 105b) it arises by two heads, a superior and an inferior; the fibers of the 
two unite anteriorly, whereupon insertion is made on the palatine part of the 
palatoquadrate cartilage. 



Superficial Constrictors op Pharynx 



The superficial constrictor muscles vary considerably from those studied in 
Heptanchus. These muscles in pentanchid Elasmobranchs are six in number. 



102 



THE ELASMOBRANCH FISHES 



In the rays, the fibers of the dorsal constrictors are not continuous with those 
of the ventral constrictors, the two sets being separated by a horizontal tendon. 



DOESAL CONSTRICTORS 



The first dorsal constrictor in Acanthias (fig. 104), as has been said, is sepa- 
rated from the levator maxillae only at the ventral part of the spiracle where 
its fibers curve posteriorly around the base of the spiracle to be attached to 




Fig. 104. Lateral pharyngeal muscles, Acanthias. (From Vetter.) 

a.md., adductor mandibulae ; csd.-, second dorsal constrictor ; csv., ventral constrictor ; 
Us., levator labii superioris ; \.mx., levator maxillae ; tr., trapezius. 

the quadrate. In a boiled specimen of Mu^telus californicus the first dorsal 
constrictor can readily be distinguished from the levator maxillae by a differ- 
ence in its color. Furthermore its anterior ventral margin folds in under the 
levator maxillae. In the rays (fig. 105) the first dorsal constrictor is a clearly 
marked band which forms a crescent in front of the spiracle. 

In Elasmobranchs which have a nictitating membrane the muscles con- 
trolling that membrane are derivatives of the first dorsal constrictor. The nic- 
titator in Mnsielus californicus may here be described with the other accom- 
panying muscular slips. It has its origin posterior to that of the first dorsal 
constrictor and under the second dorsal constrictor. From its origin it passes 
anteriorly and downward in front of the spiracle {nt., fig. 106) to be inserted 
on the nictitating membrane (tj.). Crossing the nictitator almost at right 
angles is a deeper muscle, the depressor (dp.) of the upper eyelid, which takes 
origin somewhat posterior and dorsal to the spiracle and over the second con- 
strictor. This passes forward and upward to be inserted at the tail of the eye 
on the upper eyelid. Arising in the common mass with the depressor of the 
upper lid but superficially, is a rudimentary retractor palpebrae superioris 
(r.p.) which ends anteriorly against the nictitator so that the nictitator oper- 
ates between it and the depressor. Arising from the anterodorsal margin of 



THE ELASMOBRANCn FISHES 



103 



the spiracle is the dilator {(U.) of tlie spiracle, which passes anteriorly and nj)- 
ward between the reti-actor and dej)ressor finally to join the nictitator. Sur- 
rounding the spiracle (sp.) sui)erficially is the constrictor spiraculae (c.s.). 
The second dorsal constrictor (cs(]r, fig. 104) in the sharks extends from the 
supraotic crest of the cranium back to and even perforating the anterior end 
of the trapezius (tr.) ; other fibers arise from the seam dorsal to the cleft. The 




a.md. 




A B 

Fig. 105. Dorsal pharyngeal muscles. (Prom Tiesing.) A. Torpedo. B. Baja. 

a.md., adductor mandibulae; csd., dorsal constrictor muscle; l.hm., levator hyoman- 
dibularis; lls.'^-', first to fifth slips of levator labiales; l.mx., levator maxillae; /.r., levator 
rostri. 



insertion of this muscle in Acanthias is interesting. Part of its fibers touch the 
hyomandibula and a number of other fibers impinge on the ceratohyoid ; while 
still a third set joins a horizontal tendinous bridge, separating dorsal and 
ventral constrictors. Back of this tendon other fibers are continuous with the 
ventral constrictors. 

Those fibers inserted on the hyoid in Heterodontus and Squatina are sepa- 
rated into a kind of levator hyomandibularis. In the rays the levator hyo- 
mandibularis (l.hm., fig. 105) is a muscle of great importance. It may be in- 
serted on the hyomandibula at the angle {Torpedo, fig. 105a, l.hm.; Raja (fig. 
105b) , or on the lower part as in Rhinohatis. In the rays those fibers back of the 
levator hyomandibularis and in front of the first cleft form the second dorsal 
constrictor proper. These fibers run practically in a horizontal direction. 

The remaining dorsal constrictors are similar to the second. A point or two, 
however, may be added. From the third to the fifth constrictors in the sharks 



104 



THE ELASMOBRANCH FISHES 



(Acanthias), the fibers dorsally arise largely from the seam above the cleft 
and from the extrabranchial cartilages. These fibers pass do^\^lward and 
those more anterior are inserted on the seam in front and on its underlying 
extrabranchial cartilage (fig. 104). The posterior fibers pass on and are con- 
tinuous with the ventral constrictors. The sixth dorsal constrictor in addition 
has some of its fibers arising from the scapular part of the pectoral girdle. 

In the rays there is present a levator rostri (l.r., fig. 105). This muscle has 
its origin dorsally in the fascia of the longitudinal muscle, near the posterior 

seam at the sides of the dorsal con- 
strictor, and extends to the margin of 
the rostrum (Rhinohatis) where it is 
inserted on heavy tissue. 

The trapezius, which is not a part of 
the constrictor system, may here be 
considered. In sharks it arises from the 
fascia of the dorsal bundles, and from 
the cranium above and at the sides of 
tlie ninth foramen (Acanthias), its 
origin extending almost from the cra- 
nium to the region of the scapula. In 
Acanthias (#r., fig. 104) it is perforated 
by slips of the dorsal constrictors and 
has its insertion largely on the scapula. 
In the rays several slips of muscle are 
found in the location of the trapezius, 
but it is not certain that these are comparable with the trapezius of the sharks. 




Fig. 106. Muscles to the nictitating mem- 
brane and associated parts, lateral view, 
Mustelus calif ornicus. 

C.S., constrictor spiraculae; dl., dilator 
spiraculae; dp., depressor of upper lid; 
I., upper eyelid ; n., nictitating membrane ; 
?i^.,nictitator muscle ; r.p., superficial fibers 
which do not reach upper lid ; sp., spiracle. 



VENTEAL CONSTRICTOES 



In Acanthias, according to Marion (1905) there may be two parts to the first 
ventral constrictor, the more anterior of which is the smaller and bridges the 
anterior symphysis of the lower jaw. The more posterior part is much wider 
and is like that in Heptanchus. These parts take origin from a midventral 
seam and are inserted on the posterior margin of the mandible. In the rays the 
second part acts as a depressor mandibulae {cl.mcl., fig. 107b). 

The second ventral constrictor in the sharks is similar to that in Heptanch lis. 
This also arises from the midventral seam and is inserted on the border of the 
hyoid cartilage. In the rays the second ventral constrictor is divided into an 
anterior and a posterior part. The anterior part acts as a depressor hyomandib- 
idaris {d.hm., fig. 107b) , a muscle antagonistic to the levator hyomandibularis 
previously mentioned as derived from the second dorsal constrictor. The more 
posterior and narrower ventral part is the second ventral constrictor. 

Each of the remaining ventral constrictors in the rays (fig. 107b) consists 
of two parts, a large outer and a small inner portion. 



THE ELASMOBRANCII FISHES 



105 



Superfieial to this ventral region in tlie rays, arising from the fascia of the 
ventral longitndinal nmscle (the coraeomandihnlaris), is a depressor rostri 
{d.r., fig. 107). This is inserted far forward between the end of the propter- 
ygium and the tip of the rostrum {Raja). It has a balancing effect on the 
levator rostri (see fig. 105, l.r.) previously described. 

Deeper Muscles of Pharynx 



The interbranchials of more specialized Elasmobranchs are usually more 
completely separated from the constrictors into distinct bundles than are 



c.md 



cbr.. 





Fig. 107. Ventral pharyngeal muscles. (From Tiesing.) A. Torpedo. B. Eaja. 

a.mcl., adductor mandibulae ; cl., tirst gill cleft; c.hr., coracobranchial muscle; c.hy., cora- 
cohyoideus; c.md., coracomandibularis ; csv., ventral constrictor muscle; d.hm., depressor 
hyomandibularis ; d.md., depressor mandibulae; d.r., depressor rostri; lls.^, median levator 
labii; Us.'^, lateral levator labii; Us.*~', fourth and fifth slips of levator labii; n.a., nasal 
aperture. 

those of HeptancJiKS. In Acanihias (fig. 108a), as is true in general, they lie 
between the demibranchs, in front of and against the branchial rays of all the 
whole gills. The dorsal fibers arise from the seams, separating the dorsal con- 
strictors above the cleft, and from the dorsal extrabranchial cartilages. The 
shorter, inner fibers are attached to the outer margin of the epibranchial seg- 
ment of the branchial arch, while the outer fibers pass over to ventral inter- 
branchial fibers (sharks) . The ventral fibers of the interbranchials arise from 
the fascia around the coracomandibularis muscle and from the ventral extra- 



106 



THE ELASMOBRANCH FISHES 



branchial cartilages. The shorter or inner fibers join the eeratobranchial carti- 
lage, and the outer fibers in the sharks pass into the dorsal interbranchial 
fibers. 

In the rays the interbranchials are somewhat like those of the sharks. Here, 
however, the dorsal and ventral fibers do not form a continuous muscle even 
at the outer margin. The fibers both dorsally and ventrally arise as in the 
sharks. The inner dorsal fibers are inserted also on the epibranchial {eh., fig. 





A B 

Fig. 108. Interbranchial musculature. (From Marion.) A. Acanthias. B. 2?«m erinacea. 

ad., adductor muscle; br., branchial ray; ch., ceratoliranchial cartilage; eh., epibranchial 
cartilage; ex.b., extrabranchial cartilage; td., longitudinal tendon. 

108b) and the inner ventral fibers on the eeratobranchial cartilages (ch.). The 
outer fibers are attached to the cartilaginous branchial ray which proceeds 
outward at the angle between the epibranchial and eeratobranchial segments. 
Some of the most median fibers ventrally take origin from the fascia of the 
coracomandibularis muscle. 

INTEEARCUALES 



The interarcuales, as in Heptanchus, are divided into two systems, a dorsal 
(medial of Marion) and a lateral system. The subspinalis, which is considered 
li}^ some as the first of the dorsal systems, arises from the base of the cranium, 
the ventral side of the vertebrae, and the fascia ventral to the longitudinal 
bundle, and tapers back to be inserted near the tip and laterally usually on the 
first pharyngobranchial cartilage. In Scyllium, as in Heptanchus, the sub- 
spinalis is divided into two bands; in Sqtiatina it is very slender; and in Sci/m- 
nus and in the rays it is generally absent. 

The true dorsal interarcuales {ia.d., fig. 109) unite succeeding pharyngo- 
branchial segments. The dorsal interarcuales vary in number; five in Hep- 
tanchus, four in Hexanchus, three in Acanthias and Heterodontus, and two 
in Raja. In Scijmni(s the tips of the pharyngobranchial segments are bound by 
strong ligaments to a sheet of connective tissue ventral to the spinal column. 

In Acanthias (fig. 109a), as in Heptanchus and many other sharks, the 
lateral interarcuales system (ia.l.) consists of V-shaped muscles, one limb of 



THE ELASMOBRANCH FISHES 



107 



the V arising on tlie posterior side of the base of the first pharyngobranchial, 
and tlie other linih, on the anterior side of the base of the following pharyngo- 
branchial. The fibers of the two limbs of the V pass ventrally and anteriorly 
and unite before they are attached on the posterior side of the epibranchial 
cartilage. The first to the third interarcuales of the lateral system in pentachid 
sharks are alike, but the fourth is undivided in its origin. 




Fig. 109. Interarcuales muscles. A. AcaniJiias. (From Vetter.) B. Heterodontus francisci. 
(Lucile Graham, orig.) 

eh., epibranchial cartilage; ia.d., dorsal interarcuales muscles; ia.J., lateral interarcuales; 
ph., pharyngobranchial cartilage. 

In Heptanchus maculatus (p. 93, fig. 94) the second or posterior branch of 
the first lateral interarcualis is seen to be closelj^ related to the dorsal system ; 
while in the second to the fifth, the lateral interarcuales come to be more and 
more widely separated from the dorsal system. In Heterodontiis (fig. 109b) 
the dorsal system consists of very broad bands, and the lateral system is also 
composed of wide bands, the inserting heads of which are not divided. 

ADDUCTORES ARCUS 



Under this head are included the adductor mandibulae and the branchial ad- 
ductors. The adductor' mandihidae in a type like Acanthias {a.md., fig. 104) 
may be described as a mass of muscle at the outer angle of the jaw, which arises 
from the quadrate and passes over to the whole of the side of the posterior 
part of the mandible. According to Vetter a small slip of the adductor mandi- 
bulae in Acanthias may also pass downward and join the ventral constrictor 
already described. In a form like Heterodontus the adductor is of unusual 
size. In Torpedo {a.md., fig. 107a) it is divided into a large median and a 
smaller lateral part; and in Eaja (fig. 107b) and Rhinohatis the median part 
is small and the outer part is subdivided into a large inner and an immense 
outer division. 

Adductors are present on all the gill-bearing arches, except the hyoid. They 
arise from grooves on the inside of the epibranchials, and are inserted on simi- 
lar grooves inside of the ceratobranchials (fig. 108b, ad.). In the branchial 



108 



THE ELASMOBRANCH FISHES 



region of Chlamydoselachus the adductors diminish in size from behind for- 
ward, so that, in addition to the absence of one from the hyoidean arch, there 
is but slight evidence of a first branchial adductor. 

Hypobranchial Musculature 

The hypobranchial or ventral longitudinal muscles extend from the coracoid 
cartilage forward under the branchial basket, and consist of the arcus com- 
munes which continue forward from the pectoral girdle, the coracomandi- 





Fig. 110. Hypobranchial muscles. (From Max Fiirbriiigcr.) A. Scymnus. B. Heterodonlus 
philippi. 

car., coracoarcualis muscle; c.hr., coracobranehialis; c.hy., coracohyoideus muscle; c.md., 
coracomandibularis. 



hularis, lying in the middle line, the coracohyoideus at the sides of the cora- 
comandibularis, and the deeper coracohranchiales extending to all the bran- 
chial arches. All these muscles except the coracobranchials arise from the 
first five trunk myotomes (fig. 102) as buds which migrate forward and me- 
diad and take up their position under the branchial and buccal areas. The 
coracohranchiales in Scyllium, according to Edgewortli (1903), are developed 
from head myotomes. In the sharks the hypobranchials usually come to be 
heavy round muscles, while in the rays they are more or less flattened. 

The arcus communes (see coracoarcualis, car., fig. 110) are separated by 
mj^osepta (scriptores tendiniae) , much like other ventral bundles, into a vary- 
ing number of segments in different forms. In Heterodontus philippi (fig. 
110b) , for example, a single segment is produced, in Raja two, and in Scymnus 
(fig. 110a) and a number of other types, five are present anterior to the girdle. 



THP] ELASMOBRANCH FISHES 



109 



Tlie eoracomandibularis {c.md.) in the shark is seen ui)()n removing the 
skin and the first and second ventral constrictors. In the rays it lies directly 
under the skin. In 8cynin\(s {c.md., fig. 110a) it is an unpaired muscle which 
originates directly from the coracoid cartilage and is inserted near the man- 
dibular symphysis. That the muscle is of a paired nature, however, is shown 
by its being innervated by paired nerves. In Raja its paired condition is indi- 
cated anteriorly, and in Aetolxitis the two muscles are entirely separate. 

The coracohyoideus muscle {chij., fig. 110) is seen lying at the sides of the 
eoracomandibularis {Heterodontus,&g. 110b) . It arises from the fascia around 
the arcus communes and is inserted on the 
basihyoid and lower part of the hyoid 
arch. In Scymnus (fig. 110a) it passes 
more directly forward from the most an- 
terior arcual segment than it does in Hep- 
tanchus. In Raja (see fig. 107b) the coraco- 
hyoideus is small and lies more or less over 
the eoracomandibularis. It is inserted on 
the ventral part of the basihyal segment. 

The coracobranchiales (c.hr.) are the 
deepest of the hypobranchial muscles and 
support the floor of the pericardial cavity. 
They consist of five separate parts in pen- 
tanchid sharks, six in Hexanchns, and 
seven in Heptanchiis. In the rays they 
arise in a common mass. The first coraco- 
branchialis (c.hr.) is large in Scymnus; 
and the remaining muscles, except the last 
which is tlie largest, are smaller. They are 
normally inserted on the ceratobranchial 
segment of the branchial arches. 




Muscles op the Fins 



Fig. 111. Development of the muscles 
to the pectoral fin, Acanthias. (From 
Erik Miiller.) 

1-33, muscle buds; I-XII, nerves. 



In Elasmobranchs the muscles of the 
paired fins arise in the embryo as buds 

from the ventral downgrowth of the myotome. Varying numbers of these buds 
take part in the formation of the fins in different species of Elasmobranchs. 
After the dorsal somite is separated off from the lateral plates its myotome 
grows rapidly both dorsally and ventrally. As the ventral downgrowth 
passes the fin area it gives off the muscle buds as lateral growths. The muscle 
bud separates into an anterior and a posterior primary bud, each of which in 
turn divides into an upper and a lower secondary bud so that from one myo- 
tome four buds arise. The two of these l)uds which are dorsal in position go to 
make up dorsal fin muscles, while the two ventral in position compose the 
ventral musculature of the fin (fig. 111). 



110 



THE ELASMOBRANCH FISHES 



It is evident that the number of segments tliat take part in the formation 
of buds for the pectoral fin is fewer in the sharks than in the rays. This fact is 
clear when we consider two types like Mustelus and Torpedo, in the former of 
which the fin is relatively narrow and in the latter is of great extent. Accord- 
ing to Maurer (1912), in the embryo of Mustelus only ten segments contribute 
to the formation of the musculature of the pectoral fin ; while in Torpedo there 
are twenty-six such segments. 

The further course of the development of these buds in two forms like the 

above has been studied in great 
detail because of the bearing which 
such development has on the lateral 
fin-fold theory. That in a type like 
Mustelus segments (myotomes) an- 
terior to the pectoral fin and between 
the pectoral and pelvic fins form 
buds which atrophy without enter- 
ing the fin is taken by those wdio 
accept the lateral fin-fold theory to 
mean that the fin previously had a 
much greater anteroposterior extent 
than at present; and it is hence in 
agreement with what would be ex- 
pected from that theory. 

Muscles of the unpaired fins are 
formed in essential respects like 
those of paired fins. As the myotome 
grows dorsally to the middorsal line 
it gives off buds in the regions of the 
dorsal fins and the dorsal lobe of the 
caudal fin. Each bud for the un- 
paired fins divides into an anterior 
and a posterior bud but no further 
division takes place since the buds from opposite sides unite to form the muscu- 
lature of the unpaired fins. Ventral buds arise from the tip of the tail forward 
to the anal region. Tlie more posterior of these supply the ventral lobe of the 
caudal fin, while those in the region of the anal fin, in forms in which an anal 
fin develops, provide musculature for that fin. 




Fig. 112. Adult pectoral muscles, Sqtialus 
sucJdii. (Evelyn Forsythe, orig.) 

cL, gill cleft; d.b., dorsal bundle; d.r.m., 
dorsal radial muscle of pectoral fin; l.h., lat- 
eral bundle ; U., lateral line ; ms., myoseptum ; 
tr., trapezius muscle ; v.h., ventral median 
muscle. 



MUSCLES OF THE CLASPERS 



The muscles which control the claspers are usually more complex than those 
described for Heptanchus. In Chlamydoselachus, however, few points of modi- 
fication are shown. The principal change is noted in the area of the adductors. 
While the adductor in the notidanids is a long muscle, in Chlamydoselachus 
(ad., fig. 113) it is relatively broad and fan-shaped. Here, too, the external 



THE ELASMOBRANCH FISHES 



111 





Fig. 113. Muscles of the claspers, Chlamydoselachus. A and B, ventral and dorsal views. 
(Trom Goodey.) ad., adductor; cp., compressor; dl., dilator; f.e., external flexor; f.i., in- 
ternal flexor. 





Fig. 114. Muscles of the claspers, Eaja. (From Bachman.) A and B, ventral and dorsal 
views, ad., adductor; cp., compressor; dl., dilator; f.e., external flexor. 



112 THE ELASMOBRANCH FISHES 

flexor (f.e.) differs in that its point of origin is far removed from that of the 
internal flexor (/.?".). 

In the rays, where the skeleton of the fin is much more complex, the muscles 
have undergone a relatively high degree of specialization. In figure 114a of 
Raja clavata the adductors (ad.) are shown on the ventral side as diverging 
fibers passing toward the sac. Surrounding the sac is the large compressor, a 
part of which also appears in dorsal view {cp., fig. 114b). The large dilator 
(dl.) in the same view is divided into a ventral and a dorsal part, the latter 
being very heavy. Continuing from this muscle on the dorsal side is the large 
external flexor (f.e.) and at its sides is the margin of the adductor seen also in 
ventral view. 



ELECTRIC ORGAN IN ELASMOBRANCHS 

One of the most highly specialized organs found in the animal kingdom is 
present in the rays. This is the electric organ by means of which electric shocks 
can be generated. While this organ is found in its perfected form in Torpedo, 



/Ids. 



Fig. 115. The relation of the electric organ to the muscles, Eaia batis. (From Ewart.) 
ds., electric discs; mf., muscle fibers. 

it is also present in the genus Baja. In Torpedo it consists of vertically placed 
discs located on the dorsal part of the pectoral fin. In the rays, on the other 
hand, it is made up of a series of cones located in the tail. 

Electric Organ of Rays 

The electric organ of the ray is spindle-like and extends throughout the greater 
length of the tail. It is spindle-shaped, however, only in part. While it tapers 
gradually at both ends, in the middle region it is not always cylindrical, since 
it is subject to pressure from the surrounding ligaments, muscles, and the 
vertebral column, resulting often in deep grooves in the organ. (Ewart.) 

Figure 115 shows the relation of the electric organ of the ray to the sur- 
rounding muscular tissue. From this it is seen that the organ is continuous 
with the lateral row of muscle cones. In fact it is clear that the organ itself is 
formed as a series of cones altogether similar to those of the muscle, with the 
single exception that the direction of the muscle fibers (mf.) in the muscle is 
more ol)lique to the myosepta than are the discs (ds.) of the electric organ. 




THE ELASMOBRANCH FISHES 



113 



m 



't/f 





Fig. 116. Development of electric organ of Baia hatis. Stages A-C. (From Ewart.) 
ds., electric disc; el.c, electric club; mf., muscle fiber; ms., myoseptum; n., nerve. 



114 



THE ELASMOBRANCH PISHES 



The electric discs (ds.), although directed more at right angles to their con- 
nective tissue septa, have otherwise the same general relations as the fibers of 
the muscle. Like them they are smaller and less regular at the tip and base of 

the cone. They are, however, much larger 
than the muscle fibers. As individual units 
they are more or less quadrangular in 
shape, their walls being formed of connec- 
tive tissue. A glance will show that there 
are a great number in a single organ. It has 
been estimated that as many as 20,000 discs 
are present in an adidt Rata hatis. 

One of the most interesting things about 
the electric organ is the fact that, whether 
it be in Raja or in Torpedo, it is formed as 
a series of metamorphosed muscle fibers. 
The organ as described by Ewart in Raia 
hatis first appears when the emlu-yo is 
about an inch long. Here it is confined to 
the tail and only those muscle fibers are af- 
fected Mhich belong to the lateral bundles, 
as described for Lamna (fig. 101). These 
fibers undergo complete change of form 
and assume an entirely secondary func- 
tional role. 

A group of such fibers (fig. 116a) shows 
the beginning stages in this metamorphosis. 
The anterior fibers, near their attachment 
to the nn'oseptum, are beginning to enlarge 
into club-shaped structures {el.c), while 
the posterior fiber (inf.) is still of the mus- 
cle type. By further growth and difi^erentiation each incipient electric club 
comes to assume the form of a cone, in the enlarged end of which in time a con- 
cavity forms. A nerve entering this concavity breaks up into many branches 
{n., fig. 116b). At this stage, striation characteristic of the muscle fiber has 
decreased on the cups, but striations are still present on the body of the cone. 
In a later stage (fig. 116c) the organ has acquired essentially the adult char- 
acteristics. In this stage it is seen that the disc (ds.) has greatly enlarged and 
that the terminal part of the cone has lost something of its muscle-like appear- 
ance. From a somewhat more mature organ we may study the detail of its 
finer structure. 

FIXER AXATOMY OF ELECTRIC ORGAN 




Fig. 117. Finer structure of electric 
disc. (From Ewart.) 

av., alveolar layer; o.e., outer elec- 
tric layer; str., striate layers. 



The disc may be divided into three well defined layers (fig. 117), an outer 
electric layer (o.e.), a middle striated layer (str.), an inner alveolar layer 
(av.). The outer layer is in fact composed of two layers, the more superficial 
of which is made up of a net of nerves, and the next layer is characterized by 



THE ELASMOBRANCH FISHES 115 

the presence of cells with enormous nuclei. The middle striated layer consists 
of fibers probabl}^ of connective tissue which take a transverse and wave-like 
course. In this, nuclei are rarely seen. The inner or alveolar layer is at first 
composed of granular tissue which later gives rise to long, backward directed 
projections. At the base of these three layers is a thick cushion of gelatinous 
tissue which is contained in the connective tissue walls of the cone. 















f|if jj/ " 




Fig. 118. Electric organ, Torpedo. (After Scbimkevitsch from Krupski's atlas.) 

eJ.c, electric columns. 

In the adult organ of Torpedo, the discs, as previously noted, are located in 
the body in the region of the pectoral fins. Removal of the skin from this area 
{el.c, fig. 118) shows the organ to be made up of multitudes of hexagonal 
columns resembling the cells in honeycomb. Each column further consists of a 
series of discs piled one upon another, ten to twelve of these being present in 
each column. Each disc may be considered as having two surfaces, one ventral, 
the other dorsal. The ventral surface bears a negative charge of electricity 
while the dorsal is positively charged. 

The innervation of the electric organ will be considered in Chapter IX. 



THE ELASMOBRANCH FISHES 

BIBLIOGRAPHY 
Chapter IV 

MUSCULATUEE 

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9 text figs. 

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1909. Luther, Alex., Untersuchungen iiber die von n. trigeminus innervierte Muskulature 
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nachbarten Organen. Acta Soc. Sci. fenn., T. 36, No. 3, pp. 1-176, Taf. 1-5, 23 
text figs. 




118 THE ELASMOBRANCH FISHES 

1909. Luther, Alex, Beitrage zur Kenntnis von Muskulatur und Skelet des Kopfes des 
Haies Stegostoma tigrinum Gm., und der Holocephalen mit einem Anhang iiber die 
Nasenrinne. Acta Soc. Sci. fenn., T. 37, No. 6, pp. 1-60, 36 text figs. 

1905. Mahion, G. E., Mandibular and Pharyngeal Muscles of Acanthias and Raia. Amer. 
Nat., Vol. 39, pp. 891-924, 15 text figs. 

1892. Maurer, F., Der Auf bau und die Entwicklung der ventralen Eumpfmuskulatur bei 
den urodelen Amphibien und deren Beziehung zu den gleichen Muskeln der Selaehier 
und Teleostier. Morph. Jahrb., Bd. 18, pp. 76-179, pis. 4-6, 6 text figs. 

1906. Mauker, F., Die Entwickelung des Muskelsystems und der elektrischen Organe. Hert- 
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1912. Maurer, F. Die ventrale Eumpfmuskulatur der Fisehe (Selaehier, Ganoiden, Teleos- 
tier, Crossopterygier, Dipnoer). Jena. Zeitschr. Naturwiss., Bd. 49, pp. 1-118, Taf. 
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1911. Muller, Erik, Uutersucluingen iiber die Muskeln und Nerven der Brustflosse und der 
Korperwand bei Acanthias vulgaris. Aiiat. Hefte, Bd. 43, pp. 1-147, pis. 1-26, 11 
text figs. 

1834. Muller, J., Vergleichende Anatomic der Myxinoiden. Abh. Akad., Berlin. 

1897. Neal, H. v., The Development of the Hypoglossus Musculature in Petromyzon and 
Squalus. Anat. Anz., Bd. 13, pp. 441-463, 2 text figs. 

1918. Neal, H. V., The History of the Eye Muscles. Jour. Morph., Vol. 30, pp. 433-453, 20 
text figs. 

1891. Platt, Julia B. (See Bibliography, Chap. I.) 

1899. RiDEWOOD, W. G., On the Eyelid-Muscles of the Carchariidae and Seyllium. A contri- 
bution to the Morphology of the Nictitatijig Membrane of Sharks. Jour. Anat. and 
Physiol., Vol. 33, pp. 228-242, 7 text figs. 

1897. RuGE, G., Ueber das peripherische Gebiet des nervus facialis bei "Wirbelthieren. 
Festschr. f . Gegenbaur, Vol. 3, pp. 195-348, 76 text figs. 

1896. TiESiNG, Berthold, Ein Beitrag zur Kenntnis der Augen-Kiefer und Kiemenmuske- 
latur der Haie und Rochen. Jena. Zeitschr. Naturwiss., Bd. 50 (N. F. 23), pp. 75-126, 
pis. 5-7. 

1874. Vetter, B., Untersuchungen zur vergleichenden Anatomic der Kiemen- und Kiefer- 
musculatur der Fisehe. Jena. Zeitschr. Naturwiss., Bd. 8 (N. F. 1), pp. 405-458, 
pis. 14-15. 

1882. Wijhe, J. W. VAN, Ueber die Mesodermsegmente und die Entwickelung der Nerven 
des Selachierkopfes. Verh. d. k. Akad., Amsterdam, Deel 22, pp. 1-41, Taf. 1-5. 

ELECTRIC ORGAN 

1876. B.iBUCHix, A., Uel)crsieht der neuen Untersuchungen iiber Entwickelung, Bau und 
physiologische Verhaltnissc der elektrischen und pseudoelektrischen Organe. Arch. f. 
Anat. u. Physiol., Bd. 1876, pp. 501-542, Taf. 11-12, 2 text figs. 

1893. Ballowitz, Emil, Ueber den Bau des elektrischen Organes von Torpedo mit beson- 
derer Beriicksichtigung der Nervenendigungen in Demselben. Arch. mikr. Anat., Bd. 
42, pp. 459-568, Taf. 29-31. 

1897. Ballowitz, E., Ueber die Uebereinstimmung des feiueren Baues der elektrischen Or- 
gane bei den starkelektrischen und schwachelektrischen Fischen. Anat. Anz., Bd. 13, 
pp. 124-126. 

1897. Ballowitz, E., tiber den feineren Bau des elektrischen Organs des gewohnlichen 
Rochen (Raja clavata L.). Anat. Hefte, Bd. 7, pp. 283-375, Taf. 19-29. 

1873. Boll, F., Beitrage zur Physiologic von Torpedo. Arch. f. Anat. u. Physiol., Bd. 1873, 
pp. 76-102. 



THE ELASMOBRANCn FISHES 119 

1874. Boll, F., Die Structur dor elektrischen Flatten von Torpedo. Arch, niikr. Anat., Bd. 
10, pp. 101-121, Taf. 8. 

1876. Boll, F., Neue Untersueliungen iiber die Structur der elektrischen Flatten von Tor- 
pedo. Arch. f. Anat. n. Fhysiol., Bd. 1876, pp. 462-479, Taf. 8. 

1898. Crevatin", F., Ueber das sogenannte Stabchennetz im elektrischen Organ der Zitter- 
rochen. Anat. Anz., Bd. 14, pp. 243-250, 2 text figs. 

1849. EcKER, A., Einige Beobachtungen iiber die Entwicklung der Nerven des electrischen 
Organs von Torpedo galvanii. Zeitschr. wiss. Zool., Bd. 1, pp. 38-47. 

1888. EwART, J. C, The Electric Organ of the Skate. On the Development of the Electric 
Organ of Raia batis. Phil. Trans. Boy. Soc. Lond., Vol. 179B, pp. 399-409, pi. 68. 

1888. EwART, J. C, The Electric Organ of the Skate. On the Structure of the Electric Organ 
of Raia cireularis. Phil. Trans. Roy. Soc. Lond., Vol. 179B, pp. 410-416, pi. 68. 

1888. EwART, J. C, The Electric Organ of the Skate. The Electric Organ of Raia radiata. 
Phil. Trans. Roy. Soc. Lond., Vol. 179B, pp. 539-552, pis. 79-80. 

1892. EwART, J. C, The Electric Organ of the Skate — Observations on the Structure, Rela- 
tions, Progressive Development, and Growth of the Electric Organ of the Skate. Phil. 
Trans. Roy. Soc. Lond., Vol. 183B, pp. 389-420, pis. 26-30. 

1883. Fritsch, G., Bericht iiber die Fortsetzung der Untersuchungen an elektrischen 
Fischen. Beitrage zur Embryologie von Torpedo. Sitzber. Akad. Berlin, I, pp. 205- 
209, pi. 4. 

1883. Fritsch, G., Die elektrischen Fische in Lichte der Descendenzlehre. Samm. gemein. 
Wissen. Vortr., Bd. 18, Ser. 1, pp. 835-898, 7 text figs. (Virchow Holtzendorff Yor- 
trage.) 

1801. Geoffroy, E., Memoire sur I'anatomie coniparee des organes eleetriques de la Rale 
torpille, du Gymnote engourdissant, et du Silure trembleur. Ann. du Mus., T. 1, pp. 
392-407, pi. 26. 

(l845. GoODSiR, J., Observations on Electric Organs (Ray). Ann. Mag. Nat. Hist., Ser. 1, 
Vol. 15, p. 122. 

n. GoTCH, Francis, The Electromotive Properties of the Electrical Organ of Torpedo 
iiarmorata. Phil. Trans. Roy. Soc. Lond., Vol. 178B, pp. 487-537, 4 text figs. 

88. (OTCH, F., Further Observations on the Electromotive Properties of the Electrical 
(rgan of Torpedo marmorata. Phil. Trans. Roy. Soc. Lond., Vol. 179B, pp. 329-363, 
is. 51-52, 3 text figs. 
1861. Iabtm ANN, RoBT., Bemerkungeu iiber die elektrischen Organe der Fische. Arch. f. 
Kn&t. u. Physiol., Bd. 1861, pp. 646-670, Taf. 16. 

1911. Laguesse, E., Tin Exemple bien net d'architecture lamellaire du tissu conjonctif lache. 
0. E. Soc. Biol. Paris, T. 71, pp. 328-329. 

1861. MTonnell, R., On an Organ of the Skate which appears to be a Honiologue of the 
Electrical Organ of the Torpedo. Nat. Hist. Review, pp. 57-60. 

1906. PoBTiER, P., Les Poissons eleetriques. Bull. Mus. Ocean. Monaco, No. 76, 1906, pp. 
1-23, 18 text figs. 

1873. Reiciienheim, Max, Beitrage zur Kenntnis des elektrischen Centralorgans von Tor- 
pedo. Arch. f. Anat. u. Physiol., Bd. 1873, pp. 751-759, Taf. 15-16. 

1856. Remak, R., Ueber die Enden der Nerven im elektrischen Organ der Zitterrochen. Arch. 

f. Anat. u. Physiol., Bd. 1856, pp. 467-472. 
1898. Retzius, G., Ueber die Endigung der Nerven im elektrischen Organ von Raja clavata 

und Raja radiata. Biol. Untersuch. (n. s.), Bd. 8, pp. 83-93, pis. 19-21. 

1846. Robin, Ch., Recherehes sur un organe particulier qui se trouve sur les poissons du 
genre des Raies (Raia cuv.). C. R. Acad. Sci. Paris, T. 22, pp. 821-822. 



120 THE ELASMOBRANCH FISHES 

1865. Robin, Ch., Memoire sur la demonstration experimentale de la production d'electricite 
par un appareil propre aux poissons du genre des Raies. Jour, de I'Anat. et Physiol., 
T. 2, pp. 507-535, pis. 33-35. 

1888. Sanderson, J. B., and Gotch, Francis, On the Electrical Organ of the Skate. Jour. 
Physiol., Vol. 9, pp. 137-166, 5 text figs. 

1889. Sanderson, J. B., and Gotch, Francis, On the Electrical Organ of the Skate. Part II. 
Jour. Physiol., Vol. 10, pp. 259-278, pi. 22, 1 text fig. 

1858. SCHULTZE, Max, Zur Kenntnis des den electrischen Organen verwandten Schwanz- 
organes von Eaja clavata. Arch. f. Anat. u. Physiol., Bd. 1858, pp. 193-214, Taf. 9. 

1906. SCHULZE, O., Zur Frage von dem f eineren Bau der elektrischen Organe der Fische. 
Biol. Centralbl., Bd. 26, pp. 640-656. Festschr. f. J. Rosenthall, Leipzig, pp. 101-118. 

1845. Stark, Dr., On the Existence of an Electrical Apparatus in the Flapper Skate and 
Other Rays. Ann. Mag. Nat. Hist., Vol. 15, p. 121. 










Fig. 119. Body cavity opened to show viscera, Heptanohu,s maculatus. (C. G. Potter, orig.) 

d., cloaca ; co., colon ; c.s., cardiac stomach ; ??'., liver ; pn}~-, dorsal and ventral lobes of 
pancreas; /;..s'., pyloric stomach; re, rectum; rc.g., rectal gland; sp.L, valvular intestine. 




Fig. 120. Digestive tract and its mesenteries, 11 cpldnchiis niacuhiiu.s. 
(Euth Jeanette Powell, del.) 
CO., colon ; c.s., cardiac stomach ; d.ch., bile duct; fZw., duodenum ; /;.2J.,hei:)atie portal vein; 
m.j)., median fold of mesentery (omentum) ; mr., mesorectal mesentery; or., oesophagus; 
pn.^'-, dorsal and ventral lobes of pancreas; p.s., pyloric stomach; r.f., right fold of mesen- 
tery; re, rectum; rc.g., rectal gland; sp.i., spiral intestine; spL, spleen; spl.^, anterior exten- 
sion of spleen. 



V 

DIGESTIVE TRACT 

DIGESTIVE TRACT OP HEPTANCHUS MACULATUS 

Mesenterial Structures 

The digestive tract of HeptancJius (figs. 119 and 120) is suspended from the 
dorsal body wall by mesenteries, which are present in two general areas, an 
anterior and a posterior (fig. 120). The anterior mesenteries are somewhat 
complex; while those which suspend the posterior part of the tract are simpler. 
For couA^enience of description the mesentery in the anterior region may be 
considered as made of a right, a median, and a left fold. The right fold of the 
mesentery {r.f., fig. 120) passes from the middorsal line and from the right 
suspensory ligament to the hepatic portal vein (h.p.) and to the spiral intes- 
tine (sp.i), the extent of its origin, in other words, being along the middorsal 
line from the liver in front, back to the region of the superior mesenteric ar- 
tery. Since the anterior part of the right mesentery is attached along the 
hepatic portal vein and back to the suspensory ligament of the spiral intestine, 
it is largely hidden in figure 120. 

The second or median division extends from the right suspensory ligament 
of the liver and the ligament along the hepatic portal vein, across to the 
oesophagus and down the lesser curvature of the stomach to the proximal tip 
of the duodenum. The larger part of this is shown in figure 120 (m.p.) with a 
portion removed to show underlying organs. The left mesentery extends from 
the middorsal line to the dorsal side of the oesophagus and the stomach. Along 
its line of origin the left fold lies against the right and the two are firmly 
fused together. Remnants of this part of the mesentery appear on the outer 
angle of the stomach separating the stomach from the spleen (spl.) and from 
the pancreas (pn.^). 

These three parts of the mesentery may be considered as loosely enveloping 
the digestive tract and extending from the middorsal line on the right side to 
the suspensory ligament and hepatic portal vein. From here the middle seg- 
ment (omentum) stretches across to the lesser curvature of the stomach; the 
left segment continuing around is attached to the middorsal line on the left. 

The posterior mesentery or mesorectum (mr.) extends from the middorsal 
line to the rectal region ; it suspends the rectal or digitif orm gland, and reaches 
forward to the posterior mesenteric artery. 

Buccal Cavity 

The mouth or entrance to the buccal cavity is ventral in Heptanchus. If seen 
from below (fig. 119) it presents the appearance of a large crescentic slit with 
regular margins, except near the angles where there are enlarged folds. The 

[121] 



122 



THE ELASMOBRANCH FISHES 




Fig. 121. Teeth of Heptanchus indicus. 
donald and Barron.) 



(From Mac- 



mouth and nasal pits in He/ptanchus are not connected by oronasal grooves 
characteristic of some of the Elasmobranchs. The buccal cavity proper is 
large and spacious. Its floor is lifted up by the basihyal cartilage, forming a 
skeleton for the so-called tongue. The mucous membrane lining the mouth is 
provided both dorsally and ventrally with numerous stomodeal denticles (see 
p. 24, fig. 27c) which, as we have seen in a study of the integument, are modi- 
fications of placoid scales. In the region just within the crescent of teeth 
the lining of the mouth is thrown into heavy folds, which lie over the concen- 
tric rows of tooth buds. 

The teeth of Heptanchus (see fig. 48, facing p. 44) consist of a heavier lower 
series and a cuspidate upper series. The first tooth in the lower row of Hep- 
tanchus maculatus, like that 
for H. indicus (fig. 121), is 
unpaired and without a me- 
dian cusp. On either side of 
it are seven teeth, the most 
posterior of which is cusp- 
less and is followed by sev- 
eral rows of smaller flat 
nodules not shown in figure 
48. The first of the large 
teeth is provided with a se- 
ries of three conules on the median margin and usually six larger conules on 
the lateral margin. Other lower teeth including the sixth, though differing in 
size, are essentiallj^ like the first paired tooth. Unlike Heptanchus indicus (fig. 
121) there is usually no unpaired upper tooth in Heptanchus maculatus. The 
first paired tooth above bears a long fang which is directed downward; at its 
sides are small basal denticles like those in Heptanchus indicus. The second 
tooth and the ones following, although larger, differ from the first only in that 
they possess median conules and outer cusps, several of which are present on 
each tooth. 

Pharynx and Associated Structures 

The pharyngeal part of the tract is wide from side to side and depressed or 
flattened dorsoventrally. Through its ventrolateral walls are perforations, the 
internal branchial openings by means of which the respiratory current reaches 
the gill pockets (see fig. 142, facing p. 148). These openings are of interest 
in a consideration of the respiratory system. The lining of the pharynx, like 
that of the buccal cavity, is provided with denticles, but these on the roof in 
the posterior part are confined to a narrow strip just above the internal bran- 
chial clefts. The pharynx narrows toward the region of the oesophagus, which 
is closed except during the passage of food. By this closure of the oesophagus 
the buccal cavity and pharynx form a relatively large room. 

In connection with the pharynx are the thymus and thyroid glands. The 
thymus in Heptanchus, figure 122, lies dorsal to the first six gill clefts and 
takes the form of bunches of grapes. Van Wijhe has made the remarkable dis- 



THE ELASMOBRANCH FISHES 



123 



covery tliat in the young embryo of H. cinercus the thymus is not a ductless 
gland. In this type ducts lead from the first six lobes of the thymus into the 
pharynx. The thyroid gland is located at the symphysis of the lower jaws 
between the coracomandibularis and coracohyoideus muscles (see p. 200, fig. 
184, th.). It is a mass of semitransparent glandular tissue slightly crescentic 
in shape and surrounded by a capsule of connective tissue. 

Oesophagus 



The digestive tract is continued posteriorly from the pharynx by a short, 
thick-walled portion, the oesophagus (oe., fig. 120) . The lining of the oesopha- 
gus is thrown into whitish longitudinal 
folds, some of which may be continuous 
with those of the stomach. The oesopha- 
gus is distinguished from the stomach, 
however, by the character of its folds, 
the folds of the latter being much more 
pronounced. The epithelium of the 
oesophagus differs from that of the 
pharynx in that it does not contain 
stomodeal denticles. 



Stomach 




Fig. 122. Section through thymus gland 
of 63 mm. embryo Heptanchus cinereus. 
(From Van Wijhe.) 

d., duct of thymus; e., epithelial body; 
lim., hyomandibular cartilage; oi., otic 
area; p-q., palatoquadrate cartilage; //(., 
thymus nodules; //, second gill cleft. 



The stomach of Heptanchus is U- 
shaped, the larger left limb being the 
cardiac end (f..s\, fig. 120), and the 
smaller right limb, the pyloric division 
(p.s.). Superficially the cardiac por- 
tion of the stomach appears as a more 
or less distended bag. while the pyloric 

division is thick-walled. Within the cardiac stomach is usually found a consid- 
erable amount of undigested food material in the form of pieces of fishes, the 
shell and claws of crabs, and the like. 

Internally the pronounced folds of the cardiac stomach continue in a more 
or less longitudinal direction from its union with the oesophagus throughout 
two-thirds of its length where they become more or less tortuous. At the distal 
end of this part of the stomach there is present a blind sac in which some of 
the folds terminate. Some of the small folds abut against a circular fold which 
in a way separates the cardiac from the pyloric division. The pyloric lumen 
of the stomach is long and narrow, and its folds are not especially marked at 
the proximal end. As the terminus of the pylorus is reached, however, they 
become much higher. Three-fourths of an inch from the termination of the 
pyloric linil) there is a slightly enlarged portion which opens into the duod- 
enum through the pyloric valve. 



124 THE ELASMOBRANCH FISHES 



SPLEEN 

The spleen (spl.) although unconnected with the digestive tract may be con- 
sidered here. In Heptanchus this organ, when its relation to that in other 
forms is studied, is very instructive. It consists of a long, more or less lobate 
band extending from the ventral lobe of the pancreas over the greater (outer) 
angle of the stomach. It then crosses over the stomach dorsally and is con- 
tinued in the lesser curvature, one of its branches extending as far forward as 
the tip of the cardiac portion {spl}, fig. 120) . 

Duodenum or Middle Intestine 

The part of the digestive tract immediately following the pylorus, the duo- 
denum or middle intestine, is well defined in Heptanchus. In figure 120, this 
segment {du.) is covered in part by the ventral lobe of the pancreas {pn}). 
As in several other forms, the valve of the spiral intestine extends forward 
throughout the length of the middle intestine and touches the pyloric valve. 
Into this segment of the intestine the ducts of the liver and the pancreas empty. 

The liver {Iv., fig. 119) consists of a right and a left lobe between which is a 
small caudate lobe, not shown in the figure. In the caudate lobe is located the 
gall bladder which is emptied by a long duct (d.ch., fig. 120) into the duod- 
enum. 

The entrance of the duct into the duodenum is of interest. It reaches and 
enters the wall on the dorsal side only a short distance from the pyloric ter- 
minus of the stomach. It then runs in the wall backward and outward on a line 
almost at right angles to the attachment of the first whorl of the spiral valve. 
After having encircled about one-fourth of the duodenal circumference it 
empties into the duodenum a short distance from the first loop of the valvular 
intestine. 

The pancreas is composed of two compact and connected lobes. The smaller 
of these is dorsally placed {pn}, fig. 120), while the more compact lobe lies 
ventrally on the angle between the pyloric division of the stomach and the 
duodenum (pn.-) . The two lobes empty by a common duct which leaves the ven- 
tral lobe and, passing through the duodenal wall, runs almost parallel with the 
first annular artery. As it passes backward and outward it approaches the first 
fold of the valvular intestine and finally empties into the duodenum only a 
short distance to the left of (dorsally to) a line drawn from the entrance of 
the bile duct at right angles to the first fold of the valvular intestine. 

Valvular Intestine 

The most interesting part of the intestine (figs. 120, sj^.i., and 123) is the spiral 
valve contained within its cavity. The attachment of the valve to the intestine 
may be seen from the outside as a series of annular folds traversed by blood 
vessels, seventeen or eighteen turns of which are present in Heptanchus macu- 
latus. By making a window in the vahailar intestine (fig. 123) it may be 



THE ELASMOBRANCH FISHES 



125 




Fig. 123. Valvular intestine, with middle segment omitted, Heptanchus maculatus. (Dun- 
can Dunning, del.) 

pv., pyloric valve. 



126 



THE ELASMOBRANCH FISHES 



observed that the folds are far apart anteriorly and very much closer together 
posteriorly. The valve is formed as an ingrowth into the intestine and extends 
from the duodenum throughout the large intestine to the region where the 
opening of the rectal gland enters the intestine posteriorly. This fold is con- 
siderably broader than the diameter of the intestine and is thrown into a series 
of cones having their apices pointing anteriorly. The surface of the valve, if 
seen under the microscope, shows numerous finger-like villi which serve for the 
absorption of digested food. 

Colon and Rectum 



The part of the large intestine immediately following the valve is known as the 
colon (co., fig. 120). It is a muscular segment and superficially appears as 
slightly bulbous. Its lining, together with that of the part succeeding it, the 
rectum (re), is thrown into longitudinal folds. In the most posterior part, 
however, the walls of the rectum are smoother. The line of demarcation be- 
tween the colon and the rectum is formed by a 
lumen from the rectal or digitiform gland. 

EECTAL GLAND 

The rectal or digitiform gland (rc.g., fig. 120 and 
fig. 124) in Heptanchus is a finger-like structure 
which is composed of multitudes of gland cells 
and which empties by a central lumen into the in- 
testine. It is so arranged, however, that the lumen 
does not enter immediately at the point at which 
it reaches the intestine, but passes sharply for- 
ward and downward emptying on a level with the 
terminus of the spiral valve. 




CLOACA 



Fig. 124. Sagittal section 
through rectal gland, Hep- 
tanchus. (From Howes.) 

CO., colon; fd., fold of spi- 
ral intestine; lu., lumen of The rectum empties into an enlarged room, the 
gland ; re, rectum. , i • i • t -, ^.^ , i 

cloaca, which is lined with a smooth mucous mem- 
brane (see fig. 252, facing p. 290). Into the anterior part of the cloaca empty 
the products from the digestive, the urinary, and the genital systems, and in its 
posterior part are two finger-like processes, the cloacal papillae (p.). 



ABDOMINAL PORES (POEI ABDOMINALES) 

As a usual thing the cloacal papillae in Heptanchus are imperforate. Occa- 
sionally, however, as on the left side in figure 252, they are perforate, forming 
the so-called pori abdominales. These pores connect the abdominal cavity or 
coelom with the exterior. 



THE ELASMOBRANCII FISHES 



127 



DIGESTIVE TRACT OF ELASMOBRANCHS IN GENERAL 

The digestive tract const itutes a tube in which food is digested and through 
the walls of which it is absorbed into the circulatory system. In the adult, as 
we have seen in Heptanchus, the tract is folded upon itself so that when seen in 
ventral view it takes the form of an |. A median line parallel to the body axis 
may bisect the oesophagus and cloaca leaving the stomach to the left and the 
spiral intestine to the right. 

Mesenteries 

The mesenteries of Heptanchus are generalized when the Elasmobranchs as a 
group are considered, but they are more specialized than are those described 




Fig. 125. Development of teeth in lower jaw of Spinux niger. (From Laaser.) 

d.r., dental ridge; e., enamel; e.o., enamel organ; md., mandibular cartilage; n.f., outer 
furrow ; p., tooth papilla. 

by Howes (1890) for Hypnos suhnigrum., the Australian torpedo. In Hypnos, 
which has the most generalized type of any adult Elasmobranch with which I 
am acquainted, the mesentery extends as an almost unbroken sheet along the 
entire digestive tract. Only in the region of the spleen and of the rectal gland 
are there any indications of a break in the folds. In the adult of most other 
Elasmobranchs, however, there is a more pronounced tendency than in Hep- 
tanchus toward a loss of parts of the folds. In Acanthias, for example, rela- 
tively small parts of the folds which we have described as right and left remain. 
In its development the digestive tract of the Elasmobranch fishes is a more 
or less simple tube. It consists of a median segment which is put into com- 
munication with the outside : ( 1 ) l)y an anterior invagination which finally, 
as the stomodeum, ])reaks through to join the middle segment, and (2) by a 
posterior pit which also reaches the middle segment as the proctodeum. There 
is thus formed a tube including three parts: (1) the anterior stomodeum, 
lined with the outside ectoderm, which in the adult becomes the buccal and 



128 



THE ELASMOBRANCH FISHES 




pharyngeal?) regions; (2) a long middle portion lined with entoderm which 
includes in the adult the segments from the oesophagus to the end of the 
rectum, and finally (3) a posterior proctodeimi or cloacal area also lined with 
ectoderm. From these simple beginnings the complex tract 
results. As growth proceeds a series of dilations and con- 
strictions divides the tract into parts characteristic of the 
adult. These we shall next consider in order. 

Buccal Cavity 



The mouth in Elasmobranchs is a large crescent which is 
usually ventral, although in certain types it is terminal, in 
position. It is bounded by membranous folds or lips and 
leads into a voluminous buccal cavity. 

The floor of the buccal cavity is raised up into a heavy 
fold, the "tongue," which in some forms (Lamna) is well 
developed; in others it is less pronounced. The buccal cav- 
ity is lined with a smooth or papillated mucous membrane 
(Mustelus, Scyllium, Chlamifdoselachus) , the cells of 
which secrete mucin; but it is devoid of all glands which 
are characteristically present in higher forms. Perforating 
the lining of the cavity are two structures which, although 
differing in form, are essentially identical : the stomodeal 
denticles and the teeth. The former we have considered in 
Chapter II, page 38. The latter may be discussed here more 
in detail. 

TEETH 



The teeth^ characteristic of the Elasmobranchs are of two 
types : sharp or prehensile teeth and pavement or crushing 
teeth. Between these extremes multitudes of patterns, more 
or less complex, occur. In the early stages the two types 
are essentially alike, but as development proceeds each 
takes on its specific character. The general mode of devel- 
opment we may examine before considering the types 
further. 

A sagittal section made through the lower jaw of Spinax 
niger by Laaser (fig. 125) shows the ectoderm sinking in to form a dental ridge 
(d.r.). In this ridge several tooth germs are developing, and cells are collect- 
ing at the papilla (p.) to form still another tooth. In the tooth germs which are 
more mature than those just mentioned the papillae have gro\\ai outward 
carrying caps of epidermis, the enamel organ (e.o.), over them. In their for- 
mation the teeth are much like the saw tooth already studied (p. 35) . 

Teeth thus formed in other Elasmobranchs may be long and fang-like, 
1 Tor bibliography on the teeth, see Chap. II. 




B 

Fig. 126. Tooth pat- 
terns. A. ChJamy- 
doseJachus. (From 
Bos©.) B. Mylio- 
batis. (From Gar- 
man.) 



THE ELASMOBRANCII FISHES 



129 




Fig. 127. Tooth of extinct Carcharodon. (Photograph natural size.) 



130 THE ELASMOBRANCH FISHES 

resulting from a single narrow papilla, of which several may fuse at the base 
into a simple triconodont tooth as in Chlamydoselachus (fig. 126a) ; or in the 
rays a most complex arrangement of large median and smaller lateral hexag- 
onal plates as in Myliohatis (fig. 126b) may result in pavement or crushing 
teeth; or the plates may reach from side to side in an unbroken line as in 
Aeiobatis. The teeth of Heterdontus (fig. 128) are particularly interesting in 
that they represent both anterior prehensile and posterior crushing teeth. 
Furthermore, in the area between these two, transitional teeth are present 









Fig. 128. Median view of teeth in jaws, Heterodontus francisci. 
(Duncan Dunning, del.) 

which have characteristics of both. The teeth in the sixth row from the front 
are flattened out and are provided with cusps essentially like the pavement 
teeth of Miisielus henlei. 

The finer structure of a tooth (fig. 129) is somewhat similar to that of a 
scale. In both there is an outside harder cap usually of structureless enamel 
(e.) over a heavier inside layer of dentine. The enamel in some teeth is of a 
coarse gi"ain, closely resembling dentine and by some held to be identical with 
it. The dentine (d.) is of the tubular type formed by the odontoblasts of the 
dentinal papilla. In the connective tissue cells at the base of the tooth vaso- 
dentine [vd.) is formed, which is somewhat softer than dentine proper. In the 
adult Myliohatis the whole core of the tooth becomes filled with dentine as 
does the core in the tooth of the saw (Pristis), leaving only central canals from 
which run numerous dentinal canals (d.c). 

It is generally held that enamel results from the basal layer of the epidermis, 
with the exception that in Carcharias, according to Tomes (1898) it arises 
from a special amelioblastic layer formed by the corium. All agree that den- 
tine is produced in and by the corium. Here the odontoblasts send out pro- 
cesses along which are deposited the lime salts which harden as dentine. The 
processes become surrounded and are retained within dentinal canals {d.c, 
fig. 129) which ramify throughout the dentine. In some forms the dentinal 



THE ELASMOBRANCH PISHES 



131 



tubes end in enlarged islands continuous with the enamel (Galeus) . In others 
the tubules ])enetrate far outward into the enamel and either divide into 
branches {Carcharias) or are lost as fine single tubules (Lamna). 

EEPLACEMENT OF TEETH 

Teeth which have been injured or lost are replaced by new ones. To gain a 
notion of the provision made for repairing injury or loss it is only necessary 
to examine the mouth of a form like Carcharias, or Heterdontus (fig. 128) . In 





B 



Fig. 129. Finer structure of a tooth, Myliohatis. (From Eose.) A. Sagittal section of whole 
tooth. B. Transverse section cutting a few canals ; highly magnified. 

CO., central canals ; d., dentine ; d.c, dentinal canals ; e., enamel layer ; vd., vasodentine. 

these forms the teeth are arranged horizontally in crescentic rows, several of 
the outer rows functioning at the same time; upon injury or loss the outermost 
teeth are replaced by others which migrate outward over the jaw to take their 
places. It may thus happen that the teeth in several rows may be lost in en- 
counter or in capture of food. In such an event, provision has been made for 
their replacement, for at the base of the innermost row are other deeply buried 
and less mature teeth ranging in development down to flabby tooth-buds. 



Pharynx 

The pharynx is that part of the tract which leads from the buccal cavity and 
which is characterized by branchial perforations or clefts opening through 
its walls. In addition to the spiracle seven such apertures perforate the 
pharynx in Heptanchus, six in Hexanchus and Chlamydoselachus, and five in 
pentanchid Elasmobranchs. In the adult the spiracle opens into the hyoidean 
pocket and is of small size. The remaining perforations are large but de- 



132 



THE ELASMOBRANCH FISHES 



crease in size posteriorly. The lining of the pharynx is a continuation of that 
of the buccal cavity, being made up of a deeper layer of comiective tissue and 
a superficial epithelium; in the latter are located the mucous cells. Special 
interest attaches to the pharyngeal region because of two structures associated 
with it, the thymus and the thyroid glands. 

The thymus gland is an embryonic structure appearing as a series of nodules 
connected into a chain above the gill pockets. We have seen that six such 








B 



Fig. 130. Developing thymus gland, Spinax. (From Fritsche.) A. Transverse section. 
B. Section of gland magnified. 

a.G.s., anterior cardinal sinus; did., notochord; c?.^*, third and fourth clefts; ht., heart; 
my., myotome; n.t., neutral tube; pli., pharynx; tlu, thymus gland. 

nodules may be present in Heptanchus. In Spinax, Acanthias, 3Iu.sfeli(s, and 
Scylliiini four of these, corresponding to the first four branchial clefts, are 
present, and, in the embryo of Spinax, transitional thymus buds have also 
been found over the spiracular and the fifth clefts. In the rays thus far studied 
a similar number is present lying back of the spiracle and between the gill 
pockets and the lateral line. Figure 22d gives a dorsal view of the embryo of 
Urolophus in which the nodules of the thymus are seen between the gill 
pockets and the lateral line canal. 

In development the thymus arises as an anterodorsal thickening of the 
epithelium of the gill pouches (th., fig. 130a). These thickenings as we have 
said may represent the spiracular and the five branchial pockets in pentanchid 
Elasmobranchs, but the first and the last never pass the rudimentary stage. 

Figure 122 of Heptanchus cinereus shows that each thymus nodule has the 
form of a bunch of grapes. 

A highly magnified section through the thymus of Spinax (fig. 130b) shows 
two types of cells, one outer and larger, and the other a more deeply located, 
small, round cell. Among them are to be found occasional lymphocytes, and it 
has been questioned whether the smaller cells of the thymus are not trans- 



THE ELASMOBRANCH PISHES 



133 



foi'med directly into lyiii|)li()(*ytos. It is generally believed, however, that the 
function of the thyniu.s, whatever it may be, is not that of a l,vniphoid organ 
(Fritsehe, 1910). 
That the thynuis possesses a tlnct in Heptanvlius, as has been demonstrated 





Fig. 131. Thyroid follicles making up the gland. (From Ferguson.) A. Carcharias Uttortilis. 
B. Eaia erivacea. 



by Van Wijhe, is an unusually interesting fact, although the significance of 
the duct has not yet been made out. 

The thyroid- gland in the Elasmobranch fishes is a gelatinous mass of tissue 
surrounded by a connective tissue capsule. In the sharks it is located in the 
region behind or below the basihyal cartilage, 
between the coracomandibular and the coraco- 
hyoideus muscles. It may be creseentic in «ii'T^^e 
or it may be more or less irregular {Acanthias) . 
Where the basihyal cartilage is broad it occupies 
its ventral side, resting in a depression {Acanfhias, 
Mustelns) or in a deep groove {Carcharias Uito- 
ralis) . It occupies the space between the basihyoid 
and the bifurcation of the ventral aorta. Where 
the cartilage is narrow as in Raia erinacea the 
thyroid may lie farther posterior on the terminal 
bifurcation of the ventral aorta. 

Upon removal of the connective tissue capsule 
from the thyroid it is seen to be made up of groups 
of follicles of various forms. They may be small 
(Carcharias, fig. 131a ; Mustelus; Squat ina) or 
large (Raia, fig. 131b). A section through an in- 
dividual follicle shows an outer wall of epithelium 
enclosing a mass of colloidal substance (Fergu- 
son, 1911). 

The history of the thyroid in forms lower than the Elasmobranchs is of in- 
terest. In Amphioxus, the Ascidians, and Ammocoetes, there is present in the 
floor of the pharynx a median groove, the ciliated endostyle, the walls of which 

2 Thyreoid. 




Fig. 132. Sagittal section 
through the thyroid gland of 
CMamydoselachus anguineus. 
(From Goodey.) 

fl., follicle ; s.d., remnant of 
stomodeal denticle; t.d., thy- 
roid duct. 



134 



THE ELASMOBRANCH FISHES 



possess groups of cuneiform secreting cells. By the beating of the cilia, food 
caught in the mucous secretion is directed into the intestine. 

Marine (1913) has shown for Ammocoetes of the brook lamprey that the 
groups of cuneiform secreting cells degenerate at the time of metamorphosis, 
and that the follicles of the thyroid gland arise from certain areas in the walls 
surrounding these columns. 

In the embryo of the Elasmobranchs the thyroid arises similarly as an 
evagination of the pharyngeal floor, or as a solid block of cells in which a lumen 
may form (Acanthias) . Either way, in development, it sinks deeper. While it 
may retain its connection with the pharynx practically until the period of 
birth, later than this all relation with the pharynx is usually lost and the 
thyroid becomes a "ductless gland." 

Especially significant in this regard is the discover}' by Goodey (1910) that 
in Chlaniydoselachus the duct (t.d., fig. 132) retains its connection with the 
pharynx. In this form the aperture of the duct enters the pharynx, through 
a perforation in the basihyal cartilage, and is lined with the pharyngeal epi- 
thelium as is the endostyle of Amphioxus. Within this invaginated lining of 
Chlaniydoselachus numerous scale-like structures are present, the remnants 
of stomodeal denticles (s.d.). 

Oesophagus 




In most of the Elasmobranchs the segment of the digestive tract known as the 
oesophagus {oe., figs. 173 and 175) is short. In some forms it is easily distin- 
guished from the stomach liy its smaller 
size, but in others it is wide and ]>asses al- 
most imperceptibly over into the stomach. 
Denticles which may be present on the 
])haryngeal lining cease more or less ab- 
ruptly at the beginning of the oesophagus. 
The mucous membrane lining the oesoph- 
agus may be covered with long, finger- 
like papillae as in Acanthias, but usually 
it is thrown into numerous folds. Ante- 
riorly these folds are low and regular; 
posteriorly they may run transversely 
marking a boundary between the oesopha- 
gus and the stomach. Anteriorly, too, the 
mucous membrane is similar to that of the pharynx, but posteriorly it consists 
mainly of ciliated and goblet cells. 

A section of the membrane lining the oesoi^hagus of Squatina, by Petersen 
(1908), shows the two main types of cells (fig. 133). One of these is of long 
ciliated cells (cil.), among which are scattered the mucous cells (mc). The 
mucous cells are extremely large and vacuolated, and each has its nucleus w^ell 
toward the basal end of the cell. 



rig. 133. A section through the lin- 
ing of the oesophagus of Squatina 
(From Petersen.) 

cil., long ciliated cell; m.c, mu- 
cous cell. 



THE ELASMOBRANCH FISHES 



135 



Stomach 



Tlie stoinacli of Elasmobranchs when seen in ventral view is a U- or J-shaped 
tube (for types see figs. 173 to 175, c.s. and p.s.). the left arm of which, as in 
Heptanchus, is the cardiac and the right arm the pyloric division. The great 
variety in shape of the stomach found among the Elasmobranchs is due largely 
to variation in the relative length of the pyloric arm. In some, although the 
pyloric division is small in diameter, in length it 
is practically equal to the cardiac (leopard shark, 
fig. 173a; Raja); in others this arm is less than 
one-half the length of the cardiac, so that the 
shape of the stomach in these is better described 
as a J (Acanthias) . In still others the pyloric limb 
is onl,y a small projection from the cardiac divi- 
sion of the stomach (Scynmus licliia, fig. 135b ; 
Laemargus rostratus, fig. 136b). In the latter 
there is a so-called "blind sac" (sc.-) at the angle. 

The mucous lining of the adult stomach is 
thrown into folds which, as we have said, may be 
continuous with those of the oesophagus. The folds 
on the walls of the cardiac division are high and 
may extend in part as the finer folds into the py- 
loric limb, but those of the two regions are usually 
distinct. 

A section through the mucosa of the stomach of 
Squalus acanthias according to Ringoen (1919; 
fig. 134) shows a gland as long and flask-shaped. 
The superficial epithelial cells (ejJ.) are somewhat 
like those found in the oesophagus, that is, they 
are columnar or pyramidal cells, the upper part 
of which contains a plug of mucus (pi.). The cells 
lining the median part of the crypt are large and 
their nuclei are vertical in position. In the deeper 
crypts are the gastric cells that are peculiar to the 
cardiac stomach. They lie at the base of the crypts and are large and granular 
and of a polygonal shape with their nuclei taking a more or less horizontal 
position. These are the true peptic cells (pc), which secrete the digestive 
substances. 

The function of gastric juice, which contains hydrochloric acid and a fer- 
ment, pepsin, is the digestion of protein matter. Such digestion takes place at 
a much lower temperature in the stomach of the shark than in the stomach of a 
higher animal. While, in the latter, digestion is carried on at body temperature 
(37° C), in the former it takes place at 10° to 15° C, although the pepsin may 
also be active at as high as 40° C. 








Fig. 134. Section through lin- 
ing of the stomach, Squalus 
acanthias. (From Eingoen.) 

ep., epithelial cell ; pc, pep- 
tic cells; ph, mucous plug. 



136 



THE ELASMOBRANCH FISHES 



At the terminus of the pyloric division of the stomach is a circular band of 
muscle fiber, the pyloric valve, separating the pyloric stomach from the duo- 
denum or middle intestine. This valve varies considerably in extent in differ- 
ent forms. In some types it shows only slight signs of constriction, as for ex- 
ample in Heterodonfus, where it may allow food of considerable size to pass 
into the spiral intestine. In Heptanchus we have seen that it projects as a well 
defined circular band into the duodenum; and in Laemargus the valve is 
greatly extended. 

In some of these forms an interesting condition obtains in which a room, the 
bursa entiana, may be formed and into which the pylorus may empty. This 






A B C 

Fig. 135. Relation of the terminal part of the pylorus to the next segment, the duodenum. 
A. Galeus. (From Eedeke.) B. Scym7ius. (From Helbing.) C. Hypnos. (Australian tor- 
pedo.) (From Howes.) 

h.e., bursa entiana; d.ch., bile duct; prt., intraintestinal partition; p.v., pyloric, valve; 
py., pylorus; spL, spleen; sp.v., spiral valve; y.s., yolk stalk. 

occurs in Galeus {h.e., fig. 135a) and in Cetorhinus. In Scymrius (fig. 135b) 
such an enlargement is formed in a different way. Here the pylorus enters a 
special room the upper part of which is the liursa entiana, and the lower is 
continued into the intestine as a valve-free portion. In the Australian torpedo 
Hypnos (fig. 135c) there is an interesting modification of this plan. The en- 
larged room is separated into an anterior and a posterior chamber by an intra- 
intestinal partition (pr^.). Into the former the pylorus (?jy.) empties and into 
the latter the ductus choledochus (d.ch.) enters. 

Duodenum or Middle Intestine 



The duodenum may be encroached upon by the spiral valve through its entire 
course, as in Heptanchus, or it may be free as in a few of the sharks and rays. 
In a type like Spinax niger (fig. 136a) the valve-free portion is long. This 
segment is present in Bhinohatis, but, of all the rays, it is best developed in 
Trygon. In the great majority of types, however, the valve has so encroached 
upon the pylorus that no free portion exists {Galeus, fig. 135a, Carcharias, 
Lamnidae, Notidanidae, Scyllidae, Rhinidae, and some of the Rajidae). 

The proximal part of the duodenum shows different stages of complexity. 
This may be illustrated by Spinax niger, where the pyloric tip is small and 



THE ELASMOBRANCH FISHES 



137 



enters the duodenum in such a way as to form two miniature blind sacs (sc.^"^) . 
In Laemargus rostratus (fig. 136b) the blind sac is divided so that the proxi- 
mal part of the intestine appears as a bilobed structure. Into one of the lobes 
the pylorus enters. 

The ducts from tlie liver (d.ch., fig. 137b) and pancreas (p.d.) enter this 
middle or duodenal part of the intestine as in Heptanchus, and their entrance 





Fig. 136. The duodenum. A. Spinax niger. (From Eedeke.) 'B. Laemargus rostratus. (From 
Helbing.) 

d.ch., bile duct ; p.v., pyloric valve ; py., pyloric stomach ; sc.^~-, first and second blind 
sacs; spl., spleen. 

marks the region from which the liver and the pancreas arose in the embryo, 
the length of the ducts in the adult showing how far the two organs have ])e- 
come separated from their place of origin. 

LIVER 



The liver in the adult consists of a right and a left lobe, connected solidly an- 
teriorly as in Heptanchus (see fig. 119) . From the posterior part of the union 
of the two lobes a caudate lobe may arise. In this or in either of the other lobes 
the gall bladder may be located. The main lobes of the liver are character- 
istically large in the sharks and may extend the entire length of the coelom. 
In some of the larger sharks (Cetorhinus) as much as five barrels of oil are 
reported to have been obtained from the liver of a single specimen. In the 
rays the lobes are less well developed, but here, too, they are often large. 

Bile secreted by the liver is collected by a series of tubules some of which 
empty into the gall bladder. The gall bladder is drained by a duct w^hich, 
joined with other ducts from the liver, is the ductus choledochus. This duct 
in Squalus sucklii (d.ch., fig. 137b) reaches the intestine at its proximal part 



138 



THE ELASMOBEANCH FISHES 



and on the dorsal side, but unlike that of Heptanchus it passes backward 
within the intestinal wall, making a half-loop before it empties into the ventral 
side of the duodenum. In its course it becomes thrown into a series of ridges 
much like the ridges in the seminal vesicles of some Elasmobranchs. 

PANCEEAS 

The pancreas (fig. 137) in the adult consists of two lobes. One of them, the 
dorsal lobe (pw.^) , runs parallel with and over the terminal part of the cardiac 
stomach and near its middle part sends a bridge over the pylorus to join the 
ventral lobe (pn.'^) which is closely bound to the ventral surface of the proxi- 





..pn; 



A B 

Fig. 137. The pancreas and associated structures. A. Acanthias, dorsal view. (From Kan- 
torowicz.) B. Squalus sucMii, dorsal view. (Chester Stock, orig.) 

d.ch., bile duct or ductus choledochus from the liver; ia., intraintestinal artery; iv., intra- 
intestinal vein; pn.^, dorsal lobe of pancreas; pn.^, ventral lobe of pancreas; p.d., pan- 
creatic duct ; sph, spleen. 

mal part of the middle intestine. In all specimens which I have examined the 
pancreatic duct (p.d., tig. 137b) empties into the duodenum much as was de- 
scribed ior Heptanchus (p. 124). 

SPLEEN 

The spleen, although not connected by ducts leading to the digestive tract, 
is located in the mesentery and may be regarded as an organ made up of cells 
specialized from the connective tissue of the mesentery. In sharks it is often 
triangular in shape and its characteristic position (spl., fig. 137a) is on the 
greater curvature of the stomach, that is, on the outer angle between the 
cardiac and pyloric divisions. In the rays (see p. 187, fig. 175, spl.) it is a lobu- 
lar or round structure located in the angle of the lesser curvature of the 
stomach. In Heptanchus we have observed that the spleen takes a generalized 
form in which it extends along the outer angle of the stomach and across the 



THE ELASMOBRANCH FISHES 



139 



mesentery into the lesser angle. In other words, in addition to the type of 
spleen on the onter angle characteristic of sharks, Heptanchus also has splenic 
tissue on the inner angle of the stomach like that of the rays. 

Valvular Intestine 

The valvular intestine receives its name from the fact that it contains within 
its lumen a membranous fold or valve. This fold as a general thing in the 




A B C 

Fig. 138. Valvular intestine. (From T. J. Parker.) A. Attachment in Zijgaena. B. Valve in 
Scyllium. C. Valve in Zygaena. 

Elasmobranchs is spiral in nature, and produces a spiral valve. In a few forms, 
however {Zijgaena), it is of the scroll type, with the line of attachment along 
the wall of the intestine only slightly curved (fig. 138a). This valve in Zy- 
gaena (fig. 138c) is rolled up within the intestine and has a width two-thirds 
of its length. 

Seen from the outside the spiral valve presents the aspect of a screw over 
the threads of which has been wrapped some thin substance through which the 
threads are evident superficially. But such a likeness would be correct only if 
the threads grew from and were a part of the outer covering. 



140 



THE ELASMOBRANCH FISHES 



In development the valve first appears as a ridge or fold of the intestinal 
mucosa along the intestinal wall. Whether the increase in the width of the 
valve is due to the growth of the mucosa pulling the tissue within the folds as 
in Ammocoetes (Daniel, 1931), or to the growth of connective tissue which 
forces the mucosa downward as a cap, is not clear. The likelihood is that 
both of these processes take place. This fold in a .young Zygaena hangs down 
as a simple longitudinal plate which, upon reaching the opposite side of the 

lumen, rolls up into a scroll valve as would a sheet 
of paper similarly droj^ped down. In the spiral 
valve, however, another factor enters. Here a tor- 
sion occurs in the lining of the intestine which 
throws the scroll into a spiral. Figure 139 from 
Riickert (1896) represents the mucosa of the in- 
testine in the formation of the spiral. The turns in 
figure 139b are more numerous in the posterior re- 
gion, and the intestine in the older stage is seen to 
be relatively shorter. 

The number of turns of the valve in the adult 
varies greatly among the Elasmobranchs. In fact, 
slight variation in the number within a single spe- 
cies is common. In some the number is as low as 
four (Prionace), or eight (Raja), or ten (Muste- 
liis). In others it is increased but slightly (twelve 
in Heterodontus). In some, however, great num- 
bers are present, twenty in Lamna and forty-five 
in Alopias. 

In width the valve in the adult may approach or 
extend beyond the middle of the lumen [Hep- 
ia7ichus). When it is less than one-half the width 
of the intestine there appears in end view a canal 
which would represent the core of the screw. If it 
exceeds one-half the diameter of the intestine the 
free edge representing the core of the screw rolls 
up so as to appear from side view as a series of con- 
centric cones pointing anteriorly or posteriorly 
(Scyllium, fig. 138b). In some forms the valve anteriorly may thus be thrown 
into cones, while posteriorly the free edge does not reach the middle line. 
Hence an end view from the posterior would present a central lumen (Raja, 
fig. 140). 

The lining of the valvular intestine from the duodenum to the end of the 
spiral valve consists of cylindrical and goblet cells (Carcharias, Mustelus) 
and is thrown into multitudes of tiny points or villi (fig. 140) . From the type 
of lining it is apparent that the function of the spiral valve, in addition to pre- 
venting the too rapid passage of food, is the absorption of digested substances. 



A B 

Fig. 139. Two stages A-B to 
show the development of the 
spiral valve, Prts^mrw^. (From 
Riickert.) 



THE ELASMOBRANCH FISHES 



141 



Colon and Rectum 

The segments of the intestine succeeding the valvular intestine, the colon and 
the rectum, usually differ in their lining from the valvular intestine in the ab- 
sence of villi. Separating these two areas and emptying into the dorsal side of 
the digestive tract is the rectal gland. 

The rectal or digitiform gland varies in size from a tiny structure one-half 
an inch long, as in some of the rays, to one three or four inches long in some of 
the larger sharks. It is a compound, tubular 
structure, the secreting cells of which are 
surrounded by a strong, fibremuscular layer. 
These cells empty their secretion through 
tubules into a median lumen which, as we 
have seen in Heptanchus, enters the intes- 
tine between the colon and the rectum. In 
some of the rays the lumen is large, that part 
at which it enters the intestine being espe- 
cially expanded (Raja, fig. 141b) . In most of 
the sharks, however, the gland is constricted 
at its base, and its lumen is small. As a gen- 
eral thing the lumen is prolonged forward 
so that the entrance to the intestine is ante- 
rior to the position of the gland (Zygaena, 
fig. 141a). Although the rectal gland has 
been studied in a great number of forms, its 
function has not yet been made out. 

CLOACA 

The cloaca is the common receptacle into 
which the digestive tract and the ui'inary 
and genital systems empty. It is generally an 
enlarged room, the walls of which are more 
or less loosely folded. In some types, trans- 
verse crescentic folds from the dorsal wall of 
the cloaca separate it into an anterior and a 
posterior division on each side. In the ante- 
rior division two types of structure may be 
found. One of these is a pair of finger-like 

papillae (see fig. 252, 2^., facing p. 290) ; the other is a pair of cloacal pits. The 
papillae, when present, usually contain prolongations of the lining of the body 
cavity. The cloacal pits are often located lateral and posterior to the papillae. 
In some forms both the pits and the papillae are present. In connection with 
the pits or with the papillae may occur apertures, the abdominal pores, which 
put the abdominal cavity into connection with the outside. 




Fig. 140. Valvular intestine of Eaja. 
(From Paul Mayer.) 



142 



THE ELASMOBRANCH FISHES 



ABDOMINAL POEES 

The abdominal pores in the Elasmobranchs may perforate the tips of the 
papillae or, as was just said, they may pass through the cloaeal pits. Some of 
the types having perforated papillae are Triakis semifasciatus, Carcharias 
glaucus, Zygaena, Mustelus. In some the papillae are imperforate (Squatina) . 
The slit-like type of abdominal pores is in connection, not with the papillae, 
but with the cloaeal pits. This type is especially common among the rays. 





A B 

Fig. 141. Sections through rectal gland. (From Howes.) A. Zygaena. B. Baja. 

The abdominal pores vary so greatly in different species and in fact in 
different individuals of the same species as to suggest that they are vestigial 
structures. In the Cyclostome fishes they function as apertures through which 
the sex cells leave the body; but in the Elasmobranchs, if the pores have ever 
been thus used, this function has been lost upon the appropriation of special 
channels formed for the passage of the sex cells. It hence becomes a question 
whether or not in the Elasmobranchs the openings subserve any particular 
function at the present time. 



THE ELASMOBRANCH FISHES 143 

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THE ELASMOBRANCH FISHES 145 

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1898. Yung, emile, De la digestion gastrique chez les Squales. C. R. Acad. Sci. Paris, T. 126, 

pp. 1885-1887. 

1898. Yung, emile, Sur les fonctions du pancreas chez les Squales. C. R. Acad. Sci. Paris, 
T. 127, pp. 77-78. 

1899. Yung, emile, Recherehes sur la digestion des poissons. (Histologie et physiologie de 
I'intestin.) Arch, de Zool. exper. et gen., Ser. 3, T. 7, pp. 121-201, pi. 9. 



VI 

RESPIRATORY TRACT 

RESPIRATORY TRACT OP HEPTANCHUS MACULATUS 

HeptancJuis is characterized by having the greatest number of gill clefts or 
external branchial apertures of any kno\vii Elasmobranch. In fact it is from 
the number of clefts, as we have said, that it has received its name. These per- 
forations in Heptanchns are lateral in position and lie between the cranium 
and the pectoral fin. The first branchial cleft, not the spiracle, is the largest of 
the series, while the one farthest from the cranium is the smallest. By opening 
one of the clefts both dorsally and ventrally we may study the structures of 
the gill pouch or pocket. 

Gill Pouch or Pocket 

If the second gill pocket of Heptanchus (fig. 142, facing p. 148) be opened 
as suggested above, the cavity is seen to be shaped somewhat like a funnel 
flattened from side to side, with the apex pointing inward and opening into 
the pharynx as the internal branchial aperture. The internal branchial aper- 
ture of the second pocket is supported by the first branchial arch anteriorly 
and by the second branchial arch posteriorly. On the margin of the first arch, 
and pointing backward, are one or two low projections which resemble in- 
cipient gill rakers. On the anterior branchial arch are also numerous eleva- 
tions, the bases of the cartilaginous branchial rays (b.r.), which support the 
gill septum. The tips of these cartilages are visible on the posterior surfaces. 
The walls of the pocket, both anteriorly and posteriorly, are covered with nu- 
merous radiating gill filaments or folds (fl. ) , the effective organs of respiration. 
The remaining pockets, excepting the seventh, are essentially similar to the 
first. The seventh external branchial cleft has the same funnel-shaped arrange- 
ment with its smaller apex connecting with the pharynx. On its anterior wall 
filaments similarly appear although these are fewer in number than in the 
preceding pockets. The posterior wall, however, is perfectly smooth, all sem- 
blance of gill filaments being entirely wanting. 

Spiracle 

The spiracular pocket upon being opened is found to be similar to the last 
pocket, that is, it bears filaments only on the anterior wall. But it differs from 
the others in the reduction in size of both its external and internal apertures 
and in the reduction in the number of its filaments. 

The spiracle in Heptanchus is not a straight tube, but as Ridewood (1896) 
says, it bears diverticula on its walls. The most important of these diverticula 

[147] 



148 THE ELASMOBRANCH FISHES 

is located dorsally and extends inward until it meets and joins the cartilage of 
the auditory capsule. A second smaller diverticulum also extends dorsally 
from the spiracular walls near their union with the pharynx. 

Gill or Holobranch 

A holobranch consists of the tissue between two gill pockets; for example, the 
tissue between the second and third pockets constitutes the second gill or holo- 
branch. This second holobranch hence includes all filaments on the posterior 
wall of the second pocket, and those on the anterior wall of the third gill 
pocket, as well as the supporting structures between the two. The filaments on 
the anterior and posterior sides of a whole gill are also designated as the an- 
terior and posterior demibranchs. Thus the filaments on the posterior side of 
the second pocket form the anterior demibranch of the second gill, and the fila- 
ments on the anterior side of the third pocket constitute the posterior demi- 
branch of the second gill. 

With this understanding of a gill there are thus present in Heptanchus six 
whole gills, located between pockets 1-2, 2-3, 3-4, 4-5, 5-6, 6-7. In addition 
there is present on the anterior wall of the first pocket a half -gill, the hyoidean 
demibranch. It is often more convenient to consider the number of demi- 
branchs rather than the number of holobranchs. Thus there are present in ad- 
dition to the first unpaired hyoidean demibranch, a second and third, a fourth 
and fifth, a sixth and seventh, an eighth and ninth, a tenth and eleventh, and a 
twelfth and thirteenth branchial demibranch, these pairs representing the 
first, second, third, fourth, fifth, and sixth gills or holobranchs, respectively. 

The gill filaments of the adult differ from certain external filaments present 
in the embryo. The external filaments of the emliryo arise from the tissue on 
the posterior part of the hyoidean and branchial arches, and in Heptanchus 
are of particular interest because of their great numbers, a mark of a general- 
ized condition. Those filaments arising on the hyoid in Heptanchus cinereus, 
according to Braus (1906), consist of 14 filaments; those from the first bran- 
chial opening, 39; from the second to the fifth, 29 each; from the sixth, 26, and 
from the last, 18. 

Gill Supports 

The arches supporting the gills of the adult (see fig. 48, facing p. 44, and 
fig. 49, p. 45) are the hyoidean and all the branchial arches except the last. 
From epi- and ceratobranchial segments of all the visceral arches, except the 
mandibular and the last branchial, cartilaginous branchial rays (b.r.) extend 
outward toward the integument. The branchial rays on the hyoidean arch 
(fig. 48) are often branched and complex, but on the other visceral (branchial) 
arches they are simple unbranched rods. In the latter they are separated from 
the gill filaments anteriorly by the interbranchial musculature and extend as 
supports beyond the ends of the filaments. 

The septum of each holobranch, or wall between two sets of filaments, ex- 



>>\\h, 









Fig. 142. Second gill pocket opened, Eeptanchus maculatus. (Duncan Dunning, del.) 
h.r., branchial rays; ex.h., extrabrancliial cartilage; fl., gill filaments. 



THE ELASMOBRANCH FISHES 



149 



tends from the visceral skeleton outward and is attached to the inte<4'unient; 
it is further secured in its outer margin by the dorsal and ventral extra- 
branchial cartilages, where such exist. These extrabranchials (see p. 45, fig. 49, 
ex.b.) extend around the margin of the septum roughly at right angles to the 
branchial rays. At their dorsal and most ventral angles the extrabranchials 
pass posteriorly across the pockets so that 
in the opened pocket their transected ends 
show on both sides of the incision (ex.h., fig. 
142). 

Gill Filaments 



csd. 



Fig. 143. Section 
cutting parallel 
to branchial fil- 
nments tlirougli 
second holo- 
branch, Heptan- 
chus maculatus. 
(H. M. Gilkey, 
del.) 



m 



The respiratory membrane is formed of 
the series of folds or filaments {fl.) attached 
to the anterior and posterior sides of the sep- 
tum of each whole gill. Anteriorly these folds 
compose the respiratory surface of the an- 
terior demibranch; posteriorly, that of the 
posterior demibranch. The filaments of the 
posterior demibranch in Hepfanchus macu- 
latus extend considerably farther distally on 
the septum than do those of the anterior set, 
but they also arise farther out from the base. 
In both they consist of a series of plates flat- 
tened from side to side. The longest of these 
plates is located medially and from this point 
they get shorter and shorter until at both the 
dorsal and ventral angles of the pocket they 
are relatively minute. Above and below the 
epi- and ceratobranchial segments filaments 
are absent. 

A section taken at right angles to the in- 
ternal branchial arch and cutting through 
the epibranchial segment, parallel with an 

anterior and a posterior filament, is seen in figure 143. In this, the fourth dor- 
sal constrictor muscle (csd.) is thickened toward the margin of the septum, 
where it is supported by the extrabranchial cartilage (ex.b.). It is continued 
toward the internal branchial aperture as an interbranchial muscle (ib.d.) 
directly in front of the cartilaginous branchial ray (b.r.), the upper part of 
which in figure 143 has been cut off. Anterior to the branchial ray is the third 
afferent artery (af.) which is surrounded by the nutrient vein (sinus) of the 
arch, and anterior to the union of the branchial ray with the internal branchial 
arch is the large anterior efferent-collector artery (efc.^) ; just posterior to 
this efferent-collector is the pretrematic division of the branchial nerve (cross- 
hatched), and below the posterior filament is the small posterior efferent- 
collector artery (efc.^). 



ad.-^ 



ad., adductor 
muscle; a/., third 
afferent artery ; 
b.r., branchial 
ray cut short; 
csd., fourth dor- 
sal constrictor 
muscle; eb., epi- 
branchial seg- 
ment of internal 
branchial arch; 
efc.*''% fourth 
and fifth effer- 
ent-collector ar- 
teries; ex.b., ex- 
trabranchial car- 
tilage; fl.a., an- 
terior filament ; 
fl.p., posterior 
filament; ib.d., 
dorsal inter- 
branchial mus- 
cle ; 11., posterior 
division of the 
branchial nerve. 



150 THE ELASMOBRANCH FISHES 



RESPIRATORY TRACT OF ELASMOBRAXCHS IN GENERAL 

The respiratory tract or passageway through which the water current passes 
in bathing the gills of Elasmobranchs, unlike the tract in higher forms in 
which the respiratory current may enter external nares, begins with the 
mouth and terminates with the external branchial clefts. The following parts 
are included in the respiratory tract : the buccal cavity, the enlarged pharynx, 
and, in the wall of the pharynx, the gill pockets. The gill pockets, as in Heptan- 
chus, are reached from the pharynx by the internal branchial apertures and 
open to the exterior through the external branchial apertures or clefts. We 
shall describe first the external branchial apertures. 

Many of the Elasmobranchs are of the pentanchid type, that is, those 
possessing five gill clefts; only Chlamijcloselachus and those sharks belonging 
to the notidanids exceeding this number. Record is also made of a greater 
number occasionally occurring in other forms. In Chlamydoselachus and in 
Hexanchus six clefts are present and in Heptanchus, as we have seen, there 
are seven, the greatest number known for any gnathostome or jaw-possessing 
vertebrate. 

The position of the external branchial apertures or clefts has long been used 
in separating the sharks from the rays. Characteristic of the former the clefts 
are lateral in position, while in the latter they are ventrally located. In the 
raj'S, however, there enters the element of time, for while a ventral position of 
the clefts is characteristic of the adult, the position for a considerable period 
of time in the embryo is lateral (see p. 11) . 

The external branchial apertures in the sharks vary greatly in relative size. 
In a type like Acanthias (fig. 5) they extend as slits practically one-half the 
height of the bod3\ In Heterodontus (fig. 17) the first cleft is large, but the 
last is so small as to be of slight functional value. Many other forms are like 
Heterodontus in this regard. In some of the other sharks the clefts are of 
immense size. This condition is found especially in the lamnoids and in Ceto- 
rhinus and Rhinodon (fig. 3), in all of which the apertures are practically 
the height of the pharj-ngeal diameter. In the rays the clefts are relatively of 
a much smaller size. 

The first cleft to open and one of the most important in the emliryo is the 
spiracle. As growth proceeds, however, the spiracle fails to keep pace ^^^.th the 
other clefts, so that at its maximum development in types like Acanthias (see 
p. 11, fig. 22) and Squatina it is relatively small. In others, as for example 
Lamna and Carcharias, the spiracle of the adult is often minute; and in still 
others all superficial trace of it may be lost. In the rays, on the contrary, it 
has assumed a secondary function and hence has l^ecome enlarged. 

Gill Pouch or Pocket 

In shape the gill pockets of the sharks are generally like those of Heptanelius. 
In types like Lamna, Cetorhinus, or Rhinodon, however, the external clefts 
are so large that the normal form of the closed pocket is more like that of the 



THE ELASMOBRANCII FISHES 



151 



opened jioeket in HcptdiicIiHs. In tlie rays the pocket is somewhat like an in- 
verted U, in which the internal and external branchial apertures are repre- 
sented by the ti])s of the IT close together. 

The first indication of a pocket in the embryo appears as an evagination or 
outpocketing of the pharyngeal wall toward the exterior (gp., fig. 144). As 
this approaches the surface it meets a slight pitting in from the outside, and 
the wall between the two breaks through to form the external branchial cleft 
or aperture. 

In pentanchid types six of these pouches, including the s])iracular, are 
formed; but accessory ]>ockets are often indicated. 
Thus in Acanthias a small i^oucli on the left side 
(or a pair of pouches) is produced as an evagina- 
tion from the floor of the pharynx just back and 
mediad of the sixth pouch. This pouch does not 
reach the outside layer but comes in contact with 
the roof of the pericardial cavity. Here it forms 
numerous tubules and becomes the so-called post- 
branchial or suprapericardial body. These bodies 
have also been described for Scyllium, Galeus, 
Pristiurus, and Raja. 

In figure 144 the anterior clefts have thus broken 
through. Between the pockets are columns, from 
which all the tissues of the holobranchs are later 
produced including their supporting cartilages 
and musculature. On the posterior wall of the 
hyoidean cleft will appear a little later the begin- 
nings of the embryonic or external gill filaments. 

The whole column from the internal branchial 
arch toward the exterior lengthens out and the 
central core becomes a plate, the septum or dia- 
phragm of the gill. This plate supports the fila- 
ments or respiratory membrane, and is peculiar in 
the Elasmobranchs in that it extends outward 
beyond the filaments which are attached to it. It is 
from the peculiar attachment of the filaments to the septa that the Elasmo- 
branchs have received their name ('EXacrjuos: blade- or strap-like; ^payxt-o-: 
gilled). The septum may be said to attach internally on the internal visceral 
arches, and to extend outward to the lower layer of the integument. Each 
septum is limited anteriorly and posteriorly by a heavy layer of tissue which 
is continuous with the mucous membrane of the pharynx. Terminally the an- 
terior and posterior plates of this tissue run parallel and are separated only 
by the interbranchial musculature, connective tissue, and cartilaginous bran- 
chial rays {Heptanchus) which support the septum. 

The interbranchial muscles (ib.m., fig. 145) are located on the anterior sur- 




Fig. 144. Horizontal section 
through pharynx to show for- 
mation of the gill pockets, 
Hcterodontus francisci. (Mar- 
shall Williamson, del.) 

h.v., blood vessels; c, cav- 
ity; gp., gill pocket. 



152 THE ELASMOBRANCH FISHES 

face of, and run at right angles to, the cartilaginous branchial rays of each 
whole gill. These muscles form thin sheaths dorsally and ventrally by which 
attachment is made to the fascia and to the extrabranchial cartilages; medially 
a part of the interbranchial joins the epi- and ceratobranchial segments of the 
internal branchial arches. Some of the outer fibers in their course are attached 
to cartilaginous branchial rays, but in the sharks most of them are continuous 
above and below. In the rays these muscles are regularly attached to the bran- 
chial rays (see p. 106, fig. 108b) . They are thus able by contraction to decrease 
the size of the pocket. 

The filaments of the adult gill are to be distinguished from those of the early 
embryo which we may first consider. Upon the breaking through of the ex- 
ternal clefts in the embryo of all Elasmobranchs a series of nodules arises from 
the posterior margins of the hyoidean and the branchial arches. These grow 
outward as long external gill filaments which sometimes exceed the entire 
body in length {JJrolophus, fig. 22d) ; in others they are shorter [Acanthias, 
fig. 22c). Such filaments are characteristically more numerous in the more 
generalized types. In Heptanch xs we have seen the greatest number known in 
Elasmobranchs. In Acanthias they are more numerous than in Scylliuni, and 
in Scyllium they exceed in number those of most rays. 

These filaments in the living embryo are particularly noticeable from the 
fact that they are kept constantly in motion and are filled with blood, giving to 
them their striking color. Under the microscope the circulation of blood can 
here be seen to remarkable advantage. Each embryonic filament is a more or 
less flattened plate with a blood vessel encircling it near its outer border. The 
whole tissue of the filament acts as the respiratory membrane. 

External branchial filaments thus developed in the embryo are early ab- 
sorbed, giving place to permanent or internal filaments. In some forms the 
filaments of the adult may be of great length reaching practically to the outer 
margin of the septum (Lamna). In many others, however, they are of medium 
length (Scyllium); while in a few they are relatively short (Sguatma). 
Usually in the Elasmobranchs the filaments of the posterior demibranch are 
longer than those of the anterior; and those of the middle of the septum are 
the longest of the series. 

Sections through the septum parallel to the filaments give a clear notion of 
the structure of a gill. Droscher (1882) gave such a section through a holo- 
brancli of Torpedo (fig. 145) which shows that here the filament of the pos- 
terior demibranch, like that in Heptanchus, is longer than that of the anterior, 
and that both extend only two-thirds the length of the septum. Running 
transversely through the central part of the septum, from the internal bran- 
chial arch toward the exterior is the interbranchial muscle; back of this is the 
cartilaginous branchial ray. It is observed further that the filaments, instead 
of being round as in face view, are flattened from side to side. A section of the 
filaments of Torpedo, so far as considered, is essentially like the one studied 
of Heptanchus. It differs from it somewliat in the position of its blood and in 
its nerve supply. 



THE ELASMOBRANCH FISHES 



153 



IMi®. 



ib.m.j. 



e.b.= 






eb. 



.a.b 



_w-.efc. 



::;^.._b.r. 



The finer structure of the gill of Torpedo shows that the relation of the blood 
system to the gill is much like that of Mustelus and Heptanchus. Passing 
through the base of the septum, and anterior to the cartilaginous branchial 
ray (b.r.), is an afferent ])ranchial artery ((//.)• From this an arteriole (a.h}) 
passes to the anterior filament and another (a.h.-) to the posterior filament. 
These arterioles give off smaller l)ranches which break u)) into a net of capil- 
laries which, in turn, form a complex web over 
the larger ])art of the surface of the filament. 
The capillaries are continued to efferent bran- 
ehials {e.h.^ and e.h.^) as efferent arterioles 
which carry the oxygenated blood down the 
filament into the efferent-collectors (efc), an 
anterior and a posterior of which are present 
at the base. 

In the spiracular pocket as in branchial 
pockets the anterior wall is usually also pro- 
vided with filaments. These are numerous in the 
rays but few in number in the sharks, as in Het- 
erodontus. In a type like Carcharias, in which 
the spiracle is minute or wanting, they are en- 
tirely absent. 

From the anteromedial wall of the spiracular 
pocket and dorsally located there is an evagina- 
tion which may reach the auditory capsule and 
be attached to it above the postorbital groove 
(dc, SciiUiuni, fig. 146, Mustelus, Gaelus, 
Squatina, Rhinobatis, Zygaena). At its begin- 
ning the diverticulum may be practically closed 
(Oaleus), but as it approaches the auditory 
capsule it becomes enlarged. In Squalus acan- 
thias, Norris and Hughes (1920) have recently 
shown that this evagination may be divided 
into two or three diverticula each of which is 
supplied with a branch of the ramus oticus 
VII nerve. The organ is considered to be a 
modified ampulla of Lorenzini. A second diver- 
ticulum on the anteromedial wall of the spiracular pocket is located ventrally 
near the union of the pocket with the pharynx. This in ScylUum is shallow, 
but in some of the other Selachians it is connected to the spiracular pocket by 
a neck. In the spiracle of rays there is a well developed valve on the anterior 
side of the cleft. It is composed of a stiff crescentic fold of connective tissue 
which is constantly opened and closed in respiration. Serving as a support for 
the fold is the strong crescentic spiracular cartilage which at each end is fixed 
by ligament. The closure of the spiracle is due to the contraction of the first 




-ad. 



C 



Fig. 14.5. Section of branchial 
filaments, parallel to branchial 
ray. Torpedo. (From Droscher.) 
a.b.^~-, afferent branchial ar- 
terioles from afferent artery; 
ad., adductor muscle; af., affer- 
ent artery; h.r., branchial ray; 
e.b.^~-, efferent branchial arte- 
rioles to efferent-collectors ; efc, 
efferent-collector; ih.m., inter - 
branchial muscle. 



154 



THE ELASMOBRANCH FISHES 



dorsal constrictor muscle. A similar valve, although less well developed, 
occurs in some of the sharks, as for example in Acanthias and Musteliis. 

The internal branchial apertures of the branchial pockets may be simple 
slits or the aperture may be modified by the presence of certain gill rakers. 
The gill rakers in Squalus sucklii (gr., fig. 147) form a series of processes from 
the pharjaigeal arches across the internal branchial apertures. On the first 

two branchial arches they project 
only from the anterior surface of 
the arch, while on the third and 
fourth they extend from both the 
anterior and the posterior sur- 
faces. These projections are cov- 
ered with caps of the mucous lin- 
ing, on which stomodeal denticles 
are present, and are supported 
internally by cartilages, the ante- 
rior ones of which practically 
touch the internal branchial arch 
and the posterior ones are directed outward from the adductor muscle (ad.). 
In Cetorhinus a straining apparatus is differently formed. As we have 
remarked (p. 37) the gill rakers in Cetorhinus and Rhinodon are modifica- 
tions of placoid scales which arise from semilunar bases and are continued as 
long filaments across the internal branchial aperture. Upon opening a gill 
pocket in Cetorhinus (fig. 148a) myriads of these rakers or filaments (g.r.) 
are seen to be attached to the internal branchial arches and to extend inward 
so that those from the arch in front of the pocket overlap those from the arch 
behind it (fig. 148b). There is thus formed of these rakers a V-shaped strain- 




rig. 146. Transverse section through spiracles, 
ScyUium. (From Eidewood.) 

dc, dorsal caecum. 




Fig. 147. Horizontal section cutting through the gill pockets to show gill rakers, Squalus 
sucMii. (H. M. Gilkey, del.) 

ad., adductor muscle; af., afferent artery; h.r., branchial ray; efc.\ anterior efferent- 
collector; gr., gill rakers; ia., internal branchial aperture; ih.m., interbranchial muscle. 

ing apparatus which points into the pharynx and completely covers the inter- 
nal branchial aperture. By means of this strainer small organisms, prevented 
from passing out with the respiratory current, are collected in great numbers 
and passed down the digestive tract as food. 



THE ELASMOBRANCH FISHES 



155 



PRODUCTION OF RESPIRATORY CURRENT 

For tlie sharks and rays in general the respiratory current is produced by 
the interaction of the complicated series of buccal and ])haryngeal muscles 
which insure that when the current enters the mouth the external clefts close 
and when the clefts oi)en. the mouth closes. In general the action is as follows : 
By the contraction of the ventral, longitudinal, or hyj^obranchial musculature 





Fig. 148. A. Part of a gill pocket of Cetorhinus. (From Pavesi.) B. Diagram of a section 
parallel to the gill rakers. 

&./•., branchial ray; fl., filaments; g.r., gill rakers; g.p., gill pocket; ia., internal branchial 
aperture. 



the floor of the mouth and pharynx is lowered, thus enlarging the Ijuccal and 
pharyngeal rooms, at the same time that the mouth is opened. Into this cavity 
the water rushes. The adductors then act, closing the mouth and at the same 
time flexing the epi- and ceratobranchial segments of the arches, thereby 
spreading apart the cartilaginous branchial rays and causing the pockets to 
enlarge. The water now enters the pockets and is then forced out through 
the external clefts by the contraction of the constrictor and interbranchial 
muscles. By this action of the muscles a rhythm is produced which under 
conditions of rest is about thirty-five respirations a minute (Heterodontus 
francisci) . 



156 



THE ELASMOBRANCH FISHES 



DIEECTION OF RESPIRATOEY CUREENT 




Fig. 149. Sagittal section through buccal cav- 
ity of Baia erinacea to show valves (v.). (From 
Eand.) 



In the free-swimming sharks the current enters the mouth, from which it 
passes through the pharynx and into the gill pockets, the external clefts, in- 
cluding the spiracle, at the same time remaining closed. The mouth then closes, 
the external clefts open, and the water is forced out. 

In the rays, which spend most of their time on the bottom and hence often 
in mud or sand, there is an interesting change in the direction of the current. 
In these the greater part of the current enters through the spiracle and but 
little through the mouth. The valve of the spiracle then closes and the water 
is forced out ventrally through the external branchial clefts. At the expulsion 
of the water the mouth does not entirely close, but only a little of the current 

is able to gain exit through it because 
of valves wdiich are located on its 
roof and floor (v., tig. 149). 

In ra^ys there often occurs a rever- 
sal of the current, by which the water 
is spouted outward through the spir- 
acle. Rand (1907) has shown that 
this may be brought about experi- 
mentally in several ways. In the first 
place spouting may be produced by 
putting the ray into water over- 
charged with carbon dioxide, or it may be the result of fatigue as is shown by 
compelling the ray to keep in rapid motion for a period of time. Again, spout- 
ing may be produced by putting a soft substance, as for example sea moss, into 
the spiracle. To any of these experiments the ray responds by ejecting a col- 
umn of water through both spiracles. Rand has shown further that by striking 
the margin of the spiracle or of the eye on a single side, spouting may be pro- 
duced by a single spiracle on the side thus irritated. 

While spouting is characteristic of the rays it is not confined to them. In 
Squatina, a form which spends much time on the bottom, I have also found 
spouting to occur. But in this type it is not so much an indication of a reversal 
of the current. Here as in other sharks water enters the mouth and passes 
out through the clefts. To study the spouting behavior and to note the direc- 
tion of the current, I have observed Squaiiyia (a shark) and Rhinobatis (a ray) 
in the same large aquarium. Under such conditions both are seen to spout 
occasionally. With exercise the spouting occurs more frequently. If now the 
water in the aquarium is let off so as to expose the spiracular clefts, Rhino- 
batis becomes greatly agitated while Squatina is little disturbed. These 
observations show that the spiracle in Squatina is not, as it is in the rays, the 
principal intake. Darbishire (1907) says for Squatina, however, that carmine 
liberated in the region of the spiracle enters, and furthermore, that it enters 
not rhythmically but in a constant stream. This regularity of the current is 
produced by a rhythmic action of the free margins of the gill septa. In the 



THE ELASMOBRANCH FISHES 157 

experiments on Squat ina, above mentioned, I have also observed the phenome- 
non of the outgoing current, but that the spiracle is not of great importance 
as an aperture for the intake of the current is shown by my experiment. 

Something of the same rhythmic current occurs in the rays, although it may 
be produced differently. In s{)ecimens of the small sting-ray, Urolophus, which 
are buried in the sand with only the spiracular clefts and the outline of the 
bodj' discernible, the s])iracular clefts open and close regularly. The only evi- 
dence of the outgoing current, however, is seen in a regular geyser of sand 
grains arising to an inch or two in height at the anterior margin of the pec- 
toral fin. 

CIECULATION OF BLOOD IN FILAMENTS 

In a later study of the blood system ( Chapter VII, p. 161 ) it will be found that 
the afferent arteries bear non-oxygenated blood from the heart to all the 
demibranchs and give off to them smaller branches which run outward toward 
the tips of the filaments; the branches in turn give off thin-walled arterioles 
which in their course break up into capillaries. It is in these capillaries that 
the exchange of oxj^gen and carbon dioxide takes place. From the capillaries 
efferent branchial arterioles convey the oxygenated blood toward the base of 
the filaments into either an anterior or a posterior efferent-collector (efc). By 
means of these collectors oxygenated blood is removed from the region of the 
gills. In other words, while a single afferent artery supplies both anterior and 
posterior series of filaments with non-oxygenated blood, two efferent-col- 
lectors, one at the base of each demibranch, carry away the blood which has 
been oxygenated. 

RESPIEATION OR THE EXCHANGE OF GASES 

The respiratory current thus brought into the gill pockets is separated from 
the blood contained in the capillaries of the filaments only by the thin capil- 
lary wall. The free oxygen of the water passes by osmosis through this wall 
into the blood to be distributed to the body, and the carbon dioxide brought to 
the capillaries from the body tissues passes outward into the water to be 
eliminated by the respiratory current. The exchange of gases takes place with 
extreme rapidity as is evident from the fact that the blood makes its complete 
transit of the capillaries in a very short time. 



158 THE ELASMOBRANCH FISHES 

BIBLIOGRAPHY 
Chapter VI 

1840. AlesAjVNDRINI, Axtonti, Observationes super iiitima branchiarum structura piscium 
cartilagineorum. Novi Comment. Acad. Bonon., Vol. 4, pp. 329-344. 

1886. Bemmelen, J. F. van, tjber vermuthliche rudimentare Kiemspalten bei Elasmo- 
branc.hieni. Mitt. Zool. Stat. Neapel, Bd. 6, pp. 165-184, pis. 11-12. 

1818. Blainville, H. D., tJber den Bau der Kiemen bei den Foetus der Haifische. Arch. f. 
Physiol., Bd. 4, pp. 275-296. 

1906. Braus, H., Ueber den embryonalen Kiemeuapparat von Heptanchus. Anat. Anz., Bd. 
29, pp. 545-560, 2 text figs. 

1917. Camp, W. E., The Development of the Supraperieardial (Post-branchial, Ultimo- 
branchial) Body in Squalus acanthias. Jour. Morph., Vol. 28, pp. 369-415, pis. 1-2, 
29 text figs. 

1857. Cornalia, Emilio, Sulle branchie transitorie dei feti Plagiostomi. Gior. dell' I. E. 
Istit. Lombardo (n. s.), T. 9, pp. 256-258. 

1870. Cornish, Thomas, On a Shark Captured in Mount's Bay on June 11, 1870, Supposed 
to be Identical with the Basking Shark of Pennant and the Broadheaded Gazer of 
Couch. Zoologist, August, 1870, pp. 2253-2260. 

1907. Darbishire, A. D., On the Direction of the Aqueous Current in the Spiracle of the 
Dogfish : together with some observations on the Respiratory Mechanism in other 
Elasmobranch Fishes. Jour. Linn. Soc. Lond., Vol. 30, pp. 86-94, 3 text figs. 

1886. DoHRN, A., Studicn zur Urgeschichte des Wirbelthierkorpers. XI. Spritzlochkieme 
der Selachier, Kiemendeckelkieme der Ganoiden, Pseudobranchie der Teleostier. Mitt. 
Zool. Stat. Neapel, Bd. 7, pp. 128-176, Taf. 2-5. 

1882. Droscher, Wilhelm, Beitrage zur Kenntnis der histologischen Struktur der Kie- 
men der Plagiostomen. Diss. Leipzig, 1881, pp. 12-176, pis. 9-12. 

1890. EwART, J. C, On the Spiracles of the Porbeagle Shark (Lamna cornubica). Jour. 

Anat. and Physiol., Vol. 24 (n. s., Vol. 4), pp. 227-229. 
1909. Hyde, Ida H., A Study of the Respiratory and Cardiac Activities and Blood Pressure 

in the Skate following Intravenous Injections of Salt Solutions. Kansas Univ. Sci. 

Bull., Vol. 5, No. 4, pp. 29-63, pi. 10, 49 text figs. 
1836. Leuckart, F. S., Untersuchungen liber den ausseren Kiemen der Embryonen von 

Rochen und Haien. Ein Beitrag zur Entwickluugsgeschichte der Abtheilung der 

Knorpelfische angehorenden Plagiostomen. Stuttgart, pp. 1-44, pis. 1-5. 
1880. M'Kenkrick, J. G., On the Respiratory Movements of Fishes. Jour. Anat. and 

Physiol., Vol. 14, pp. 461-466, pi. 28. 
1894. Mayer, P., ttber die vernieintliche Schwimmblase der Selachier. Mitt. Zool. Stat. 

Neapel, Bd. 11, pp. 475-478, 1 text fig. 
1920. NoRRis, H. W., and Hughes, Sally P., The Spiracular Sense-Organ in Elasmo- 

branchs. Ganoids and Dipnoans. Anat. Rec, Vol. 18, pp. 205-209, 1 text fig. 
1874. Pavesi, p., Contribuzione alle storia naturale del genere Selache. Ann. Mus. Civ. 

Storia nat., Genova, Vol. 6, pp. 5-72, pis. 1-3. 
1911. PoLiMANTi, Osw., Uber den Beginn der Atmung bei den Eml)ryonen von Scyllium 

(Catulus Cuv., Canicula L.). Zeitschr. Biol., Bd. 57, pp. 237-272, 2 text figs. 
1904. QuiNTON, Rene, Communication osmotique chez le poisson Selacien marin, entre le 

milieu vital et le milieu exterieur. C. R. Acad. Sci. Paris, T. 139, pp. 995-997. 



THE ELASMOBRANCII FISHES 159 

1907. Rand, H. W., Tlie Functions of the Spiracle of tlie Skate. Amcr. Nat., Vol. 41, pp. 
287-302, 3 text figs. 

1896. RiDEwoOD, W. G., On the Spiracle and Associated Structures in Elasniobranch Fishes. 

Anat. Anz., Bd. 11, pp. 425-433, 2 text figs. 
1875. ScHENK, S. L., Die Kiemenfaden der Knorpelfische wiihrend der Entwnckelung. Sitz- 

ber. Akad. Wien, math.-naturwiss. Klasse, Vol. 71, pp. 227-238, Taf. 1. 

1875. TtTRNER, Wm., On the Presence of Spiracles in the Porbeagle Shark (Lamna cor- 
nubica). Jour. Anat. and Physiol., Vol. 9, pp. 301-302. 

1879. Turner, Wm., The Structure of the Comb-like Branchial Appendages and of the Teeth 
of the Basking Sliark (Selache maxima). Jour. Anat. and Physiol., Vol. 14, pp. 273- 
286, pi. 12. 

1898. ViRCHOw, H., Ueber Oberflachenbilder von Selachierkiemen und Mesodermursprungs- 
zone. Anat. Anz. (Verh.), Bd. 14, pp. 43-49, 4 text figs. 

1876. Wright, E. P., The Basking Shark. Nature, Vol. 14, pp. 313-314, 2 text figs. 



VII 
CIRCULATORY SYSTEM 

CIRCULATORY SYSTEM OF HEPTANCHUS MACULATUS 

The heart in Heptanchus (fig. 150) is located over, and in front of, the sternal 
symphysis of the pectoral arch in the region between the gills, and is retained 
within a relatively large pericardial cavity. It is made up of two rooms 
proper, an auricle (atrium) (au.) and a ventricle (vn.), into the former of 





Fig. 150. A heart of Heptanchus macuJatus. (Marie Weldt, orig.) B. Valves of conus, ven- 
tral view. (From Garman.) 

ap., aperture of last afferent artery; au., auricle (atrium) ; tr.af}''^, first and sixth bran- 
chial afferent arteries; c.a., conus arteriosus; cr.l., left coronary artery; hy.af., hyoidean 
aft'erent artery; p.c, pericardial; v.a., ventral aorta; v.c, valves of conus; vh., ventricle. 

which the blood enters from the sinus venosus (see p. 203, fig. 188, s.v.), and 
from the latter of which it is expelled through the conus arteriosus {c.a., 
fig. 150). 

The auricle, or atrium, is a greatly enlarged, thin-walled sac which lies dor- 
sall,y over the ventricle. Connecting it with the sinus venosus is the sinu- 
auricular opening. At the sides of this opening are the two sinu-auricular 
valves (sa., fig. 188), which prevent the backward flow of the blood into the 
sinus venosus upon the contraction of the auricle. Connecting the auricle with 
the ventricle is an auriculoventricular opening guarded by valves of the same 
name attached to the ventricular walls. 

The more or less triangular ventricle forces the blood by waj- of the conus 
arteriosus through the gill capillaries and as a consequence of the strain 



[160] 



THE ELASMOBRANCH FISHES 161 

imposed iii)Oii it, its walls have become greatly thickened. Inside, the lining 
has become thrown into numerous irregularities and is richly i)rovided with 
supporting chordae tendineae stretched from the walls of the ventricle to the 
auriculoventricular valves. These tendinous cords prevent the valves from 
being forced into the auricle ui)on the contraction of the ventricle. 

The conus arteriosus connects the ventricle and the ventral (ascending) 
aorta (vm.). Its walls are muscular so that it serves to keep the blood sent out 
from the ventricle at a more constant pressure. A longitudinal section through 
the conus of Heptanchiis shows its thickened walls and, on the sides of the 
lumen, a series of pocket-like valves {v.c, fig. 150b), which prevent blood 
from reentering the ventricle. These are arranged in three longitudinal rows, 
one dorsal and two ventrolateral in position. The anterior and much larger 
valves in each row are located just back of the exit of the most posterior 
afferent branches of the ventral aorta. Following these are other small valves, 
behind which are three valves of medium size, in each row. 



ARTERIES 

Ventral Aorta 

The ventral aorta {v.a., fig. 150a) continues forward from the conus and, in 
the region back of the mandibular symphysis, divides into right and left 
halves. Along its course it gives off paired branches, the afferent arteries 
{hy.af., and hr.af}~'^, fig. 150a) which distribute blood to the gills. 

AFFERENT ARTEEIES OF ADULT 

In Heptanchus these branches are seven in number. An anterior pair, con- 
sisting of the hyoidean afferent (hij.af.) and the first branchial afferent 
{hr.af}), arises from a common trunk formed by the bifurcation of the an- 
terior end of the ventral aorta. The first or hyoidean supplies the hyoidean 
demibranch, entering in front of the first gill pocket. The second enters the 
first whole gill l)etween the first and second pockets, supplying both of its 
demibranchs. Following these on each side are given off the second to the sixth 
branchial afferents, the last two of which arise close together from the ventral 
aorta (fig. 150b, ap.). The second to the sixth branchial afferents enter and 
supply the fourth-fifth, sixth-seventh, eighth-ninth, tenth-eleventh, and 
twelfth-thirteenth demibranchs. Branches from the afferents break up into 
smaller and smaller arterioles and finally as capillaries supply the filaments 

of all the gills. 

EFFEREN T-COLLECTOES 

From the capillaries the blood passes down the filaments into efferent-col- 
lectors. If the most anterior gill pocket of an injected specimen be opened, as 
is the second pocket in figure 142, an efferent-collector artery would be seen to 
drain the demibranch in front of the cleft, the hyoidean demibranch (see also 



162 



THE ELASMOBRANCH FISHES 



fig". 151) , and another, the one behind the cleft. These two arteries, the first and 
second efferent-collectors, unite both ventrally and dorsally, forming a loop 
around the cleft (fig. 151). The third and fourth efferent-collectors encircle 
the second pocket as the first and second encircle the first pocket, but the one 
forming the anterior part of the loop, which is the posterior efferent-collector 
of the first holobranch, is small and has an irregular course. As it passes veu- 




Fig. 151. Efferent-collector arteries, Eeptanchus maculatus. (R. T. Trotter, orig.) 

br.ef.^'^, first and sixth branchial eiJerent arteries; cm.^'^, second and sixth commissural 
arteries; c.tr., cross-trunk; d.a., dorsal aorta; e.c, external carotid; efc}'^, first and thir- 
teenth efferent-collector arteries; hy.ef., hyoidean efferent; ps., pseudobranchial artery; 
th., posterior thyroid artery. 

trally it may increase somewhat in size and is connected with the efferent- 
collector in front of it by numerous cross-trunks. Ventrally it joins the larger 
efferent-collector behind the cleft, the anterior efferent-collector of the second 
holobranch, and dorsally it loops back also making connection with the same 
vessel. Similar efferent-collectors encircle the remaining pockets, except the 
last, where a complete loop is not formed due to the lack of gill filaments on 
the posterior wall of the pocket. The blood from the last (thirteenth) efferent- 
collector is drained by means of cross-trunks {c.tr.) into the twelfth efferent- 
collector through the greater part of the holobranch; and ventrally the last 
efferent-collector joins the one in front of it directly. 

Branches of Efferent-Collectors 
HYPOBEANOHIAL ARTERIES 



From the ventral angles of the second to the fifth (or sixth) efferent-collector 
loops in Heptanchus, large commissural arteries (cm.-'°, figs. 151 and 153) 
pass inward toward the middle line. Near their origin they are usually con- 
nected by small vessels which, joined together, may be called the lateral hypo- 



THE ELASMOBRANCH FISHES 



163 



branchial artery {l.hb., fig. 153); or the lateral liypobranchial vessels may 
arise from tlie ventral parts of the eiferent-colleetor loops; or again a lateral 
hypobranchial segment may be incomplete between certain of the commis- 
surals. Jnst before reaching the midventral line the commissurals of each side 
form a larger longitudinal vessel, the median hypobranchial {m.Jib.),i\i.e right 
and left median hypobranchials being connected by several small connectives 
{en.) which may or may not unite right and left pairs of commissurals. The 
tAvo median hypobranchial trunks are continued posteriorly above the peri- 




Fig. 152. Arterial branches of first efferent-collector, Heptanchus macuJatus. (Marie Weldt, 
orig.) 

a.c, anterior cerebral ; a.sp., arteria spinalis ; hr.ef}, first branchial efferent; d.a}, paired 
dorsal aorta; d.a., dorsal aorta; hy.ef., hyoidean efferent; i.e., internal carotid; m.c, median 
cerebral; ns., nasal artery; o.vi., ophthalmica magna; or., orbital artery; p.c, posterior 
cerebral; ps., pseudobranchial artery; r.a., ramus anastomoticus ; rs., rostral artery; sg., 
segmental artery. 



cardial roof as the two large pericardial arteries (^^c). The pericardials may 
be of essentially the same size or the right one may be the better developed 
(see fig. 154, r.pc). The epigastric artery (epg., fig. 154), which supplies 
branches to the oesophagus and the stomach, is a branch off of the pericardial 
or is the direct continuation of the right one. 

A large vessel is given off from the median hypobranchial between commis- 
sures two and three, or three and four, which passes to the midventral line and 
joins a similar artery from the opposite side. This united trunk extends poste- 
riorly, and at the sixth commissural divides into the right and left coracoid 
arteries (co.a.), which join the subclavian arteries. Similarly, near the sixth 
commissural, an artery arises from the median hypobranchial on each side to 
pass posteriorly and toward the midventral line. This artery, however, does 
not reach or fuse with its fellow from the opposite side, but continues as the 
coronary artery (cr.L, figs. 153 and 150) to the heart. 

In Heptanchus maculatus the left coronary (cr.l., fig. 150a) runs along the 
dorsal and left side of the conus arteriosus, supi^lying it and the dorsal side of 



164 



THE ELASMOBRANCH FISHES 



the ventricle. About midway of the eoniis it sends a branch to the ventral side 
of the ventricle. The right coronary passes around to the ventral side of the 
conus, supplying branches to the tissues on the right side and to the ventral 
and dorsal parts of the ventricle and to the auricle. A posterior coronary 




m.hb 




Fig. 154 



Fig. 153 

Fig. 153. Hypobrancliial arteries, Heptanchus maculatus, dorsal view. (Marie Weldt, orig.) 
a.dl., anterior dorsolateral artery; a.l., anterior lateral artery; Ir.a., brachial artery; 
ce., coeliac axis; C7n.-'^, second and sixth commissural arteries; en., connective; co.a., cora- 
coid artery; cr.l., left coronary artery; cr.p., posterior coronary artery; e.c, external ca- 
rotid; L/i b., lateral hypobranchial; -m./i &., median hypobranchial; mi., metapterygial artery ; 
pc, pericardial artery; p.dl., posterior dorsolateral artery; p.s., posterior scapular; s.cl., 
subclavian artery; th.^-", anterior and posterior thyroid arteries; I -VI, first and sixth bran- 
chial clefts. 

Fig. 154. Epigastric artery (epg.), Heptanchus maculatus, ventral view. (Cecil Rowe,orig.) 
mJih., median hypobranchial ; r.pc, right pericardial; VI-VII, sixth and seventh bran- 
chial clefts. 

{cr.p., fig. 153), arising from each coracoid artery, passes inward to supply 
the sinus venosus. 

From the ventral angle of the first efferent-collector loop a posterior thyroid 
(th.-, fig. 153) is given oi¥ which in general position takes the place of a first 
commissural. From about the same position or a little more anterior than the 
posterior thyroid the large external carotid artery' {e.c, figs. 151 and 153) 
passes forward and the stem then divides, one part passing inward to the sym- 



THE ELASMOBRANCH FISHES 165 

pliysis (»f the Iowcm- jaw aiul giving l)ranches to the coracomandibularis mus- 
cle; and the other upward between the hyoid and mandibular cartilages to 
supply the structures around the mandibular and hyoid regions. The external 
carotid or one of its branches gives off a small branch (th.\ fig. 153) which 
supplies the anterior part of the thyroid gland. 

From the upper third of the first efferent-collector another large artery, the 
pseudobranchial (jjs., figs. 151 and 152) , runs forward to break up into a num- 
ber of strong branches in the spiracle. From the spiracle this artery is con- 
tinued inward and forward as the ramus anastomoticus {r.a., fig. 152), which 
passes through a foramen in the orbit (see fig. 47, f.r.a., facing p. 44) to join 
the internal carotid artery inside the cranial wall. Before entering the orbit, 
however, this ramus gives off the ophthalmica magna (o.m., fig. 152) to the 

eye region. 

EFFEEENT AETERIES 

The first efferent artery, the hyoidean efferent (hy.ef., fig. 152), extends as a 
continuation of the first efferent-collector from the anterodorsal angle of the 
first efferent-collector loop forward and mediad to the paired dorsal aorta 
{d.a.'). The remaining efferents, branchial efferents {br.ef.^-'^, fig. 151) simi- 
larly arise from the dorsal angles of efferent-collector loops, but they, as con- 
tinuations of the anterior efferent-collector, extend backward and inward to 
join the unpaired dorsal aorta. In Heptanchus five of these efferents, above 
the first to the fifth holobranchs, reach the unpaired dorsal aorta and a sixth 
joins the fifth. 

Near the union of the hyoidean efferent and the paired dorsal aorta an or- 
bital (stapedial) artery {or., fig. 152) is given off laterally and extends for- 
ward through the orbit. The paired dorsal aorta is continued forward as the 
internal carotid (i.e., fig. 152) which enters the cranium from the midventral 
line to supply the brain. After perforating tlie cartilage the right internal 
carotid joins the left and the two run for a short distance as a common trunk. 
They then separate and pass forward and slightly outward where they are 
joined by the right and the left ramus anastomoticus, respectively (r.a., fig. 
152) . Given off after the union of the ramus anastomoticus and internal caro- 
tid are the three cerebral arteries to the brain. The anterior cerebral (a.c, 
fig. 152) passes anteriorly between the hemispheres of the brain and is usually 
joined to its fellow from the opposite side by an intercommunicating artery. 
The median cerebral (m.c.) passes forward to supply the olfactory tract and 
bulb. The posterior cerebral (p.c.) passes backward, and its two branches join 
to form the arteria spinalis (a.sp., fig. 152) which extends down and ventral 
to the spinal cord. There is thus formed by the union of the anterior and the 
posterior cerebrals a complete circle around the ventral part of the brain. 

Dorsal Aorta 

The dorsal aorta (fig. 152) is composed of a short anterior paired part (d.a.^) 
and a long posterior unpaired part (d.a.) . The paired part receives the hy- 
oidean efferent, and the unpaired part receives the first four pairs of efferents 



166 THE ELASMOBRANCH FISHES 

and the united trunk of the fifth and sixth efferents. The unpaired dorsal aorta 
then passes backward ventral to the spinal column, becoming in the tail region 
the caudal aorta. In its course through the body it gives off arteries to the di- 
gestive tract and its appendages, to the extremities, and to the body muscula- 
ure and deeper structures. 

AKTERIAL SUPPLY TO DIGESTIVE TRACT 

The arteries given off from the dorsal aorta to the digestive tract consist of 
three large trunks, the coeliac axis, the anterior mesenteric, and the posterior 
mesenteric arteries. 

COELIAC AXIS AND ITS BRANCHES 

In Heptanchus maculatus the coeliac axis (ce., fig. 155) arises as a single trunk 
from the ventral side of the dorsal aorta, only a short distance posterior to the 
union of the last efferents. It passes downward and backward as a relatively 
long artery and at the place where it strikes the portal vein it di^ddes into : 
(1) a very short gastrohepatic which bifurcates into a small hepatic branch 
(k., fig. 155) to the liver and a large gastric branch to the stomach, and (2) a 
large anterior intestinal artery (a.i.a.) which is continued along the valvular 
intestine as the ventral intestinal artery. 

The gastric artery separates into two main divisions, the anterior gastric 
(a.g.) and ventral gastric (v.g.) arteries. The anterior gastric sends a branch 
to the ventral union of oesophagus and stomach and also supplies a branch to 
the dorsal side of the anterior part of the cardiac stomach. The ventral gastric 
artery (v.g.), which is the posterior of the gastric divisions, passes down the 
ventral side of the cardiac stomach and at the angle between the cardiac and 
the pyloric arms of the stomach breaks up into numerous branches some of 
which pass along the pylorus and anastomose with a posterior gastrosplenic 
artery. 

The hepatic artery (h.) passes toward the liver, and after giving branches 
to the anterior segment of the spleen, bifurcates, giving off a smaller artery to 
the left lobe of the liver and a larger branch to the right lobe. These two he- 
patic divisions follow the course of the larger hepatic veins almost to the tip of 
the liver, giving off numerous branches as they go. 

The anterior intestinal division of the coeliac axis {a.i.a., figs. 155 and 156) 
runs posteriorly and strikes the anterior part of the duodenum. Before enter- 
ing the duodenum as the intraintestinal artery (see fig. 156, i.a.) it gives off: 
(1) a posterior gastro-pancreaticosplenic artery (p.gps.), (2) the ventral in- 
testinal artery (v.i.a.), and (3) the gastroduodenal artery (gd.). The poste- 
rior gastro-pancreaticosplenic supplies a short branch to the distal part of the 
pylorus (py.a.) and a long branch which after supplying branches to the pan- 
creas (pn.) and spleen passes along the pyloric arm as the posterior gastro- 
splenic {p.gs., fig. 155) . This branch finally reaches the posterior side of the 
pylorus where it gives off splenic branches and receives certain anastomosing 




•^;L.l 



Fig. 155. Vascular supply to the digestive tract, Heptanchus maculatus. (Duncan Dunning, 
del.) (Dorsal and ventral intestinal arteries and veins drawn relatively close togetlier in 
order that they may be seen.) 

a.g., anterior gastric artery; a.gps., anterior gastro-pancreaticosplenic artery; a.gps.v., 
anterior gastro-pancreaticosplenic vein ; a.i.a., anterior intestinal artery ; a.i.v., anterior 
intestinal vein; ce., coeliae axis; d.i.a. and d.i.v., dorsal intestinal artery and vein; /;., he- 
patic artery; h.p., hepatic portal vein; /.hi., inferior mesenteric; p.gs., posterior gastro- 
splenic artery; p.gs.v., posterior gastrosplenic vein ; p.i.a., posterior intestinal artery ; p.i.v., 
posterior intestinal vein; r., rectal artery; v.g., ventral gastric artery; v.g.v., ventral gastric 
vein ; v.i.a., and v.i.v., ventral intestinal artery and vein. 



THE ELASMOBRANCII FISHES 



167 



brandies from the ventral gastric artery. It then extends to the cardiac 
stomacli wliere it anastomoses with the anterior gastrosplenic artery which 
sup})lies a more anterior segment of the stomach. 

The ventral intestinal artery (v.i.a.) passes over the distal end of the py- 
lorus and the ventral lobe of the pancreas, to appear on the ventral side of the 
valvular intestine. On the intestine it supplies the distal part of the duodenum 
and furnishes the ventral side of the valvu- 
lar intestine with numerous paired annular 
branches which run on the attached edge of 
the spiral valve more or less nearly encir- 
cling the intestine (see fig. 155). 

The gastroduodenal artery {gd., fig. 156) 
in addition to supplying the proximal part 
of the duodenum sends a short branch to the 
tip of the pylorus. 

SUPEEIOE (ANTERIOR) MESENTERIC AND 
ITS BRANCHES 

The superior mesenteric artery (s.m., fig. 
157) as such is usually absent in Heptanchus 
moculatus and when present it is never more 
than a short stem given off from the dorsal 
aorta a little more than one-half the way 
back in the body cavity. (In figure 155 the 
superior mesenteric branches [a.gps. and 
p.i.a.] have been displaced forward.) It im- 
mediately divides into: (1) an anterior gas- 
tro-pancreaticosplenic {a.gps., fig. 155) and 
(2) a posterior intestinal (p.i.a.). 

The anterior gastro-pancreaticosplenic 
(a.gps.) sends branches to the dorsal and distal parts of the cardiac stomach, 
to the dorsal lobes of the pancreas, and to the spleen in and on the angle of the 
stomach. 

The posterior intestinal artery runs back over the mesentery to the valvular 
intestine which it joins at about the middle of its length. Then, as the dorsal 
intestinal (d.i.a.), the artery continues backward to supply the dorsal poste- 
rior half of the spiral valve with annular branches. Some of these branches 
anastomose with similar annular branches from the ventral intestinal artery. 
Finally the dorsal intestinal artery traverses the region of the colon and ter- 
minates in the rectal gland where it joins branches of the inferior mesenteric. 




Fig. 156. Arterial supply to the du- 
odenum. Dorsal view, Keptanclius 
maculatus. (Marie Weldt, orig.) 

a. La., anterior intestinal artery; 
du., duodenum; gd., gastroduodenal 
artery; i.a., intraintestinal artery; 
pn., pancreatic branch ; pn.^'", dor- 
sal and ventral lobes of pancreas; 
p.gps., posterior gastro-pancreatico- 
splenic artery ; py.a., pyloric artery ; 
v.i.a., ventral intestinal artery. 



INFERIOR (POSTERIOR) MESENTERIC ARTERY 



The inferior mesenteric artery (i.m., figs. 157 and 155) arises as a single trunk 
a few segments behind the superior mesenteric region. It runs along the an- 
terior margin of the mesorectal mesentery to the anterior part of the rectal 



168 



THE ELASMOBRANCH FISHES 



gland where it divides, the anterior branch joining the dorsal intestinal artery 
to which reference has been made. The posterior part supplies numerous 

branches to the rectal gland and rectum. 



AETEEIAL SUPPLY TO EXTREMITIES 




Fig. 157. The dorsal aorta and its 
branches, Heptanchus maculatus. 
(Marie Weldt, orig.) 

a.dl., anterior dorsolateral artery; 
a.l., anterior part of lateral artery; 
hr.a., brachial artery; ce.. coeliac axis; 
co.a., coracoid artery; ic, intercostal 
artery; il., iliac artery; i.m., inferior 
mesenteric; od.a., oviducal artery ; p.dl., 
posterior dorsolateral artery; p.l., j)os- 
terior part of lateral artery; r., asym- 
metrical rectal artery ; s.cl., subclavian 
artery ; sg., segmental artery ; s.m., su- 
perior mesenteric artery. 



The arterial supply to the extremities in- 
cludes a pair of subclavian arteries (s.cl., 
fig. 157) carrying blood to the pectoral 
fins, and a pair of iliac arteries (il.) lead- 
ing toward the pelvic fins. 

The subclavian arteries in Heptanchus 
maculatus are unusual in that they often 
are no better developed than are the com- 
mon intercostals (ic). They pass from the 
dorsal aorta near the union of the last ef- 
ferents and out toward the pectoral re- 
gion. A short distance out each subclavian 
gives off a dorsolateral artery, one rela- 
tively large division of which passes for- 
ward (a.dl., fig. 157) and one backward 
(p.dl.). From about this point a smaller 
artery passes dorsally supplying the area 
posterior to the scapula. The brachial ar- 
tery (hr.a.) leaves the subclavian and con- 
tinues through a foramen in the pectoral 
girdle to the pectoral fin. From the 
brachial foramen the subclavian is con- 
tinued forward by a larger brachioscapu- 
lar vessel (see also hsc, fig. 169) to join 
the coracoid artery (co.a., fig. 153) at a 
place where it meets the lateral (abdomi- 
nal) artery (a.l.). The coracoid in turn 
joins the median ventral unpaired artery 
(see fig. 153), described with the hypo- 
branchial system. 

Arising from the brachioscapular ves- 
sel is a branch (nit., fig. 153) , which passes 
along the metapterygium of the pectoral 
fin. The lateral artery (a.l., figs. 153 and 
157) is a continuation of the coracoid; it 
passes backward, hidden by the lateral 
abdominal vein, to join the iliac artery 



(il.) from the posterior region. Also leav- 
ing the coracoids, but farther anterior than the origin of the lateral, is the pos- 
terior coronary artery (cr.p., fig. 153), which has been previously described. 



THE ELASMOBRANCH FISHES 169 

The iliac artery arising? from the dorsal aorta in the pelvic region is larger 
than tlie subclavian. The first branch of the iliac on the left side, the rectal, is 
asjanmetrical and passes forward to supply the wall of the rectum (see r., figs. 
155 and 157) . Before the iliac, as the femoral, passes through a foramen in the 
pelvic girdle to supply the jielvic fin, it joins the posterior part of the lateral 
artery (pi.) . 

ARTERIAL SUPPLY TO DEEPER STRUCTURES 

The third set of paired arteries arising from the dorsal aorta consists «of seg- 
mental arteries {sg., fig. 157) . The first of these segmentals may arise from the 
paired dorsal aorta (sg., fig. 152) throughout the pharyngeal region and per- 
forate the deeper musculature around the spinal column. 

In the region between the subclavians and iliacs about thirty pairs of regu- 
larly arranged segmental arteries leave the aorta. Each segmental sends a 
branch dorsally around the spinal column supplying a median branch to the 
spinal cord and lateral branches to the musculature, and is continued by a 
superficial intercostal branch (ic.) which runs along the peritoneum bound- 
ing the body cavity. These intercostals extend outward to supply the inter- 
septal musculature and are often of great length. Occasionally, however, some 
of these are lacking, and the interseptal spaces, especially in the region of the 
mesonephros or kidney, may be supplied by neighboring arteries. 

The renals are the most ventral branches of the segmentals. They are more 
irregular along the anterior prolongation of the kidney, but in the posterior 
region where the kidney is enlarged they are regular in position and of large 
size. 

The oviducal arteries in Heptanchus {od.a., fig. 157) arise from the third 
pair of segmentals back of the subclavians. These arteries run posteriorly, and 
at a])out the eighth segment behind the subclavians liecome tortuous in their 
course ending in segment nine or ten. 

Caudal Aorta 

The dorsal aorta, as the caudal artery, traverses the haemal canal to the tip of 
the tail giving off segmentals similar in general to those of the trunk. These 
pass from, the caudal aorta laterally through interhaemal spaces and branch 
into ventral and dorsal divisions; the ventral branches go to the ventral mus- 
cles and the dorsal branches pass upward and around the body of the centrum. 
From the dorsal part, spinal arteries are given off which enter the neural canal 
to supply the spinal cord. 



170 



THE ELASMOBRANCH FISHES 



CIRCULATORY SYSTEM OF ELASMOBRANCHS IN GENERAL 



The circulatory apparatus in the Elasmobranchs in general consists of four 
structures, all formed first as simple tubes. These structures are (1) a rela- 
tively simple two-roomed heart, the walls of a part of which have become thick- 
ened for pumping; (2) arteries, which bear blood from the heart and from the 
gills; ^3) a series of terminal thin-walled capillaries which connect the arteries 

with (4) the veins. The veins in 
Elasmobranchs are relatively large 
and return the blood to the heart. 
As is true for vertebrates in gen- 
eral, the arteries and veins are dis- 
tinguished by two or three well de- 
fined characteristics. Both are made 
up of layers from the inside out as 
follows : a thin lining, the intima, 
around which is the muscularis or 
muscular layer. Surrounding the 
muscular layer is the serosa. But in 
the artery the muscular layer is 
much thicker than it is in the vein. It 
is this layer in the artery which 
keeps the blood under more constant 
pressure and forces it through the 
capillaries and veins back to the heart. A second distinction between the two 
is that the veins possess valves. These valves are numerous and are especially 
large at the ends where the vessels empty. They are so arranged that blood 
can pass freely towards the heart, but its passage in the opposite direction 
is precluded. 

The blood stream in the Elasmobranchs, as in other vertebrates, is made up 
of a relatively large amount of plasma in which is contained a small amount of 
serum. The erythrocytes or red corpuscles (fig. 158) contain only a small 
amount of haemoglobin or red coloring matter and are nucleated cells much 
greater in diameter than are the red corpuscles of man. The white corpuscles 
are also nucleated and may be filled with granules. 




Fig. 158. Ked blood corpuscles of Sqiialns 
sucMii. (M. C. Williamson, orig.) 



He.^t 



The heart in the Elasmobranchs, as was seen in Heptanch us, does not undergo 
the differentiation characteristic of the more complex heart of higher animals. 
As a usual thing, it is composed of a thin-walled auricle (atrium) (a«., fig. 159) 
and a thick-walled ventricle {vn.). The auricle receives the non-oxygenated 
blood from the sinus venosus {s.v.) and the the ventricle sends it forward 
through the conus (truncus) arteriosus (c.o.). 



THE ELASMOBRANCH FISHES 



171 



Tlie sinus voiiosus may in general be described as a delta-shaped collecting 
sac, the apex of which leads to the auricle and the base of which is posterior in 
position (see }). 203. fig. 188). Dorsally the sinus venosus is fused to the pos- 
terior part of the roof of the pericardial cavity; laterally each angle of the 
delta extends to the right or left as the duct of Cuvier. The principal change in 
the form of the sinus venosus from that just described is found in some of the 
rays (see p. 209. fig. 194b) in which the lateral angles are drawn out into the 
elongated ducts of Cuvier. 

Connecting the sinus venosus with the auricle is the sinu-aurieular aperture 
which, as in Hepta7ich us, is guarded by the sinu-auricular valves. These valves 
are nothing more than double folds of the endothelial lining of the auricle 






Fig. 159. The heart opened to show valves. (From Garman.) A. Isuriis. B. CeplialoscyUium. 
C. Mobida. 

OIL, auricle (atrium) ; c.a., conus arteriosus; s.v., sinus venosus; vn., ventricle. 

projecting into the sinus venosus. They are so arranged as to permit the free 
passage of blood into the auricle, but a flow in the opposite direction is pre- 
vented by their closure. 

The auricle (atrium) in the Elasmobranchs in general is thin-walled and 
lies over the ventricle. The walls of the sac, however, may be folded and may 
even give the appearance of possessing two rooms. Internally the auricle in 
practically all Elasmobranchs is smooth, that is, it rarely possesses tendinous 
supporting cords which pass across the cavity from one wall to the other. The 
auriculoventricular opening may be shifted sharply to the left, so that the 
communication between the auricle and ventricle is visible in ventral view, 
^riie auriculoventricular valve consists of two pocket-like flaps, the concavities 
of which are directed toward the ventricle. 

The ventricle is relatively small in all the Elasmol)ranchs, although in the 
rays it may be relatively thick. It may be described as a pyramid with the base 
posterior, two faces directed ventrally and outward, and the other dorsalty in 
position. A section through the ventricle shows its greatly thickened walls 
(vn., fig. 159) . The lining, unlike that of the auricle, is often exceedingly rough 
and irregular. The tendinous cords (chordae tendineae) present in the ven- 
tricle of the Elasmobranchs are muscular at one end and drawn out into 
longer or shorter tendons at the other. The ends are attached to opposite walls 
and prevent the vessel from spreading beyond its capacity. 



172 THE ELASMOBRANCH FISHES 

The apex of the ventricle is continued anteriorly by the conus arteriosus 
(c.a.), a short and narrow tube, the lumen of which is triangular and the walls 
muscular. Internally the conus of the Elasmobranchs is universally provided 
with three longitudinal rows of semilunar valves, corresponding to the faces 
of the triangle, one row dorsal and the other two ventrolateral in position. 
Figure 159 shows several types of valves. As a rule, in the sharks the number 
of tiers in each longitudinal row decreases in the more highly specialized 
forms. A somewhat generalized condition was seen in Heptanchus (fig. 150b) 
in which four or five tiers occur, and a fairly general condition is that of 
Isurus (fig. 159a). Much greater specialization is present in C ephaloscyllium 
(fig. 159b), in which only two tiers of valves are present. A decrease in the 
number of tiers does not necessarily indicate specialization in the rays, as is 
shown by the fact that several tiers are present in Mohula (fig. 159c), which 
is a highly specialized form. In most sharks the valves of the anterior row 
are best developed and often cover practically the entire lining of this section 
of the conus. The valves in the succeeding tiers usually decrease in size the 
farther they are located posteriorly. Not infrequently the lips of the valves 
are held in position by chordae fendineae and cliordae may also extend from 
the fold of a valve posteriorly (fig. 159a) . Among the regular valves, further- 
more, are often found smaller or accessory valves (see fig. 159c). 



ARTERIES 

Ventral or Ascending Aorta 

The ventral aorta {v. a., fig. 167) in the adult passes forward as a continuation 
of the short conus arteriosus. In all the Elasmobranchs this, also, is a relatively 
short trunk and divides anteriorly into right and left halves. The ventral 
aorta is smaller in caliber than the conus and its walls are thinner. As in 
Heptanchus, it gives off afferent arteries which carry the blood to the gill 
region to be oxygenated. 

AFFEEENT AETERIES OF ADULT 

In all Elasmobranchs the hyoidean afferent {hy.af., fig. 166b) and the first 
branchial afferent on each side {hr.af}) arise from a common trunk, this 
being a bifurcation of the ventral aorta above mentioned. In many sharks 
{Squalus, fig. 161; Mustelus antarcticus, fig. 166a) the last two afferents also 
arise from a common trunk, but when this occurs the trunk is short. In some 
forms the last two arise separately, as in Heptanchus {ap., fig. 150b) and 
Chlamijdoselachus (fig. 160). The second branchial afferent arises as a single 
outgrowth from the ventral aorta between the common trunk of the hyoidean 
and first branchial afferents and that of the last two afferents in pentanchid 
Elasmobranchs (Mnstelus, fig. 166). This is also the condition in at least one 
of the rays, Basyatis dipterura (fig. 167). In rays in general, however, the 



-> '7 



y^ 




Fig. 161. Dorsal view of afferent and efferent arteries, S(iii(ili(s- .sucllii. (Elizalietli Chris- 
tiansen, orig.) 

hr.af.^, first branchial afferent artery; hrj f., liranchial eff'erent artery ; dr., cross trunk ; 
ce.. coeliac axis; da}, paired dorsal aorta; (/.«., unpaired dorsal aorta; v.c, external carotid 
artery; Inj.af., hyoidean aft'erent; pa., pseudoljrancliial artery. 



THE ELASMOBRANCH FISHES 173 

second branchial afferent joins the common trnnk of the last two so that, in 
them, only two stems leave the ventral aorta. 

Each afferent continues around the base of the cartilaginous gill arch in 
front of the branchial rays, giving off smaller afferent arterioles both to the 
anterior and to the posterior filaments of a gill {Squalus sucklii, fig. 161, 
hr.af.). In their course upward the afferents grow smaller and smaller and 
terminate in the dorsal part of the branchial region. An interesting arrange- 
ment is reported by Allis (1911) for Chlamydoselachus (fig. 160) in which the 
afferent arteries, except the hyoidean and last branchial, instead of ending 
dorsally, bifurcate, one branch («/.") passing over the cleft anteriorly to join 




Fig. 160. Branchial arteries of Chlaviydoselachus. (From Allis.) 

af.", anterior division of afferent; af.^, posterior division of afferent; hr.af, fifth bran- 
chial afferent ; efc, efferent-collector ; hy.af., hyoidean afferent artery ; or., orbital artery, 

the afferent in front, the other (af.^) passing dorsally and back over the suc- 
ceeding cleft to join the following afferent. In this arrangement, which is not 
greatly unlike that of Squalus, the afferents are connected into a series of 
closed loops around all the clefts. 

CAPILLAEIES 

The capillaries in the gill filaments or folds connect the arterioles of the 
afferents (see fig. 145, a.h.) with a similar series of efferent arterioles (e.h.) 
leaving the gills. They form a net so complex that it is impossible to trace an 
individual capillary. The wall of each capillary is made up of a single layer of 
cells, forming the effective membrane through which the exchange of gases is 
made. If a longitudinal section could be made through the entire length of a 
single capillary it would begin where the thicker walled afferent arteriole ends 
and end with another thicker walled arteriole, the beginning of the efferent- 
collector type of vessel. 

EFFEEENT-COLLECTOES 

Blood brought to the gills by the afferents passes into the capillaries of the 
gill filaments. Here it is oxygenated and is then sent down the gill filaments 
(e.b.-, fig. 145) into efferent-collectors (efc.) lying at their bases. The efferent- 



174 



THE ELASMOBRANCH FISHES 



collector which forms the anterior part of the loop, however, is a posterior 
efferent-collector, for it drains the posterior demibranch of a whole gill ; and 
the efferent-collector posterior to the cleft is the anterior efferent-collector of 
the following gill. To make this clear, if the area between two pockets, for 
example between the first and second pockets, be considered (fig. 161), its 
anterior efferent-collector (the second) drains the demibranch just behind 




Fig. 162. Dorsal view of afferent and efferent arteries, Basyatis dipterura. (Blanche Lilli- 
bridge, orig.) 

ac, accessory branchial arteries; hr.ef., branchial efferent arteries; c.tr., cross-trunk; ce., 
coeliac axis; efc, efferent-collector; e.c, external carotid; hy.ef., hyoidean efferent; p.c, 
posterior cerebral artery; ps., pseudobranchial artery; s.cl., subclavian artery; s.m., su- 
perior mesenteric; I, first gill cleft. 



THE ELASMOBRANCH FISHES 



175 



the first pocket, and the posterior efiferent-collector (tlie tliird) drains the 
area in front of the second pocket. The anterior etferent-collector, by its con- 
nection through cross-trnnks (c.tr., fig. 161) with the posterior collector back 
of it and by its ventral continuation with the collector in front of it, receives a 
considerable amount of blood and is a much larger vessel than is the posterior 
eflferent-collector of the same gill. It is the anterior efferent-collector which is 
continued directly outward to the dorsal aorta as the branchial efferent proper 
(see fig. 161, hr.ef.). All the efferent-collector loops posterior to the one just 
described are similarly made up of posterior and anterior efferent-collectors, 
and all are emptied into the unpaired dorsal aorta similarly through branchial 
efferents (hr.ef.) which are the direct continuation of the anterior efferent- 
collectors. 



d.a'- 




Fig. 163. The developing branchial arteries, Squalus acanthias. (From Scammon.) 

a.a.^''^, first and sixth embryonic aortic arches; af., first afferent artery; cl., gill cleft; 
d.a.^, paired dorsal aorta; d.a., dorsal aorta; ef., efferent artery; e/c", anterior efferent- 
collector; efo.b, posterior efferent-collector; ps., pseudobranchial artery; sp. spiracle; v. a., 
ventral aorta; x., where break in primary arch will take place. 

We may briefly consider the formation of the afferent and efferent systems 
in the embryo of Squalus as described by Scammon (1911), The arterial 
system here consists of (1) a ventral aorta (v. a., fig. 163) running forward 
from the heart under the gill region, (2) a dorsal aorta (d.a.) extending 
backward above this and dorsal in position, and (3) six aortic arches (a.a.'^~^) 
connecting dorsal and ventral aortae in the pharyngeal or branchial region. 
Upon the formation of the gill filaments a new branch (e/c") is budded off 
which collects the blood from the newly formed gill tissues. The origin of this 
collector from the embryonic aortic arch marks the place where the embryonic 
arch later separates (see last arch at x) into two parts, the upper becoming 
the efferent (ef.) which joins the dorsal aorta, and the lower the afferent 
(a.f.), which joins the ventral aorta. The collector (e/c") (efferent-collector) 
next sends cross-trunks backward to the posterior demibranch and a posterior 
efferent-collector (efc.^) is formed. In the last arch it will be observed that the 
posterior efferent-collector has not as yet formed. In a general way it may be 
said that for every embryonic aortic arch, except the first and second, two ef- 
ferent-collectors thus arise. One of them is formed for the anterior demibranch 
(efc") , the other for the posterior (efc.^). The two collectors then continue to 
grow downward and the tip of the posterior efferent-collector now joins the 



176 



THE ELASMOBRANCH FISHES 



collector behind it. Soon the gill cleft is snrroiinded ventrally by these vessels, 
but dorsally the loop is not yet completed around the cleft. A second important 
change then takes place. A branch from the posterior efferent-collector passes 
backward above the cleft and connects with the anterior efferent-collector 
following. There is thus completed an efferent-collector loop around the entire 
cleft characteristic of the adult. 

In certain forms, even in the adult, the posterior efferent-collector may re- 



ps.... 




Fig. 165 



Fig. 164 



Fig. 164. Arteries derivative of the first efferent-collector, Squahis suoldii. (L. H. Bennett, 
orig.) 

hr.ef}, first branchial efferent ; e.c, external carotid ; ef.c}--, first and second efferent- 
collector arteries; hy.ef., hyoidean efferent; ps., pseudobranchial artery. 

Fig. 165. Cross-trunks from eighth to ninth efferent-collectors, Squalus sucklii. (L. H. Ben- 
nett, orig.) 

ct., cross-trunk ; ef.c}'^, eighth and ninth efferent-collectors. 

tain a dorsal commissural connection with the anterior efferent-collector of the 
same gill. This interesting condition obtains in the adult Chlamydoselachus 
(fig. 160). In this form the posterior efferent-collector sends a branch to join 
the efferent-collector back of it, but it also retains an embryonic attachment 
with the anterior efferent of its own holobranch. Usually, however, the branch 
connecting it to the succeeding efferent-collector becomes so important that 
the original connection of the posterior efferent-collector to its own efferent 
anteriorly is entirely lost in the adult. In any event there results in the adult 
a complete efferent circuit around the gill cleft, each circle being composed of 
the posterior efferent-collector of one gill and the anterior efferent-collector 
of the gill following. Around all the clefts (except the last) and the spiracle, 
complete loops are formed as thus described. 

Since there are no demibranchs posterior to the last cleft no efferent-col- 
lector lies back of this cleft and hence a circuit is incomplete around it. The 
last posterior efferent-collector, that one in front of the last cleft, is usually 



THE ELASMOBRANCH FISHES 



177 



separated from the anterior efferent-collector dorsally, and its blood readies 
the anterior efferent-collector in front of it only by means of cross-trunks 
{dr., figs. 162 and 165). It is by means of such cross-trunks that the last 
efferent-collector is relieved of its blood. In fact, the posterior collector of 





Fig. 166. Hypobranchial arteries. A. Mustelus antarcticus. (From T. J. Parker.) B. Mus- 
telus canis. (From Ferguson.) 

O.Z., anterior lateral (lateral abdominal) artery; hr.a., brachial artery; hr.af}'*, first and 
fourth branchial afferent arteries; co.a., coracoid artery; cm., commissural artery; cr.l., left 
coronary artery; cr.p.. posterior coronary; e.c, external carotid; hy.af., hyoidean aifereut; 
l.lil)., lateral hypobranchial; m.a., mandibular artery; m.hb., median hypobranchial ; n.a., 
nutrient artery; pc, pericardial; s.cl., subclavian; v.a., ventral aorta; III, position of 
third gill cleft. 



each gill empties a considerable amount of its blood into the anterior effer- 
ent-collector of its own gill by similar cross-trunks. These trunks may be 
numerous as in Heptanchus maculatus, few as in Squalus, or they may be 
single as in Dasyatis {dr., fig. 162). 

The circuits made by the efferent-collectors, as we have said, are drained 
into the dorsal aorta by means of the efferent arteries. These arteries we shall 
consider after discussing certain branches given off by the efferent-collectors. 



178 



THE ELASMOBRANCH FISHES 



BRANCHES OF EFFERENT-COLLECTOES 



HYPOBRANCHIAL ARTERIES 



The liypobraiichial arteries in Elasmobranchs form a most complex system of 
vessels in the ventral walls and floor of the pharyngeal area. In general the 
ventral ends of the different efferent-collector loops may be more or less com- 
pletely connected by a longitudinal vessel which, f ollo^^^ng Parker and Davis, 
I have termed in Heptanch us the lateral hypobranchial artery {l.lib., fig. 153) . 

This vessel sometimes forms a con- 
tinuous ventral chain on each side 
from the first to the fourth effer- 
ent-collector loop (Mustelus, fig. 
166) . In Rata erinacea, and some- 
times in Carcharias littoralis, 
according to Parker and Davis 
(1899) , the lateral hypobranchial 
may even include the fifth loop, 
but there is considerable irregu- 
larity about this. Whatever con- 
nections the loops may make with 
the lateral hypobranchials, how- 
ever, the tendency is to make 
them in the region of the second 
and third liranchial arches rather 
than from the first or last loops. 
In other forms the lateral hypo- 
branchial line is incomplete {Raja 
clavata), and in still others a 
lateral hypobranchial is absent 
{Dasijatis clipterura, fig. 167). 
Commissural arteries {cm.) may arise from the hypobranchial, at or pos- 
terior to the angles of the efferent-collector loops, and pass toward the mid- 
ventral line anterior to the third and fourth afferent arteries {Mustelns 
antarcticus, fig. 166a) ; or only a single one may be present as is usual for 
Squalus sucMii. The commissurals passing from the lateral hypobranchials 
medially may meet paired median hypobranchials as in Heptanchus macula- 
tus (fig. 153 and Hexanchus corinus, fig. 169) . Some evidence of paired median 
vessels is also seen in Mustelns {m.lih., fig. 166) . In certain forms the commis- 
sural may join an unpaired median hypobranchial as in Carcharias littoralis. 
The commissures may be of a dorsal or a ventral type. In the former type 
the artery passes to the median line above the ventral aorta, while in the latter 
it passes below the ventral aorta. In Mustelns canis (fig. 166b) both com- 
missures are of the ventral type; in S.cyllinm catnlns both are of the dorsal 




Fig. 167. Hypobranchial arteries, Dasyatis dip- 
tenira. (Blanche Lillibridge, orig.) (For expla- 
nation see fig. 166.) 




Fig. 168. Hypol)rancliial arteries, Tlexanchus coriinis, ventral view. (From Keys.) 
a.co., median coracoid artery ; iif.''\ third afferent artery ; co.a., coracoid artery ; La. 
lateral (abdominal) artery; l.d.r., lateral abdominal vein; ii.cr., posterior coronary artery: 
v.a., ventral aorta. 



THE ELASMOBRANCH FISHES 



179 



tyi)e. Tn Cdirluirids I it f oralis one is dorsal and the other ventral. In Zygaena 
two are ventral and a third is dorsal. 

In HexancJius corinus the median stem of the coracoid artery (co., fig. 
169) arises from the left median hyi)obranchial and is continued posteriorly 
by the coracoid artery (co/). The coracoid is continued as the lateral (ab- 
dominal) artery which follows the course of the lateral abdominal vein. From 
the lateral (abdominal) artery 
the brachioscapular artery {bsc, 
fig-. 169) carries a large supply- of 
blood to the pectoral area and 
joins the subclavian at a point 
where the brachial artery (br.) is 
given off to the fin. 

From the median hyprobran- 
eliials, or from the last commis- 
sural, the pericardial (pc, fig. 
166b) and the coronary arteries 
(cr.l.) may arise, and near the 
origin of pericardials and coro- 
naries in the rays, the coracoid 
artery (co.a.) joins the last com- 
missural {Dasyatis, fig. 167). In 
Carcharias littoralis a branch, 
designated by Parker and Davis 
as the epigastric, arises from the 
median unpaired hypobrancliial. 

The pericardial, as in Heptan- 
chus, goes to supply the dorsal 
pericardial wall, and to furnish 
branches to the oesophagus and 
one or more epigastric arteries to 
the dorsal side of the stomach. An interesting condition obtains in Lamna cor- 
nuhica (Burne. 1923) in which the pericardials, after having traversed the 
"supra-hepatic retia," supply practically the whole of the blood to the 
digestive tract. 

The coronary arteries {cr.l., figs. 166b and 167) in the Elasmoliranchs are 
unusually well developed. They may consist of a median ventral artery aris- 
ing from the ventral type of commissure and a single dorsal arising from the 
dorsal type as in Carcharias littoralis. Or they may consist of a pair of vessels, 
the left one of which may go to the ventral side of the conus and ventricle, and 
the right to the dorsal side. In Heterodontus two pairs of coronaries are pres- 
ent, one of which is dorsal, the other ventral. In the rays, as in Heptanclius, a 
posterior pair of coronaries {cr.p., fig. 169) arises from the coracoid arteries 
and runs forward to the sinus venosus and ventricle. These coronaries are 
especially interesting in Dasyatis. While the right posterior artery extends 




Fig. 169. Eelations of coracoid to lateral (ab- 
dominal) artery, Hexanchus corinus, ventral 
view. (From Keys.) 

CO., median stem of coracoid artery; co.', cora- 
coid artery; br., brachial artery; hsc. brachio- 
scapular artery; cr.p., posterior coronary ar- 
tery; l.a., lateral (abdominal) artery; l.a.v., 
lateral abdominal vein; .s.cl.v., subclavian vein. 



180 



THE ELASMOBRANCH FISHES 



only to the sinus venosiis, the larger left one passes across the ventricle to unite 
with the right (dorsal) coronary forming a strong continuous vessel. A branch 
arising from this trunk on the ventricle passes across the conus to the left 
ventral coronary'. 

It will be observed that at the ventral angles of the efferent-collector loops 
are certain smaller nutrient vessels (n.a., fig. 166a) which supply arterial 




Fig. 170. The carotids and associated arteries. (From Hyrtl.) 

A. Acanthias. B. Zygaena. C. Haja. 

a.clJ., anterior dorsal artery; ce., coeliac axis; d.a., unpaired dorsal aorta; d.a}, paired 

dorsal aorta "vertebral artery"; hy.ef., hyoidean efferent; i.e., internal carotid; ml., mye- 

lonal artery to cord; or., orbital (stapedial) artery; ps., pseudobranchial artery; r.a., ramus 

anastomoticus; s.cl., subclavian; s.m., superior mesenteric. 

blood to the gills and surrounding tissue. In Dasyatis arteries are well devel- 
oped both at the ventral and the dorsal angles of the loops. Here they are ac- 
cessory efferent-collectors {ac, fig. 162) from accessory gills. 

AETEEIAL SUPPLY TO HEAD 



In the Elasmobranchs in general the two branches from the first efferent- 
collector to the head are essentially like those noted in Heptanchiis. They often 
differ, however, in extent of distribution. The first, the external carotid artery 
(e.c, figs. 164 and 166a) , arises from the ventral angle of the hyoidean efferent- 
collector and its branches are similarly distributed, as in Heptanchus. The 
mandibular artery {m.a., fig. 167) extends toward the symphysis of the lower 
jaw and supplies structures in this area. The hyoid artery runs upward be- 
tween the hyoid and the mandibular arch, giving oft' twigs along its course. 

The second branch, the pseudobranchial, arising from the middle of the first 
efferent-collector, differs greatly in the sharks and rays {ps., fig. 164) . Only in 



THE ELASMOBRANCn FISHES 181 

the sharks may it be spoken of as a true anastomoticns. In a type like Scyllium 
it courses by the spiracular region almost uninterruptedly and passes through 
a foramen in the orbit to join the internal carotid. In some of the other forms 
(Acanthias, Zxjgaena, fig. 170) instead of being straight it may pursue a most 
tortuous course. In these, and in many other forms it is interrupted at the 
spiracle. In CetorJiinus (fig. 171b), for example, it forms the so-called "won- 





B 

Fig. 171. Arteries associated with the spiracle. 
A. Raja. (From Hyrtl.) B. Cetorhinus. (From Carazzi.) 
i.e., internal carotid artery; op., optic artery; p.s., pseudobranchial artery; r.a., ramus 
anastomoticns. 

der net," the wonder net being composed of a coil of arteries connecting the 
part going to the spiracle with the part leaving it. 

A different condition is found in this artery in the rays. In these the spiracle 
is usually large and the blood supply to the filaments is better developed than 
in the sharks. The artery here takes its origin similarly from the first efferent- 
collector and as a large vessel (ps., fig. 171a) passes toward the pseudobranch. 
Before reaching the latter, however, it gives off a large branch to the adductor 
mandibularis muscle. At the pseudobranch it separates into numerous fila- 
mentous arteries, and then continues as a smaller artery (r.a.) to join the 
internal carotid artery (i.e.), as in the sharks. On its way it gives off the 
ophthalmica magna to the eye. 

]Much attention has been given to the function of the ramus anastomoticns. 
In some of the sharks in which the spiracle is closed and in w^hich the artery 
passes almost directly from the collector to the internal carotid, it is a true 
ramus anastomoticns. But in the rays it is composed of an afferent and an 
efferent part. It has been urged by Hyrtl (1858) that in the rays the branch 
connecting the internal carotid and the spiracular gill is the afferent branch 



182 THE ELASMOBRANCH FISHES 

to the spiraciilar pseudobranch and that it carries, at least in part, non-oxy- 
genated blood from the eye; and further that the branch extending from the 
pseudobranch to the first elf erent-collector is the true efferent pseudobranchial. 
In the embryo of Squalus acanthias the pseudobranchial (ps., fig. 163) is 
seen in relation to the remnant of the first embryonic arch, which at this stage 
has broken, and the segment from the ventral aorta is only a nodule (a.a}). 
The pseudobranchial itself is somewhat like a cross-trunk in that it is attached 
to the posterior efferent-collector, but it crosses a relatively long span through 
the posterior demibraneh of the hyoidean gill and through what would be the 
anterior demibraneh of the hyoidean gill and the posterior demibraneh of the 
spiracular gill were such demibranchs present. 

EFFEEENT AETEEIES 

We may now continue the description of the efferent arteries (see figs. 161- 
162 ) . In the large majority of forms five efferent arteries are present (pentan- 
ehid sharks). These represent the dorsal parts of the second to the sixth 
embryonic aortic arches and consist of the hyoidean and four branchial effer- 
ents. The hyoidean efferent artery passes forw^ard and inward to join the 
paired dorsal aorta when such persists, or is continued into the head 1)y the 
orbital (stapedial) artery {Heptanehus, fig. 152). In the embryo of Squalus 
acamtliias (fig. 163) this vessel is relatively large where it joins the paired 
dorsal aorta. The four branchial efferents may reach the aorta as four arteries 
on each side (Selachians) ; or the first maj^ fuse with the second so as to give 
only three pairs of branches (most rays, fig. 170c). In Hexauchus and in 
Chlamydoselachus (fig. 160) the fifth branchial efferent joins the fourth 
before entering the aorta just as, in Heptanehus, the sixth branchial joins 
the fifth. 

The orbital (stapedial) artery maj- arise at the union of the hyoidean effer- 
ent and paired dorsal aorta (or., fig. 152) or it may arise near or anterior to 
this union (fig. 170a). When the latter condition obtains there is formed a 
common stem from which the orbital and the internal carotid spring. In the 
adult rays where the paired dorsal aortae may be absent the hyoidean efferent 
may continue directly into the common stem. 

The orbital (stapedial) artery may reach the orbit without perforating the 
basis cranii as in Heptanehus, or it may enter bj^ a foramen in the margin of 
the foramen through which the hyomandibular nerve enters {Heterodontus, 
}). 59, fig. 66). The orbital gives off one or two branches {Chlamydoselaehus, 
fig. 160) which supply the muscles of the eye, and a second important branch 
which passes backward to the hyoid area where it may anastomose with the ex- 
ternal carotid system. The main stem then passes forward under the eyeball 
and leaves the orbit through the orbitonasal canal. This stem gives off' a buccal 
artery which turns downward and backward to supply the adductor mandi- 
l)ulae muscle and finally the stem divides into the nasal and rostral arteries. 

The internal carotid {i.e., left side, p. 163, fig. 152), which may be consid- 
ered as the direct continuation of the paired dorsal aorta, enters the cranium 



THE ELASMOBRANCH FISHES 



183 



tliroui>li a foramen or pair of foramina in or near the middle line. Witliiii the 
craninm it may be joined to the eorres|ionding internal carotid of the opposite 
side by a cross-connective, as in S(/u<ifina; or the two may run for a short dis- 
tance as a fused connnon trunk. The internal carotid then passes forward, and 
after receiving the ranuis anastomoticus, gives off the optic artery (op., fig. 






Fig. 172. Cerebral arteries. A. Sqnalns sticMn. (E. H. Barbera, orig.) B. Cetorhinun. 
(From Carazzi.) C. Haja cJavata. (From Hyrtl.) 

a.c, anterior cerebral artery; a.sp., spinalis artery; hs., basal artery; i.e., internal carotid 
artery; m.c, median cerebral; ml., myelonal artery; p.c, posterior cerebral; tn., terminal 
nerve. 



171a), which runs with the optic nerve to the eye. Each internal carotid then 
turns upward, as in Heptanchus, and divides into the three cerebral arteries 
which vary considerably in the different Elasmobranchs. 

The cerebral arteries, consisting of an anterior, a median, and a posterior 
pair of arteries are, as we have seen in Heptanclms, derivatives of the right 
and left internal carotids. The anterior cerebral arteries {a.c, fig. 172a) pass 
forward ventrally around the lobi inferiores of the brain and over the optic 
chiasma; in front of this, right and left arteries may be put into communica- 
tion by a cross-trunk. From here forward great variation ensues. In some 
types these arteries pass as single strands between the right and left divisions 
of the telencephalon. In others they extend forward as numerous branches 
{Heterodontus, rays). The median cerebrals (m.c.) extend under the telen- 
cephalon and along the olfactory tracts as fairly simple vessels (Squalus 



184 



THE ELASMOBRANCPI FISHES 



sucliii, fig. 172a). In certain forms they may extend forward in a number of 
strands {Cetorhinus, fig. 172b). The posterior cerebrals in Squalus (p.c, fig. 
172a) loop around the inferior lobes of the brain and fuse into a single strand. 
By the union of right and left posterior cerebrals behind and the anterior cere- 
bral in front a circle of Willis is formed somewhat like that in man. Further- 





Fig. 173. Vascular supply to the digestive tract. A. TriaTcis semifasciatus, ventral view. 
(Elizabeth Christiansen, orig.) B. Mustelus antarctieus, dorsal view. (From T. J. Parker.) 
a.g., anterior gastric artery; a.g.v., anterior gastric vein; a.gps., anterior gastro-pancre- 
aticosplenic artery; a.gs., anterior gastrosplenic artery; a.i.a., anterior intestinal artery; 
a.i.v., anterior intestinal vein; ce., coeliac axis; co., colon; c.s., cardiac stomach; d.g., dorsal 
gastric artery; d.i.a., dorsal intestinal artery; d.i.v., dorsal intestinal vein; gh., gastro- 
hepatic artery; Ji., hepatic artery; h.'p., hepatic portal vein; i.a., intraintestinal artery; i.v., 
intraintestinal vein; i.m., inferior mesenteric artery; Iv., cut end of liver; oe., oesophagus; 
V-gs., posterior gastrosplenic artery; p.gs.v., posterior gastrosplenic vein; p.i.a., posterior 
intestinal artery; pn., pancreatic artery; pn.^~^, dorsal and ventral lobes of pancreas; re, 
rectum; spl., spleen; v.g., ventral gastric artery; v.g.v., ventral gastric vein. 



THE ELASMOBRANCH FISHES 



185 




Fig. 174. Vascular supply to the digestive tract, Heterodontus francisci. (Duncan Dunning, 
del.) 

a.g., anterior gastric artery; a.g.v., anterior gastric vein; a.gs., anterior gastrosplenic 
artery; a.gs.v., anterior gastrosplenic vein; a.i.a., anterior intestinal artery; aw., annular 
artery; ce., coeliac axis; co., colon; c.s., cardiac stomach; dch., ductus choledochus; d.i.a., 
dorsal intestinal artery; d.i.v., dorsal intestinal vein; dw., duodenum; ep., epigonal artery; 
gli., gastrohepatic ; h., hepatic artery; Ji.p., hepatic portal vein; i.m., inferior mesenteric; 
oe., oesophagus; p.^s., posterior gastrosplenic artery; p.gs.v., posterior gastrosplenic vein; 
p.i.a., posterior intestinal artery ; p.i.v., posterior intestinal vein ; pn., pancreatic artery ; 
p.s., pyloric stomach; ?"C., rectum; s.m., superior mesenteric artery; v.g., ventral gastric 
artery ; v.g. v., ventral gastric vein ; v.i.a., ventral intestinal artery. 



186 THE ELASMOBRANCH FISHES 

more, upon the posterior fusion of right and left cerebrals a midventral artery, 
the basilaris (hs.), is produced, which supplies the medulla of the brain and as 
the spinalis (a.sp., fig. 172a) continues ventrally down the spinal cord. The 
spinalis will again be considered in our description of the arteries of the spinal 
cord. In Raja (fig. 172c) the posterior cerebrals form broad arches on the 
brain stem before uniting. Here the spinalis is made up of several strands. 
According to Hyrtl it receives the myelonal vessel (ml.) from the united first 
and second efferents, as was seen in figure 170c. 

Arterial Supply to Trunk 

DORSAL AORTA 

The dorsal aorta in all the Elasmobranchs is supplied with blood from the 
branchial efferent arteries; it extends through the body and is continued into 
the tail as the caudal aorta. It arises in the embryo as a pair of arteries. Evi- 
dence of this condition may be absent in the adult, but in forms like Acanthias 
and Zygaena the anterior part of the aorta, "the vertebral," to which the 
second embryonic aortic arch is attached, indicates the paired condition. In 
Acanthias the paired aortae {da}, fig. 170) , as widely separated arteries, pass 
forward and are joined by the hyoidean efferents; while in Zygaena (fig. 
170b) the two are fused far forward. In the adult ray, on the contrary (fig. 
170c), all indication of the paired aortae is lacking. In the region of the trunk 
the paired embryonic aortae early fuse into a single median dorsal aorta, 
which is the source from which many vessels arise. These arteries, as in Hep- 
tanchus, spring from the aorta either as paired or as unpaired vessels. 

UNPAIRED ARTERIES 

The unpaired arteries in anteroposterior direction are the coeliac axis {ce., 
figs. 173-175), the superior mesenteric {s.m.), and the so-called inferior mes- 
enteric {i.m.) arteries. When a fourth is present, as is usual in the sharks, it 
is due to a failure to form a superior mesenteric, the two branches of which 
arise separately from the dorsal aorta. 

COELIAC AXIS AND ITS BRANCHES 

The coeliac axis {ce., figs. 173-175) arises posterior to the union of the fourth 
branchial efferents and behind the paired subclavians. In some forms this is 
a relatively short trunk {Mustelus), while in others it is long {Acanthias) . 
Normally as in Heterodontvs {ce., fig. 174) it supplies the gonad and then 
divides into two parts, one of which, the gastrohepatic {gh.), supplies the 
stomach and the liver; the other, the anterior intestinal, {a.i.a.), passes back- 
ward to the region of the intestine. 

The gastrohepatic may be a fairly well developed segment as in Triahis (fig. 
173a). It is, however, usually a short trunk as in Acanthias; or it may be en- 



THE ELASMOBRANCH FISHES 



187 



tirely absent {Dasyatis, fig. 175). The gastric arteries follow much the same 
plan in the Elasniobranchs generally as lias been descril)ed for Heptanchus. 
In certain forms, however, a dorsal gastric may be well developed. 

The hepatic artery may arise as a single trunk, Heptanchus, or a hepatic 
l)ranch may be given off from the anterior intestinal artery, as in Mustelus ant- 
arcticus (fig. 173b). Occasionally a second hepatic branch may be given off 
from the ventral gastric in Triakis. The hepatic artery (or arteries) supply 
twigs to the ductus choledochus, to the sail bladder, and to the liver. 




Fig. 175. Arteries to the digestive tract, Dasyatis dipterura, ventral view. (Blanche Lilli- 
bridge, orig.) 

Fig. 176. Duodenal supply, Dasyatis dipterura, dorsal view. (Blanche Lillibridge, orig.) 
a.g., anterior gastric artery; a.g.v., anterior gastric vein; a.gps., anterior gastro-panerea- 
tieosplenic artery; a.gs., anterior gastrosplenic artery; a.i.a., anterior intestinal artery; 
ce., coeliac axis; c.s., cardiac stomach; d.i.a., dorsal intestinal artery; d.ch., ductus choled- 
ochus; d.i.v., dorsal intestinal vein; du., duodenum; gd., gastroduodenal artery; h., hepatic 
artery; h.p., hepatic portal vein; i.a., intraintestinal artery; oe., oesophagus; p.gps., pos- 
terior gastro-pancreaticosplenic artery ; p.gs., posterior gastrosplenic artery ; p.gs.v., pos- 
terior gastrosplenic vein; p.i.v., posterior intestinal vein; pn., pancreatic branch; pn}~'^, 
dorsal and ventral lobes of pancreas; py., pylorus; s.m., superior mesenteric artery; spA., 
valvular intestine; v.g., ventral gastric artery; v.g.v., ventral gastric vein. 



188 THE ELASMOBRANCH FISHES 

The anterior intestinal arterj^ (a.i.a., figs. 173 and 174) is a relatively large 
vessel in the sharks, while in the rays it is usually less important. In both it is 
continued as the intraintestinal (i.a.) along the free edge of the spiral valve. 
In sharks before it enters the intestine as the intraintestinal it gives off the 
ventral intestinal artery which runs along the ventral side of the vahoilar in- 
testine to which it gives numerous annular branches. In the rays a ventral 
intestinal artery is characteristically absent. 

In Dasyatis the anterior intestinal gives off an anterior gastro-pancreatico- 
splenic artery (a.gps., fig. 175), which in sharks is given off from the dorsal 
aorta. In Rhinohatis and Raja this artery is a branch from the superior mesen- 
teric arter3^ This artery sends a strong branch to the dorsal lobe of the pan- 
creas (fig. 175), and then, as the anterior gastrosplenic (a.gs., fig. 173b) 
divides, supplying the spleen and the dorsal side of the cardiac stomach. Just 
before the anterior intestinal disappears as the intraintestinal it gives off a 
posterior gastro-pancreaticosplenic (p.gps., fig. 176), which artery after sup- 
plying pancreatic branches to the ventral lobe of the pancreas passes in the 
gastrosplenic omentum around the outer margin of the pyloric stomach as the 
posterior gastrosplenic artery (p.gs., fig. 175). In the rays, where there is no 
spleen on the angle of the stomach, all the branches of this artery go to the 
stomach. A second branch, the gastroduodenal artery (gd.), is given off at 
about the same place or farther posteriorly (Raja) from the anterior intes- 
tinal artery. This supplies the duodenum and may also supply the dorsal side 
of the pyloric arm of the stomach as did a similar artery in Heptanchus. 

There is usually considerable anastomosing of the branches on the pyloric 
angle of the stomach. Branches of the anterior gastrosplenic join with those 
from the posterior gastrosplenic as also do branches from the ventral gastric 
artery. 

SUPERIOE MESENTERIC ARTERY 

The superior or anterior mesenteric artery is variable as to its place of origin. 
It may arise in the midbody far removed from the coeliac axis, as in Hetero- 
dontus (s.m., fig. 174), or it may more nearly approach the coeliac as in Scyl- 
lium, Galeus, Triakis (fig. 173a). In the rays the superior mesenteric and the 
coeliac are often in close proximity (Raja clavata; Dasyatis, fig. 177b). The 
office of the superior mesenteric in the sharks is to supply blood to two general 
areas. The main arterj' to one of these areas is the anterior gastro-pancreatico- 
splenic (fig. 173b), which supplies the distal part of the cardiac stomach 
(a.gs.), the pancreas, and the spleen. The other is the posterior intestinal 
(p.i.a.), which is continued along the dorsal wall of the vah^lar intestine as 
the dorsal intestinal artery (d.i.a.) . 

A superior mesenteric, as just described, is present in Heterodontus {sm., 
fig. 174) and may occasionally be found also as a short trunk in Acanthias (out 
of 500 specimens examined in Squalus sucMii seven had short superior niesen- 
teric trunks) . In this form, then, while a short trunk is occasionally present, in 
the majority of occurrences the branches of the superior mesenteric arise sepa- 



THE ELASMOBRANCH FISHES 



189 



rately i'l-om the dorsal aorta. In such, the posterior intestinal arises anterior 
to and crosses over the anterior gastro-i)anereaticos])lenic. This condition is 
characteristic of many ty])es of which it should be said that no true superior 
mesenteric artery exists. 





Fig. 177. Dorsal aorta and its segmentals. A. ScyUnim. (Prom Carazzi.) 
B. Dasyatis. (Blanche Lillibridge, orig.) 
hr.a., braeliial artery; ce., eoeliae axis; cL, cloacal branch; da., branch to clasper; d.a., 
dorsal aorta; d.l., dorsolateral artery; f.in., iliac artery; mt.a., metapterygial artery; o.d., 
oviduct; od.a., oviducal artery; p.i.a., posterior intestinal artery; p.l., posterior lateral 
artery; pr.a., propterygial artery; r., rectum; s.cL, subclavian artery; s.m., superior mesen- 
teric; t*. v., urinary vesicle. 

The anterior gastro-pancreaticosplenic (a.gps.) is contrasted with the pos- 
terior gastro-pancreaticosplenic {p.gps.), previously described, by its supply- 
ing a more anterior (proximal) segment of the digestive tract. While, as we 
have seen, the posterior artery of this name is in relation to the pyloric part 



190 



THE ELASMOBRAXCH FISHES 



of the stomach, the anterior gastro-pancreaticosplenic supplies the cardiac 
stomach. Its gastric part divides into two branches which supply the dorsal 
and distal halves of the cardiac stomach. The branch supplying the dorsal 
half may anastomose with twigs from the dorsal gastric artery which in Mns- 

telus may be a derivative of the 
anterior gastric (a.g., fig. 173b). 
The splenic branch of the anterior 
gastro - pancreaticosplenic leaves 
the gastrosplenic stem near the 
union of cardiac and pyloric limbs 
and passes to the spleen on the 
angle of the stomach. 

The posterior intestinal divi- 
sion of the superior mesenteric 
uniformly in the sharks is con- 
tinued as the dorsal intestinal ar- 
tery along: the dorsal side of the 
intestine. In the rays a posterior 
intestinal artery as such is absent, 
for in them the superior mesen- 
teric is a long stem which reaches 
entirely to the valvular intestine 
(see s.m., figs. 175 and 176). As 
in the sharks, its continuance 
along the dorsal side of the intes- 
tine may be designated as the 
dorsal intestinal artery. 

In higher vertebrates the coe- 
liac and superior mesenteric ar- 
teries combine into a coeliacomes- 
enteric. This, how^ever, does not 
occur in Elasmoln-anchs. In a ray 
like Dasyatis (fig. 175) part of 
the function of the superior mes- 
enteric may be performed by the 
coeliac, that is, the anterior gastro-pancreaticosplenic which in sharks, as a 
branch of the superior mesenteric, arises from the anterior intestinal artery, 
which is a derivative of the coeliac axis. 




Fig. 178. Arteries of the pectoral fin, Acanthias, 
dorsal view. (From Erik Miiller.) 

br., bracliial artery; l.p., lateral pterygial ar- 
tery ; m.p., median pterygial. 



INFEEIOE MESENTERIC ARTERY 



The inferior or posterior mesenteric artery {i.m., figs. 173 and 174) usually 
arises as a single vessel, except occasionally as in Acanthias (see fig. 179) , and 
is more or less removed from the anterior mesenteric. It supplies branches to 
the epigonal organ or non-functional part of the sex gland in forms in which 
such exists {Heierodontus, fig. 174, ep.; Triakis, and Mustelus antarcticus) , 



THE ELASMOBRANCH FISHES 



191 




and then runs to the rectal gland and the surrounding parts of the digestive 
tract, where it l)reaks up into a network of vessels. Unlike Heptanchus, how- 
ever, it does not usuallj' anastomose with the dorsal intestinal artery. 

Occasionally median vessels arise from the dorsal aorta posterior to the 
origin of the inferior mesenteric. They are characterized by passing directly 
downward to the region of the rectum and, occasionally, to the oviducts. Such 
vessels may be present in Acanthias and one or several of them may be present 
in Raja. These vessels have been held by 
Howes (1891) to represent the true inferior 
mesenteric comparable to that in mammals. 

Paired Branches op Aorta 

SUBCLAVIANS AN^D ILIACS 

The subclavians of the more specialized Se- 
lachii are similar in position to the same ar- 
teries in Heptanchus (s.cl., fig. 157) and 
Hexanchus (fig. 169). Considerable varia- 
tion in the origin of the subclavians may oc- 
cur. As a rule they are given otf from the 
dorsal aorta at the region between the third 
and fourth pairs of efferent arteries, but 
they may arise farther back at the base of 
the fourth pair of efferents {Dasyatis, fig. 
162, s.cl.). While their origin is usually sym- 
metrical, sometimes right and left arteries, 

as Monroe (1785) long ago claimed for the ray, arise asymmetrically. Such a 
condition is found in Dasyatis (fig. 177b). 

The subclavians (fig. 166a) are usually much stronger vessels than are those 
of Heptanchus where they take little blood from the dorsal aorta. In some 
specimens of Heptanchus twigs given oflf from the subclavians indicate that 
the blood is flowing toward the dorsal aorta. In other types, as the subclavians 
pass outward along the pectoral girdle they give off important branches which 
differ considerably in the sharks and the rays. Usually in both, after the dorso- 
lateral {a.cll., fig. 170c) is given off it divides into a relatively large branch 
which passes forward across the scapula, and another which passes backward 
between the bundles of muscles. The brachial leaves the subclavian and enters 
the foramen of the pectoral girdle. As it passes through the girdle it fol- 
lows the ventral canal and consequently enters the fin on the ventral side 
(Squalus). 

In the fin of Acanthias the brachial artery separates into a median pterygial 
{m.p., fig. 178) and a lateral pterygial artery (l.p.). In the rays the brachial 
separates into a strong branch to the propterygium and another to the meta- 
pterygium. From these branches numerous smaller arteries run to the radial 
muscles of the fin. 



Fig. 179. Eelations of rectal artery, 
Squahis sucJclii. (C. E. Bird, orig.) 

d.a., dorsal aorta; fm., femoral 
artery ; il., iliac artery ; i.rn., inferior 
mesenteric ; r., rectal artery ; r.g., 
rectal gland. 



192 THE ELASMOBRANCH FISHES 

The subclavian is continued ventrally along the coracoid segment of the 
girdle by the coracoid artery {co.a., fig. 166a), which at the ventral part of 
the girdle gives off posteriorly the anterior lateral artery (a.l.) which is the 
anterior part of the lateral (abdominal) artery. This artery, as in Heptanchus, 
runs under the walls of the lateral abdominal vein and continues posteriorly 
past the iliac arter3^ As the coracoid is followed forward and inward it is seen 
to meet its fellow from the opposite side in the midventral line and to give off 
the posterior coronary (cr.p., figs. 153 and 169) to the heart. By the union of 
right and left coracoids a median stem is formed. This stem joins the commis- 
surals from the ventral efferent-collector loops. 

The iliacs have the same form and take the same direction as the renal ar- 
teries but their terminal parts extend into the pelvic fins as the femoral ar- 
teries. The iliac artery arises from the posterior part of the lateral (abdomi- 
nal) artery (p.L, figs. 157 and /.m., 177b) . In the rays Parker has figured renal 
arteries arising from the iliacs. These renal arteries apparently are not com- 
parable to the rectal artery in Heptanchus. A rectal (hypogastric) artery in 
Squalus sucMii {r., fig. 179) may arise from the femoral or from the iliac. As 
in Heptanchus maculatus it runs to the digestive tract and anastomoses with 
the posterior intestinal. The iliac is continued to the pelvic fin as the femoral 
which distributes smaller arteries both to the dorsal and the ventral sides of 
the fin. 

SEGMENTALS 

For convenience the segmentals may be divided into three groups. One group 
consists of the musculospinal arteries (fig. 170a), anterior to the subclavian 
arteries; the second group is situated between the subclavians and the iliacs; 
and the third set is posterior to the iliacs in the region of the tail. 

The musculospinal arteries are usually paired, but sometimes they are more 
or less irregularly arranged. The first pair of these in Heterodontus passes 
upward and around the spinal column and through small canals in the poste- 
rior walls of the cranium (see p. 54, fig. 61) . The intercostal branches of these 
segmentals may divide just over the pharynx as in Zygaena (fig. 170b), or 
they may be regular as in Acanthias (fig. 170a) . In Baja the myelonal artery 
(ml., fig. 170c) supplies the spinal cord. 

A trunk segmental sends a large vertebromuscular branch dorsally around 
the vertebra and up the dorsal septum to the middorsal line. As seen in a trans- 
verse section of Squaliis sucklii {d.vm., fig. 180) this artery gives rise to a 
number of branches, the most dorsal of which ( elms. ) passes upward along the 
myoseptum to supply the musculature of the dorsomedian bundle. In the re- 
gion of the dorsal fins large branches of this artery extend into the fins. The 
next branch given off, the vertebrospinal artery, passes mediad through the 
neural arch to supply the spinal cord. Inside of the neural canal this artery 
divides into a dorsal branch, the ramus dorsalis {Acanthias, fig. 181, d.r.), and 
a ventral branch, the ramus ventralis {v.r.). The smaller ramus dorsalis forms 
the tractus arteriosus lateralis (tr.l.), which passes both backward and for- 



THE ELASMOBRANCH FISHES 



193 



ward on the cord. The ramus ventralis passes to the inidventral line where it 
enters the arteria spinalis (a.sp.). Branches, especially from the tractns la- 
teralis, enter the substance of the cord and terminate in large part around the 
grey matter (Sterzi, 1904) . Other branches given off laterally from this dorsal 
branch of the segmental pass outward in a spiral direction along the myosep- 
tum dorsally and laterally to the muscle bundles. 

The segmental, as the intercostal (i., fig. 180), 
is continued laterally to supply the musculature 
encircling the peritoneum. The intercostals in all 
recent Elasmobranchs are reduced in number (fig. 
177) . While in Heptanch us there are about thirty 
pairs, in the rays they may be reduced to only a 
few pairs, and those remaining may present great 
irregularities in size and position. This irregu- 
larity is especially noticeable in the trunk region 
where there is a considerable crowding of the vis- 
cera. 

The most ventral branch of the segmental, the 
renal artery (rn.), turns ventrally and enters the 
tissue of the kidney. In the posterior region where 
the kidney is enlarged the vessels come to be strong 
trunks which pass downward, then sharply back- 
ward, to break up into numerous branches. 

The renal divisions of the segmental artery may 
be modified as the oviducal arteries. In Scyllium 
several pairs of the segmentals just posterior to 
the subclavians {od.a., fig. 177a) pass to the ovi- 
duct and shell glands. The fourth to the sixth are 
the main ones, which pass to the oviduct and along 
the greater part of its length. In Heterodontus 
francisci two sets of these arteries are present. An 
anterior set arises from the eleventh and twelfth 
segmentals, and a posterior set from the nine- 
teenth and twentieth segmentals. The oviducals 
anastomose on the oviduct and in viviparous types 
send arteries to the inner lining of the uterus to 
supply the villi. In those forms in which the young 
are carried for a considerable time the oviducal 
arteries may be remarkably developed [Acanthias, see p. 306, fig. 267a ; 
Rhinohatis) . 

CAUDAL SEGMENTALS 




Fig. 180. Transverse section 
through trunk region, Squalus 
sucklU, showing branches of 
a segmental artery, (From 
Coles.) 

C, central muscle bundle ; 
c, artery to central bundle; 
d.a., dorsal aorta ; DMS., dor- 
somedial septal bundle of 
muscles; dms., dorsomedial 
septal artery; DS., dorsal sep- 
tal bundle; ds., dorsal septal 
artery; d.vm., dorsal verte- 
bromuscular artery; i., inter- 
costal artery ; L., lateral 
bundle ; Ja.v., lateral abdomi- 
nal vein ; U., lateral line ; N., 
neural muscle bundle ; n., ar- 
tery to neural buiidle ; rn., 
renal artery; vs.a., vertebro- 
spinal artery. 



The segmentals in the region of the tail differ somewhat from those of the body 
just described. The principal change is the result of the haemal arch. The 
segmental, in addition to sending a vertebromuscular branch upward, sends 



194 



THE ELASMOBRANCH FISHES 



a similar vertebromuscular branch downward to supply the segment at the 
sides of the haemal arch. Branches from this ventral artery supply the ventral 
muscles in the caudal region. From the ventral branch of the vertebromuscu- 
laris, the anal fin, where such is present, and the ventral lobe of the caudal fin 



Vs.a 




a.sp. V.I. 



Fig. 181. Arteries and veins of the spinal cord, Acanthias. (From Sterzi.) 

a.sp., arteria spinalis; d.r., ramus dorsalis artery; d.r.v., dorsal ramus vein; tr.l., tractus 
lateralis; v.l., vena limitans; v.r., ramus ventralis artery; v.r.v., ventral ramus vein; vs.a., 
vertebrospinal artery; vs.v., vertebrospinal vein. 



are supplied with arterial blood. The ventral branches in the region of the fins 
are better developed than are those of the dorsal series. In a type like Squatina 
where a dorsal fin is located on the tail the dorsal branches similarly supply 
this fin as well as the dorsal lobe of the caudal fin. 



THE ELASMOBRANCH FISHES 195 

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1912. Allis, E. P., Jr., The Branchial, Pseudobranchial and Carotid Arteries in Heptan- 
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1889. Ayers, K., The Morphology of the Carotids, based on a study of the Blood-Vessels 
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1904. Cabazzi, D., Sulla circolazione arteriosa cardiaca ed esofagca dello Scyllium eatulus 
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1905. Carazzi, D., Sul. sistema arterioso di Selache maxima e di altri Squalidi (Acanthias 
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1928. Coles, Esther M. The Segmental Arteries in Squalus sucklii. Univ. Calif. Publ. Zool., 

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1930. CoRRiNGTON", J. D., Morphology of the Anterior Arteries of Sharks. Acta Zoologica, 

Bd. 11, pp. 185-261, 24 text figs. 
1918. Daniel, J. Frank, The Subclavian Vein and Its Relations in Elasmobranch Fishes. 

Univ. Calif. Publ. Zool., Vol. 18, No. 16, pp. 479-484, 2 text figs. 

1925. Daniel, J. Frank, La Signification de I'artere coraco-laterale chez les poissons Elas- 
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1926, Daniel, J. Frank, The Lateral Blood Supply of Primitive Elasmobranch Fishes. 
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1909. DiAMAiiE, v., Sui rapporti della vena porta e delle arterie splanchiche in Scyllium 
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1910. DiAMARE, v., I vasi splancnici e loro relazioni topografiche in Scyllium eatulus e 
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moria I, Arch. zool. Napoli, Vol. 4, pp. 437-488, 1 tav. Also: 1916, Publ. Stat. Zool. 
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1884. DoHRN, A., Die Entwicklung und Differeuzirung der Kiemenbogen der Selachier. 

Mitt. Zool. Stat. Neapel, Bd. 5, pp. 102-192, Taf . 5-11. 
1886. DoHRN, A., Entstehung und Differeuzirung des Zungenbein- und Kieferapparates 

der Selachier. Mitt. Zool. Stat. Neapel, Bd. 6, pp. 1-92, pis. 1-8. 
1835. DuvERNOY, G. L., Sur quclques particularites du systeme sanguin abdominal et du 

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T. 3, pp. 274-281, pis. 10-11. 

1911. Ferguson, J. S., The Anatomy of the Thyroid Gland of Elasmobranchs, with remarks 
upon the Hypobranchial Circulation in these Fishes. Amer. Jour. Anat., Vol. 11, pp. 
151-210, 20 text figs. 



196 THE ELASMOBRANCH FISHES 

1866. Gegenbaur, C, Zur vergleielienden Anatomie des Herzens. I. Ueber den Bulbus ar- 
teriosus der rische. Jena. Zeitschr. Naturwiss., Bd. 2, pp. 365-375. 

1891. Gegenbaur, C, iJber den Conus arteriosus der Fische. Morph. Jahrb., Bd. 17, pp. 
596-610, 7 text figs. 

1903. Greil, Ueber die Entwickelung des Truncus arteriosus der Anamnier. Anat. Anz., Bd. 
23 (Verb.), pp. 91-105, 11 text figs. 

1929. Grodzinski, Z., Entwicklung der Blutgefasse bei Seyllium conicula. Memoire Bull. 

I'Aead. Polonaise Sci. et Litt., (Ser. B Math.-Natur.), pp. 417-454, pis. 26-28, 3 

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1902. Grynfeltt, Ed., Vascularisation des corps surrenaux cliez les Seyllium. C.R. Acad. 

Sci. Paris, T. 134, pp. 362-364. Also: C.R. Soc. Biol. Paris, T. 54, pp. 144-146. 
1902. Grynfeltt, Ed., Les corps suprarenaux chez quelques Squales et leurs rapports avec 

le systeme arterial. IV' Congres de I'Assoc. des Anat. Montpellier, Nancy, pp. 31-34. 

1845. GuiLLOT, NATAiiis, Sur un reservoir particulier que presente I'appareil de la circula- 
tion des Raies. C.R. Acad. Sci. Paris, T. 21, pp. 1179-1180. 

1906. Hochstetter, P., Die Entwickelung des Blutgefassystems. Hartwig's Handb. vergl. 
u. expt. Entwick., Bd. 3, Tell 2, pp. 21-115. 

1892. Hoffmann, C. K., tiber die Entstehung der endothelialen Anlage des Herzens und 
der Gefasse bei Hai-Embryonen (Acanthias vulgaris). Anat. Anz., Bd. 7, pp. 270-273, 
3 text figs. 

1893. Hoffmann, C. K., Zur Entwicklungsgeschiclite des Herzens und der Blutgefasse bei 
den Selachiern. Ein Beitrag zur Kenntnis des unteren Keimblattes. Morph. Jahrb., 
Bd. 19, pp. 592-648, pis. 22-25, 6 text figs. 

1900. Hoffmann, Max, Zur vergleichenden Anatomie der Gehirn- und Riickenmarksarte- 
rien. Zeitschr. Morph. Anthropol., Vol. 2, pp. 247-322, pis. 7-10, 7 text figs. 

1818. Home, Everard, Additions to an Account of the Anatomy of Squalus maximus, con- 
tained in a former paper; with observations on the Structure of the Branchial Ar- 
tery. Phil. Trans. Roy. Soc. Lond., Pt. I, pp. 227-241, pis. 16-22. 

1891. Howes, G. B., On the Intestinal Canal of the Ichthyopsida, with especial reference to 

its Arterial Supply and the Appendix Digitiformis. Jour, and Proe. Linn. Soc. Lond., 

Vol. 23, pp. 381-410, pis. 1-2. 
1858. Hyrtl, Joseph, Das arterielle Gefass-system der Rochen. Denkschr. Akad. Wiss. 

math.-naturw. Wien, Bd. 15, pp. 1-36, Taf . 1-5. 
1872. Hyrtl, Joseph, Die Kopfarterien der Haifische. Denkschr. Akad. Wiss. math.-naturw. 

Wien, Bd. 32, pp. 263-275, Taf. 1-3. 

1845. Jones, T. W., The Blood-Corpuscle considered in Its Different Phases of Development 
in the Animal Series. Phil. Trans. Roy. Soc. Lond., Vol. 136, 1846, pp. 63-102, pis. 1-2. 

1868. JouRDAiN, S., Coup d'oeil sur les systemes veineux et lymphatique de la Raie bouclee. 
Ann. Acad, la Rochelle, T. 8, pp. 21-34. 

1928. Keys, Ancel B., The Derivatives of the Hypobranchial Arteries in Hexanchus cori- 
nus. Univ. Calif. Publ. Zool., Vol. 31, pp. 111-130, pis. 5-8, 5 text figs. 

1898. Lafite-Dupont, Note sur le systeme veineux des Selaciens. Soc. Sci. d'Arcachon Stat. 
Zool., Ann. 3, pp. 86-93. 

1887. Mayer, Paul, tiber die Entwicklung des Herzens und der grossen Gefasstamme bei 
den Selachiern. Mitt. Zool. Stat. Neapel, Bd. 7, pp. 338-370, Taf. 11-12. 

1888. Mayer, P., tJber Eigenthiimlichkeiten in den Kreislaufsorganen der Selachier. Mitt. 
Zool. Stat. Neapel, Bd., 8, pp. 307-373, pis. 16-18. 

1894. Mayer, P., Ueber die ersten Stadien der Gefasse bei den Selachiern. Anat. Anz., Bd. 9, 
pp. 185-192. 



THE ELASMOBRANCH FISHES 197 

1912. Majximow, Alexander, Untersuehuiigi'ii iiber Blut und Bindegewebc. V. Uber die 
•embryouale Entwicklung der Tliymus bei Selacliiern. Arch. mikr. Aiiat., Bd. 80 
(Abt. 1), pp. 39-88, Taf. 4-8. 

1903. MtJLLEE, E., Morphologie der Cefasstiimme. Aiiat. Ilefte, Bd. 22, pp. .379-568, Taf. 
17-36, 24 text figs. 

1897. Nexjville, H., Sur les vaisseaux intra-intestinaux dcs Selaciens. Bull. Mus. d'Hist. 

Nat. Paris, No. 3, p. 317. 
1901. Neuville, H., Contribution a I'etude de la vascularisation intestinale cliez les Cyclo- 

stomes et les Selaciens. Ann. Sci. Nat. Zool., Ser. 8, T. 13, pp. 1-115, pi. 1, 22 text figs. 

1928. O'DoKOGHUE, C. H., and Abbott, Eileen, The Blood-vascular System of the Spiny 
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burgh, Vol. 55, pp. 823-890, 20 text figs. 

1886. Parker, T. J., On the Blood-Vessels of Mustelus antarcticus: a contribution to the 
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don, Vol. 177B, Pt. 2, pp. 685-732, pis. 34-37, 2 text figs. 

1899. Pajiker, G. H., and Davis, F. K., The Blood Vessels of the Heart in Careharias, Raja, 
and Amia. Proc. Boston Soc. Nat. Hist., Vol. 29, pp. 163-178, pis. 1-3. 

1874. PAVE.SI, P., Contributione alia storia naturale del genere Selaehe. Ann. Mus. Civ. 

Storia nat. Genova, Vol. 6, pp. 5-72, tav. 1-3. 
1905. PlTZOR.NO, Marco, Richerche di morfologia eomparata sopra le arterie succlavie ed 

ascellare, Pt. Ill, Selaci. Monit. Zool. Ital. Firenze, Vol. 16, pp. 94-103, 3 text figs. 
1882. PoTJCH^T, G., Terminaisons vasculaires dans la rate des Selaciens. Jour, de I'Anat. et 

Physiol., T. 18, pp. 498-502, pi. 28. 
1892. Rabl, C, Ueber die Entwicklung des Venensystems des Selachier. Festschr. Rudolf 

Leuckart, pp. 228-235, 3 text figs. 
1892. Eaffaele, F., Ricerche sullo sviluppo del systema vascolare nei Selacei. Mitt. Zool. 

Stat. Neapel, Bd. 10, pp. 441-479, tav. 29-31. 
1867. Robin, Ch., Memoire sur I'anatomie des lymphatiques des Torpilles comparee a celle 

des autres Plagiostomes. Jour, de I'Anat. et Physiol., T. 4, pp. 1-34, pis. 1-3. 

1900. Rodier, E., Sur la pression osmotique du sang et des liquides internes chez les pois- 
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1890. Rose, C, Beitrage zur vergleichenden Anatomic des Herzens der Wirbelthiere. Morph. 

Jahrb., Bd. 16, pp. 27-96, Taf. 4-5. 
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1904. Sterzi, G., Die Blutgefasse des Riickenmarks. Untersuchungen iiber ilire vergleich- 
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1876. Stohr, p., Ueber den Klappenapparat im Conus arteriosus der Selachier und Ganoi- 

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Sitz. Ges. nat. Freunde Berlin, No. 5, pp. 98-103. 



VIII 



CIRCULATORY SYSTEM (Continued) 
CIRCULATORY SYSTEM OF HEPTANCHUS MACULATUS 






mi 



Veins 

Blood distributed by the arteries is re- 
turned to the heart by several important 
systems of veins. We shall consider these 
vessels for Heptanchus maculatus in the 
particular regions which they occupy. 



VEINS OF HEAD 

The anterior cardinal sinus {a.c.s., figs. 
182 and 183) drains the blood from the 
region around the eye, and as a large ves- 
sel passes backward dorsal to the gills. At 
the pectoral girdle it drops downward and 
enters the duct of Cuvier. The vessel may 
be thought of as arising from two terminal 
vessels, the anterior cerebral and the an- 
terior facial veins. 

The anterior cerebral vein (see fig. 182) 
collects blood from the forward part of 
the brain by two main branches; the ante- 
rior lu'anch runs over the telencephalon 
receiving numerous venules; the posterior 
division drains the dorsal part of the dien- 
cephalon and receives a few branches from 
the optic lobes. The anterior facial or or- 
bitonasal vein returns the blood from the 
nasal region (see fig. 190, a.f.v.). Both the 
anterior cerebral and the anterior facial 
vessels empty into the large orbital sinus 
(o.s.) back of the eyeball. Connecting the 
two orbital sinuses is the interorbital vein 
which passes through the cranium by way 
of the interorbital canal (see fig. 47, i.o., 
facing p. 44). 
From the orbital sinus the anterior car- 
dinal passes backward through the postorbital groove and, in the region of 
the hyoidean arch, broadens out as the anterior cardinal sinus proper (a.c.s., 
fig. 183) . In the pharyngeal region it receives certain nutrient veins (nil., figs. 




Fig. 182. Anterior cardinal sinus, Hep- 
tanchus maculatus. (Mast Wolfsohn, 
orig.) 

a.c.s., anterior cardinal sinus ; d.s., 
Danielian sinus; nu., nutrient vessel; 
O.S., orbital sinus. 



[198] 



THE ELASMOBRANCH FISHES 



199 



182-184) from the hyoidean demibranch and from all the holobranchs. Pos- 
teriorly, the anterior cardinal sinus drops suddenly downward into the duet 
of Cuvier (f7.r., fig. 183). 

A second vessel, dorsal to the brancliial region but nearer the middle line, is 
the Danielian sinus (d.s., figs. 182 and 183) discovered by Mast Wolfsohn. 
Anteriorly this sinus extends almost to the vagus foramen and posteriorly it 
reaehes practically to the end of the anterior cardinal sinus. The anterior ex- 
tremity ends in a blind sac (hs.) and similarly the posterior extremity may 
end l)lindly. The Danielian sinus is connected with the anterior cardinal by 
numerous openings (ap., fig. 183) the most anterior of which is near the en- 




Fig. 183. Danielian sinus, Hci^ianchus maculatus. (Eiith Conrad, orig.) 
ap., apertures connecting anterior cardinal (a.c.s.) and Danielian sinuses (d.s.) ; 1)8., 
blind sacs leading from Danielian sinus; d.c, entrance to duct of Cuvier; nu., nutrient 
vessels ; po.o., posiorbital process. 

trance of the hyoidean vein. Other openings (aj).) between the anterior cardi- 
nal and Danielian sinuses, posterior to this point, are arranged more or less 
segmentally as may be seen by lifting up the vagus nerve. 

The inferior jugular vein (i.j., fig. 184) drains the ventral area of the phar- 
ynx and extends as an enlarged vein or sinus back to the heart. Anteriorly it 
receives a tributary from the symphysis of the lower jaw (smt.) and another 
from the thyroid region (th.v.). It next receives the hyoidean vein or sinus 
(h.s.) and. at regular intervals back of this, ventral nutrients (nv.) which are 
continuous with the dorsal nutrients of the anterior cardinal. Notwithstand- 
ing the fact that it receives veins from all the holobranchs, the inferior jugu- 
lar decreases in caliber in its course backward. Near its termination it curves 
laterally and then passes backward to enter the duct of Cuvier just mediad of 
the entrance of the subclavian vein (fig. 188) . 

VEINS OF TAIL 

The caudal vein (cd.v., fig. 185) passes forward in the haemal canal of the 
tail, and back of the cloaca divides into the renal portal veins (r.j).) . It receives 
branches from the dorsal and posterior ventral cutaneous veins (fig. 189) and 
numerous segmental veins from the tail; other segmentals join the renal 
portals as they pass along the dorsolateral margins of the kidney. The ventral 
rami of the segmental veins receive the blood from the musculature of the 
ventral lobe of the caudal fin, and each dorsal ramus collects venous blood 
from dorsal musculature and from the spinal cord. The renals finally break up 
into a net in the tissues of the kidney. 



200 



THE ELASMOBRANCH FISHES 



VEINS FROM KIDNEY OR MESONEPHROS 



Blood is returned from the kidneys and the back by the two large postcardinal 
veins {p.c, fig. 185) which run along the sides of the spinal column just under 
the lining of the body cavity. The right one of these veins may be traced as 
a single vessel from the posterior tip of the kidney forward to the region of 

the inferior mesenteric artery, where it 
is joined by the left postcardinal. Just 
behind the origin of the posterior in- 
testinal artery the two postcardinals in 
Heptanchus are joined by two or more 
cross-trunks (dr.). 

The postcardinals receive numerous 
revehentes {rv., fig. 186b) from the kid- 
ney, which have collected the blood dis- 
tributed by the advehentes (av., fig. 
186a) of the renal portal veins. As the 
two postcardinals pass forward they 
receive segmental veins from the body 
wall and before reaching the heart in- 
crease in size to form the enlarged post- 
cardinal sinuses (p.c.s., fig. 185). The 
walls of right and left sinuses freely 
intercommunicate posteriorly and an- 
teriorly and are held in place by multi- 
tudes of tendinous cords. The posterior 
cardinals enter the duct of Cuvier {d.c, 
fig. 183) posterior and mediad of the 
aperture for the anterior cardinal sinus. 




Fig. 184. Diagram of inferior jugular 
vein, Heptanclnts maculatus. (Mast Wolf- 
sohn, orig.) 

CO., coracoid cartilage ; h.s., liyoidean 
vein or sinus; hy.af., hyoidean afferent 
artery; i.j., inferior jugular vein; nu., 
nutrient vein ; smt., submental vein ; tli., 
thyroid gland; th.v., thyroid vein; v.a., 
ventral aorta. 



VEINS FROM DIGESTIVE TRACT 



Venous blood is returned from the di- 
gestive tract to the heart by the hepatic 
portal system of veins (see fig. 155, 
/«.?;., facing p. 166) .The principal veins 
making up this system in Heptanchus are : the posterior intestinal, the intrain- 
testinal, the anterior intestinal, the gastrics, the portal, and the hepatic veins. 
The posterior intestinal vein, as the dorsal intestinal {d.i.v., fig. 155), drains 
the rectal gland and passes forward along the colon and valvular intestine. 
At about the place where the posterior intestinal artery (p.i.a.) reaches the 
valvular intestine the vein, as the posterior intestinal proper, leaves the in- 
testine and passes over the bridge of the pancreas. Before joining the anterior 
intestinal vein it receives the large anterior gastro-pancreaticosplenic vein 
formed by a gastric branch from the dorsal side of the cardiac stomach, splenic 



THE ELASMOBRANCH FISHES 



201 



branches from the sphMMi on and in the angle of the stomach, and smaller pan- 
creatic veins. As it passes the anterior segment of the spleen it receives one or 
more additional strong branches. 

The anterior intestinal vein is a forward continuation of the intraintestinal 
vein and, as such, drains the free margin of the valve within the valvular intes- 
tine. Near the place where it leaves the intestine the anterior intestinal vein re- 
ceives several branches. The first of these 
branches is the ventral intestinal vein 
( I'. ?.!•., fig. 155) which arises on the rectum 
and passes over the colon and along the 
ventral side of the valvular intestine par- 
allel with and not far from the dorsal in- 
testinal vein. At the distal part of the val- 
vular intestine the ventral intestinal vein 
is usually connected by a transverse ves- 
sel with the dorsal intestinal vein (d.i.v.), 
and along the valvular intestine it re- 
ceives annular branches. The ventral in- 
testinal in continuing over the ventral lobe 
of the pancreas receives certain branches 
from the pancreas. It next receives a 
long gastrosplenic branch (p.gs.v.) which 
drains the posterior side of the pyloric 
stomach along which it travels from the 
cardiac division where it receives splenic 
branches. The anterior intestinal may 
next be joined by the large anterior branch 
of the gastric which drains the most ante- 
rior part of the cardiac stomach and, in 
part, the main division of the anterior seg- 
ment of the spleen ; or the anterior gastric 
vein may join the ventral gastric and 
empty with it into the portal. 

After the union of the anterior and the 
posterior intestinal veins the large ventral 
gastric (v.g.v.) joins the stem of the he- 
patic portal system {h.p.). The hepatic 
portal vein extends a short distance for- 
ward and divides into right and left 
halves to the lobes of the liver. The blood 
thus distributed to the liver is finally col- 
lected by right and left hepatic trunks 
which enter the sinus venosus of the heart 
a short distance from the middle line 
(/i.r., fig. 188). 




Fig. 185. General view of veins in Hep- 
taiiclms maculatus. (C. G. Potter, orig.) 

a.c, anterior cardinal ; Tyr.v., brachial 
vein ; dr., cross-trunk ; cd.v., caudal 
vein ; cl.v., eloacal vein ; co.v., coracoid 
vein; f.v., femoral vein; i.j., inferior 
jugular vein; l.a.v., lateral abdominal 
vein; p.c, postcardinal vein; p.c.s., 
postcardinal sinus; r.p., renal portal; 
s.cl.v., subclavian vein ; s.sc.v., subscap- 
ular vein ; s.v., sinus venosus. 



202 



THE ELASMOBRANCH FISHES 



VEINS OF BODY WALL 



The veins of the bodj^ wall empty into the lateral abdominal system, the chief 
vessels of which are the lateral abdominal veins {l.a.v., fig. 185) which run 
along the sides of the body just under the lining of the body cavity. In the re- 
gion of the cloaca, right and left lateral abdominal veins are continuous across 








A B 

Fig. 186. Finer vessels in kidney, Eeptanchus maculatus. (C. G. Potter, orig.) A. Segment 
of kidney showing renal vein and advelientes. B. Section of the kidney showing revehentes 
entering the postcardinal. 

av., advehentes; da., dorsal aorta; p.c, postcardinal vein; rn., renal vein; rv., revehentes; 
ur., ureter ; vd., vas deferens. 

the midventral line. Each lateral abdominal receives a cloacal vein (cl.v.) 
from the rectal region and a femoral vein (f.v.) from the pelvic fin. Between 
the pelvic and the pectoral regions the lateral abdominals receive numerous 
tributaries from the thinner musculature of the body wall, which are not 
shown in figure 185. In the pectoral region it receives the brachial vein {hr.v., 
fig. 187) from the pectoral fin, the subscapular (s.sc.v.), including the lateral 

cutaneous vein {l.c.v., fig. 187), 
and the coracoid vein (co.v.) 
from the ventral part of the 
girdle; the last-named vessel re- 
ceives the ventral cutaneous vein 
from the skin. As the subclavian 
vein, the lateral abdominal turns 
sharply upward and enters the 
duct of Cuvier {s.cl.v., fig. 187). 

VEINS OF SKIN 




S.CI.V. 



Fig. 187. The subclavian vein and its relations, 
Heptanchus maculatus, lateral view. 

br.v., brachial vein; co.v., coracoid vein; d.c, 
duct of Cuvier; l.a.v., lateral abdominal vein; 
/.C.V., lateral cutaneous vein; p.c.s., postcardinal 
sinus ; s.cJ.v., subclavian vein ; s.sc.v., subscapular 
vein. 



The dorsal cutaneous vein in Hep- 
tanchus maculatus (p.d.c, fig. 
189) runs in the connective tis- 
sue of the skin along the mid- 
dorsal line. For convenience of 
description it may be divided 
into a posterior and an anterior 



THE ELASMOBRANCH FISHES 



203 



part. The posterior part is first found al)out halfway between the pectoral and 
pelvic segments and continues from this point posteriorly almost to the tip of 
the tail. In the region of the dorsal lobe of the caudal fin the dorsal cutaneous 
vein is double, and around the dorsal fin right and left dorsal cutaneous veins 
form a loop from which a strong intercommunicating branch passes to join the 
lateral cutaneous vein. From the unpaired vein just back of the loop a medium 




Fig. 188. Sinus venosus opened, Heptanchus maculattis. (Euth Jeanette Powell, del.) 

a.c, anterior cardinal; au., auricle; h.v., hepatic vein; i.j., inferior jugular vein; p.c.s., 
postcardinal sinus; sa., sinu-auricular valve; s.cl., subclavian vein; s.v., sinus venosus; v., 
valves of anterior cardinal sinus. 

deep vein (m.v.) passes to the right of the spinal column to join the caudal 
vein (cd.v.). The anterior part of the dorsal cutaneous vein ends anteriorly 
in a V-shaped sinus over the brain case. 

The lateral cutaneous vein (Lev., fig. 189) runs directly under the skin, 
parallel with and ventral to the lateral line groove. It extends from the middle 
region of the caudal fin forward, and joins the subscapular sinus near its tip 
(fig. 187). In its course forward it receives numerous segmentally arranged 
cutaneous branches, and in the region of the dorsal fin it has strong intercom- 
municating branches which join the cloacal vein (cl.v.) of the lateral ab- 
dominal system. 




Fig. 189. Cutaneous veins, Heptanchus macidatus. (Helen Hopkins, orig.) 
cd.v., caudal vein; cJ.v., cloacal vein; l.c.v., lateral cutaneous vein; 77!.^;., median unpaired 
vein ; p.d.c, posterior dorsal cutaneous vein ; p.v.c, posterior ventral cutaneous ; s.sc.v., sub- 
scapular vein. 

The ventral cutaneous vein lies in the subcutaneous tissue in the midventral 
line. In the region of the anal fin it forms a loop and like a V its right and left 
branches run backward on the caudal fin. The posterior part of the vessel 
empties into the cloacal vein (cLv.) and the anterior segment of this vein joins 
the coracoid vein (co.v., fig. 187) . 



204 THE ELASMOBRANCH FISHES 



CIRCULATORY SYSTEM OF ELASMOBRANCHS IN GENERAL 

Blood which has been distributed by the arteries to tlie capillaries of the 
tissues is collected and returned to the heart by the veins. The veins, as we 
have said, differ as a rule from the arteries in possessing thinner walls. In sec- 
tion this is seen to be due especially to a lack in the muscular layer. A back- 
ward flow of blood, which in the arteries is prevented by the muscular elas- 
ticity of the larger proximal arteries and by the rhythmic action of the heart, 
is prevented in the veins by valves. These valves are present at irregular in- 
tervals throughout the course of some of the veins and are especially marked 
at the junction of principal trunks, as for example at the entrance of the an- 
terior cardinal to the duct of Cuvier (v., fig. 188). The valves are formed by 
the lining of the veins as loose crescentic folds, the concavity of which is 
directed toward the heart. These permit a free course of the blood toward the 
heart but prevent its backward flow by filling with blood and thus blocking 
the lumen. 

The Elasmobranch veins frequently become greatly enlarged sinuses. An 
incision through the postcardinal sinus shows that its walls, though similar in 
other respects to those of the veins, differ from them especially in two ways. 
In the first place if they possess any musculature it is exceedingly thin, and 
secondly they have, passing from wall to wall, numerous tendinous supporting 
cords. Many such enlarged sinuses are present in the Elasmobranclis espe- 
cially in the region of the head and in the proximal part of the veins near 
the heart. 

Veins 

For convenience of description the veins of the Elasmobranchs in general may 
be grouped into seven systems as follows: (1) those which return blood from 
the head, the anterior cardinal sj'stem; (2) veins which bring blood from the 
caudal region to the kidnej', the renal portal system ; ( 3 ) the vessels draining 
the kidneys along the dorsal body wall, the posterior cardinals; (4) the veins 
which carry the blood from the digestive tract and its appendages to the liver, 
the hepatic portal system; (5) those veins which return blood from the ex- 
tremities and sides, the lateral abdominal system; (6) a system of veins drain- 
ing the walls of the heart; and (7) the cutaneous veins or veins of the skin. 

AXTEEIOR CARDINAL SYSTEM 

The anterior cardinal system consists of the anterior cardinal or jugular veins 
and the inferior jugulars together with their tributaries. The anterior cardinal 
vein {a.c.s., fig. 190), like that of Heptanchus, passes from the orbital sinus 
{o.s.) back over the branchial basket. The orbital sinus receives the anterior 
facial (a.f.v.) or orbitonasal and the anterior cerebral veins {a.c.v., fig. 191), 
together with certain cutaneous veins of the head. Right and left orbital sinuses 



THE ELASMOBRANCH FISHES 



205 



are connected bj- the interorbital vein which traverses the interorbital canal 
(see fig-. 47, facing p. 44). The blood thus collected in the orbital sinus passes 
through the postorbital groove and then backward in the enlarged anterior 
cardinal sinus over the pharyngeal region. The sinus receives nutrients from 
the gills. At the level of the most posterior gill arch the anterior cardinal drops 
down and enters the duct of Cuvier or the postcardinal sinus (Scyllium, fig. 
190b, a.c.s.). The branches to the anterior cardinal may be considered further. 





Fig. 190. Anterior cardinal system. 
A.Mustelus. (From T. J. Parker.) B. Scyllium. (From O'Donoghue.) 

a.c.s., anterior cardinal sinus; a.c.v., anterior cardinal vein; a.f.v., anterior facial vein; 
tr.v., brachial vein; h.s., hyoidean sinus; h.v., hyoidean vein; i.j., inferior jugular; l.a.v., 
lateral abdominal vein ; l.c.v., lateral cutaneous; n.s., nasal sinus; o.s., orbital sinus; p.c.s., 
postcardinal sinus; p.c.v., posterior cerebral vein; s.cl.v., subclavian vein; s.sc.v., subscapu- 
lar vein. 

A supraorbital vein (so.v., fig. 192a) collects blood from the skin and the 
jelly-like tissue overlying the cranium, and passes forward from the segment 
of the parietal fossa (p.f.) over the dorsal surface of the nasal capsule. At the 
place where the superficial ophthalmicus nerve {fo.VII, fig. 192a) perforates 
the cartilage the left supraorl)ital receives a small dorsomedian rostral vein 
{dm.v.). 



206 



THE ELASMOBRANCH FISHES 



The supraorbital then extends forward, receiving a dorsolateral branch 
(dl.v.), and then passes through the foramen in the roof of the olfactory 
capsule and continues as the nasomaxillary (nm.v. ) . As this vein curves down- 
ward within the capsule and along its posterolateral aspect it receives a fairly 
large branch (n.v., fig. 192b), which is the result of a remarkable leash of ves- 
sels coming from the folds of the olfactory organ. Just after the nasomaxillary 





A B 

rig. 191. Cerebral veins. (From Kex.) A. Scyllium catulus. B. Eaja asterias. 
a.c.v., anterior cerebral vein; my., myelonal vein; p.c.v., posterior cerebral vein. 

enters the cartilage in the postero ventral region of the capsule it is joined by 
the orbitonasal (on.v.) or anterior facial vein (a.f.v., fig. 190), which serves to 
connect the veins under discussion wath the orbital sinus {o.s., fig. 190). The 
nasomaxillary vein next emerges from the nasal cartilage just laterad of the 
point where the maxillary nerve first passes under the basal fenestra (fn., 
fig. 2, Wells, 1917) . Here it receives twigs from the skin and tissue outside of 
the capsule, and a subrostral vessel (sr.v.) from the tip of the snout. In the 
posterior part of its course the nasomaxillary vein swings mediad in front of 
the superior labialis muscle (Us.) to join its mate from the opposite side to 
form a dorsal sinus (s.) . From the sinus (s.) the buccopharyngeal veins {hp.v., 
fig. 192b) lead backward, and at the basal angle of the cranium, right and left 
vessels swing outward following the margin of the orbits. At the postorbital 
process each buccopharyngeal receives one or two lateral tributaries from the 
sides of the upper jaw, and then right and left vessels take an almost parallel 
course posteriorly finally to empty into the anterior cardinal sinus. 

The anterior cerebral vein (a.c.v., fig. 191a) , which, as we said above, enters 
the orbit as the principal vein from the anterior part of the brain, may vary 



THE ELASMOBRANCII FISHES 



207 



considerably from the condition seen in Heptanchus. It is formed by an 
anterior vessel on the olfactory bulb which receives a branch from the olfac- 
tory lobe, a median branch from the ventral olfactory and diencephalic areas, 
and a posterior vein which in the middorsal line joins a similar vein from the 
opposite side as the mesencephalic vein. In other words, the first branch of 




fo. VII 



po p. 




Fig. 192a. Veins dorsal to roof of 
cranium, Squalus sucMii. 



Fig. 192b. Veins in roof of buccal 
cavity, Squalus sucldii. 



bp.v., buccopharyngeal vein ; dl.v., dorsolateral rostral vein ; dm.v., dorsomedian rostral 
vein ; fn., basal fenestra ; fo. VII, exit of superficial ophthalmic nerve ; Us., attachment, 
superior levator labialis muscle ; n.a., nasal aperture ; n.c, nasal capsule ; nm.v., nasomaxil- 
lary vein; n.v., nasal vein; on.v., orbitonasal (facial) vein; p.f., parietal fossa; po.p., post- 
orbital process; s., sinus; so.v., supraorbital vein; sr.v., subrostral vein. 



the anterior cerebral drains the olfactory lobe and tract, the second the whole 
of the ventral area back to the optic chiasma, and the third the whole of the 
remaining dorsal region anterior to the cerebellum. It is through the last- 
named vessel that the roof of the third ventricle is drained. 

The anterior cardinal sinus (fig. 190) receives the hyoidean vein and the 
nutrients (not shown) from all the holobranchs. These nutrients, as in Hep- 
tanchus, are continuous with ventral nutrients. 

The posterior cerebrals (p.c.v., fig. 191) from the brain enter the anterior 
cardinal sinus. In a type like Scyllhim (fig. 191a) they collect the blood from 
the cerebellum and medulla and pass it posteriorly in large right and left 
dorsal veins through the foramina with the vagus nerves. In Raja asterias 
(fig. 191b) the posterior cerebral is usually a single vessel. Continuing pos- 



208 



THE ELASMOBRANCH FISHES 



terior from this vessel in both Scyllium and Raja is the dorsal myelonal vein 
(my.). This vein is absent, however, in Acanthias (fig. 181). A ventral 
myelonal vein, arising posterior to the optic chiasma, drains the vascular sacs 
and passes on down the cord. 

The hyoidean vein in Mustelus enlarges ventrall}' into the hyoid sinus. Each 
sinus (h.s., figs. 190 and 193) is triangular and of large size, the base of the 

triangle extending from the tip of the 
lower jaw in front to back of the first 
afferent artery. In Acanthias the fore- 
most of the communicating vessels be- 
tween right and left hyoidean veins 
forms the hyoidean sinus. In Car- 
charias and Raja a thyroidean sinus of 
considerable size is formed in the mid- 
line ventral to the thyroid gland (Fer- 
guson, 1911). 

From the postero ventral angle of the 
hyoidean sinus, the inferior jugular 
vein passes backward to enter the duct 
of Cuvier (figs. 190 and 193). On its 
way it receives the ventral nutrient 
veins (nu.) which are connected with 
the dorsal nutrients of the anterior car- 
dinal sinus. 

EENAL PORTAL SYSTEM 

In the embryonic condition a su])intes- 
tinal vein extends from the tail to the 
heart. It later separates into an ante- 
rior and a posterior part, the anterior 
part becoming the line of the hepatic 
portal system and the posterior part 
that of the renal portal system now 
under consideration. 

The caudal vein, as the basis of the 
renal portal system of the adult, ex- 
tends from the tip of the caudal fin through the haemal canal to the cloaca. 
It represents the stem of a Y, the arms of which pass to the sides of the cloaca 
as the renal portal veins (r.p., fig. 194a) . Each renal portal continues forward 
and upward dorsal to and along the lateral margin of the kidneys, giving to 
each numerous advehentes. 

In its course as a single median vessel, the caudal receives dorsal and ven- 
tral segmental veins on each side, which are of large size at the place where the 
dorsal and ventral lobes of the fin are deepest. The two renal portals also re- 
ceive segmental veins from the body wall in the region of the kidneys and from 




Fig. 193. Veins ventral to pharynx, Mus- 
telus antarticus. (From T. J. Parker.) 

a.c.s., anterior cardinal sinus; hr.v., 
brachial vein; li.s., hyoid sinus; h.v., hyoi- 
dean vein; i.j., inferior jugular; l.a.v., 
lateral abdominal vein; nu., nutrient; 
p.c.s., postcardinal sinus. 



THE ELASMOBRANCn FISHES 



209 



the oviduct of the female. In certain forms, as we shall see later, the tips of 
the ovidncal veins are in communication with other veins in the cloacal region. 

POSTERIOR CARDINAL VEINS 

The posterior cardinal veins in the Elasmobranchs in general are two enlarged 
vessels located at the sides of, and ventral to, the spinal column and ventro- 
lateral to the dorsal aorta. In some types right and left postcardinals may 





rig. 194. General view of veins of body. 

A. MuMelus antarcUcus. (From T. J. Parker.) B. Eaia erinacea. (From Rand.) 

a.o.s., anterior cardinal sinus; hr.v., brachial vein; ed.v., caudal vein; cl.v., cloacal vein; 
f.v., femoral vein; i.j., inferior jugular vein; il.v., iliac vein; l.a.v., lateral abdominal vein; 
p.c, postcardinal vein; p.c.s., posteardinal sinus; r.p., renal portal vein; s.sc.v., subscapular 
vein; s.cl.v., subclavian vein. 

form a fused vessel posteriorly (Scijllium and Raja) . In others the postcardi- 
nals extend as separate vessels from the posterior tip of the kidney behind to 
the sinus venosus in front. As a usual thing, liowever, only the right one ex- 
tends backward the whole length of the kidney, while the left is attached to 
the right {Mustelus, fig. 194a, p.c). 

In the sharks the anterior third of the posterior cardinals forms the enlarged 



210 THE ELASMOBRANCH FISHES 

postcardinal sinuses, the two usuallj^ being freely intercommunicating. At the 
middle of the postcardinals in the rays (fig. 194b) a spacious sinus (p.c.s.) is 
formed which is prolonged backward by a narrower outpocket toward the 
rectal gland. In most forms the thin walls of the sinuses are strengthened by 
the unusual development of the trabeculae. 

The postcardinals may enter the posterior inner angle of the duct of Cuvier 
a considerable distance from the entrance of the hepatic veins, as in Hep- 
tancJms, or they may empty as in Mustelus (fig. 190a) . 

The first and most posterior branches received by the posterior cardinals 
are the revehentes draining the kidneys; while the tributaries from the an- 
terior region are usually the large genital sinus draining the gonads, and the 
subscapular vessel coming from under the scapula. Between these two areas 
and throughout the greater part of their course forward they receive seg- 
mental veins from the body walls. The ventral branches of the segmentals 
drain the blood from the interseptal spaces, and the dorsal branches the blood 
from the deep musculature of the back. Into the dorsal rami the veins {vs.v., 
fig. 181) from the spinal cord enter. 

In a type like Acanthias the vertel)rospinal veins leave the neural canal 
through the foramina of the dorsal nerve roots. Within the canal each vein 
divides into a dorsal ramus {d.r.v.) which drains the dorsal part of the cord 
and a ventral ramus {v.r.v.) draining the ventral part. The ventral ramus is 
also connected with a vena limit ans which extends longitudinally along the 
ventral side of the cord. In Scyllmm the dorsal rami of each side of the cord 
form plexuses of veins each of which is more or less united into a longitudinal 
dorsolateral tract. In the rays, longitudinal tracts form a dorsal spinal vein 
of large size which as we have seen joins the posterior cerebrals anteriorly. 

HEPATIC PORTAL SYSTEM 

Blood which has been distributed to the digestive tract by the coeliac axis and 
the mesenteric arteries is returned from the tract by branches of the hepatic 
portal system. Two such branches in both the sharks and the rays are of 
special interest. These are the intraintestinal, including the anterior intestinal 
vein, and the posterior intestinal or mesenteric veins. To these branches should 
be added the gastric veins mentioned for Heptanchiis. 

The intraintestinal vein (see fig. 173b, i.v.) represents a part of the anterior 
segment of the subintestinal vein of the embryo. It was discovered first on the 
free margin of the scroll valve of Zygaena where it is of so large a size that 
Duvernoy (1833) described it as a "venous heart." Such, however, is not its 
nature. As it emerges from the anterior end of the valvular intestine it is 
continued as the anterior intestinal vein. 

The anterior intestinal vein (see p. 184, fig. 173b, a. i.v., and figs. 174 and 
175) is usually well developed in the sharks, but is small or relatively insig- 
nificant in the rays. As it continues forward from the intraintestinal vein it 
is joined by the ventral intestinal which arises on the ventral side of the 
valvular intestine, and receives annular branches from the attached side of 



THE ELASMOBRANCH FISHES 211 

the valve. At the anterior end of the valvular intestine it receives branches 
from the lobes of the pancreas and the posterior gastro-pancreaticosplenic 
vein (p.gs.v., fig. 173b) . 

The posterior gastro-pancreaticosplenic is continued in the mesentery 
(omentum) between the spleen and the stomach as the gastrosplenic vein 
{p.gs.v., figs. 173-175). While in Heptanchus it is relatively small, although 
a long vessel, in most of the other sharks it is well developed, and in the ray 
(Dasyatis, fig. 175) is of relatively immense size. Here, in the absence of a 
spleen on the greater curvature of the stomach, it drains only the stomach 
and pancreas. 

The anterior intestinal then passes forward to join the posterior intestinal. 
The segment of the anterior intestinal vein as it passes forward to join the pos- 
terior intestinal varies greatly in length. In ScylUuDi it is exceedingly short, 
but in Acanthias and Miistelus (fig. 173b) it is a relatively long segment. 

The posterior intestinal vein is the direct continuation of the dorsal intes- 
tinal vein (d.i.v., fig. 173) which arises within the tissue of the rectal gland, 
from the tip of the gland (leopard shark, fig. 173a) , or from a sinus which runs 
longitudinally along the lumen of the gland to its base (Acanthias, Mustelns, 
fig. 173b). The dorsal intestinal passes along the dorsal side of the valvular 
intestine, receiving annular branches. In some forms the posterior intestinal 
vein leaves the intestine at the place where the posterior intestinal artery 
strikes it, that is, at about the middle of the intestine {Acanthias, Dasyatis, 
fig. 175), or it may leave it farther forward {Heterodontus, fig. 174; Triakis, 
fig. 173) . It extends forward by the spleen, from which it receives the anterior 
gastrosplenic vein {Squalus sucklii; Heterodontus, fig. 174, a.gs.v.). The pos- 
terior intestinal vein then proceeds forward to join the anterior intestinal 
vein to form the portal. 

The anterior gastro-pancreaticosplenic vein, which is an important vessel 
in Heptanchus, is much simpler in Mustelus (fig. 173b) and in ScylUum. In 
all types it is divided into gastric and splenic parts, and as in Triakis (fig. 
173a) it usually receives a branch from the dorsal lobe of the pancreas. 

The portal or hepatic portal is formed by the union of the posterior intes- 
tinal vein, anterior intestinal trunk, and one or more gastrics. It passes to the 
liver usually as a vessel of large size, receiving on its w^ay the large ventral 
gastric {v.g.v., figs. 174 and 175), two or more branches of which drain the 
ventral surfaces of the cardiac and pyloric stomach. Upon reaching the liver 
the portal divides into two branches, one to each of the lobes. These branches 
extend to the tips of the lobes, giving off in their course numerous other 
branches which break up into a net. 

The blood thus distributed to the liver by the hepatic portal vein and by the 
hepatic arteries is re-collected by the hepatic veins and taken to the heart. The 
hepatic veins may empty near the median line by a right and left vein, as in 
Acanthias; or these veins may break up into a more or less complex net before 
entering the sinus venosus {Lamna) . In other forms the two hepatic vessels 
join and enlarge in the anterior part of the liver, forming immense hepatic 



212 



THE ELASMOBRANCH FISHES 



sinuses. Where the two vessels fuse together the walls between the two sides 
are more or less broken down and the remaining walls are supported by 
trabeculae. These hepatic sinuses may empty by relatively small apertures 
near the middle line into the sinus venosus (Scyllium). In certain types the 
hepatic veins enter the outer tips of the duet of Cuvier (Torpedo, Raja) . 

DEVELOPMENT OF HEPATIC POETAL SYSTEM 



In the embryo, the vitelline veins from the yolk sac (v. v., fig. 195) are among 
the first vessels to appear. These are followed by the subintestinal vein (s.i.), 
previously mentioned, which unites with the vitelline to form an omphalo- 



^ 



i 






C 



D 



Fig. 195. Diagram of development of hepatic portal system in Elasmobranchs, ventral view. 
(From Eabl, modified.) 

a.c.v., anterior cardinal vein; cd.v., caudal vein; 7;.^;., hepatic vein; Iv., liver capillaries; 
om., omphalomesenteric vein; p.c, postcardinal ; p.i.v., posterior intestinal vein; r.p., renal 
portal vein; s.i., subintestinal (intraintestinal) vein; s.v., sinus venosus; v.v., vitelline vein. 

mesenteric. The right vein remains rudimentary, but the left omphalomes- 
enteric becomes an important vessel (om..). When the developing liver comes 
in contact with the omphalomesenteric the latter vessel sends branches into 
the tissue of the liver and divides into two parts, each of which forms a series 
of capillaries in the liver (Iv. fig. 195c) . After the absorption of yolk, and the 
consequent loss of the vitelline veins, the main stem of the hepatic portal sys- 
tem is along the subintestinal line. This vein in the adult is carried in with the 
develo])ing valve into the valvular intestine, as the intraintestinal, to drain the 
free margin of the valve. There is next formed the posterior intestinal or 
mesenteric vein (p.i.v., fig. 195b-c). Blood now empties by the united sub- 
intestinal and posterior intestinal into the hepatic portal and this empties into 
the liver. From the liver the blood is collected and passes to the sinus venosus 



THE ELASMOBRANCH FISHES 



213 



(s.v., fig. 195c) by the liopatic veins (h.v.), which were previously the anterior 
ends of the vitelline veins. In figure 195c and d the cardinal and renal systems 
are also well developed. 

LATERAL ABDOMINAL SYSTEM OF VEINS 

The lateral abdominal veins {l.a.v., fig. 194) extend from the pelvic to the 
pectoral segments of the body just under the peritoneimi in the sides of the 
body wall. Posteriorly each vein may arise from a net of fine veinlets on the 
side of the rectal and cloacal walls 
{Raja,&g. 194b) ; or right and left 
veins may be continuous across 
the pelvic cartilage {Mustelus 
antarcticus, fig. 194a ; Scyllium 
canicula). Posteriorly a rectal 
branch joins the lateral abdomi- 
nal of Scyllium near the midven- 
tral line. The first important trib- 
utary (or tributaries) to the lat- 
eral abdominal system of veins is 
the iliac, resulting from a fusion 
of the cloacal and femoral veins 
from the cloacal and pelvic areas, 
respectively {Mustelus, fig. 194a) . 
In certain forms the cloacal and 
femoral veins join the lateral 
abdominal independently, as in 
Heptanchus. In Raja an acces- 
sory femoral vein also empties 
into the lateral abdominal (fig. 
194b). The femoral veins (f.v.) 
are formed in the pelvic fin from 
numerous veinlets, while the cloa- 
cal veins (cl.v.) drain the sides of 
the cloacal region. 

Blood collected from the deeper 
structures in the posterior region, 
then, whether from the cloaca or the pelvic fin, is carried forward by the 
lateral abdominal vessel. As this vessel passes anteriorly many veins from 
the body wall enter it. 

At the pectoral girdle the lateral abdominal vein receives important tribu- 
taries. The first of these is the brachial vein. In the sharks the brachial arises 
from the union of a dorsal, a median pterygial {m.p.v., fig. 196), and a lateral 
pterygial vein {l.p.v.) of the fin. In rays where the pectoral fin is large in 
extent, a much larger median pterygial branch is present and is joined by 
the smaller lateral, ventral vein. In addition a large anterior branch from the 




Fig. 196. Veins of pectoral fin, Acanthias. (From 
Erik Miiller.) 

l.p.v., lateral pterygial vein; m.p.v., medial 
pterygial vein. 



214 



THE ELASMOBRANCH FISHES 



propterygium joins the lateral abdominal vein independent of the brachial 
(Raia erinacea, fig. 194b) . In Raja nasuta two independent brachial branches 
join the lateral vein. 

In Heptmichus it was seen that the subscapular vein (s.sc.v., fig. 187) is an 
important tributary of the lateral abdominal, emptying, in common with the 
brachial as a brachioscapular vessel, blood from the pectoral girdle and from 
the lateral cutaneous vessel. In Mustelus kcnlei a short subscapular trunk 
joins the brachial but all blood from the lateral cutaneous reaches the heart 

through the postcardinal. Squalus 
sucklii is of interest as a type 
which actually bridges these two 
extremes. In it the lateral abdom- 
inal {l.a.v., fig. 197), just before 
entering the duct of Cuvier, re- 
ceives the brachioscapular trunk 
which includes the subscapular 
vein (s.sc.v.). Now the subscapu- 
lar vein dorsally comes in contact 
with, and has an opening into, 
the postcardinal sinus (p.c.s.). 
The lateral cutaneous vein (Lev.) 
empties into the subscapular near 
the union of the subscapular with 
the postcardinal sinus, so that the 
blood from the lateral cutaneous 




Fig. 197. Diagram of relations of postcardinal 
to lateral abdominal system, Squalus sucTclii. 

'br.v., brachial vein; h.s.c, brachioscapular; 
co.v., coracoid vein ; d.c, duct of Cuvier ; l.a.v., 
lateral abdominal vein; l.c.v., lateral cutaneous 
vein; p.c.s., postcardinal sinus; s.cl.v., subclavian 
vein; s.sc.v., subscapular. 



vein after entering the subscapular may pass dorsally into the postcardinal 
sinus or ventrally into the lateral abdominal vein. In other words, if the sub- 
scapular vein of Squalus sucklii had no connection with the postcardinal sinus, 
Squalus would be in all essentials of the type of Heptanchus. If, however, that 
segment of the subscapular between the entrance of the lateral cutaneous 
(Lev., fig. 197) and the brachial vein (hr.v.) were dropped out, then the 
lateral cutaneous would be independent of the lateral abdominal system and 
the type would be like that of Mustelus or Scyllium. 

After receiving the brachioscapular trunk (brachial and subscapular), the 
lateral abdominal vein as the subclavian (s.cl.v.) turns sharply upward in the 
pericardio-peritoneal wall and across the scapular cartilage to empty into the 
duct of Cuvier (d.c.) as in Heptanchus. 

The history of the lateral abdominal system is of interest. In origin it is one 
of the earliest of the systems to appear. Furthermore it occupies a position 
which would have been of particular value had a lateral fin-fold been present, 
for such a vessel would have drained this fold directly as it does those parts 
of the fold which remain, that is, the paired fins. 

The lateral abdominal vein has often been considered in relation to the 
ventral abdominal vein of the amphibians which in its anterior and posterior 
sections drains the paired appendages, but in the middle region is a single 



THE ELASMOBRANCH FISHES 



215 



ventral vessel. Anteriorly, in the embryonic amphibian, it enters tlie duct of 
Cuvier, but later, by secondary twigs, it comes to empty directly into the liver. 
From these characteristics it appears likely that the ventral abdominal in 
Amphibia is homologous with the lateral abdominal of Elasmobranchs. 



VEINS or HEART 



Three sets of vessels return blood, distributed ])y the coronary arteries, from 
the heart itself. These are a small right coronary, a median cardiac vein, and a 
larger left coronar}- vein. These veins enter the sinus venosus, near the sinu- 
auricular opening, usually bj^ two or more apertures. The right and left 
systems in Acanthias, however, join and empty into the sinus venosus by a 
single large aperture. 




Fig. 198. Cutaneous system of veins, Squalus sucTclU. (Helen Hopkins, orig. ) 

cd.v., caudal vein; ?.c.f. and l.c.v}, superior and inferior lateral cutaneous veins; 'm.v., 
median vein; p.c, posteardinal vein; p.d.c, posterior dorsal cutaneous vein ; p.v.c, posterior 
ventral cutaneous ; s.sc.v., subscapular vein. 

The right coronary may drain the right side of the ventricle and the dorsal 
side of the conus {Carcharias lit f oralis), emptying into the sinus venosus by 
its own aperture at the right side of the sinu-auricular opening; or the right 
may arise as two vessels on the dorsal and ventral sides of the conus and on the 
ventral side of the ventricle. These two continue separately and open inde- 
pendently {Raia erinacea). The left coronary in Raia erinacea is of consider- 
able size and drains the ventral and lateral parts of the ventricle. In other 
forms it is a vessel of importance {Carcharias littoralis, Cetorhi^ius, Scyl- 
lium), draining the ventral and lateral parts of the ventricle. 

The cardiac veins drain the dorsal part of the ventricular wall. They may 
form as a double vessel and unite to enter the sinus venosus with the left 
coronarj^ {Carcharias littoralis), or they may empty independently into the 
sinus venosus by a half-dozen smaller mouths {Raja rubens) . In Raia erinacea 
numerous vessels receive blood from the large triangular area lying parallel 
to the posterior margin of the sinus venosus and empty it directly into the 
sinus venosus. 

The vessels of Thebesius in the Elasmobranchs, according to Parker and 
Davis (1899), are deep in the walls of the heart and are connected with the 
coronary veins. They may be detected by immersing the heart in water and 



216 



THE ELASMOBRANCH FISHES 



then inflating the left coronary vein with a blow pipe, whereupon bubbles of 
air emerge from the left atrial wall into the atrium {Acanthias) . No bubbles, 
however, appear upon the inflation of the right coronary vein. The same may 




Fig. 199a. Veins of, and in the region of, first dorsal fin, Squalus sucTclii. (L. H. Bennett, 
orig.) 

c, vein connecting dorsal and lateral cutaneous systems ; c.v., vena circularis ; d.c, dorsal 
cutaneous vein ; dfv., vein draining dorsal fin. 

])e demonstrated in the coronary arteries but with more difficulty. In Raia 
evinacea by inflating either right or left vein a similar bubbling occurs from 
the inner surface of the atrium, although none occurs from the ventricle. In 

the Elasmobranchs, then, the superfi- 
cial veins of the heart empty into the 
sinus venosus and the deeper Thebesian 
vessels enter the atrium direct. 

CUTANEOUS SYSTEM OF VEINS 



The cutaneous veins consist of the dor- 
sal, ventral, and lateral vessels of the 
skin. 

The dorsal cutaneous vein collects 
blood from the skin on the back 
{Squalus sucklii, fig. 198, p. d.c). It ex- 
tends along the middorsal line from the 
caudal fin to the endolymjihatic ducts, 
and surrounds both dorsal fins in closed 
loops {c.v., fig. 199a). Posterior to the 
loop surrounding the first dorsal in 
Mustelns antarcticus according to T. J. 
Parker (1886) a median vein passes 
downward to the left of the column to 
join the left renal portal. This is essen- 
tially the condition found in Squalus 
except that this deep vessel arises as a 




Fig. 199b. Cutaneous veins in region of 
cloaca, Squalus sucMii. (Edith Stoker, 
orig.) 

l.a.v., lateral abdominal vein; p.c, post- 
cloacal segment of ventral cutaneous ; pi., 
pelvic vein from ventral cutaneous to 
sinus (s.). 



THE ELASMOBRANCH FISHES 217 

doul)le vein tlio parts of wliicli later unite to form the rena profunda of Mayer. 
Further, in Squaliis tlie voia profunda joins the i)ostcardinal vein. In Mus- 
felus henlei if the deep vessel ])asses to the right of the column it breaks up 
in a leash of vessels and may join the renal portal vein. In Heptanchus macu- 
latus a similar vessel in the region of the single dorsal fin, which is com])aral)le 
to the second dorsal fin of Squalus, passes to the right of the column and joins 
the caudal vein. The vena profunda of the second dorsal loop in Sqiialus and 
in Musfelus Jienlei joins the caudal vein. 

The lateral cutaneous veins (l.cv., fig. 198) accompany the lateral line sys- 
tem, the main trunk of the lateral cutaneous passing just mediad of and ven- 
tral to the lateral line. They collect blood from dorsal and ventral branches 
to the corium and underlying connective tissue and empty anteriorly into the 
subscapular sinus (s.scv.), which in turn enters the postcardinal sinus except 
in types like Heptanchus. A system of cross-trunks in the regions of the dorsal 
fins puts the lateral cutaneous in connection with the dorsal cutaneous vein 
(c, fig. 199a). In a number of Elasmobranchs (Squalus sucklii, fig. 198) an 
accessory lateral cutaneous vein (l.c.v.'^) extends from the pelvic to the pec- 
toral segments, parallel with the lateral cutaneous proper. This vessel has 
segmental connections with the lateral cutaneous above it and the ventral 
cutaneous below it. 

The ventral cutaneous vein is divided into pre- and postcloacal parts 
ip.v.c. ) . The precloacal part, not shown in figure 199b, extends as an unpaired 
median vein between the pelvic and pectoral regions. In the region of the 
pectoral it bifurcates, sending a right and a left branch to the lateral abdomi- 
nal vein. These branches seem to be the same as those which I described for 
Heptanchus maculatus as the coracoid veins (Daniel, 1918) . Posteriorly in the 
pelvic area, the vessel also divides (pi., fig. 199b). Here its right and left 
branches in Squalus sucklii empty into a sinus (s.), from which the blood 
enters the lateral abdominal vein {l.a.v.). 

The postcloacal division of the ventral cutaneous extends from the tail to 
the cloacal area. It arises as a pair of vessels on the right and left sides of the 
caudal fin, which do not meet around the tip of the ventral lobe of the fin. These 
vessels communicate with the caudal vein. If an anal fin be present (Scyllium) 
the ventral cutaneous forms a loop around the anal fin and then extends to 
the cloaca (p.c, fig. 199b) , bifurcating into right and left branches. The blood 
collected by this vein also passes into the sinus (.s-. ) and thence into the lateral 
abdominal system. 

NATUEE OF CUTANEOUS SYSTEM OF VESSELS 

It was early observed that the cutaneous vessels in Elasmobranchs are not 
accompanied by arterial trunks. From this fact it was argued that they are 
not blood vessels but are lymphatic in nature. Work done by Miss Coles (1928) 
in this laboratory, however, showed that while no longitudinal arterial trunks 
accompany these vessels in Squalus sucklii, yet they are provided with a rich 
arterial supply which is given off by branches from the segmental arteries. 



218 THE ELASMOBRANCH FISHES 

Moreover, Burne (1923) has shown for Lamna that well marked cutaneous 
arteries do accompany some of these veins. Parker (1886) called attention to 
the presence of blood in these vessels and Mayer (1888) proved that they are 
provided with valves. Furthermore, the cutaneous vessels are put into direct 
connection wutli the deeper veins, for example, the caudal vein. Also, Mayer 
reported the circulation of blood in these vessels in a semitransparent embryo. 
All the above points made it likely that these vessels are true blood-vessels. 
The actual proof that they are haemal and not lymphatic in nature was made 
by Edith Stoker (see Daniel and Stoker, 1927) in the following way. The 
shark was first anaesthetized and a glass canula was inserted into a cut 
through the lateral cutaneous vessel. In this experiment the blood will circu- 
late through the canula, demonstrating that the vessels are for the circulation 
of blood. 

Lymphatic Vessels 

Lj-mphatic vessels have been difficult to demonstrate in Elasmobranchs. 
Within recent years, however, Hoyer (1928) has succeeded in injecting these 
vessels, and reports that they have the following arrangement. Right and left 
trunks (thoracic ducts) accompany the dorsal aorta from the tip of the tail 
to the head. These trunks collect lymph from the muscles of the tail and trunk 
through tiny branches which run with the intersegmental arteries. They re- 
ceive lymph from other vessels which run with the arteries and veins along 
the myosepta and the longitudinal septa separating dorsal and ventral 
bundles. A net of these vessels is present under the peritoneum and over the 
kidney. Lymphatics from the intestine unite in a plexus at the base of the 
mesentery and these plexuses are put into connection with the thoracic ducts 
through cross-trunks. In addition to these right and left ducts in the tail and 
trunk, there are two jugular trunks in the area of the head and pharynx, 
which accompany and finally enter the cardinal sinuses. Further, accompany- 
ing the subclavian arteries there are two branches which also enter the cardi- 
nal sinuses. 



THE ELASMOBRANCH FISHES 219 

BIBLIOGRAPHY 
Chapter VIII 

1878. Bai^four, F. M., A Monograiah on the Development of Elasmobraneh Fishes. London, 

pp. 1-295, 9 pis., 9 text figs. 
1923. BuRNE, E. H. (see p. 195). 
1918. Daniel, J. Frank, The Subclavian Vein and Its Relations in Elasmobraneh Fishes. 

Univ. Calif. Publ. ZooL, Vol. 18, pp. 479-484, 2 text figs. 

1927, Daniel, J. Frank, and Stoker, Edith, The Relations and Nature of the Cutaneous 
Vessels in Selachian Fishes. Univ. Calif. Publ. Zool., Vol. 31, pp. 1-6, 4 text figs. 

1931. Daniel, J. Frank, and Bennett, L. H., Veins in the Roof of the Buccopharyngeal 
Cavity of Squalus sucklii. Univ. Calif. Publ. Zool., Vol. 31, pp. 35-40, 3 text figs. 

1833. DuvERNOY, G. L., Sur quelques particularites du systeme sanguin abdominal et du 
canal alimentaire de plusieurs poissons cartilagineux. Ann. Sci. Nat. Zool., Vol. Ill 
(1835), Ser. 2, pp. 274-281, pis. 10-11. 

1887. HocHSTETTER, F., Beitrage zur vergleichenden Anatomie und Entwicklungsgeschichte 
des Venensystems der Amphibien und Fische. Morph. Jahrb., Bd. 13, pp. 119-172, 
pis. 2-4, 7 text figs. 

1893. HocHSTETTER, F., Eutwicklung des Venensystems der Wirbeltiere. Ergebn. d. Anat. 
u. Entwick., Bd. 3, pp. 460-489, 24 text figs. 

1906. HocHSTETTER, F., Die Entwieklung des Blutgefassystems. Hertwig's Handb. vergl. 
u. expt. Entwick., Bd. 3, Teil 2, p. 116. 

1893. Hoffman, C. K., Zur Entwicklungsgeschichte des Venensystems bei den Selachiern. 
Morph. Jahrb., Bd. 20, pp. 289-304, pi. 12. 

1901. Hofmann, Max, Zur vergleichenden Anatomie der Gehirn- und Eiickenmarksvenen 
der Vertebraten. Zeitschr. Morph. u. Anthropol., Bd. 3, pp. 239-299, pis. 16-20, 6 
text figs. 

1859. JouRDAiN, S., Reeherches sur la veine port renale. Ann. Sci. Nat., T. 12 (Ser. 4), pp. 
134-188, 321-369, 5 pis. 

1910. Lafite-Dupont, Sur le developpement de la paroi des sinus veineux des poissons car- 
tilagineux. (Reun. biol. Bordeaux.) C.R. Soc. Biol. Paris, T. 68, p. 694. 

1785. Monroe, A., The Structure and Physiology of Fishes. Edinburgh, 128 pp., 44 pis. 

1900. Neuville, H., Note preliminaire sur I'endothelium des veines intestinales chez les 
Selaciens. Bull. Mus. Hist. Nat., No. 6, pp. 71-72. 

1914. O'DoNOGHUE, C. H., Notes on the Circulatory System of Elasmobranchs. I. The Ve- 
nous System of the Dogfish (Scyllium canicula). Proc. Zool. Soc. Lond., 1914, pp. 
435-455, pis. 1-2, text figs, 1-4. 

1928. O'DoNOGHUE, C. H., and Abbott, Eileen (see p. 197). 

1881. Parker, T. J., On the Venous System of the Skate (Raja nasuta). Trans. N. Z. Inst., 
Vol. 13, pp. 413-418, pi. 15. 

1899. Parker, G. H., and Davis, F. K. (see p. 197). 

1892. Rabl, C, Ueber die Entwieklung des Venensystems der Selachier. Festschr. z. 70. 
Geburtstag R. Leuekarts, pp. 228-235, 3 text figs. 

1905. Rand, H. W., The Skate as a Subject for Classes in Comparative Anatomy; Injec- 
tion Methods. Amer. Nat., Vol. 39, pp. 365-379, 1 text fig. 

1905. Rand, H. W., and Ulrich, J. L., Posterior Connections of the Lateral Vein of the 
Skate. Am. Nat., Vol. 39, pp. 349-364, 5 text figs. 



220 THE ELASMOBRANCH FISHES 

1891. Eex, H. Beitrage zur Morphologic der Hirnvenen der Elasmobranehier. Morph. Jahrb., 

Bd. 17, pp. 417-466, Taf. 25-27. 
1845. EoBiN, Ch., Le Systeme veineux des poissons cartilagineux. C.R. Acad. Sci. Paris, T. 

21, pp. 12-82. 
1845. EoBiN, Ch., Note sur le systeme lymphatique des Eaies et des Squales. L'Institut Paris, 

T. 13, pp. 144-145, 232-233. 
1867. EoBiN, Ch., Menioire sur les dispositions anatomiqucs des lymphatiques des Torpilles, 

comparees a celle qu'ils presentent chez les autres Plagiostomes. C.E. Acad. Sci. Paris, 

T. 64, pp. 20-24. 
1911. ScAMMON, E., Normal Plates of the Development of Squalus acanthias. Normenta- 

feln z. Ent.Av. d. Wirbel., Heft 12, pp. 1-123, 4 pis., 25 text figs. 
1902. ViALLETON, L., Les Lymphatiques du tube digestif de la Torpille (Torpedo marmo- 

rata, Eisso). Arch. d'Anat. micr., T. 5, pp. 378-456, j)ls. 13-14. 
1908. WiDAKOWiCH, v., Wie gelangt das Ei der Plagiostomen in den Eileiter? Ein Beitrag 

zur Kenntnis des Venensystems von Scyllium canicula. Zeitschr. wiss. Zool., Bd. 91, 

pp. 640-662, Taf. 29, 2 text figs. 
1906. Woodland, W., A Suggestion concerning the Origin and Significance of the Eenal- 

portal System, with an Appendix relating to the Production of the Subabdom- 

inal Veins. Proc. Zool. Soc. London, 1906 (2), pp. 886-901, 1 text fig. 

LYMPHATICS 

1928. HoYER, Henry, On the Lymphatic Vessels of Scyllium canicula. Anat. Eecord, Vol. 40, 

pp. 143-145. 
1928. Hoyer, Henry, Eecherches sur les vaisseaux lymphatiques des Selacieus. Bull. 

Internat. Acad. Polon. Sci. et Lett. cl. Sci. (Math.-Natur. B), pp. 79-104, pis. 7-10. 




Fig. 200a. Tlie brain and associated sense organs, JI(pia)ic]nis maculalvs, dorsal view. 
(Duncan Dunning, del.) 

iu.VII, buccal branch of facial nerve; ch., cerclielluni ; el., ciliary nerve; c.r., restiform 
body; di., diencephalon ; hmd., hyomandil»ular division of the facial nerve; i.l., inferior 
lobe; md.V, mandibular division of the fifth nerve; m.n., median olfactory nucleus; med., 
medulla; ms., mesencephalon; mx.V, maxillary division of trigeminal nerve; ol.h., olfactory 
bulb; ol.l., olfactory lobe; oJ.t., olfactory tract; op.l., optic lobe; op.V, ophthalmicus pro- 
fundus division of the trigeminal nerve; os.V and VIl, ophthalmicus superficialis of tri- 
geminal and facial nerves; /?., telencephalon; in., terminal nerve; v.s., vascular sac; w-z, 
occipitospinal nerves ; I, olfactory nerve ; //, optic nerve ; III, oculomotor or third nerve ; 
IV, trochlearis or fourth cranial nerve; T'7, abducens or sixth cranial nerve; Till, auditory 
or eighth cranial nerve; IX, glossopharyngeal nerve; A', vagus nerve. 



IX 



NERVOUS SYSTEM 

NERVOUS SYSTEM OF HEPTANCHUS MACULATUS 

Central Nervous System 
BRAIN 

The brain of Heptanchus maculatiis may be described as made up of five 
divisions, as is common for the Elasmobranchs. These divisions, beginning 
anteriorly are : the telencephalon, the diencephalon. the mesencephalon, the 
metencephalon, and the mj^elencephalon. 

The telencephalon ( //., fig. 200a ) , if seen 
from the dorsal side, appears as a bilobed 
mass which is continued forward by long 
olfactory tracts (ol.t.). Between the tracts 
and projecting slightly anteriorly is the 
median olfactory nucleus (m.n.), better 
seen in ventral view (fig. 200b). Dorsally 
the telencephalon is raised up into the so- 
called pallial eminences. At the angle be- 
tween the median olfactory nucleus and 
the pallial eminence is the recessus neuro- 
porieus, at the sides of which arises the 
terminal nerve (tn.). The telencephalon 
is continued posteriorly by the diencepha- 
lon (fZf, fig. 200b). 

The diencephalon is provided with a 
thin roof through which the pineal stalk 
passes as a slender thread upward and 
forward to the cartilaginous roof. This 
segment of the brain continues posteriorly 
as a gradually narrowing mass back to the 
place where the optic nerves (//) form 
the optic chiasma. From the posterior and 
ventral part of this division arises the in- 
fundibulum, at the sides of which are the 
inferior lobes (i.l.) and the vascular sacs 
(v.s.) of the brain. In the middle line and ventral to the vascular sacs are the 
lobes of the pituitary (see fig. 215) . 

The mesencephalon is well developed (ins., fig. 200b) . Dorsally it consists of 
two hollow optic lobes (op.l., fig. 200a) or corpora bigemina. These in their 
posterior part are overlapped by the cerebellum (cb.) and ventrally by the 
infundibnlum and its associated structures. Through the posterior roof of 




md. V. 



Fig. 200b. Brain and cranial nerves, 
Heptanchus maculafus, ventral view. 
For explanation see fig. 200a. 



[221] 



222 



THE ELASMOBRAXCH FISHES 




Fig. 201. Cross-section of cord, Hep- 
tanchus cinereus. (From Sterzi.) (Grey 
matter stippled.) 

d.h., dorsal horn; v.h., ventral horn. 



the mesencephalon and near the cerebellnm the fourth cranial or trochlear 
nerve arises {IV). Ventrally the mesensephalon is composed of large fiber 
tracts through which the third cranial or oculomotor nerve (7/7) emerges and 
passes forward to muscles of the eye. 

The metencephalon consists, in large part, of the cerebellum (c6.), a large 
shield-shaped mass, separated dorsally into right and left halves by a median 
groove. The cerebellum extends anteriorly over the posterior half of the optic 

lobes, and posteriorly it overlies the fol- 
lowing division of the brain. Ventrally 
and under the cerebellum are heavy fiber 
tracts which also belong to the segment 
of the metencephalon. 

The myelencephalon is the last seg- 
ment of the brain. It comprises the me- 
dulla oblongata (»ief?.) which from above 
extends upward on each side almost to 
the tip of the cerebellum as the resti- 
form bodies (corpora restiforme, c.r.). 
Back of the restif orm bodies the medulla 
grows smaller and smaller in diameter 
until it joins the cord. Within the myelencephalon and the metencephalon is 
the fourth ventricle which is covered over by a thin roof. The myelencephalon 
gives rise to all the cranial nerves back of the fourth. 

SPINAL COED 

The spinal cord extends from the medulla practically to the tip of the tail. 
Externally, as is seen in figure 201, it presents no evidence of a dorsal or a 
ventral groove which in some Elasmobranchs (see fig. 217b) may mark the 
boundaries of the cord into right and left halves. Superficially the cord con- 
sists of numerous fiber tracts surrounding masses of nerve cells. 

In the section (fig. 201) the central, more solid part of the cord constitutes 
the grey matter (cells) roughly in the form of an X, the upper arms of which 
are the dorsal horns {d.h.) and the lower arms, the ventral horns {v.h.) of the 
cord. The less conspicuous dorsal horns lie near the middorsal line, while the 
ventral horns extend outward and downward as an expanded mass on each 
side. Furthermore there is a median ventral mass extending toward the mid- 
ventral line from the union of the ventral horns. Wliere the crossing of the X 
takes place there is a considerable thickening, in which is located the neuro- 
coele or cavity of the spinal cord. It will be observed that the neurocoele is 
practically at the middle of the section and is circular in outline. 



Peripheral Nervous System 

The cranial and spinal nerves arising from the central nervous system per- 
forate the cranium and spinal column respectively. Also perforating the cra- 
nium but back of the last cranial nerve are certain accessory nerves. 



THE ELASMOBRANCH FTSHER 223 



CRANIAL NERVES 

The first cranial or olfactory nerve has its end organs in the mucous membrane 
of the olfactory organ. The nerve passes backward in two short bundles, one 
median and the other lateral in position (7, fig. 200a) to the olfactory bulbs 
(ol.h.) The bulbs are joined to the olfactory lobes (ol.l.) of the brain by means 
of long olfactory tracts {ol.t.) . 

Running parallel with this tract is the terminal nerve {tn.), which in Hep- 
tanchus arises near the recessus neuroporicus and passes along the tract as a 
slender strand. Near the olfactory bull) it runs laterad and enters the fissure 
dorsally between the lateral and median divisions of the olfactory nerve. To 
the median nerve it gives an exceedingly slender strand and continues with 
the lateral division of the nerve, finally reaching the mucous membrane of 
the cup. 

The second or optic nerve (77, figs. 200a and 200b) has its origin from the 
retina of the eye. As a thick stem it passes inward, crosses in the optic chiasma, 
and enters the diencephalon. 

The third cranial nerve or oculomotor (777) springs from the ventral side 
of the midbrain and passes to muscles of the eye. In the orbit it divides so as to 
supply branches to the inferior, the superior, and the anterior rectus, and 
to the inferior oblique muscles of the eye. 

The trochlearis or fourth nerve (7F) also arises from the mesencephalon, 
but the fibers perforate the roof of the brain instead of the floor. The fibers 
then as a small band pass under the overhanging cerebellum and down over the 
mesencephalon forward and outward to the superior oblique muscle of the eye. 

The trigeminal or fifth cranial nerve in Heptanchus maculatus arises from 
the brain in common with, but slightly anterior and ventral to, the seventh 
nerve. A short distance from its origin it separates into its main branches. 
The first of these nerves, the ophthalmicus profundus (op. V, fig. 200a), runs 
to the median side of the eyeball. Before reaching the latter, however, it 
gives off the ciliary branch (cl.) to the eye; it then perforates the cartilagi- 
nous capsule surrounding the eyeball and continues forward under the supe- 
rior rectus, next emerging from the cartilage to pass forward under the ante- 
rior rectus muscles and between the superior and inferior obliques. It leaves 
the orbit by a separate foramen, and finally breaks up in the skin over the 
nasal capsule. The ophthalmicus superficialis of the fifth (os.V) is a small 
branch or branches running with the ophthalmicus superficialis of the seventh 
nerve. The maxillary branch (inx.V, fig. 200a) of the trigeminal breaks up 
into three or four main divisions which go to supply the region of the upper 
jaw; while the mandibular division of the fifth (md.V) passes backward and 
downward around the angle of the mouth to the mandibular region. 

The abducens or sixth nerve {VI, fig. 200b), after leaving the mid ventral 
line of the myelencephalon back of the fifth-seventh complex, passes under the 



224 



THE ELASMOBRANCH FISHES 



main stems of this complex and out from the cranium through its own fora- 
men. It enters the base of the external rectus muscle. 

The facial or seventh cranial nerve like the fifth is composed of four im- 
portant branches. These are first the superficial ophthalmic nerve {os.VII, 
figs. 200a and 200b) which runs above all the eye muscles through the orbit, 
gives branches dorsally to the supraorbital sensory canal, and then leaves the 
orbit by the large anterodorsal ophthalmic foramen (f.o.VII, fig. 47). Out- 
side of the orbit it supplies branches to the supraorbital canal and to certain 
groups of the ampullae of Lorenzini. The second or buccal division of the 







Fig. 202. Branches of the facial nerve, HeptancMis maculatus. (W. R. Dennes, orig.) 

bu.VII, buccalis of facial; c.t., chorda tympani; limd., hyomandibular (postspiracular) 
division of seventh nerve; md.e., superior and inferior branches of external mandibularis of 
seventh; md.i., internal division of mandibularis of seventh; os.VII, ophthalmicus super- 
ficialis of seventh nerve; pl.VII, palatinus; po.s., postspiracular twigs; pr.s., prespiracular 
nerve ; sp., spiracle. 

facial nerve {hu.VII) passes from the brain stem just dorsal to the maxillary 
division of the fifth (mx.V). In the orbit it divides much like the maxillary 
division of the fifth. It goes to supply the infraorbital canal and the ophthal- 
mic and buccal groups of ampullae. The palatine division of the seventh 
(pl.VII, fig. 202) leaves the main stem of the hyomandibular nerve and passes 
ventralward, dividing into an anterior and a posterior branch, to the palate 
of the mouth. The most posterior division of the facial, the hyomandibular 
{hmd., figs. 200a and 202), after giving off the palatine branch, passes 
sharply backward around the spiracle, downward around the angle of the 
jaw and forward along the mandible. It first gives a prespiracular branch 
(pr.s., fig. 202) to the anterior wall of the spiracle; other twigs (po.s.) are 
next given off to the posterior wall of the spiracle. Back of the angle of the 
jaw a superficial branch runs forward toward the angle of the jaw, and two 
branches (md.e.) pass along the external side of the mandible to the hyoman- 
dibular canal and mandibular groove (hmc. and mg., fig. 228) . A deep branch 
(md.i., fig. 202) given off at the angle runs along the body of the hyoid, and 
another branch, the chorda tympani (c.t.), passes forward between the hyoid 
and the mandible. 

The auditory or eighth nerve in Heptanchus (VIII, figs. 200a and 200b) is 
more or less clearly separated from the seventh. It has a large ganglion from 



THE ELASMOBRANCH FISHES 225 

which run two main branches; the anterior branch passes outward and divides 
into two divisions; the other passes backward and separates into numerous 
twigs. Tliis nerve supplies the ampullae of the semicircular canals and sends 
divisions to other parts of the ear. 

The glossopharyngeal or ninth nerve (IX, figs. 200a and 203) arises back 
of the sixth and farther from the middle line and passes posteriorly under 
the floor of the auditory capsule. Before reaching the surface it gives off a 
branch (st.IX, fig. 203) from which sui)ratemporal branches are sent off pre- 
sumably to the anterior segment of the lateral line canal. Another branch 
[dr. IX) passes upward and backward from the supratemporal but its desti- 
nation has not been determined. After leaving the cranium the glossopharyn- 
geal bears a ganglion ign.) dorsal to the first branchial cleft, from which pro- 
ceeds the main pretrematicus nerve ipri.) ; this nerve runs down the hyoidean 
demibranch in front of the first gill pocket. A pharyngeal branch (ph. IX) 
arises from the ganglion over the dorsal angle of the first cleft and passes for- 
ward to supply the dorsal pharyngeal wall. From the pharyngeal in Hep- 
tanchus macula t us there is given off an internal pretrematic (prt.i.). The 
third division of the nerve is the post-trematicus (po.t.) which passes back 
of the first cleft, in the anterior demibranch of the first whole gill. In Hep- 
tanchus maculatus the post-trematicus is divided into two distinct bundles, 
of which the anterior is sensory and the posterior is mixed. The posterior 
branch of the post-trematicus continues to the ventral pharyngeal region, 
supplying motor branches to muscles and sensory fibers to the mucous mem- 
brane. One of its sensory branches curves forward and runs dorsally past the 
end of the anterior sensory root. 

The vagus or tenth cranial nerve {X, figs. 200a and 200b) leaves the brain 
stem by a number of roots. Some of these roots arise ventrally close to the 
origin of the ninth. Others arise in a crescentic line from this almost to the 
middorsal line. The nerve leaves the cranium through a large foramen and 
outside appears in three main divisions. One of these, the lateralis nerve {ll.X, 
fig. 203), supplies the lateral line and extends practically to the tip of the 
tail. The branchial divisions ( 6 r.X^"*^ ), consisting of six nerves, pass backward 
and then ventrally to supply the gill area not supplied by the glossopharyn- 
geal; while the third division, the visceral (6/.X), passes on to the viscera. 

The lateralis nerve (ll.X) is the most anterior bundle of the vagus arising 
from the brain (figs. 200a and 200b). It continues backward more or less 
separate and distinct even in the canal where it gives off two branches, one of 
which (st.X, fig. 203) passes to the supratemporal and lateral sensory canals, 
the other (dr.X) being distributed to the area of the pit organs along the back. 
The lateralis then runs along the wall of the anterior cardinal sinus and 
finally passes posteriorly between the dorsal and lateral bundles giving off 
branches to the lateral line. 

The branchial nerves to the gills (hr.X^~^) lie in the floor of the anterior 
cardinal sinus. Distal to their ganglia each of these nerves comprises the same 
number of parts as the ninth, pretrematicus, post-trematicus, and pharyngeal 



226 



THE ELASMOBRANCH FISHES 






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THE ELASMOBRANCII FISHES 



227 



brandies. The ])liarynu:eal is unlike tliat of the ninth only in that it first passes 
))osteriorly and then cni-ves forward. The nerves posterior to the seventh cleft 
have undergone considerable cliange. The third division of the vagus, tlie 
ramus intestinalis (bi.X), breaks up into a numl)er of important divisions, 
some of which go to the lieart, others to the oesophagus, and still others to the 
stomach and associated parts. 

OCCIPITOSPINAL NERVES 



Arising ])osterior to the vagus are certain small nerves known as the occipi 
tospinales (w-z, figs. 200a. 200b, and 204) . These nerves are unusually numer 
ous in Heptanchiis and con- 
sist of both ventral and 
dorsal branches. The first 
pair to arise ventrally {w, 
fig. 200b) is near the mid- 
ventral line and not unlike 
the sixth or abducens nerve 
in origin. Posterior to this 
are three others (.r, y, z), 
making in all four ventral 
pairs. Dorsally in Heptan- 
chiis maculatus (fig. 200a) 
there are two pairs, the first 
of which joins the "(/" divi- 
sion of the ventral nerve. 
These nerves supply the subspinalis and dorsal interarcuales muscles (fig 
204) and are like the spinal nerves with which .r, ;/, and z form a plexus. 




Fig. 204. Cervical and brachial plexus, IJcptancJtiis 
cinereus. (From Max Fiirbringer.) 

hr.p., brachial plexus; cr.p., cervical plexus; d.r., dor- 
sal or sensory nerve; ^., dorsal root ganglion; sp., spinal 
(mixed) nerve; v.r., ventral or motor nerve; «', x, y, s, 
occipitospinal nerves. 



SPIXAL NERVES 



A spinal nerve arising from the spinal cord in Heptanchus cinereus (M. 
Fiirbringer, 1897) is composed of a single ventral (motor) and a dorsal 
(sensory) root. The ventral root comes from cells in the ventral horn of the 
cord* {v.h., fig. 201) and passes outward to be joined by the dorsal root (fig. 
204) ; while the dorsal root extends to the dorsal horn of the cord (<:/./(., fig. 
201) through the dorsal root ganglion (g., fig. 204). After the union of dorsal 
and ventral divisions the mixed nerves thus formed, in the region of the neck 
and the pectoral and pelvic fins, are pressed together. 

The cervical {cr.p., fig. 204) and pectoral plexuses {hr.p.) may be consid- 
ered together since they form a continuous bundle. In Heptanchus cinereus 
these plexuses (fig. 204) are made up of the first seven spinal nerves together 
with occipitospinales which were above mentioned. At the region of the scapula 
the nerve trunk separates, the cervical nerves {cr.p.) going to the hypobran- 
cliial or ventral longitudinal muscles, and the pectoral plexus {hr.p.) to the 
pectoral fin. 



228 



THE ELASMOBRANCH FISHES 



Beginning with the twenty-fifth spinal nerve {sp. 25, fig. 205) , the terminal 
divisions of some of the spinal nerves are connected at the region of the lateral 
abdominal vein (l.a.v.) into a nervous collector. This in Hepfanchus cinereus 
as described by Braus (1898) forms a collector of iiniisnal length, which 
continues to the thirty-eighth spinal nerve {sp. 38), but does not perforate 
the cartilage of the pelvic girdle. 

Posterior to the pelvic girdle a pelvic plexus of nerves {pi. p., fig. 205) is also 
found in Heptanchus. This plexus in the adidt Heptanchus cinereus usually 
involves only the forty-eighth and forty-ninth nerves, and the plexus thus 
formed is considerably back of the pelvic girdle. 




Fig. 205. Nervous collector, Heptanchus cinereus. (From Braus.) 

l.a., lateral artery; l.a.v., lateral abdominal vein; php., pelvic plexus; sp. 25, 38, twenty - 
fifth and thirty-eighth spinal nerves. 




Fig. l.'(l(i. A multipolar inutdi- cell. (From Jlouser.) 
o..r., axoiic ; (?/., (Innlritc ; /(.. luu-lcus ; )(/(., micli'olu.s. 



THE ELASMOBRANCH FISHES 



229 



NERVOUS SYSTEM OF ELASMOBRANCHS IN GENERAL 

The central nervous system, comprising the brain and the spinal cord, is com- 
posed of neurones. A neurone consists of a nerve cell from which proceed the 
axone and dendrites. The nerve cells themselves are of three general types. 
One of these is a multipolar cell (fig. 206) with numerous dendritic processes 





Fig. 20 



Fig. 208 



rig. 207. Bipolar cells of cord, Pi-istiurus. (Modified from Yon Leiihossek.) 

a-c, stages in development of a bipolar cell; ax., axone or fiber; c, adult type of cell; 
d.r., dorsal root; g., dorsal root ganglion. 

Fig. 208. Supporting elements, Mustelus cants. (From Houser.) A. Neurogleal cell. B. 
Ependymal cell. 



(dt.) an axone (ax.), and a large nucleus (n.); another is the bipolar type 
with two processes to the cell body. Figure 207 shows the development of 
some bipolar cells in the dorsal root ganglion of Pri.stiiirus. An early stage (a) 
shows the two processes, one of which, the axone proper, passes in toward the 
cord; the other process extends toward the periphery. Stage (c) shows the 
union of the two processes to form the stem of a T. In this stage the cell appears 
to be unipolar or with a single process extending from it. 

In addition to the cells and fibers found in the central nervous system, cer- 
tain supporting elements are also present. The first of these are the ependymal 



230 



THE ELASMOBRANCH FISHES 



cells (fig. 208b) which are modifications of the cells bounding the central canal 
or neurocoele. The processes from these cells often reach entirely across to 
the external margin of the cord. The second type of supporting cell is the 
neurogleal cell which maj^ take the form shown in figure 208a, or its processes 
ma^- be long. 

Development of Central Nervous System 



In its origin the nervous system is laid down as a flattened, horseshoe-shaped 
plate of cells, of ectodermal origin, which later becomes spatulate (fig. 209). 
The broad end of the spatula, which will later form the brain, differs consider- 
ably from the narrower handle which is the 
, '' rudiment of the spinal cord. The margins of the 

plate (i?./., fig. 209) fold up and the whole 
l)late then sinks down along the middorsal line. 
In the region of the brain, where the plate 
widens, the li]« of the plate close slowl.y; but in 
the region of the cord the closure goes on with 
more rapidity. Even before the closure of the 
plate, however, outpocketings to form the primi- 
tive optic vesicles (op.r.) appear. The plate 
then closes into a tube, the point at which 
closure last takes place remaining as the neuro- 
pore (np., fig. 212). 

The tube thus formed is divided into the 
])rimitive forebrain, the midbrain, and the 
hindbrain. By further development the fore- 
brain and the hindbrain are separated into four 
of the permanent segments of the adult brain, 
while the mesence^^halon does not divide 
further. The prosencephalon or forebrain becomes the telencephalon and 
diencephalon and the hindbrain or rhombencephalon becomes the metenceph- 
alon and myelencephalon. Thus the five divisions of the adult brain which we 
have described for Hepfanchus are formed. 




Fig. 209. L'tvelojiment of medul- 
lary plate, Acanfhias. (From 
Locy.) 

«./., neural fold ; op.v., optic 
vesicle. 



GEOSS FORM OF BRAIN 



Superficially, different Elasmobranch ])rains present very different appear- 
ances. This difference is due largely to the condition of the olfactory tracts and 
their appended olfactory bulbs. The bulbs, arising as outgrovv^ths from the 
prosencephalon, may still be practically in contact with it in the adult. In 
Scijmvus (fig. 213a) they may extend but a short distance out from the pros- 
ence})halon. In other types, however, as in Hepfanchus, they may be drawn 
far forward so as to be remotely removed from their place of origin (Squatina, 
Laemargus horeolis, and especially Hexanchus) . In addition the olfactory 
tracts may be widelj' divergent as in Raja (fig. 211b) and Zygaena, or they 



THE ELASMOBRANCII FISHES 



231 



may take a coiii'sc more or less nearly ])arallel as in (Uirvharias and i\I i/liobatis 
(fig. 212). 

In the greater nnmber of Elasniobranchs the brain is elongated so that in a 
dorsal view at least parts of all the five segments are visible. In a few, how- 
ever, the brain may appear as a more or less compact mass. As a normal thing 





A B 

Fig. 210. Brain of Reterodonius francisci. (Mildred Bennett, del.) A. Dorsal view. B. 
Ventral view. 

ch., cerebellum; c.r., restiform body; dL, diencephalon or thalameneephalon ; id., inferior 
lobe; im.n., median olfactory nucleus; mcd., medulla; ms., mesencephalon; old)., olfactory 
bulb; oJd., olfactory lobe; old., olfactory tract; opd., optic lobe; p.c, pallial eminence; tl., 
telencephalon; v.s., vascular sac; 7, olfactory nerve; //, optic nerve; IV, trochlearis or 
fourth nerve; VI, al»ducens or sixth cranial nerve; IX. glossopharyngeal nerve; X, vagus 
nerve. 



the compactness is the resnlt of the enlargement of the cerebellnm {ch., fig. 
212) wherel)y it overlies the mesencephalon or, occasionally, even a part of the 
diencephalon {Scoliodon, or Myliohat is). 

We may now consider in more detail the different segments of the brain. In 
form the telencej^halon (fl.) depends largely npon the proximity of the right 
and left halves. In Heterodontus francisci (fig. 210) these are clearly sepa- 
rated by a median anterior sulcus so that the lobes have undergone but slight 
fusion in the middle line. In Heptanchus we have noted a similar condition 



232 



THE ELASMOBRANCH FISHES 



although in it fusion is greater. From Heterodontus to Scoliodon (fig. 211a) 
a series of forms may be obtained, showing a progressive specialization to a 
condition in which practically no sign of right and left lobes remains. 

In the component parts of the telencephalon there is also great diversity. 
The median olfactory nucleus (m.n.) produces very different effects depend- 
ing on the degree of its own development and also on the degree of fusion of 
the right and left telencephalic lobes. In Scymnus these nuclei constitute a 






Fig. 211. Dorsal view of brain. (From Locy.) A. Scoliodon. B. Haja. 

cb., cerebellum ; c.r., restif orm body ; di., diencephalon ; m.n., median olfactory nucleus ; 
med., medulla; np., neuropore ; ol.b., olfactory bulb; oJ.l., olfactory lobe; ol.t., olfactory 
tract; op.L, optic lobe; tJ., telencephalon; /«., terminal nerve; I, olfactory nerve; II, optic 
nerve ; IV, trochlearis or fourth nerve ; X, vagus nerve. 



pair of rounded masses located terminally between the olfactory tracts (m.n., 
fig. 213) and extending ventrally. Much the same condition obtains in Squalus 
sucklii (see p. 183, fig. 172a), but in this form the median nucleus is not pro- 
jected forward into so sharp a mass. In a type like Heterodontus it stands out 
in a particularly striking way. In many otlier forms, on the contrary, the 
median olfactory nucleus is poorly developed. 

The pallial eminences also are very differently developed in different Elas- 
mobranclis. In Scynuiu.^ again they are sharply defined as prominent dorsal 
mounds l)uilt up of masses of cells (p.e., fig. 213a). In other forms these lobes 
are less well developed (Acanthias) ; while in still others they are scarcely 
discernible. 

The diencephalon (di.) in general maj^ be said to be narrow from side to side 
and show no marked swellings. In some forms it is well seen from above, as in 
Heterodontus (fig. 210) and Raja (fig. 211b) . In all such occurrences the brain 
stem is well drawn out in this region. In those Elasmobranchs in which a 



THE ELASMOBRANCH FISHES 



233 



greater compaetness of brain obtains, or in those where the eerebellnm is 
especially well developed, the diencephalon may be more or less completely 
hidden (Scoliodon, fig. 211a ; Mijliohatis, fig. 212). 

Both dorsally and ventrally the diencephalon is characterized by out- 
growths which are of interest. From the roof arises the chimney-like epiphysis 
(pineal stalk) {ep., fig. 213a) which passes upward and forward to the roof of 
the cranium. In general the stalk terminates at 
the roof immediately posterior to the anterior 
fontanelle and is usually spread out terminally 
into the disc-like pineal body. 

In development the pineal region in the early 
embryo of Acanthias (Minot, 1901) shows a 
series of arches in the roof of the brain which 
are separated by a series of projections. The 
long anterior projection (v., fig. 214) is the 
velum, an important landmark separating 
telencephalon from diencephalon. The velum in 
figure 214b separates the paraphysial arch 
(pa.) from the small postvelar arch (p.v.). 
Back of the postvelar arch is a projection in 
which the superior commissure (s.c.) runs. An 
early stage of the epiphysis (cp., fig. 214a) is 
shown behind this projection, and in the projec- 
tion is the posterior commissure (p.c). Figure 
214b is a later stage in the development of the 
pineal stalk in which the surrounding struc- 
tures have reached a definitive form. The pineal 
stalk (ep.) has almost reached the surface; the 
paraphysis (pa.) is enlarged; and the velum 
and commissures are well marked. 

From the floor of the diencephalon an evagi- 
nation, the infundibulum (in., fig. 213b), drops 
downward and backward, and at its sides are 
the inferior lobes (lobi inferiores, i.l., fig. 213a) 
and the vascular sacs (sacci vasculosi, v.s.). The infundibulum meets and 
fuses with the hypophysis, an outgrowth from the buccal cavity, to form the 
pituitary. The hypophysis may present a complex appearance, or it may be 
comparatively simple. In Scymnus, as in Hepfanchua, it is composed of three 
well defined divisions, an anterior terminal, a median, and a paired posterior 
division, the posterior division being considerably removed from the body of 
the infundibulum and connected with it only by a narrow strand. 

Figure 215 is a detailed drawing in side view of the pituitary and surround- 
ing structures, in the adult of Squalus sucklii. In this area, in addition to the 
inferior lobes (i.l.b.) and vascular sacs (v.s.) of the brain, there are three 
unpaired parts of the hypophysis lying in the midventral line. These are the 




Fig. 212. Dorsal view of brain, 
Myliohatis calif ornicus. (For ex- 
planation see fig. 211.) 



234 



THE ELASMOBRANCH FISHES 



anterior (a.l.), intermediate (i.l.), and inferior lobes (i.l.h.). To these parts 
are to be added the paired suj^erior lobes (s.l.) which lie at the sides of and 
above the intermediate lobes. 

The mesencephalon (fig. 213, op.l.) is a conservative segment and yet it 
varies considerably in different forms. In many it is relatively inconspicuous 
because of the extreme development of the cerebellum, while in others it comes 




Fig. 213. Brain of Scymnus. (From Burckhardt.) A. Side view. B. Median sagittal view. 
ch., cerebellum; c.r., restiform body; (q)., pineal stalk; ffin., median longitudinal bundles; 
in., infundilnilum ; i.L, anterior lolie ; Lv., lobe of the vagus ; m-.n., median olfactory nucleus ; 
op.l., optie lobe ; p.e., pallial eminence ; p.L, posterior or inferior lobe of hypophysis ; v.s., 
vascular sacs ; //, optic nerve. 

to be unusually large. In all forms the roof of the mesencephalon is composed 
of a right and a left optic lobe {op.l., figs. 210 and 213) which are hollow out- 
pockets from the dorsal side of the mesencephalic segment. It is largely among 
the cells of these lobes that the fibers of the optic nerve terminate. The ventro- 
lateral part of this segment of the ])rain (see fig. 213b) is enlarged by longi- 
tudinal swellings, the lateral fiber tracts of the mesencephalon. Through the 
ventral walls the third cranial or oculomotor nerve ])asses, and from the roof 
the fourth nerve leaves the brain stem (IT, figs. 210 and 212). 

The metencephalon as a segment is usually well developed in the Elasmo- 
branchs. Dorsally it consists of the cerebellum (ch.) and ventrally it is swollen 
by large fiber tracts {Scy)iinus, fig. 213b). The cerebellum is usually rhomboid 
in shape and divided dorsally by a median longitudinal furrow into right and 
left halves (fig. 210a). These may be further separated into anterior and 
posterior parts l)y a second furrow at right angles to the first. In some forms, 
as has been said, the cerebellum comes to be very complex and of inniiense size. 



THE ELASMOBRANCII FISHES 



235 



It is so in LainiKi, Scoliodon (fig. 211a), Galeus, Tvijcjon, and Myliobaiis (fig. 
212). In all these the surface is thrown into numerous irregular folds or 
convolntions. 

The myelenc'ei)halon (medulla) {nied., figs. 211-21:]) when seen in dorsal 
view is shaped like the letter Y, the anterior limits of the Y being made by the 
restiform bodies, corpora restiforme {cr.). These in many of the simpler types 
of sharks, such for example as Scymnus, appear as prominent structures. In 
others the corpora restiforme are entirely hidden by the enlarged cerebellum 
(Myliohafis, fig. 212) . Both in dorsal and in ventral view the medulla is conical 





Fig. 214. Stages A and B in the development of the pineal region of Acanthios. (From 
Minot.) 

ep., epiphysis; pa., paraphysis; p.c, posterior commissure; p.v., postvelar arch; s.c, 
superior commissure; v., velum. 

in shape and tapers gradually l)ack to the spinal cord. It is from this segment 
that most of the cranial nerves arise; all in fact except the first four take 
their origin here. 

INTEENAL VIEW OF BRAIN 



A sagittal section through the brain ol Scymnus (fig. 213b) shows within the 
medulla the large cavity of the fourth ventricle. The floor and sides compose 
the fossa rhomboidealis. Along each side of the middle line of this fossa are 
two median longitudinal bundles, fasiculi longitudinales mediales (flm.). 
Running parallel to these in the posterior part of the fossa are the ventro- 
lateral bundles, fasiculi lateroventrales. Above these bundles and in the side 
wall are the lobes of the vagus (l.v.) which vary in numl)er of segments from 
types in which only a few are present to those in which there are several 
nodules {Heptanchns and Hexanchus). Above the lobes is a dorsolateral 
bundle which continues forward as the bundle to the tuberculum acusticum. 
Above this are other bundles which continue into the ridge of the restiform 
bodies or corpora restiforme. 

In the region of the mesencephalon the cavity is so large that it is an aque- 
duct of Sylvius only in name. It connects the cavity of the metencephalon and 



236 



THE ELASMOBRANCH FISHES 



myelencephalon, the fourth ventricle above mentioned, with the cavity of 
the diencephalon, the third ventricle. In the wall of the diencephalon is the 
optic thalamns, and, in its roof, the habennlar ganglion from which the pineal 




Fig. 215. The pituitary, Squohis siiclUi, side view. (Marie Carlson, orig.) 

a.L, anterior lobe; i.l., intermediate lobe; i.l.l)., inferior lobe of brain; i.l.h., inferior lobe 
of pituitary; inf., infundibulum ; s.L, superior lobe; v.s., vascular sac of brain; //, III, 
second and third nerves. 

stalk arises. In the floor of the diencephalon is the infundibulum (in.). The 
cavities in the right and left lobes of the telencephalon are the lateral 
ventricles. 

Figure 216 (Houser, 1901) is of a transverse section through the medulla of 
Mustelus to show something of its finer structure. In its median ventral mass 
lies the abducens nucleus, the fibers of which form the abducens or sixth 
cranial nerve. Around the nucleus of the sixth nerve the tract cells are scat- 
tered. These cells are of interest in that they are exceeded in size only by the 
cells in the roof nucleus of the mesencephalon (see fig. 206) . The lobes of the 
vagus (l.v.) are made up of masses of cells which receive visceral sensory 
fibers from the seventh, ninth, and tenth cranial nerves. Some of the axones 

from the cells of these lobes pass down- 
ward a short distance to the viscero- 
motor nucleus (vm.n.). The cells in this 
large nucleus give rise to the motor 
fibers of the fifth, seventh, ninth, and 
tenth nerves. 

Houser states that the general cuta- 
neous nucleus (g.c.n.) is the terminus 
for the somatic sensory fibers of the 
fifth, ninth, and tenth nerves, but ac- 
cording to Norris and Hughes (1920) 
the ninth and tenth nerves contain no 
somatic sensory elements. 

The tuberculum acusticum {t.a., fig. 

Fig. 216. Transverse section through the 216) forms a swelling on the lateral 

medulla, MusteJ^. (From Houser.) ^^^^ ^^ ^j^^ ^^^^^^j^ ventricle (r.^) and 

flm., median longitudinal bundles; f.r., 

formatio-reticularis ; l.v., lobes of vagus; is principally a center for fibers from 

g.c.n., general cutaneous nucleus; t.a., ^j^^ j^^g^,^^ fj^^g ^^ ^^^^ f^,^^^ ^^le 

tuberculum acusticum; rm.?i., visceromotor _ 

nucleus ; v.\ fourth ventricle. internal ear. 




THE ELASMOBRANCII FISHES 



237 



SPINAL COED 



The spinal cord extends from the })rain practically to the tip of the tail. Unlike 
that in the higher forms it does not possess marked pectoral and pelvic swell- 
ings produced by the i)assage of the nerve bundles to the limbs. In its external 
aspect and in its internal structure the cord may be spoken of as a simplifi- 
cation of the medulla. The place occupied in the medulla by the nucleus of the 





A B 

Fig. 217. Transverse sections of the spinal cord. (From Sterzi.) A. Acanthias vulgaris. B. 
Baja clavata. 

ca., calcification; d.li., dorsal horn; d.r., dorsal root; er., endorachis; nc, neurocoele; pm., 
paracentral mass; ps., perimeningeal space; v.h., ventral horn. 

sixth nerve and the grey matter of the formatio-reticularis {f.r., fig. 216) is 
occupied in the cord by the ventral horn {v.h., fig. 217) ; while the general 
cutaneous nucleus of the medulla (g.c.n.) gives place to the dorsal horn of the 
cord (d.h., fig. 217) ; and the lobes of the vagus and the visceromotor nuclei 
are supplanted by the paracentral mass (pm.). Furthermore, the enlarged 
fourth ventricle of the medulla becomes the small neurocoele of the cord (tic.) . 

A transverse section shows the spinal cord lying within the neural canal 
from which it is separated by a considerable perimeningeal space, especially 
in the rays (fig. 217b). The endorachis (er.) lines the neural canal and is 
further surrounded by calcification (ca.). Directly surrounding the cord is 
the meningeal lining, through which ventrally the spinal blood-vessels pass. 
The cord itself (Acanthias, fig. 217a) is much like that of Heptanchus (fig. 
201) in that the dorsal horns (d.h.) are close together and the ventral horns 
(v.h.) are almost at right angles to the central mass. Above the ventral horns 
are the outgrowing paracentral masses (pm.) . In the rays, however, the cen- 
tral grey matter of the cord is more diffuse. 

The relation of the grey and white matter, cells and fibers, in the Elas- 
mobranchs is essentially like that in higher forms, although the proportion 



238 



THE ELASMOBRANCH FISHES 



of one to the other is greatlj^ altered. Burckhardt (1911) has estimated that 
in Scymnus the grey to the white is as 1 to 17, wliile in man it is as 5 to 12. 

Peripheral Nervous System 




The i)eripheral nervous system in general, like that in Heptanclins, is made up 
of cranial and spinal elements. Both the cranial and the spinal nerves origi- 
nate from cells in or derived from the central nervous system. AVe may briefly 
review their development in a type like Acanthias. 

At the time when the neural tube closes, the cells which make up its walls 
form a single layer (fig. 218) . These cells later become pear-shaped and collect 

in groups in the region of the ventral horns. 
In the adult the motor fibers extend from 
these cells. 

The cpiestion has often arisen as to the 
formation of the nerve fiber or axone. Is it 
formed in place from preexisting proto- 
plasmic strands, or is the axone an out- 
growth from the cell body ? An examination 
of a section by Neal (1914) taken from the 
posterior region of the cord (fig. 218) shows 
that the fiber or axone (ax.) here has just 
reached the myotome (m.) and that its end, 
which is the actively growing part of the 
fiber, is rhizopod in appearance. As the 
muscle bud grows outward toward the 
fin, the fiber l)ecomes attached to it and is 
drawn out with it, forming a so-called end 
plate. Such a growing fiber is protected by 
a thin covering, the neurilemma, and sometimes a medullary sheath is formed 
between the neurilemma and the axis cylinder of the fiber. The axis cylinder 
itself becomes differentiated into numerous fibrillae. 

In the sensory (dorsal) nerves an overgrowth, the neural crest, is pro- 
duced on each side along the neural tube at the place of closure. From each 
crest, dorsal root ganglia result {g., fig. 207) which migrate slightly farther 
down the sides of the tubes. Composing these ganglia are multitudes of bipolar 
cells, the fibers from which pass from both ends or poles of the cell. Several of 
these cells (fig. 207, a to <:•) are here seen in different stages of development; 
(c) represents the mature cell in which the two poles have fused at the base 
into the stem of a T. One of the fibers thus formed by the arms of the T enters 
the central system wliile the other extends outward and receives sensation. 



Fig. 218. Transverse section througli 
developing cord, notochord (chd.), 
and myotome (i5(. ), Squal2is acan- 
thias. (From Neal.) 

ax., motor axone growing out to 
myotome (m.) from cell in the neu- 
ral tube. 



CEANIAL NEEVES 



The first cranial or olfactory nerve extends from the epithelium of the olfac- 
tory capsule as a double nerve backward to the olfactory bulb. In extent it 



THE ELASMOBRANCH FISHES 



239 




varies greatly. In some t'orins the nerve is so sliort as hardly to be observed 
from the outside. This is observable in types like Heptanchus (fig. 200a), 
Galeus. and Scifiiiniis. In Seoliodon (fig. 211a) the olfactory bulbs are farther 
sei)arated from the nasal ei)ithelium so that the two divisions of the nerve (I) 
are distinct. In Echinoyhinus the nerves are of an extreme length, rarely met 
with in any other Elasmobranch. 

A sagittal section through the anterior part of the olfactory bulb would 
show how the fibers come to masses of cells (glomeruli) in the bulb from the 
olfactory membrane. These are arranged in two 
bundles. One of these is median and ventral in posi- 
tion; the other is lateral. The fibers from the cells in 
the glomeruli of the olfactory bulb are continued to 
the olfactory lobe of the brain by the olfactory tracts 
(ol.t.). These tracts similarly vary in extent through 
extremes shown in Eckinorhinus and Hexafichus. In 
the former the bulb and the lobe are practically con- 
tinuous; in the latter the tracts are greatly drawn out. 

Accompanying the olfactory nerve is the terminal 
nerve {tn., fig. 211) studied in detail by Locy (1905) . 
This nerve in Acanthias as a type {t.n., fig. 172a) 
ends in the thickened lamina terminalis. The right 
and left nerves at the entrance of the brain would 
thus be separated by the median fissure. Through 
growth of the brain and fusion of the median olfac- 
tory nuclei it so happens that the terminal nerve 
instead of passing directly forward may arise dor- 
sail}' or ventrally. Those forms in which the nerve 
has a connection with the brain dorsal to the neuro- 
pore are : Raja, Trijgon, M[jliobatis. In others the 
connection is more nearly dorsal than ventral {Acan- 
thias, Squatina, Hexanchiis) . In still others the con- 
nection is ventral, as in Galeus, Scoliodon (fig. 211a) , 

Scyllium, Pristiurus, and Carcharias littoralls. In all the al)ove forms the 
nerve passes forward as a double or triple strand on which is a ganglion (or 
a series of ganglia). The nerves join the olfactory fila as in Heptanchus and 
go to the mucous lining of the olfactory cup. 

The optic or second nerve (77, figs. 210 and 213a) is sufficiently similar in 
the different forms to need but little description. It arises from cells in the 
retina and its fibers cross to form the optic chiasma. They then extend to, and 
terminate in, the diencephalon and mesencephalon. 

The oculomotor nerve (777, figs. 200a and 200b) arises in a nucleus under 
the aqueduct of Sylvius and leaves the brain stem near the middle line of the 
mesencephalon. It enters tlie orbit and gives otf a dorsal branch to the superior 
rectus and the anterior rectus and a ventral branch which divides into an an- 



-md.V. 



Fig. 219. Ganglia of tri- 
geminal (F) and facial 
( VII) nerve, median view, 
Chlamydosclachus. ( From 
Mrs. Hawkes.) 

hu.VII, buccal division 
of seventh nerve; g., gas- 
serian ganglion; md.F, 
mandiljular nerve; tiix.V, 
maxilliary l)rancli of fifth 
nerve; op.V, ophthalmicus 
profundus nerve; os.V, 
oi)hthalmieus superficialis 
of fifth; os.ril, ophthal- 
micus superficialis of 
seventh. 



240 



THE ELASMOBRANCH FISHES 



terior and a posterior division. These divisions supply the inferior oblique and 
the inferior rectus muscles as in Heptanchns. 

The relation of the nerve to the ciliary ganglion in some forms is interesting. 
Sometimes this ganglion (or ganglia) is on the oculomotor nerve and is called 
the ganglion oculomotoris. Or, again, it may be on a branch joining the gan- 
glion of the ophthalmicus profundus with the oculomotor nerve. 

The trochlearis nerve, as we have said, has its nucleus in the mesencephalon. 
Its fibers cross and pass out through the roof between the mesencephalon and 
the cerebellum and go to innervate the superior oblique muscle of the eye. 

The trigeminal or fifth cranial nerve is composed of an anterior portio 
minor and a posterior portio major. "While both of these roots contain motor 




Fig. 220. Cranial nerves, Squalus acanthias. (From Norris and Hughes.) 
br.p., brachial plexus; bu.VII, buccalis of seventh; d.X, ramus dorsalis of tenth nerve; 
gri., first spinal ganglion; hb., hypobranchial bundle; limd., hyomandibularis ; U.X, lateral 
line nerve ; md.e.VII, mandibularis externus of seventh ; md.i.VII, maudibularis internus of 
seventh; md.V, mandibularis of fifth; mx.V, maxillaris of fifth; op.V, ophthalmicus pro- 
fundus; os.V, and os.FII, ophthalmicus superficialis of fifth and seventh; ph. IX, pharyn- 
geal branch of ninth; pl.VII, palatinus of seventh; po.t., post-trematicus of ninth; pr.t., 
pretrematicus of ninth; sp., spiracle; st.IX, supratemporalis of ninth; st.X, supratem- 
poralis of tenth nerve ; vi.X, visceral nerve ; 1/ and 2, occipitospinal nerves ; II, optic ; ///, 
oculomotor; IV, trochlearis; VI, abducens; VIII, auditory nerve. 

and sensory portions, motor fibers predominate in the anterior part and 
sensory in the posterior root. The motor fibers arise from the visceromotor 
nucleus in the medulla (vm.n., fig. 216) and are distributed principally to the 
muscles of the jaws. The sensory fibers arise from various ganglia, such as the 
ganglion of the ophthalmicus profundus, the ophthalmicus superficialis (Mus- 
telus calif ornicus), and the gasserian ganglion. Sensation brought from the 
region around the nose passes by the ganglion cells and on to the brain. 

In the Elasmobranchs the ganglia of the trigeminal and the buccal division 
of the facial nerves are so intimately associated as often to be inseparable; 
usually the former are more or less covered up by the latter. In Chlamydosel- 
aclius, however (fig. 219) , the two are distinct medially (Hawkes, 1906) . From 
the gasserian ganglion {g.) the large maxillaris and the mandibularis divisions 
of the fifth nerve arise and from the small ganglion on the inner side of the 
gasserian, the two smaller nerves, the ramus profundus {op.Y) and the ramus 
superficialis {os.Y) of the fifth, take their origin. 



THE ELASMOBRANCH FISHES 241 

The oplithahiiicus profundus (op.V, fig. 220), after leaving its ganglion 
passes forward deep in the orbit, between superior and inferior rectus muscles 
and out anteriorly under the anterior rectus and between the obliques. On its 
M^ay past the eyeball it gives off the ciliary nerves to the eye and then passes 
to the dorsal and lateral parts of the snout (Squaliis), or also to the ventral 
part (Mustelus). 

The ophthalmicus superficialis of the fifth (os.V, fig. 220) may have an ex- 
tracranial ganglion as in Mustelus calif ornicus or it may be connected directly 
with the gasserian ganglion as in Sqnalns acanthias. In either form the nerve 
is closely associated with the superficial ophthalmic of the seventh on its way 
through the orbit. In Squalus acanthias according to Norris and Hughes 
(1920) there are three or four branches of this nerve which apparently ter- 
minate in the skin over the supraorbital crest. In Mustelus calif ornicus the 
nerve is larger and continues farther anteriorly. 

Both the ophthalmicus profundus and the ophthalmicus superficialis of the 
fifth nerve are sensory. 

The maxillaris of the fifth {nix.Y, fig. 220) originates in the gasserian gan- 
glion and passes to the jaw and on to the skin ventral to the snout. In its course 
over the floor of the orbit it is accompanied by the buccalis of the seventh. 
Near its ganglion it is often mediad of the buccalis but farther distally it may 
lie dorsal to it. As a usual thing the maxillaris branches into two or three main 
divisions, the most anterior of which passes forward almost to the tip of the 
snout. 

The mandibularis may accompany the maxillaris over the floor of the orbit 
and separate from it late to turn sharply back around the angle of the jaw as 
in Laeniargus. Or, if the angle of the jaw is relatively far posterior, the 
mandibularis may leave the maxillaris early and pass over the posterior wall 
of the orbit to reach the angle of the mouth (Chlamydoselachus, Acanthias) . 
In either form certain sensory bundles from the intracranial part of the gas- 
serian ganglion, after passing the angle of the jaw, turn forward and supply 
the skin of the lower jaw to the symphysis of the mandible. Motor fibers, dorsal 
to the sensory fibers, supi^ly the levator maxillae, the first dorsal constrictor, 
the adductor mandibulae, and a considerable part of the first ventral con- 
strictor muscle (Norris and Hughes). 

The abducens or sixth cranial nerve is a motor nerve. It arises, as w^e have 
seen, from a nucleus in the medulla and passes forward and outward to the 
posterior rectus muscle, entering it at its base. 

The seventh or facial nerve like the fifth is composed both of motor and of 
sensory components. Its motor fibers arise in the visceromotor nucleus (vm.n., 
fig. 216) just posterior to that of the fifth nerve and extend to the lower jaw 
and the facial region. Its sensory fibers are connected with large ganglia and 
are distributed essentially as in Heptanchus. In Acanthias, the nerve may join 
the brain in common with the branches of the eighth, and it is usually united 
above with the fifth. In Mustelus canis, however, it may be more or less clearly 
separate from the fifth (fig. 221). 



242 



THE ELASMOBRANCH FISHES 




For convenience of description we may consider the facial nerve as made up 
of two groups of fibers. One of these supplies the sensory canal system; the 
other belongs to the facial i)roper. Three great nerves are in the service of the 
sensory canal system. These are : the ophthalmicus superficialis {os.YII, figs. 
220 and 221) which goes to the supraorbital canal and associated ampullae of 
Lorenzini; the buccalis, to the infraorbital canal and associated ampullae of 
Lorenzini; and the external mandibular, a branch of the hyomandibular 
nerve. This supplies the hyomandibular and the mandibular canals (figs. 220 

and 245), and ampullae 
of Lorenzini. 

The ophthalmicus su- 
perficialis of the facial 
{os.YII) is a somewhat 
large nerve which arises 
from a large ganglion 
(figs. 220 and 221) and 
runs forward into the or- 
bit. Generally it enters 
the orbit through the or- 
bital fissure in common 
with the trigeminal, but 
in certain types (Mus- 
i elus henlei) it may enter 
through its o^\^i foramen 
above and in front of 
the orbital fissure. As it 
passes through the orbit 
superficially, it gives off 
numerous branches dorsally to the supraorbital canal and associated ampul- 
lae and leaves the orbit by a large anterodorsal foramen. It extends forward 
giving off a large branch which passes downward in front of the eye to supply 
the lower part of the supraorbital canal and numerous small branches which 
pass toward the tip of nose. 

The buccalis may arise from a large ganglion which is continuous with that 
of the ophthalmicus superficialis VII {Acanthias, fig. 220, hu.VII). In Mus- 
telus calif ornic us these two ganglia are distinct but the ganglion of the buc- 
calis is in close relation to the ganglion of the external mandibular nerve. The 
buccalis, as was previously mentioned, runs across the floor of the orbit closely 
associated with the maxillaris of the fifth. In the anteroventral angle of the 
orbit it divides into two or three main divisions, the twigs of which go to sup- 
ply the infraorbital canal and associated ampullae of Lorenzini. From the 
ganglion the buccalis sends off branches which supply the part of the infra- 
orbital canal posterior to the eye (see p. 279, fig. 245) , and from the dorsalmost 
part of the ganglion it gives the important ramus oticus VII to a short seg- 
ment of the most anterior part of the lateral line canal. 



Fig. 221. Eoots of fifth and seventh nerves, Mustelus canis. 
(From Green.) 

hu.VII, buccalis of seventh; ct., chorda tympani; g., 
geniculate ganglion; 7(7nd., hyomandibular nerve; mx.md.V . 
maxillaris and mandibularis of fifth ; F777, auditory nerve ; 
op.V, ophthalmicus profundus; os.V, ophthalmicus super- 
ficialis of fifth; os.V II, ophthalmicus superficialis of the 
seventh; pl.VII, palatinus of seventh; prt., pretrematicus ; 
sp., spiracle. 



THE ELASMOBRANCn FISHES 243 

The external iiiaiulil)ularis is in relation to a ganglion by the same name. It 
leaves the cranium through the facial foramen (see fig. 47, f.VIP, facing 
p. 44), and passes upward around the spiracle as a part of the ramus hyo- 
mandibularis. In Squalns acanthias the external mandibularis (md.e.VII, fig. 
220) divides into an anterior and a posterior division. The anterior division 
goes to the hyomandibular and mandibular canals. The posterior division 
supplies the hyoidean ampullae and pit organs. 

The first divisions of the facial which are not connected with lateral line or 
ampullary organs are certain branches of the hyomandibularis. 

The hj'omaiidibular trunk after leaving the brain stem passes backward, 
the main stem arching posteriorly around the spiracle as was described above 
for its external branch. Before reaching the spiracle, however, the palatine 
nerve separates from the hyomandibular trunk. It arises in the geniculate 
ganglion (g., fig. 221) and passes as a sensory nerve to the roof of the mouth 
where, as in Heptanchus, some of its fibers pass forward and others extend 
over the posterior part of the roof to supply "taste buds." After having given 
off the palatine, the hyomandibularis supplies a small pretrematicus to the 
anterior side of the spiracle. 

At about the place where the external mandibular VII, previously de- 
scribed, is given off from the ramus hyomandibularis the internal mandibular 
(md.i.VII) separates from the main trunk. This, like the palatine nerve, is 
sensory and goes to the mucous membrane along the inner side of the man- 
dibular arch. 

The ramus hyoidius VII (fig. 220) is a motor branch which in Squalus acan- 
thias divides into an anterior and a posterior division. The anterior branch 
supplies the second, and the posterior, a part of the first ventral constrictor 
muscles. 

The chorda tympani is variously interpreted for the Elasmobranchs. In 
Acanihias {ct., fig. 221) it is apparently a continuation from the palatine 
branch (pl.VII) and is therefore in front of the spiracle. In Hexanchus it has 
been considered by Ruge (1897) to be a direct continuation of the hyoman- 
dibularis and as such is therefore a post-trematicus. In both it is a sensory 
nerve and supplies the mucous lining in the floor of the mouth just mediad 
of the teeth. 

The hyomandibularis, in Torpedo, gives rise to a first electric nerve which 
goes to the anterior and inner part of the electric organ. 

The auditory nerve {VIII, fig. 220) has a large ganglion just back of the 
ganglia of the seventh nerve. From this fibers extend to the ear in two general 
groups, an anterior vestibular and a posterior saccular group. Root fibers join 
the ganglion to the medulla just back of the seventh nerve and terminate 
around cells in the tuberculum acusticum of the medulla. The distribution of 
the fibers to the ear will be discussed further in a study of the ear. 

The glossopharyngeal, as described for Heptanchus, may be taken as a typ- 
ical "mixed" nerve. It is made up of both motor and sensory fibers, the former 
arising in its visceromotor nucleus of the medulla and the latter springing 



244 



THE ELASMOBRANCH FISHES 



from a ganglion located on the nerve in or just outside the glossopharyngeal 
canal (see fig. 220). The main divisions of the nerve are a branch {st.IX, fig. 
220) to the anterior segment of the lateral line canal, and nerves in relation 
to the first and second demibranchs. 

The supratemporalis (st.IX) in Squalus acanthias, according to Norris and 
Hughes, supplies three neuromasts of the lateral line canal located between 
the neuromasts supplied by the supratemporalis of the vagus and the ramus 
oticus VII, but it is not provided with other branches to pit organs. In Chlamy- 
doselachus, Ilawkes (1906) mentioned, in addition to the twigs to the neuro- 




A B 

Fig. 222. Brachial and cervical plexuses. A. Scyllium. B. Squatina. (From Max Fiirbringer.) 
hr.p., brachial plexus; cr.p., cervical plexus; d.r., dorsal root; 0., occipitospinal nerve. 



masts of the lateral line canal, other twigs to the skin. These she suggested 
were cutaneous. They possibly supply pit organs. In Raja radiata according to 
Norris and Hughes there are no lateral line elements in the ninth nerve. 

Above the first branchial pocket, as in Heptanchus, the main part of the 
ninth nerve, as the first branchial nerve, separates into three divisions, a pre- 
trematicus {pr.t.), a post-trematicus (po.t.), and from the pretrematicus a 
pharyngeal division (ph.IX), the last-named branch being sensory and com- 
parable to the palatine division of the facial nerve. The pretrematicus is also 
sensory; it extends do^\^l the hyoidean demibranch back of the branchial carti- 
lages, supplying the mucous membrane and gill filaments. Unlike Heptanchus 
the majority of Elasmobranchs seem to lack the internal pretrematicus. The 
post-trematicus or larger division, unlike that of Heptanchus, is not always so 
clearl}^ separated into its components. The post-trematicus supplies the mu- 
cous lining along the anterior part of the first holobranch and other sensory 
fibers continue forward under the gill cleft and are distributed to the mucous 
membrane of the pharynx and buccal cavity. The motor branches supply 
muscles associated with the first holobranch. 

Accompanying the glossopharyngeal and a part of it, in Torpedo, is the 
second electric nerve. 

The vagus is the most widely distributed of any of the nerves. It is much 
like the ninth in its supply of the gills, but in addition it gives off a dis- 
tinct lateral line nerve, and its branchial divisions are bound with intestinal 



THE ELASMOBRANCH FISHES 



245 



branches into a brancliio-intestinal bnndlo. The lateral line nerve can be traced 
from the brain as a distinct stem. As it emerges from the cranium it swells out 
into a large ganglion from which this branch proceeds posteriorly practically 
to the end of the tail. From the lateral line ganglion there is given off a dorsal 
root, the supratemporalis (fig. 245) which })asses upward and forward to 
supply the suprateniporal canal, pit organs in the region of the supratemporal 
canal, and the anterior part of the lateral line canal immediately behind the 
segment supplied by the supratemporal branch of the ninth nerve. Posterior 
to this branch there is given off a second branch, the dorsalis (d.X, fig. 220) 




Fig. 223. Brachial and pelvic plexuses, Eaja vomer. (From Braus.) 



which runs posteriorly almost to the first dorsal fin. This nerve supplies sense 
organs of the anterior part of the lateral canal and also pit organs above the 
line and anterior to the dorsal fin. Many branches are given off from the lateral 
line nerve as it passes backward in the body to the sense organs along the 
lateral line canal. 

The branchial stem in the vagus is a strong bundle which divides into four 
(pentanchids), five (hexanchids), or six (heptanchids) branchial nerves. The 
first two or three of these branchial nerves, except in Torpedo, where the first 
and second branchials give off the third and fourth electric nerves, are essen- 
tially like the branchial division of the glossopharyngeal. Each branchial con- 
sists of pharyngeal and pre- and post-trematic nerves. The post-trematicus of 
the vagus is composed of two strong branches, one of which is motor, the 
other sensory. The motor division innervates the interarcual, interbranchial, 
adductor, and ventral constrictor muscles. The last branchial nerve is com- 
posed only of visceral sensory fibers (Norris and Hughes, 1920) . 

The ramus intestinalis or visceralis (vi.X) proceeds posteriorly after sepa- 
rating from the branchial stem. Its motor fibers are distributed to the 
trapezius muscle and to the digestive tract. Its sensory fibers go largely to the 
digestive tract. 

The occipitospinales (y, z, fig. 220), back of the vagus, like spinal nerves 
consist of dorsal and ventral roots, but dorsal roots to the ones most anterior 
are frequently absent. In lowly forms, as we have seen in Heptanclius, several 
pairs may be present. As many as five of the ventral occipitospinales have been 



246 



THE ELASMOBRANCH FISHES 



located on each side of the yoimg of Hexanchus and Chlamydoselachus. In 
Acanthias there are only two or three of the ventral nerves present and two 
dorsal rami. These nerves united with the first and second spinals are marked 
hb. in figure 220. In Torpedo a single ventral occipitospinal nerve as such is 
present. 

The more posterior of these nerves unite with the first group of spinal 
nerves to form the cervical plexus which in turn joins the pectoral plexus, the 
fused trunks of which may run for a short distance, with, although it is no 
part of, branches of the vagus. The whole group forming the two plexuses may 




Fig. 224. Nervous collector, Chlamydoselachus. (From Brans.) 

l.a.v., lateral abdominal vein; pl.p., pelvic plexus; sp.^^^^, twenty-fifth and thirty-eighth 
spinal nerves. 

be composed of relatively few nerves (five in Spinax) or it may include many 
(twenty in Torpedo). The nerves of the cervical plexus (cr.p., fig. 222) sepa- 
rate from the pectoral plexus and pass in front of the girdle to supply the 
hypobranchial muscle as in Scyllium and Squatina, while those of the pec- 
toral plexus (ir.p.) pass through the girdle and are distributed to the pec- 
toral fin. 

SPINAL NERVES 



In the region of the cord proper a sensory root and motor root (solid line, fig. 
220) unite to form a mixed spinal nerve, much as in Hepianclius. Each of 
these roots after leaving tlie cord passes backward within the neural canal, 
then perforates the basal or dorsal intercalary cartilages as single roots. 
Shortly before perforating the basal plate the motor branch may bifurcate 
(fig. 220) and send a branch dorsally to join a branch from the ganglion {gn.) 
of the dorsal root; or this motor root may pass by the ganglion without receiv- 
ing from it sensory fibers. This branch passes dorsally to the dorsal longi- 
tudinal bundles. When no sensory fibers join this root a sensory l)undle runs 
dorsally from the ganglion. The other branch of the l)ifurcated ventral root 
joins a sensory root from the ganglion, the two united passing as a mixed 
nerve ventralh'. 

Posterior to the pectoral fin and in the region of the lateral al)dominal vein 
the ventral rami of the various spinal nerves often form a connected strand, 



TPIE ELASMOBKANCH FISHES 



247 



the nervous collector (fig. 224) as in Heptanchus. This has been studied in a 
number of forms by Braus (1898). Extremes of variation obtain in the num- 
ber of nerves taking part in this collector. In primitive forms, as in Hep- 
tanchus, the number may vary greatly and the collector may consist of multi- 
tudes of strands wliich may or may not fuse together. A good example is 
Chlamydoselachus (fig. 224), in which the twenty-fifth to the thirty-eighth 
nerves take part. In other forms few nerves take part in its formation 
(Spinax) or no collector as such is found (Squatixa, Eaja, fig. 223) . 




Fig. 225. Sympathetie nervous system, Scyllium canicula. (From Botazzi.) 
d.a., dorsal aorta; s.f., sympathetic fibers to stomach (st.) ; s.g., sympathetic ganglia. 

The collector has been studied in great detail as to the relation wliich it 
bears to the origin of paired fins. By those who hold to the gill-arch theory 
of origin of the fins the collector is an argument for the posterior migration of 
the pelvic fins, while those who accept the lateral fin-fold theory believe that 
the collector shows principally that the paired fins formerly had a wider ex- 
tent than they have at the present time. A greater extent is indicated further 
by the fact that there may be a plexus posterior to the pelvic fin (fig. 224) . 

In Chlamydoselachus the posterior collector (fig. 224) comprises a num- 
ber of segments. In the embryo of Acanthias a posterior collector is present, 
although it is absent in the adult. The pelvic plexus in Eaja vomer is shown in 
figure 223. 

SYMPATHETIC OR AUTONOMIC NERVOUS SYSTEM 



Although the sympathetic nervous system has been studied by many workers 
but little is known concerning its form in the different Elasmobranchs. It ex- 
tends from the region of the head to the posterior part of the kidney or meso- 
nephros. 

The ciliary ganglion in the region of the oculomotor nerve represents the 
sympathetic system in the head. In various Selachians one or more small gan- 
glia are related to the third nerve. These in Acanthias are located near the 
branching of the oculomotor nerve into its dorsal and ventral rami. The gan- 
glion (or ganglia) gives rise to non-meduUated fibers which make up the short 



248 THE ELASMOBRANCH FISHES 

ciliary nerve. Norris and Hughes say that in Squalus acanfhias these ganglia 
are connected by fibers with the oenlomotoris, the ophthalmicus profundus V, 
and the palatinus VII nerves. 

In sharks the sympathetic system in the trunk is divided into two parts. The 
anterior part only is associated with the suprarenal bodies. The first trunk 
ganglion (s.g., fig. 225) of this system is the result of a fusion of several 
ganglia and farther it is fused more or less intimately with the first supra- 
renal body. It receives many fibers from the anterior spinal nerves and gives 
ofif splanchnic fibers to the viscera. The ganglia in the posterior region are 
small and are separated from the suprarenal (interrenal) bodies. Further- 
more, these ganglia of the sympathetic system seldom have connecting strands 
putting the various ganglia of a side in longitudinal communication, as occurs 
in the higher animals. The posterior ganglia are never thus connected into a 
longitudinal chain. From the posterior ganglia fine branches go to the kidneys 
and the genital ducts as well as to the posterior viscera. 



THE ELASMOBRANCH FISHES 249 

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250 THE ELASMOBRANCH FISHES 

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THE ELASMOBRANCII FISHES 251 

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252 THE ELASMOBRANCH FISHES 

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THE ELASMOBRANCH FISHES 253 

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254 THE ELASMOBRANCH FISHES 

1904. BoRCHERT, Max, Ueber Marksclieidenf ahrbung bei niederen Wirbelthieren. Arch, f . 
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1901. BoTTAZZi, p., Ueber die Innervation des Herzens von Scyllium canicula uud Maja 
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1892. Braus, H., Ueber die Rami ventrales der vordereu Spinalnerven einiger Selachier. 
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1898. Braus, H., Ueber die Innervation der paarigen Extremitaten bei Selachiern, Holo- 
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1899. Braus, H., Beitrage zur Entwicklung der Muskulatur und des Peripheren Nerven- 
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1892. DoHRN, A., Die Schwann'schen Kerne der Selachierembryonen. Anat. Anz., Bd. 7, 
pp. 348-351. 

1901. DoHRN", A., Studien zur Urgeschichte des Wirbelthierkorpers. X"V^I-XXI. Ibid., Bd. 
15, pp. 1-279, Taf. 1-15. 

1902. DoHRN, A., Studien zur Urgeschichte des Wirbelthierkorpers. XXII. Weitere Bei- 
trage zur Beurtheilung der Occipitalregion und der Ganglienleiste der Selachier. Ibid., 
Bd. 15, pp. 555-654, Taf. 24^30. 

1907. DoHRN, A., Studien zur Urgeschichte des Wirbelthierkorpers. XXV. Der Trochlearis. 
Mitt. Zool. Stat. Neapel, Bd. 18, pp. 143-436, Taf. 10-22. 

1889. EwABT, J. C, On the Cranial Nerves of Elasmobranch Fishes. (Prelim, comm.) Proc. 
Roy. Soe. Lond., Vol. 45, pp. 524^537, 2 text figs. 

1890. EwAiiT, J. C, On the Development of the Ciliary or Motor Oculi Ganglion. Proc. Roy. 
Soc. Lond., Vol. 47, pp. 287-290. 

1890. EwAiiT, J. C, The Cranial Nerves of Torpedo. (Prelim, note.) Proc. Roy. Soc. Lond., 
Vol. 47, pp. 290-291. 

1892-95. EwART, J. C, and Cole, F. J., On the Dorsal Branches of the Cranial and Spinal 
Nerves of Elasmobranchs. Proc. Roy. Soc. Edinburgh, Vol. 20, pp. 475-480, 2 text figs. 

1887. Froriep, a., tJber das Homologon der Chorda tympani bei niederen Wirbcltieren. 
Anat. Anz., Bd. 2, pp. 486-493, 1 text fig. 

1891. Froriep, A., Zur Entwickelungsgescliichte der Kopf nerven. I. Ueber die Entwickelung 
des Trochlearis bei Torpedo. II. Ueber die Kiemenspaltenorgane der Selachierem- 
bryonen. Anat. Anz. (Verb.), Bd. 5, pp. 55-65, 6 text figs. 



THE ELASMOBRANCII FISHES 255 

1901. Froriep, a., Uober die Gaiiglienloiston des Kopfcs und des Rumpfos und ilire Kreu- 
zung in der Oceipitalrcgion. Bcitrag zur Eutwickclungsgescliiclite des Relacliier- 
kopfes. Arch. f. Anat. u. Physiol. (Anat. Abt.), Jahr. 1901, pp. 371-394, pi. 17, 3 
text figs. 

1897. FuRBRiNGER, Max, ITeber die spino-oecii^italeu Norven der Selacliier und Holoce- 
phalou und ihre vergleichende Morphologie. Festschrift z. 70. Geburtstage von C. 
Gegenbaur. Bd. Ill, Leipzig, pp. 349-788, pis. 1-8, 1 text fig. 

1902. FuRBRiNGER, Max, MorpliologiscliG Streitfragen. 1. Nervus trochlearis. Rabl's Me- 
thode und Behandlung der Extremitatenfrage. Morph. Jahrb., Bd. 30, pp. 85-274. 

1909. Gast, R., Die Entwickelungen des Oculomotorins und seiner Ganglieu bei Selachier- 
embryonen. Mitt. Zool. Stat. Neapel, Bd. 19, pp. 269-444, Taf. 12-16. 

1871. Gegei^baur, C, Ueber die Kopfnerven von Hexanchus und ihr Verhaltnis zur "Wir- 
beltheorie" des Schadels. Jena. Zeitsc.hr. Naturwiss., Bd. 6, pp. 497-559, pi. 13. 

1910. Goodrich, E. S., On the Segmental Structure of the Motor Nerve-Plexus. Anat. Anz., 
Bd. 36, pp. 109-112, 1 text fig. 

1900. Greek, H. A., On the Homologies of the Chorda Tympani in Selachians. Jour. Comp. 
Neurol., Vol. 10, pp. 411-421, 3 text figs. 

1907. GuTHKE, Ernst, Embryologische Studien liber die Ganglien und Nerven des Kopfes 
von Torpedo oeellata. Jena. Zeitschr. Naturwiss., Bd. 42, pp. 1-60, Taf. 1-3, 7 
text figs. 

1906. Hawkes, O. A. M., The Cranial and Spinal Nerves of Chlaniydoselachus anguineus 
Garman. Proc. Zool. Soc. Lond., 1906 (2), pp. 959-991, pis. 68-69, 2 text figs. 

1897. HoFFMAKN, C. K., Beitrage zur Entwicklungsgeschichte der Selachii. Morph. Jahrb., 
Bd. 25, pp. 250-304, Taf. 13-14. 

1899. Hoffmann", C. K., Beitrage zur Entwicklungsgeschichte der Selachii. (Fortsetzung.) 
Morph. Jahrb., Bd. 27, pp. 325-414, Taf. 14-18, 5 text figs. 

1901. Hoffmann, C. K., Zur Entwicklungsgeschichte des Sympathicus. (I. Bei den Acan- 
thias vulgaris.) Verb. K. Akad. Wet., Amsterdam, Bd. 7, pp. 1-80, 3 Taf. 

1887. His, W., Die morphologische Betraehtung der Kopfnerven. Arch. Anat. u. Entwick., 
1887, pp. 379-453, 8 text figs. 

1898. Holmgren, E., Kurze vorlaufige Mitteilungen fiber die Spinalganglien der Selachier 
und Teleostier. Anat. Anz., Bd. 15, pp. 117-125, 11 text figs. 

1901. HousER, G. L., The Neurones and Supporting Elements of the Brain of a Selachian. 

Jour. Comp. Neurol., Vol. 2, pp. 65-175, pis. 6-13. 
1905. Johnston, J. B., The Radix Mesencephalica Trigemini. The Ganglion Istlimi. Anat. 

Anz., Bd. 27, pp. 364-379, 8 text figs. 
1914. Kappers, C. V. A., Der Geschmack, i^erifer und central, zugleich eine Skizze der 

phylogenetisclien Veranderungen in den sensibilen VII, IX,und X Wurzeln.Psychiatr. 

en neur. Bloden, Bd. 1 u. 2. 
1905. Klinkhardt, W., Beitrage zur Entwicklungsgeschichte der Kopfganglien und Sinnes- 

linien der Selachier. Jena. Zeitschr. Naturwiss., Bd. 40, pp. 423-486, Taf. 14-16, 6 

text figs. 
1857. Kolliker, A., Ueber die Ausbreitung der Nerven in der Geruchsschleimhaut von 

Plagiostomen. Verb. d. Phys.-nied. Ges. Wiirzburg, Bd. 8, pp. 31-37. 

1911. KuNTZ, A., The Development of the Sympathetic Nervous System in Certain Fishes. 
Jour. Comp. Neurol., Vol. 21. 

1891. KuPFFER, C. VON, Die Entwickelung der Kopfnerven der Vertebraten. Verb. d. Anat. 

Ges. 1891. Trans, by Strong, Jour. Comp. Neurol., Vol. 1, pp. 246-264, 315-332, 

pi. XXII. 
1916. Landacre, F. L., The Cerebral Ganglion and Early Nerves of Squalus acantliias. 

Jour. Comp. Neurol., Vol. 27. 



256 THE ELASMOBRANCH FISHES 

1892. Lenhossek, M. von, Beobachtungen an den Spinalganglien und dem Eiickenmark 
von Pristiurusembryonen. Anat. Anz., Bd. 7, pp. 519-539, 19 text figs. 

1899. LocY, Wm. a., New Facts Eegarding the Development of the Olfactory Nerve. Anat. 
Anz., Bd. 16, pp. 273-290, 14 text figs. 

1905. LocY, Wm. A., On a Newly Recognized Nerve Connected with the Fore-Brain of Se- 
lachians. Anat. Anz., Bd. 26, pp. 33-63, 111-123, 32 text figs. 

1914. McKiBBEN, Paul, Ganglion Cells of the Nervus Terminalis in the Dogfish (Mustelus 
canis). Jour. Comp. Neurol., Vol. 24, pp. 437-439, pis. 1-2. 

1881. Marshall, A. M., On the Head Cavities and Associated Nerves of Elasmobranchs. 
Quart. Jour. Micr. Sci., Vol. 21, pp. 72-97, pis. V-VI. 

1882. Marshall, A. M., The Segmental Value of the Cranial Nerves. Jour. Anat. and 
Physiol., Vol. 16, pp. 305-354, pi. 10. 

1881. Marshall, A. M., and Spencer, W. B., Observations on the Cranial Nerves of Scyl- 

lium. Quart. Jour. Micr. Sci., Vol. 21, pp. 469-499, pi. 27. 
1904. Merritt, O. a.. The Theory of Nerve Components. Jour. Anat. and Physiol., Vol. 39, 

pp. 199-241, 2 text figs. 

1893. Mitrophanow, P., Note on the Structure and the Development of Nervous Elements. 
Jour. Comp. Neurol., Vol. 3, pp. 163-167. 

1898. Neal, H. V. See Central Nervous System. 

1903. Neal, H. V., The Development of the Ventral Nerves in Selachii. I. Spinal A'entral 

Nerves. Mark Anniv. Vol., pp. 291-313, pis. 22-24. 
1907. Neal, H. V., The Morphology of the Eye Muscle Nerves. Proc. 7th Internat. Zool. 

Congr. Boston, pp. 204^214, figs. 1-10. 
1911. Neal, H. V., The Histogenesis of the "Transient" (Rohon-Beard) Cells in Selachian 

Embryos. Science (n. s.). Vol. 33, p. 273. 
1924. NoRRis, H. W., Branchial Nerve Homologies. Zeitschr. f . Morph. u. Anthrop., Bd. 24, 

pp. 211-226, 7 text figs. 
1884. ONODi, A., Ueber die Entwicklung der Spinalganglien und der Nervenwurzeln. Math.- 

nat. Ber. Ungarn, Bd. 2, pp. 310-336, Taf. 10. 
1887. ONODI, A., Neurologische Untersuchungen an Selachiern: 1. Das Ganglion ciliare. 2. 

Die Vagus Gruppe. Math.-nat. Ber. Ungarn, Bd. 5, pp. 179-188. 
1901. ONODI, A., Das Ganglion ciliare. Anat. Anaz., Bd. 19, pp. 118-124. 
1889. OsTROUMOFF, A., Ueber die Froriep'schen Ganglien bei Selachiern. Zool. Anz., Bd. 

12, pp. 363-364. 
1853. Philipeaux and Vulpian. See Central Nervous System. 
1910. Pitzorno, Marco, Sulla struttura dei gangli simpatici nei Sclaci. Monit. Zool. Ital. 

Firenze, Anno 21, pp. 53-61, pis. 3-5. 
1891. Platt, J. B., A Contribution to the Morphology of the Vertebrate Head, based on a 

study of Acanthias vulgaris. Jour. Morph., Vol. 5, pp. 79-112, pis. 4^6. 
1901. PuNNETT, R. C, On the Composition and Variations of the Pelvic Plexus in Acanthias 

vulgaris. (Abstract.) Proc. Roy. Soc. Lond., Vol. 68, pp. 140-142. Also: Zool. Anz., 

Bd. 25, pp. 233-235. 
1887. Rabl, C, tJber das Gebiet des Nervus facialis. Anat. Anz., Bd. 2, pp. 219-227. 

1878. Rohon, J. v., Ueber den Ursprung des Nervus vagus bei Selachiern mit BeriicksicH- 
tigung der Lobi electrici von Torpedo. Arb. Zool. Inst. Wien u. Triest, Bd. 1, pp. 151- 
172, 6 text figs. 

1897. RuGE, G., Ueber das peripherische Gebiet des Nervus facialis bei Wirbelthieren. 
Festsch. zum. 70. Geburtstage v. C. Gegenbaur, pp. 195-348, 76 text figs. 

1879. ScHV^ALBE, G., Das Ganglion oculomotorii. Jena. Zeitschr. Naturwiss., Bd. 13, pp. 
173-268, Taf. XII-XIV. 



THE ELASMOBRANCH FISHES 257 

1911. Sewertzoff, a. N., Die Kiemenbogen-Nerven der Fische. Anat. Anz., Bd. 38, pp. 487- 
494, 4 text figs. 

1888. Shore, T. W., Tlie Morphology of the Vagus Nerve. Jour. Anat. and Physiol., Vol. 

22, pp. 372-390. 

1889. Shore, T. W., On the Minute Anatomy of the Vagus Nerve in Selachians, with re- 
marks on the Segmental Value of the Cranial Nerves. Jour. Anat. and Physiol., Vol. 

23, pp. 428-451, pis. 20-21. 

1894. Strong, O. S., Dorsal View of the Cranial Nerves of the Leopard Shark (Galeocerdo 
maculatus Eanzani). A single plate published by Marine Biol. Lab., Woods Hole, 
Mass. 

1903. Strong, O. S., The Cranial Nerves of Squalus acanthias. (Abstract.) Science (n. s.), 
Vol. 17, pp. 254-255. 

1904. Strong, O. S., The Cranial Nerves of Squalus acanthias. Biol. Bull., Vol. 6, p. 314. 

1895. Turner, W. A., The Central Connections and Belations of the Trigeminal, Vago- 
Glossopharyngeal, Vago-Aceessory, and Hj'poglossal Nerves. Jour. Anat. and Physiol., 
Vol. 29, pp. 1-15. 

1885. A'incenzi, L., SulP origine reale del nervo ipoglosso. Atti R. Acad. Sci., Torino, Vol. 
20, pp. 798-806, tav. 7. 

1882. Wuhe, J. W. VAN, Ueber die Mesodermsegmente und die Entwickelung der Nerven 
des Selaehierkopfes. Natuurk. Verh. K. Akad. Amsterdam, Deel 22, pp. 1-50, 5 pis. 
(Also Groningen, 1915.) 

1897. WiKSTROM, D. A., Ueber die Innervation und den Bau der Myomeren der Eumpfmus- 
kulatur einiger Fische. Anat. Anz., Bd. 13, pp. 401-408. 



X 

SPECIAL SENSES 
SPECIAL SENSES OF HEPTANCHUS MACULATUS 

Under the organs of special sense in Heptanchus we may consider the olfac- 
torj^ organ, the eye, the ear, and the lateral line and associated organs. For the 
relation of the first three of these to the brain see figure 200a. 

Olfactory Organ 

The olfactory organ in Heptanchus is an almost terminal encapsulated struc- 
ture, with a convoluted mucous lining. From the outside this organ is reached 
by the nasal aperture which is separated by an overlapping median flap into 
an upper and a lower opening through which the water current circulates. 
Upon dissecting away the narial flaps the cup appears elliptical in outline and 
the lamellae are arranged in a series of folds which radiate outward from a 
central hub, like the spokes in a wheel. 

Located in the convoluted lining of the olfactory membrane are the primary 
olfactory cells (receptors). These cells send fine projections, hair cells, into 
the cup and they also send fibers posteriorly to the olfactory bulb in two 
bundles. These bundles compose the olfactory nerve, one part of which passes 
to the olfactory organ externally and the other internally {I, fig. 200a) . Olfac- 
tory sensations are carried backward by the olfactory tracts or peduncles 
(ol.t.) to the olfactory lobes (ol.l.) of the brain. 

Eye 

The eye in Heptanchus is provided with dorsal and ventral membranous and 
non-functional lids, but it is unprovided with a third eyelid or nictitating 
membrane. It appears to fit the orbit but poorly, especially in the young in 
which the orbit is unusually large. 

If the eyeball be dissected free from the orbit (figs. 91 and 200a) it is seen 
to be slightly pear-shaped and to be held in position by the optic pedicel (o.p., 
fig. 91) and the eye muscles which were described in a study of the muscular 
system (pp. 89-90) . The coat which appears externally covering the eye is the 
heavy sclera. In Heptanchus the sclera is chondrified and surrounds the ball, 
except laterally over the clear elliptical cornea; in the center of the cornea is 
the pupil surrounded by a dark colored curtain, the iris. 

Ear 

The ear proper (figs. 200a and 226) is located in the otic capsule. It is composed 
of the semicircular canals with their ampullae and the utricular and saccular 
regions. The anterior oblique and the horizontal canals have their anterior 

[258] 



THE ELASMOBRANCH FISHES 



259 



ends enlarged into ampullae (a.a. and a.h. respectively) which are placed close 
together. These two canals join the utriculus (u.) above. The ampulla of the 
posterior canal (a.p.) is located ventrally and that part of the canal lying 
near the sacculus is by some considered to be a second or posterior utriculus. 





Fig. 226. Ear of Heptanchus macuJaius. (Duncan Dunning, del.) A. Left ear, lateral view. 
B. Left ear, median view. 

a.a., ampulla of anterior oblique semicircular canal; a.h., ampulla of horizontal canal; 
aos., anterior oblique semicircular canal; a.j)., ampulla of posterior canal; en., connection 
between utriculus and sacculus; d.c., connection between sacculus and posterior oblique 
semicircular canal; e.d., endolymphatic duct; hr., horizontal canal; ?., lagena; pos., pos- 
terior oblique semicircular canal; s., sacculus; u., utriculus. 

At its base the utriculus is connected with the sacculus (s.) by a recessus 
utriculi (at en.). The sacculus is pear-shaped and continues upward in the 
chimney-like endolymphatic duct, the outer opening of which we saw in a 
study of the skull {ed., p. 42, fig. 45). From the posterior part of the floor 
of the sacculus a tongue-like sac, the lagena (?.), projects backward and 
downward. 



260 



THE ELASMOBRANCH FISHES 





Fig. 227. Cephalic canals and ampullae of Lorenzini, Heptanclms maculaUis. (G. L. TIanner, 
orig.) A. Dorsal view. B. Ventral view. 

cc, suprateniporal canal; e.d., endolymphatic duct; lime, hyomandibular canal; iha., 
inner buccal ampullae; ioc, infraorbital canal; isa., infraspiracular ampullae; IL, lateral 
line; mg., mandibular groove; mpo., pit organs in gular line (gl.) ] n.ap., nasal aperture; 
oba.^'-, outer buccal and posterior outer buccal ampullae; soa., supraophthalmic ampullae; 
soc, supraorbital canal ; sp., spiracle. 



THE ELASMOBRANCH FISHES 



261 



INNEEVATION 

The ear receives its innervation from the auditory or eighth nerve. This nerve 
as it leaves the brain stem separates into two main divisions, each of whieli 
further subdivides. Principal among the branches are the rami to the ampullae 
of the anterior oblique, the horizontal, and the posterior oblique semicircular 
canals; and those rami to the utriculus, the sacculus, and to the lagena. 

Sensory Canal System 



The sensory canal system in Heptanchiis consists of cranial canals in the head 
connected with a canal over the pharynx and a lateral open groove in the 
body region (see figs. 15, 227, and 228). The lateral canal in Heptanchus is 




Fig. 228. Cephalic canals and ampullae of Lorenzini, Heptanchus maculatus, side 
view. (G. L. Hanner, orig.) (For explanation see fig. 227.) 

especially simple for even in large specimens the canal as such is closed poste- 
riorly only to about the fifth cleft, back of which it remains an open groove 
toward the tip of the tail. 

Anterior to the spiracle and just posterior to the endolymphatic ducts a 
small transverse or supratemporal canal {cc, figs. 15 and 227a) passes off 
from the lateral canal toward the median line. This, however, does not meet 
and fuse with the similar canal from the opposite side. In Heptanchus macu- 
latus there may l-)e two supratemporal canals on a side, one posterior to the en- 
dolymphatic duct as just described, the other anterior to it. From the supra- 
temporal canal the lateral line canal passes slightly outward and forward to 
join the cranial canals. The cranial canal passing above the eye is the supra- 
orbital canal (figs. 227 and 228, soc). In front of the eye the supraorbital 
swerves outward and then sharply inward; it then turns backward and down- 
ward above the nasal aperture. The infraorbital canal {ioc.) back of the eye 
drops downward, sends the hyomandibular canal (hmc.) backward, and then 
continues forward under the eye. Before reaching the nasal aperture this 
canal is joined by the supraorbital canal (fig. 228). The main infraorbital 
next bends sharply toward the middle line, without joining the canal of the 



262 THE ELASMOBRANCH FISHES 

opposite side, and then passes outward and forward to the tip of the rostrum, 
where it^rfds blindly (fig. 227b). A mandibular groove {mg., fig. 227b) runs 
along the lower jaw parallel with the membranous lower lip. 

In the groove on the body and in the canals on the head are located sense 
organs, the neuromasts, of the sensory canal system. In Hepfanchus, as in 
other Elasmobranchs, the sense cells joining these organs are specializations 
of the cells in the wall of the canal or groove. The sense organs along the 
lateral line are supplied largely by branches of the vagus nerve. The glosso- 
pharyngeal and the otic division of the facial supply a few of the anterior 
neuromasts of the lateral line canal. The canals of the head are innervated 




Fig. 229. Pit organs, Heptanchvs macuJahis. (Madeline Marlowe, orig.) 
A., anterior; II., lateral line; p.o., pit organs; P., posterior. 

wholly by the facial nerve. The superficial ophthalmic division supplies the 
supraorbital; the buccal branch, the infraorbital; and branches of the hyoman- 
dibular nerve supply the hyomandibular canal and the mandibular groove. 

In addition to the sense organs characteristic of the sensory canal system in 
the head are other integumentary sense organs of a tubular nature. The first 
of these tubular organs are the ampullae of Lorenzini, which take the same 
relative position as do the canals of the head, and like them are similarly 
named. The ampullae, however, are grouped together, five or six such groups 
in Heptanchus maculatus being shown in figures 227 and 228. These are the 
supraophthalmic (soa.) ampullae above the supraorbital canal and in front 
of the eye; outer buccal {oba.'^), posterior outer buccal (aha-), and inner 
buccal (iha.) groups located respectively above and below the infraorliital 
canal and a part of the hyomandibular canal. A small bundle of ampullae lies 
ventral to the spiracle (isa., fig. 228). Each sense organ of an ampulla of 
Lorenzini sinks into the integument and is put into connection with the out- 
side ])y means of a longer or shorter canal (or canals) opening by a mucous 
pore (see p. 280, fig. 246). Connecting with the base of such an organ is a 
nerve twig, the twigs being furnished by the branches of the facial nerves 
which also supply the canals of the head region. 

A second kind of sense organ is the pit organ (figs. 227b, mpo., and espe- 
cially 229) . These pit organs are particularly interesting in Heptanchus macu- 
latus in that they have a wide distribution and are distinctly segmental in 



THE ELASMOBRANCH FISHES 263 

their arrangement. ]\Iost of these pits lie between the segment of the spiracle 
and the dorsal fin althongh four of them lie near the middorsal line in front 
of the apertures for the endolymphatic ducts. Further, a line extends down 
the arch back of the spiracle, as the so-called gular line (gl., fig. 228) described 
by Garman (1888) for Chlamydoselachus. This passes downward and for- 
ward in front of the first branchial cleft and ends ventrally before reaching 
the mandibular symphysis (fig. 227b). 



264 



THE ELASMOBRANCH FISHES 



SPECIAL SENSES OF ELASMOBRANCHS IN GENERAL 



The organs of special sense, olfactory, gustatory, optic, auditory, and sensory 
canal organs, although very different in the adult are, with the exception of 
the taste buds and the eye, similar in the beginning. In general, with the ex- 
ceptions made, these organs arise as thickened plates or placodes of ectoderm. 
An anterior placode gives rise to the nasal pit and a posterior placode sepa- 
rates into three parts (Mitrophanow, 1893). The 
first or anterior of these gives origin to a branchial 
sense organ over the first gill ; the second gives rise 
to the ear, and the third or posterior part produces 
the lateral line organ which, in the sharks, later 
extends to the tip of the tail. 

Olfactory Organ 

The olfactory organ in the adult is a blind sac, 
which in simpler forms like the notidanids and 
Chlamydoselach us is more or less terminal in posi- 
tion. In many other Elasmobranchs, however, its 
position is more ventral. The olfactory sac or pit 
itself varies greatly as to its shape, the nature of 
its lining, and the number and depth of the folds 
produced in it. In general it may be said to be ellip- 
tical in form, the long axis pointing anteromedially. 
The sac may be single or it may be double. In either 
case the lining is thrown into two series of ridges, 
the Schneiderian folds (fig. 231), which greatly in- 
crease the extent of its surface. The so-called sec- 
ondary folds are usually anterior and dorsal in po- 
sition while the primary folds are posterior and 
ventral. In certain forms the folds become exceedingly numerous, more than 
eighty primary folds being present. 

The sensory cells (fig. 230) have a group of hair-like processes which extend 
into the olfactory cup, and fibers which run back as the olfactory nerves to 
the glomeruli of the olfactory bulb. From the bulb, fibers extend posteriorly 
in the olfactory tract to the olfactory lobe of the brain, as we have described 
iov Heptanchus. 

When seen from the outside the aperture to the olfactory organ is usually 
separated by flaps across its middle part into two divisions. One of these 
ai)ertures is incurrent, the other is excurrent. In Heterodonfiis the excurrent 
aperture leads backward so that the water current, instead of passing out, 
passes backward and into the mouth. In some Elasmobranchs two flaps, instead 
of one, may pass over the nasal pit. When this occurs, the one passes inward 
from the ventral margin, while the other hangs down and slightly overlaps it 




Fig. 230. Types of olfactory 
sensory cells, Mustelus lae- 
vis. (rrom Asai.) 



THE ELASMOBRANCII PISHES 



265 



w^y 



from the dorsal side. In certain forms more than a single dorsal flap obtains. 
In Scyllium a double dorsal, and in Myliohatis numerous dorsals are present. 
In some of the rays (also in Squatina) these dorsal flaps extend downward as 
loose extensions of skin and are of slight value in forming a tube of the cup. 
By the meeting of dorsal and ventral flaps the normally elliptical aperture is 
changed into a figure 8, thus producing an anteromedial and a posterolateral 
opening to the olfactory organ. 

As the fish moves forward a current 
is produced mechanically through the 
olfactory cup and over the olfactory 
membrane. The importance of such a 
current in forms which have no direct 
connection from the nose to the mouth, 
that is, in which the olfactory organ is 
solely an organ of sense taking no part 
secondarily in respiration, is clearly 
seen. That the sense of smell is well 
developed in Elasmobranchs has been 
demonstrated by the experiments of 
Sheldon (1911). If for example the 
nostrils of the shark be plugged with 
cotton so as to prevent a circulation of the water over the olfactory membrane, 
the shark will swim over food without detecting it by sight; but if the nostrils 
now be unplugged, or even a single nostril, food will be found although it has 
been concealed. In fact, of all the special senses the sense of smell is probably 
of the most service. 




Fig. 231. Sagittal section of developing 
nasal pit, Acanthias. (From Berliner.) 
s.f., Schneiderian folds. 



DEVELOPMENT OF OLFACTORY ORGAN 

We may now notice briefly the development of the olfactory organ in the 
Elasmobranchs. As is common for vertebrates in general this organ first ap- 
pears as a thickened epidermal placode or plate which early pits in from the 
outside toward the brain so as to form a shallow vesicle. By further growth 
this vesicle sinks deeper and forms a blind sac (fig. 231) in which arise the 
Schneiderian folds (s.f.) of the primary and secondary types. Furthermore 
these folds have produced from their sides numerous accessory folds in which 
are found the olfactory sense-cells above mentioned (fig. 230). The two divi- 
sions of the olfactory nerve put these two areas of folds into communication 
with the olfactory luill), but each division of the nerve may be connected with 
both of the areas of the folds. 

Gustatory or Taste Organs 



Taste buds are present in the Elasmobranchs often in considerable numbers. 
These are located in the buccal cavity and pharynx and are distributed over 
the floor and tongue, along the sides, and over the roof of both cavities. An in- 



266 THE ELASMOBRANCH FISHES 

dividual organ is well illustrated in Heterodontus, in figure 34d, where it is 
seen as a papilla arising from the floor of the mouth. These organs often are 
surrounded by a more or less circular group of stomodeal denticles. 

A section through such a taste bud shows it to be placed over a cup-shaped 
base which looks something like the base of a placoid scale. The organ itself is 
made up of cells elongated in a vertical direction. These cells are of two sorts. 
One is a supporting cell and the other is a sense cell. 

Elasmobranch Eye 

Extremes to which the Elasmobranch eye may be developed may be exempli- 
fied in two t^'pes like Isistius and Sqnatina. In the former, which is a deep sea 
shark, the eye reaches a size which makes it a conspicuous structure standing 
out from the sides of the head. In the latter, which is slow moving and noc- 







A B C D 

Fig. 232. Types of Elasmobranch eyes. (From Garman.) A. Parmaturus pilosiis. B. Triakis 
henlei. C. Scoliodon. D. Carcharias milherti. 
n., nictitating membrane. 

turnal, there is but little exact vision and the eye is therefore more or less 
rudimentary. Between the two extremes are multitudes of types. 

In external view (fig. 232) the eye varies greatly in the type of its pupil 
and the nature of its lids. The pupil may be large, denoting a type of eye un- 
used to a great deal of light. Figure 233c of Spinax, a deep sea form, shows 
such a condition. In Triakis henlei the pupil is smaller and assimies a hori- 
zontal position, while in Scoliodon and CarcJiarias (fig. 232c and d) the pupil 
is vertical. 

The eyelids, especially the third or nictitating membrane (/?., fig. 232), are 
also marked characters in external view. In fact on this character alone the 
Elasmobranehs were separated by J. Miiller into two groups: (1) those hav- 
ing a nictitating membrane and (2) those devoid of it. To the former group 
belong Galeus, Mustelus, etc., and to the latter Heptanchus, Acanthias, etc. 
The nictitating membrane varies greatly in the degree of its development. In 
a type like Carcharias (fig. 232d) it reaches an optimum development where it 
can be drawn entirely over the eye. 

There has been considerable interest over the relation of the nictitating 
membrane to the lower membranous lid. The question is : Is the nictitating 
membrane the made-over lower lid with the present lower lid formed anew, or 
does it arise from the lower lid .^ In Mustelus, at least, it has been shown by 
Harman (1899) to arise as a ridge on the inner (ocular) side of the lower lid. 



THE ELASMOBRANCH FISHES 



267 



From an external view the eve is seen to be further protected by the orbit 
and in a way by upper and lower membranous lids. In Chlamydoselachus the 
lower lid is a deep fold which is interesting because a part of its inner surface 
is covered witli placoid scales. Tlie same is true of Mustelus. In fact it was from 
the possession of scales that the lower lid was formerly incorrectly supposed 
to be a newlv formed structure. 










C D 

Fig. 233. Sagittal section of eye. (From Franz.) A. Acanthias. B. CetorJdnus. C. Spinax. 
D. Raja iatis. 

ah., space for aqueous humor; c, ciliary body; ch., choroid coat; en., cornea; ir., iris; 
I., lens; l.m., lens muscle; o.p., optic pedicel; r., retina; sc, sclerotic coat; sJ., dorsal suspen- 
sory ligament; sch., suprachoroidea ; //, optic nerve. 

The eyelids are movable in only a few Elasmobranchs. In Scyllium, while 
they are so sluggish as rarely to close, yet the eye has been observed to bat. In 
many other forms the lids are more or less completely devoid of musculature 
and are therefore immovable. In others musculature may be fairly well de- 
veloped (see p. 104, fig. 106). In the rays the eyes are firmly fixed so that 
movement is impossible. 

Structure op Adult Eye 

The structure of the adult eye in a number of Elasmobranchs has been studied 
in detail by Franz (1905). In a median sagittal section through the eye of 
Acanthias striking the optic pedicel (fig. 238a) the various structures making 
up the eye appear. In the anterior part is the clear cornea (en.) and extending 
almost against it is the spherical crystalline lens (I.). The clear space between 
the two (aJi.) is filled with aqueous humor. The dark layer {ir., fig. 233c) is 
the pigmented iris, a circular curtain which contains the color of the eye; the 



268 



THE ELASMOBRANCH FISHES 



' ' -rd. 



aperture within the iris is the pupil. At (c.) is the ciliary body. The large 
cavity back of the lens is filled with the vitreous body. The lining of this cavity 
is the retina (r. ) under which is the pigmented choroid layer (ch.). Between 
the choroid and the sclerotic layers there is a suprachoroidea (sch.) of con- 
nective tissue. As an outer protective capsule, and continuing in front to the 
cornea, is the strong sclerotic layer (.sr.) through which the optic nerve passes. 
If the eye of Acanihias be compared with that of the closely allied Spinax 
niger (figs. 233a and c), which is an inhabitant of the deep sea, several im- 
portant differences will be noted. In the first place the eyeball in Spinax bulges 
out in front, and the enlarged lens extends far into the pupil. As a result, in 
Spinax the pupil is of immense size and is thus correlated with the environ- 
ment in which little light is present. The 
sclerotic layer in Spinax is thin and the 
optic ])edicel is absent. 

Two other types, Ceforhinus maximus, 
the basking shark (fig. 233b), and Raja 
hatis (fig. 233d), may be briefly noted. In 
CetorJiinus the eye is of large size and is 
marked by several distinguishing fea- 
tures. The pupil is small as is also the lens 
located well back in the cavity in the vit- 
reous body. Back of the choroid, the supra- 
choroid coat (sch.) extends dorsally and 
ventrally as a strongly vascular area. The 
sclera forms an unusually heavy carti- 
laginous capsule from which the optic 
pedicel (o.p.) is removed by a wide mass 
of connective tissue. 

In Raja (fig. 233d) the eye is of peculiar 
shape. The cornea bulges forward at the 
ventral margin, and the lens sinks deep 
within the eye. The pupil here is of minute size and the layers of the anterior 
part of the eye are exceedingly thin. On the posterior boimdary of the eye, 
however, the sclera is heawv^ and is separated from the choroid by a highly 
vascular suprachoroidea. The optic pedicel (o.p.) is wide and is joined to the 
sclera by connective tissue. 




Fig. 234. Section through the retina 
of the eye, Mustclus vulgaris. (From 
Schaper.) 

CO., cone; ei)., epithelial lining; 
i.h., inner wide layer; i.p., inner 
plexus layer; o./i., outer heavy layer ; 
rd., rod. 



FINER STRUCTURE OF RETINA 



A section through the retina of the eye of Mustelus (fig. 234) (Schaper, 1899) 
shows that this important coat of the eye is made up of numerous strata of 
cells and fibers. These may be briefly described from the inner to the outer side 
of the retina as an inner ganglionic layer containing fibers and cells, then an 
inner plexus layer (i.p.). Following this is an inner wide layer (i.h.) and an 
outer narrow heavy layer (o.h.). There then follows the important area of 



THE ELASMOBRANCH FISHES 



269 



cones (co.) and rods (rd.) which are intimately associated with vision. These 
occupy the layer farthest removed from the lens and next to the epithelial 
lining (ep.). The rods and cones together with the cells from the first or gan- 
glionic layer may be described more fully. 

The rods (rd.) and cones (co.) are characterized by the extreme length of 
the cell body. The nuclei of the rods extend almost to the heavy outer layer, 
and the slender cells reach the epithelial lining. The cones are heavier but are 
fewer in number with their nuclei staining as dark bodies also in the heavy 
outer layer. The cone cells extend toward the outer epithelium, but unlike the 
rods they do not reach the outer layer. 

It is the axones from cells of the retina that pass to the brain as the optic 
nerves. 

Development of Eye 

For convenience the development of the eye may be considered in two parts : 
(1) the development of the retina and its associated parts; and (2) the de- 
velopment of the lens. 

The first indication of the eye makes its appearance as a slight down-pitting 
of the cephalic plate even before the plate closes (see fig. 209, op.v.). Upon its 
closure these pits are directed 
outward toward the ectoderm, fl 

as the optic vesicles. The optic 
vesicles are therefore direct 
outgrowths from theforebrain. 
As each vesicle nears the ecto- 
derm it sinks in at its outer 
margin forming a tw^o-layered 
cup, the stem of which as the 
optic stalk binds the bowl of 
the cup to the brain. The outer 
layer of the bowl does not de- 
velop as nerve tissue but be- 
comes pigmented. The choroid coat develops back of this. The inside (invagi- 
nated) layer thickens to form the retina, the fibers from which pass along the 
old optic stalk to the brain as the optic nerve. 

The first indication of the lens is seen as a thickening of the ectoderm at the 
place where the optic vesicle touches it and before the vesicle invaginates to 
form the optic cup. The lens then becomes spherical, pinches off from the ecto- 
derm, and sinks into the cup. 




Fig. 235. Sagittal section through fenestra to ear, 
Haja. (From Howes.) A. Median sagittal. B. Para- 
sagittal. 

e.d., endolymphatic duct; fl., fluid; fn., fenestral 
tube; 7nZ>., tympanic membrane; ^y., tympanic cavity. 



ACCOMMODATION APPARATUS 

In almost all the Elasmobranchs. excepting types like Squatina, the eyes 
are so far separated that it would be impossible for both eyes to focus on a 
given point at the same time. In all these, binocular vision is impossible and 
monocular vision is the rule. Little adjustment of the lens is here possible. It 



270 



THE ELASMOBRANCH FISHES 



will be observed that in a type like Acanthias (fig. 233a) the lens is suspended 
by a dorsal suspensory ligament (si.) and is joined ventrally and laterally by 
a muscle (l.m.) from the iris. This might be regulatory in that it would draw 
the lens slightly forward but the lens cannot be focused with precision as it 
can in man and other higher animals. 






C D 

Fig. 236. Ear of Elasmobranchs. (Olive Swezy, orig.) 

Squalus sucldii: A. Outer view of left ear. B. Inner view of right ear. 

Heterodontus francisci: C. Outer view of left ear. D. Inner view of right ear. 

a.a., ampulla of anterior oblique semicircular canal; a.h., ampulla of horizontal canal; 

aos., anterior oblique semicircular canal; a.p., ampulla of posterior canal; d.c, connection 

from posterior canal to sacculus; e.d., endolymphatic duct; e.s., endolymphatic sac; 

/ir., horizontal canal; /., lagena; pos., posterior oblique semicircular canal; ra.a., ramus 

of eighth nerve to ampulla of anterior oblique canal; ra.h., ramus to horizontal ampulla; 

ra.n., ramus to macula neglecta ; ra.p., ramus to posterior ampulla ; ra.s., ramus to sacculus; 

ra.u., ramus to utriculus; r.u., recessus utriculi; s., sacculus; ii., utrieulus. 



THE ELASMOBRANCH FISHES 



271 



Auditory Organ 

Superfieiall.y the ear is protected by the cartilaginous auditory capsule which, 
in the embryo, joins the parachordal cartilages of the cranium (see p. 53, fig. 
58). In certain types the capsule is so thin as to give indications of the semi- 






C D 

Fig. 237. Ear of Elasmobranchs. (From Retzius.) 

Squatina : A. Outer view of left ear. B. Inner view of right ear. 

Baja ■• C. Outer view of left ear. D. Inner view of right ear. 

(For explanation see fig. 2.36.) 

circular canals superficially. Two pairs of apertures may enter the capsule 
from the parietal fossa in the middorsal line. The first and smaller of these is 
for the endolymphatic ducts (e.d., figs. 59 and 62) , while the second and larger 
pair of apertures are the fenestrae (fn.). These apertures may be considered 
more in detail. 

I have previously mentioned fenestrae for a number of sharks and for some 
of the rays (p. 55) . If we refer to figures of the dorsal view of the skull (figs. 
59 and 62) we observe that these openings are well marked and take up their 



272 



THE ELASMOBRANCH FISHES 




position posterior to the apertures for the endolymphatic duets. A sagittal 
section through the cranium of the skate as given by Howes (1883) shows that 
each fenestra enters a well defined cavity or tympanum (ty., fig. 235b) within 
the ear capsule, and further (fig. 235a) that the aperture is closed dorsally by 
a membrane (nih.). Over the membrane is a semifluid layer (fl.), above which 
is the integument. In a type in which the head is flattened (ray) this mem- 
brane fits closely over the fenestrae, and may serve to transmit sound waves 

to the cavity below it and thus to act some- 
thing after the fashion of the tympanic 
membrane of the middle ear of higher 
forms. 

The endolymphatic duct (e.d., figs. 236 
and 237) is normally small proximally 
and then enlarges into a more or less tor- 
tuous endolymphatic sac (e.s.). In Squa- 
iina, on the contrary, and to a certain 
extent in Acanthias, the mouth of the 
duct is enlarged and although bent at its 
upper part it is little changed in caliber 
throughout its course. At its base the duct 
broadens out into the sacculus (s.). In 
some forms this is relatively small (Alo- 
pias: Heterodontus, fig. 236c) , but it may 
be of large size (Sqnalus sucMii, fig. 236a, 
s. ) . It is within this cavity as well as in the 
utriculus that the otoliths or so-called ear 
stones are lodged. These in Sqnalus con- 
sist of a mass of calcareous material con- 
tained within the endolymph. In some of the other forms they are small, and 
in the embryo of Squatina they are apparently absent. In the adult Squatina 
a most interesting condition is reported. Here it is said that the place of the 
concretions of other forms is taken by sand grains which enter the wide endo- 
lymphatic duct. 

From the inferior and posterior angle of the sacculus (figs. 236 and 237) the 
lagena (l.) arises. This is usually a tongue-like projection as in Heptanchus, 
but it may assume a form greatly enlarged at the end, as in Lamna. The lagena 
is that part of the ear in the Elasmobranchs which is probably the forerunner 
of the complex cochlea of higher forms. 

At its inferior and anterior angle the sacculus is connected with the utricu- 
lus (h.) by the recessus utriculi. In figure 236a, the aperture from the utricu- 
lus to the recessus utriculi is shown as a solid ellipse. The utriculus itself is 
sometimes considered as made up of an anterior and a posterior component, 
the anterior of which is entered by the recessus utriculi, and if seen in side 
view may represent a T, the right and left arms of which are the horizontal 



B 



Fig. 238. Finer anatomy of an otic 
ampulla, Acanthias. (From Retzius.) 
A. Showing the crista acustica. B. De- 
tail of cells. 

cr., crista acustica ; cu., cupula ter- 
minalis; h., hair of hair cell (h.c.) ; 
i.e., thread cell. 



THE ELASMOBRANCH FISHES 



273 




(hr.) and anterior oblique semicircular (aus.) canals. The posterior utriculus 
is the posterior connective of the posterior oblique semicircular canal. This 
frequently, as in Heterodontus francisci {pos., fig. 236d), may be well devel- 
oped, the two parts of the utriculus being widely separated by the sacculus and 
endolymphatic duct. The posterior part of the utriculus has its connection 
with the sacculus by an elliptical aperture (d.c.) in Squalus, but this connec- 
tion is much longer in a type like Heterodontus. In Laemargus borealis the 
posterior part of the utriculus is connected with 
the sacculus by a long tube as it is also in Raja 
clavata (d.c, fig. 237d). 

The semicircular canals, although assuming dif- 
ferent degrees of compression, as is shown by a 
comparison of the compact ear of Heptanchus 
with the elongate ear of Squalus, are similarly ar- 
ranged in three planes. One of these planes is an- 
terior and oblique, another posterior and oblique, 
and the third horizontal in position. The anterior 
and horizontal canals join the utriculus proper, 
pass forward and backward respectively and then 
downward to their ampullae, which are in close 
proximity. The posterior canal is similarly a con- 
tinuation of the posterior part of the utriculus up- 
ward and backward and downward to its ampulla 
(figs. 236 and 237b and d). 

The ampullae (a.a., a.p., and a.h., figs. 236 and 
237) are interesting from their terminal relations as end organs of the nerve. 
A section through such an otic ampulla (fig. 238a) by Retzius (1881) shows 
the crista acustica {cr.) which is the terminal mass of sense cells capped by 
the cupula terminalis {cii.). A more detailed view cutting through the am- 
pulla demonstrates two kinds of cells in the crista. One of these is the support- 
ing or thread cell {t.c, fig. 238b) and the other is the sense or hair cell (h.c). 
The latter of these projects into the endolymph of the ampullary cavity and is 
capable of receiving sensations. Multitudes of cells of this sort are located in 
all the ampullae and also in the sacculus, lagena, and macula neglecta. 

INNERVATION OF EAR 

The innervation of the ear is, as we have said for Heptanchus, through the 
eighth or auditory nerve. Just before reaching the ear the nerve separates 
into two main divisions (figs. 236 and 237). The anterior division, the vestib- 
ular nerve, separates into an anterior ramus (ra.a.) to the anterior ampulla 
and a second ramus to the ampulla of the horizontal canal (ra.h.). Other 
branches from the stem supply the area of the recessus utriculi. The posterior 
of these, the saccular nerve, finally reaches the ampulla (a.p.) of the posterior 
canal. On its way it gives off a dorsal branch, the ramus neglectus (ra.n.) to 
the macula neglecta of the sacculus, and ventral branches to the lagena and to 
the sacculus (ra.s.). 



Fig. 239. Development of 
the ear of Scyllium. (From 
Krause.) 

aos., anterior oblique semi- 
circular canal; e.d., endo- 
lymphatic duct; hr., horizon- 
tal canal; s., sacculus. 



274 



THE ELASMOBRANCH FISHES 



DEVELOPMENT OF EAR 



The ear {Scyllium, fig. 239), like the nose, forms as a pit. In the development 
of the ear, however, the vesicle thus formed sinks in and, as the sacculus (s.) , 
becomes far removed from the exterior. It does not, however, lose entire con- 
nection with the outside for as it sinks inward it becomes flask-shaped, the 

long neck being the endolymphatic 
duct {e.d.). At this stage the outer 
wall of the vesicle becomes thin and 
the anterior oblique and horizontal 
semicircular canals {aos. and hr.) de- 
velop from them. 

Sensory Canal System and 

Ampullary and Pit 

Organs 

The sensory canal system as we have 
seen in Heptanchus consists of exten- 
sive sensory canals over the head and 
along the side of the body. The am- 
pullary organs associated with cer- 
tain of the canals and innervated by 
the same nerves are confined to the 
region of the head. Certain modifica- 
tions of the latter, the vesicles of Savi, 
may also be present. The pit organs 
are mainly in the anterodorsal trunk 
region but some of them are in the 
segment of the head. 

Sensory Canal System 

The sensory canals take a general 
course parallel to the long axis of the 
body. In the region of the trunk and 
tail thej^ compose the lateral line ob- 
served in our study of external form, 
and in the region of the head they 
form the cephalic canals, three or four 
main divisions of which are present in 
the sharks. One of these, the supra- 
orbital canal {soc, figs. 240-242) , runs above the eye; another, the infraorbital 
(ioc), passes back of and forward below the eye; the third or hyomandibular 




Fig. 240. Dorsal view of cephalic canals in 
Laemargus. (From Ewart.) (Drawn as 
transparent object.) 

cc, commissural or supratemporal canal ; 
lime, hyomandibular canal; ioc, infraorbi- 
tal canal; ZL, lateral canal; soc, supraorbital 
canal. 



THE ELASMOBRANCH FISHES 



275 



canal {lime.) passes backward from the infraorbital to the region of the hyoid 
arch; and a fourth, the mandibular (wic), traverses the lower jaw. Various 
modifications of these and their accessory parts will be noted later. 

The formation of the lateral and cephalic canals may first be briefly de- 
scribed. The earliest rudiment of the lateral line appears as a flattened plate 





Fig. 241. Sensory canals and ampullae of Lorenzini, Squalus suclclii. (Olive Swezy, orig.) 
A. Dorsal view. B. Ventral view. 

CO., commissural or supratemporal canal ; e.d., endolymphatic duct ; hmc, hyomandibular 
canal; iba., inner buccal ampullae; ioc, infraorbital canal; isa., infraspiraeular (hyoid) 
ampuhae ; II., lateral line ; vie, mandibular canal ; mpo., mandibular ampullae ; n.ap., nasal 
aperture; oba., outer buccal ampullae; soa., supraophthalmic ampullae; soc, supraorbital 
canal ; sp., spiracle. 

of ectoderm continuous with, and like that of, the placode for the ear, but this 
extends both forward and backward. Forward it gives rise to the long canals 
of the head region. Backward it becomes the lateral line organ. Either way, 
in order to become a canal rather than an open groove the sensory cord or 
plate of ectoderm, usually as a pocket, pushes deep into the underlying corium. 
It may be mentioned here that at regular intervals throughout its course the 
sensory cord differentiates into patches of ectoderm, the end organs or neuro- 
masts characteristic of these canals. 

In the adult Elasmobranch the lateral line canals (figs. 240-242) are simi- 
lar in distribution. They extend in or under the skin from the tip of the tail 
to the segment of the ear. The lateral line canal in Chlamydoselachns is an 



276 



THE ELASMOBRANCH FISHES 



open groove practically to the supratemporal canal, and in the notidanids it is 
open as far forward as the anterior region of the pectoral fin. In Acanthias on 
the contrary the canal is closed excepting in the region toward the tip of the 
tail. In all higher Elasmobranchs it is usually closed throughout the entire 
length. In some of these the canals run to a considerable depth, but in all such 
they still remain in communication with the exterior by tubules. The tubules 

putting the canal in com- 
munication with the out- 
side may be as numerous 
as are the branches of 
nerves (ramuli) reach- 
ing them (fig. 244). In 
certain forms, however, 
they are fewer in number. 
In its anterior segment 
the lateral line of each 
side is joined to its fellow, 
posterior to the endolym- 
phatic ducts, by the su- 
]iratemporal canal (cc). 
This canal, however, is in- 
complete in Heptanchus. 
Furthermore, on each 
side it may branch so as 
to send a part anterior to 
the endolymphatic duets 
in addition to the regular 
branch posterior to the 
ducts. In Chlamydose- 
lachus the supratemporal 
connection passes ante- 
rior to the endolympha- 
tic ducts. It seems reason- 
able to suppose that this 
branch is comparable to 
the occasional anterior 
branch of Heptanchus. 
The lateral line canal 
usually passes directly 
into the cephalic canals, but in Heterodonttts francisci at its anterior end it 
swerves sharply toward the middorsal line and joins the supratemporal canal; 
it then joins the supraorbital canal by making a sharp bend laterally. 

The cephalic canals are usually, as we have said, a direct continuation of 
the lateral line canal. In Laemargus (fig. 240) these canals are made up of the 
divisions above named. The supraorbital (soc.) passes forward above the eye 




Fig. 242. The cephalic canals, Baia hatis, dorsal view. 
(From Ewart and Mitchell, modified.) 

CO., commissural or supratemporal canal; hmc, hyoman- 
dibular canal ; ioc, infraorbital canal ; 11., lateral line ; 
mc, mandibular canal ; soc, supraorbital canal ; sc.'^'", first 
and second scapular canals; sp., spiracle. 



THE ELASMOBRANCH FISHES 



277 



and at tlie tip of the nose passes through to the ventral side. It then continues 
backward to meet the infraorbital (ioc). This condition is unlike that in Hep- 
tanchus (fig-. 228) in which the terminal part going to the infraorbital is 
usuall}^ broken in its course. The infraorbital drops back of the eye to a ventral 
position and then forward under the eye; after passing the ventral terminus 
of the supraorbital, it turns inward toward the middle line, where it approxi- 
mates or meets (Lae- 
margus) the infraor- 
bital of the opposite 
side. It then continues 
forward and slightly 
outward to the tip of 
the nose. The hyonian- 
dibular canal {hmc.) 
in the sharks branches 
oi¥ posteriorly from 
the infraorbital at the 
place where the latter 
reaches a ventral posi- 
tion back of the eye. 
Essentially the same 
plan obtains in Squa- 
lussucMii (fig. 241) as 
that here described 
for Laemargiis. 

This plan is consid- 
erably modified in the 
rays. In these, the su- 
praorbital canal (soc, 
fig. 242) in its ventral 
course is characterized 
by a peculiar loop for- 
ward and outward. It 
then meets the infra- 
orbital ventrally as in 
sharks. The hyoman- 
dibular(/tmc..fig.242) 
in the rays is greatly 
modified. It passes from the infraorbital backward, outward, and then for- 
w^ard on the ventral side of the pectoral fin, making a large ventral loop (in- 
complete in Torpedo). It then perforates the fin at the side of the olfactory 
capsule, and continues its course on the dorsal side of the fin, first inward and 
backward; then it swerves far outward and backward and then inward to join 
an anterior scapular l)ranch (sc.^) from the lateral canal, forming with the 
scapular branch dorsally on the pectoral fin a characteristic loop. 




Fig. 243. Cephalic cauals in Dicerobatis, dorsal view. (From 
Garman.) 

C.C., commissural or supratemporal canal ; hmc, hyoman- 
dibular canal; ioc, infraorbital canal; II., lateral line; soc, 
supraorbital canal. 



278 



THE ELASMOBRANCH FISHES 



In addition to what we have said of this system in a typical ray, it may be 
added that in a sluggish type like Torpedo certain of the cephalic canals may 
be lacking ventrally while in an active form like Dicerohates {Cephalopoda) 
tubules may branch off of the canal, unlike the simple tubules of Baja clavata, 
and form a complex net (fig. 243) . 

The internal structure of the canals may be studied in a transverse section 
through the lateral sensory canal of the leopard shark, Triakis semifasciatus 
(see p. 26, fig. 29, ll.c). In such a figure it will be seen that the walls of the 
canal are unequal in thickness. Both in the lateral line and the dorsal cephalic 









Fig. 244. Longitudinal section of lateral sensory canal, Mustelus canis. (From S. E. 
Johnson.) 

Grp., neuroniasts; CJvi., nerve; Lat.Cn., lateral sensory canal; HmJ., ramus of lateral 
nerve; Sn.CL, primary hair cell; Sim., supporting cells; Tub., tubule to exterior. 

canals the lumen is flattened; but in the ventral canals it is rounded. On the 
median wall of the canal the section passes through a sense organ or neuro- 
mast, composed of cells derived from the basal layer of epidermis (s.c). These 
are of two types, one a crescentic supporting cell and the other an elongated 
club-shaped sense cell. A longitudinal section through the lateral sensory 
canal of Mustelus canis (fig. 244) by Johnson (1917) shows that the neuro- 
niasts are much more numerous than are the tubes {Tub.) which open to the 
surface. Each of these groups {Grp.) is composed of primary hair cells 
{Sn.Cl.), secondary sense cells at the sides of the primary cells, and under- 
lying these the supporting cells. 

Innervation of the lateral line in the body is by means of the lateral division 
of the vagus or tenth cranial nerve. It will be observed from figure 244 that the 
ramuli {Rml.) reach the canal at about the position of the tubules, but that 
they break up into numerous fibers, which supply a multitude of neuroniasts 
{Grp.). In the most anterior part of the lateral line canal, however, a few 
twigs are received from the ramus dorsalis X {dr.X, fig. 245) and the supra- 
temporalis X. Other twigs of the supratemporalis X supply the supratemporal 
canal. The segment of the lateral canal immediately anterior to the supratem- 



THE ELASMOBRANCII FISHES 



279 



poral canal is supplied by a few twigs of the supratemporalis IX (Chlamydo- 
selaehus, Laemargus, Squalus acanthias, fig. 245, st.IX) and the most anterior 
part of this segment is supplied by a few fibers from the ramus oticus VII. 

The supraorbital canal is supplied by branches from the ophthalmicus 
superficiaJis of the facial nerve, and the infraorbital by the buccalis nerve, 
while the nenromasts of the hj-omandibular and mandibular canals are sup- 
plied by the external mandibular division of the seventh nerve. 

FUNCTION OF SENSORY CANAL SYSTEM 

The function of tlie sensory canal system has l)een made the subject of many 
studies. It was observed by early workers that the pores contained mucus. The 




Fig. 245. Innervation of the Sensory canal system and certain of the pit organs, Squalus 
acanthias. (From Norris and Hughes.) 

bu.FII, buccalis nerve; cc, suprateniporal canal; dr.X, ramus dorsalis of tenth nerve; 
liinc, hyomandibular canal; ioc, infraorbital canal; U., lateral line canal; Jl.X, lateral line 
nerve; mc, mandibular canal; mde.VII, external mandibular nerve; os.FII, ophthalmicus 
superficialis of seventh nerve; po., pit organs; soc, supraorbital canal; st.IX, supratem- 
poralis of ninth nerve ; st.X, supratemporalis of tenth nerve. 

system was therefore taken to function in the production and distribution of 
mucus and the pores were therefore called mucous pores. Later study also 
demonstrated the relation of these organs to the nervous system. 

It has been shown by G. H. Parker (1904) that a shark which has been de- 
prived of hearing and sight responds to wave movement, like that produced by 
throwing a stone into the water, so long as the nerves to the lateral line are 
intact. When these nerves are cut, however, no further response is given. 

Ampullary Organs 



The ampullary sense organs, as in Heptanchus, are confined to the head and 
are generally arranged in four or five groups. These are, in Squalus sucMii 
(fig. 241), the supraophthalmic (soa.), the inner (iha.) and outer buccal 
(oha.) groups; and the mandibular just behind the mandible; and the hyoid 



280 



THE ELASMOBRANCH FISHES 



groups {isa. and mpo.). In addition to these there is a modified ampulla in 
the spiraeular wall. In active forms the pores to these organs may be very 
numerous as in Mvstelus cams in which practically 1600 have been counted. 
In a sluggish type like Torpedo there are as few as 162 (Norris, 1929). 

Each ampullar}^ organ (fig. 246) consists of three parts: (1) a pore or 
opening to the exterior (op.) ; (2) a canal or tubule {th.) ; and (3) the ampulla 
proper (a.), located in the in- . - 

tegument. The ampulla varies 
as to pattern in the different 
Elasmobranchs. In some it is 
not divided up into ampullary 
pockets. In others it may have 
from eight to twelve pockets. 
These pockets are usually con- 
nected by a single canal with 
the outside pore. In Hexan- 
clius, however, Dotterweich 
(1932) has recently shown that 
each ampullary pocket in a 
group has its ovn\ canal and 
that a group of canals empties 
by a common pore. A trans- 
verse section through the am- 
pulla (fig. 247a) shows how 
they and the partitions sepa- 
rating them are cut. Accord- 
ing to Peabody (1897) each 
ampullary pocket ipl'.) has 
a double lining, the inner layer 
of which is of cells of large size. 
The pockets are surrounded by 
connective tissue and maj^ 
themselves surround a central part, the ampullary centrum {en.). Figure 247b 
is a sagittal section through the centrum. The section is through a pocket on 
the left and a partition between two pockets on the right. In figure 248 it is 
seen that the nerve to the ampulla enters through the centrum and spreads out 
over the ampullary pockets. 

It was formerly supposed that the cap over the centrum was the sensory 
area in which the nerve terminated, but in figure 248 it appears that, while 
the nerves lose their medullary sheaths and only the axis cylinders run toward 
the central cap, the fibers turn outward as the fibrils surround the ampullary 
sacs. Dotterweich (1932) has recently shown that the wall of an ampulla of 
Lorenzini is made of a single layer but that this layer consists of two impor- 
tant types of cells. One of these is a flask-like cell {g., fig. 249) which forms 
mucus. The other is a pyramidal or sensory cell (s.). These pyramidal re- 




Fig. 246. Ajiipullary 
organ of Lorenzini. 
(From Peabody.) 

a., ampulla; ap., 
aperture to outside ; 
tl)., tubule. 




Fig. 247. A. Transverse section. 
B. Sagittal section of an am- 
pulla, Ga/eifs. (From Peabody.) 
en., centrum ; pi:., ampullary 
pocket. 



THE ELASMOBRANCH FISHES 



281 



ceptor cells, as hexagonal plates, line the ampulla. They are apparently not 
provided with hair-like processes which extend into the ampuUary cavity but 
each pyramidal cell has a sensory nerve (o/.) leaving from the apex of the cell. 
A motor axone (ef.) extends to 
each of the secreting or flask-like 
cells. 

In their development the ani- 
pnllary organs, like the long ca- 
nals, form as pits of the epider- 
mis. These pits sink deeply into 
the integument and often extend 
far forward or backward forming 
more or less elongate tubules. At 
the end of the deepest part the 
tubule swells out, forming an am- 
pulla of Lorenzini. 

The vesicles of Savi found in 
Torpedo consist of from one hun- 
dred to two hundred hollow sacs 
in the region of the nasal pit and 

ventrally between the cartilage of the pectoral fin and the electric organ. 
Each vesicle is a transformed canal organ which, unlike an ampulla of Loren- 
zini, is unconnected with the exterior. Such a vesicle is composed of three discs, 
one of which is large and occupies a median position while at the sides of this 
are two lateral and smaller ones. 

Pit Organs 

In addition to the above-mentioned organs, pit organs found in sharks and 
rays may here be descril)ed. These organs were early seen in the rays along the 
back just mediad of the lateral line and from the suprascapular line to the 




Fig. 248. Section showing ending of nerves (h.) 
in ampulla. (From Eetzius.) 




Fig. 249. Eeceptor cells and gland 
cells in an ampulla of Lorenzini. 
(From Dotterweich.) 

af., afferent nerve ; ef., efferent 
nerve ; <;., gland cells ; s., sensory cell. 




Fig. 250. Pit organ, Haia l)atis. (From 
Ewart and Mitchell.) 



first dorsal fin. Others occur along the hyomandilndar and the infraorbital 
canals in the head. 

Ewart and Mitchell (1891) have given a section through a pit organ of the 
ray (fig. 250) which shows it to be not unlike a taste bud. The narrow neck 
leads to a group of sense cells which form a ball. Each of the sense cells is long 



282 THE ELASMOBRANCH FISHES 

and has passing from it dorsally a sensory process. A sensory nerve leaves the 
base of the organ and passes along with the lateral line nerve. 

Attention has been directed by several workers to the pit organs in sharks. 
These have l)een studied recently for Squaliis acanthias by Norris and Hughes 
(1920). Pit organs are here distributed between the lateral lines and anterior 
to the first dorsal fin, and are supplied by the dorsal ramus (dr.X) of the 
tenth nerve. In Heptanchus maculatus (fig. 229) this system of organs is 
especially worthy of note since its organs have a segmental arrangement. In 
places the lines of organs from one side to another are in almost unbroken 
continuity, while other lines are limited to one side. 



THE ELASMOBRANCII FISHES 283 

BIBLIOGRAPHY 
Chapter X 

OEGANS OF SMELL 

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1857. KoLLiKER, A., Ueber die Ausbreitung der Nerven in der Geruchsschleimhaut von Pla- 
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OEGANS OF SIGHT 

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1899. DoENECKE, F. W., Untersuchungen iiber Bau und Entwicklung der Augenlider beim 

Vogel und Haifische. Leipzig, 8°, 45 pp., 14 text figs. (Diss.). 
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Neapel, Bd. 6, pp. 432-480. 

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figs. Inaug. Diss., pp. 757-801, 3 text figs. 

1891. Froriep, Aug., Uber die Entwickelung des Sehnerven. Anat. Anz., Vol. 6, pp. 155-161, 
12 text figs. 



284 THE ELASMOBRANCH FISHES 

1899. Harman, N., The Palpebral and Oculomotor Apparatus in Fishes. Observations on 

Morphology and Development. Jour. Anat. and Physiol., Vol. 34, pp. 1-40, pis. 1-6. 
1894. LoCY, W. A., The Optic Vesicles of Elasmobranchs and Their Serial Eolations to 

Other Structures on the Cephalic Plate. Jour. Morph., Vol. IX, pp. 115-122, 6 text figs. 
1880. Matthiessen, L., Untersuchungen iiber den Aplanatismus und die Periscopie der 

Krystall-linsen in den Augen der Fische. Arch. f. d. ges. Physiol., Vol. 21, pp. 281- 

307, Taf. VI. 

1896. Neumater, Ludwig, Der feinere Bau der Selachier-Eetina. Arch. mikr. Anat., Bd. 48, 
pp. 83-111, 25 text figs. 

1909. Parker, G. H., Influence of the Eyes, Ears and Other Allied Sense Organs on the 
Movements of the Dogfish Mustelus canis (Mitchell). Bull. Bur. Fish., Vol. 29, pp. 
43-57. 

1899. SCHAPER, A., Die nervosen Elemente der Selachier-retina in Methylenblauprapara- 
ten. Nebst einigen Bemerkungen iiber das "Pigmentepithel" und die konzentrischen 
Stiitzzellen. Festsehr. z. 70. Geburtstag von Kupffer, pp. 1-10, Taf. 1-3. 

1905. ScHNAUDiGEL, O. A. F., Neurofibrillen in den Retinalganglicnzellen der Selachier. 
(Ber. Ophthalm. Ges. Heidelberg) Bd. 32, pp. 329-331. 

1898. Shearer, Creswell, On the Nerve Terminations in the Selachian Cornea. .Jour. Comp. 
Neurol., Vol. 8, pp. 209-217, 4 text figs. 

OEGANS OF HEARING 

1892. Ayers, H., Vertebrate Cephalogenesis. II. A Contribution to the Morphology of the 
Vertebrate Ear, with a Reconsideration of Its Functions. Jour. Morph., Vol. VI, pp. 
1-360, pis. 1-12, 26 text figs. 

1884. Beard, J., On the Segmental Sense Organs of the Lateral Line, and on the Morphol- 
ogy of the Vertebrate Auditory Organ. Zool. Anz., Bd. 7, pp. 123-126, 140-143. 

1899. Bethe, a.. Die Locomotion des Haifisches (Scyllium) und ihre Beziehungcn zu dem 
einzelnen Gehirntheilen und zum Labyrinth. Arch, f . d. ges. Physiol., Bd. 76, pp. 470- 
493, 2 text figs. 

1896. Everett, W. H., Anatomy of the Ear of the Dogfish (Galeus canis). (Prelim.) Trans. 

N. Y. Acad. Sci., Vol. 15, pp. 176-182, pis. 12-13. 
1902. Gaglio, G., Experiences sur I'anesthesie du labyrinthe du I'oreille chez les chiens de 

mer (Scyllium catulus). Arch. Ital. Biol., T. 38, pp. 383-392. 
1883. Howes, G. B., On the Presence of a Tympanum in the Genus Raia. Jour. Anat. and 

Physiol., Vol. 17, pp. 188-190, pi. 8. 
1901. Krause, R., Die Eutwickelung des Aquaeductus vestibuli s. Ductus endolymi^haticus. 

Anat. Anz., Bd. 19, pp. 49-59, 21 text figs. 
1907. Lafite-Dupont, Eecherches sur I'audition des poissons. C.R. Soc. Biol. Paris, T. 63, 

pp. 710-711. 

1899. Laudenbach, J., Zur Otolithen-Frage. Arch. f. d. Ges. Physiol., Bd. 77, pp. 311-320, 
1 text fig. 

1893-94. Lee, F. S., A Study of the Sense of Equilibrium in Fishes. Part I. Jour. Physiol., 
Vol. 15, pp. 311-348, 1893. Part IL Ibid., Vol. 17, pp. 192-210, 1894. 

1898. Lee, F. S., The Functions of the Ear and the Lateral Line in Fishes. Anier. Jour. 
Physiol., Vol. 1, pp. 128-144. 

1900. Lyon, E. P., Compensatory Motions in Fishes. Amer. Jour. Physiol., Vol. 4, pp. 77-82. 

1910. Maxwell, S. S., Experiments on the Functions of the Internal Ear. Univ. Calif. 
Publ. Physiol., Vol. 4 (1910-1915), p. 1. 

1911. Maxwell, S. S., On the Exciting Cause of Compensatory Movements. Amer. .Jour. 
Physiol., Vol. 29, p. 367. 



THE ELASMOBKANCH FISHES 285 

1919. Maxwell, S. S., Labyrinth and Equilibrium. I. A Comparison of the Effects of Ee- 
moval of the Otolith Organs and of the Semic-ircular Canals. Jour. Gen. Physiol., 
Vol. 2, pp. 123-132. 

1920. Maxwell, S. S., II. The Mechanism of the Dynamic Functions of the Labyrinth. 
Ibid., pp. 349-355, 1 text fig. 

1920. Maxwell, S. S., III. The Mechanism of the Static Functions of the Labyrinth. Ibid., 
Vol. 3, pp. 157-162. 

1921. Maxwell, S. S., The Equili1)rium Functions of the Internal Ear. Science (n. s.), Vol. 
53, pp. 423^29. 

1897. Morrill, A. D., Innervation of the Auditory Epithelium of Mustelus canis De Kay. 
Jour. Morph., Vol. 14, pp. 61-82, pis. 7-8. 

1903. Parker, G. H., The Sense of Hearing in Fishes. Amer. Nat., Vol. 37, pp. 185-204. 
1909. Paricer, G. H., The Sense of Hearing in the Dogfish. Science (n. s.). Vol. 29, p. 428. 
1903. Quix, F. H., Experimenten over de Functie van het Labyrinth bij Haaien. Tijdschr. 

Nederland Dierk. Ver. (Ser. 2), Deel. 8, pp. 35-61, 1 text fig. 
1881. Eetzius, G., Das Gehororgan der Wirbeltiere. Morph. -hist. Stud., Bd. 1, Pt. I, pp. 

1-217, Taf. 1-35. Stockholm. 

1883. Sew ALL, H., Experiments upon the Ears of Fishes with reference to the Function of 
Equilibrium. Jour. Physiol., Vol. 4, pp. 339-349. 

1906. Stewart, Ch., On the Membranous Labyrinths of Certain Sharks. Jour. Linn. Soc. 
Zool., Vol. 29, pp. 407-409, pi. 40. 

LATEEAL LINE AND ASSOCIATED OEGANS 

1901. Allis, E. p.. The Lateral Sensory Canals, the Eye-Muscles, and the Peripheral Dis- 
tribution of Certain of the Cranial Nerves of Mustelus laevis. Quart. Jour. Micr. Sci., 
Vol. 45, pp. 87-236, pis. 10-12. 

1884. Beard, J., On the Segmental Sense Organs of the Lateral Line, and the Morphology 
of the Vertebrate Auditory Organ. Zool. Anz., Vol. 7, pp. 123-126, 140-143. 

1868. Boll, Franz, Die Lorenzini'schen Ampullen der Selachier. Arch. mikr. Anat., Bd. 4, 

pp. 375-390, pi. 23. 
1875. Boll, Franz, Die Savi'schen Blaschen von Torpedo. Arch. f. Anat. u. Physiol., Jahrg. 

1875, pp. 456-468, Taf. 11. 

1898. Branbes, G., Die Lorenzini'schen Ampullen. Verh. deutsch. zool. Ges. 8. Vers. Heidel- 
berg, pp. 179-181. (Histologie.) 

1900. Brandes, G., Die Lorenzini'schen Ampullen. Zool. Centralbl., 7. Jahrg., p. 567. 
1908. Brohmer, p.. Die Sinneskanale und die Lorenzini'schen Ampullen bei Spinax-Embry- 

onen. Anat. Anz., Bd. 32, pp. 2.5-40, 8 text figs. 
1891. CoGGi, A., Le visicole di Savi e gli organi della linea laterale nelle Torpedini. Atti Ace. 

Lincei Eoma (4), Vol. 7, pp. 197-205, 7 text figs. Arch. Ital. Biol., Vol. 16, pp. 216- 

224, pi. 1. 

1902. CoGGi, A., Nouove richerche sullo sviluppo delle ampolle di Lorenzini. Atti Accad. 
Lincei (Ser. 5), Vol. 11, pp. 289-297, 338-340. Arch. Ital. Biol., Vol. 38, pp. 321-333. 

1902. CoGGi, A., Sviluppo degli organi di senso laterale delle ampulle di Lorenzini e loro 

nervi rispettivi in Torpedo. Arch. Zool. Napoli, Vol. 1, pp. 59-107, 2 tav. 
1905. CoGGi, A., Sullo sviluppo e la morf ologia delle ampolle di liorenzini e loro nervi. Arch. 

Zool., Napoli, Vol. 2, pp. 309-383, 3 tav., 4 text figs. 
1896. Cole, F. J., On the Cranial Nerves of Chimaera monstrosa with a discussion of the 

Lateral Line System, and of the Morphology of the Chorda Tympani. Trans. Eoy. 

Soc. Edinburgh, Vol. 38, pp. 631-680, 2 pis. 
1878. Dercum, Francis, The Sensory Organs — Suggestions with a View to Generalization. 

Amer. Nat., Vol. 12, pp. 579-593. 



286 THE ELASMOBKANCH FISHES 

1932, DOTTERWEICH, Heinz, Bau und Function der Lorenzini'schen Ampiillen. Zool. Jahrb. 

(Abt. Zool.), Bd. 50, pp. 347-418, 43 text figs. 
1891. EwART, J. C, The Lateral Sense Organs of Elasmobranelis. I. The Sensory Canals of 

Laemargus. Trans. Roy. Soc. Edinburgh, Vol. 37, pp. 59-85, pis. 1-2, 1 text fig. 

1891. EwART, J. C, and Mitchell, J. C, Tlie Lateral Sense Organs of Elasmobranchs. 
II. The Sensory Canals of the Common Skate (Raia batis). Trans. Roy. Soc. Edin- 
burgh, Vol. 37, pp. 87-105, pi. III. 

1899. FoRSELL, G., Beitrage zur Kenntnis der Anatomie der Lorenzini'schen Ampullen bei 

Acanthias vulgaris. Zeitschr. wiss. Zool., Bd. 65, pp. 725-744, Taf. 34. 
1888. Fritsch, G., Ueber Bau und Bedeutung der Kanalsysteme unter der Haut der Se- 

lachier. Sitzber. wiss. Akad. Berlin, 1888, pp. 273-306, 4 text figs. 
1895. FuCHS, ^., Ueber die Function der Organe derSeitenlinie bei denSelachiern.Centralbl. 

Physiol., Bd. 9, pp. 692-693. Also : 1895, Arch. d. Ges. Physiol., Bd. 59, pp. 454-478, 

Taf. 6. 
1888. Garman, S., On the Lateral Canal System of the Selachia and Holocephala. Bull. Mus. 

Comp. Zool. Harvard Col., 1888-89, Vol. 17, pp. 57-120, 63 pis. 

1892. Garman, S., The Vesicles of Savi. Science, Vol. 19 (n. s.), p. 128. 

1917. Johnson, S. E., Structure and Development of the Sense Organs of the Lateral Canal 
System of Selachians (Mustclus canis and Squalus acanthias). Jour. Comp. Neurol., 
Vol. 28, pp. 1-74, 83 text figs. 

1918. Johnson, S. E., The Peripheral Terminations of the Nervus Lateralis in Squalus 
sucklii. Jour. Comp. Neurol., Vol. 28, pp. 279-289, 10 text figs. 

1902. Johnston, J. B., The Homology of the Selachian Ampullae. A Note on Allis' Recent 
Paper on Mustelus laevis. Anat. Anz., Bd. 21, pp. 308-313. 

1905. Klinkhardt, Werner, Beitrage zur Entwickelungsgeschichte der Kopfganglien und 
Sinneslinien der Selachier. Jena. Zeitschr. Naturwiss., Bd. 40, pp. 423-486, pis. 14- 
16, 6 text figs. 

1678. Lorenzini, S., Osservazioni iutorno alle Torpedini. Firenze, per I'Onof ri. 

1678-79. Lorenzini, S. (1693). De anatomia Torpedinis. Ephemer. Acad. Nat. Cur., Ann. 
9 et 10, pp. 389-395. 

1901. Minckert, W., Zur Topographie und Entwickelungsgeschichte der Lorenzini'schen 
Ampullen. Anat. Anz., Bd. 19, pp. 497-527, 10 text figs. 

1929. Norris, H. W., The Distribution and Innervation of the Ampullae of Lorenzini of the 
Dogfish Squalus acanthias. Some Comparisons with Other Plagiostomes and Correc- 
tions of Prevalent Errors. Jour. Comp. Neurol., Vol. 47, pp. 449-465. 

1904. Parker, G. H., The Function of the Lateral-Line Organs in Fishes. Bull. Bur. Fish., 
Vol. 24, pp. 185-207. 

1897. Peabody, J. E., The Ampullae of Lorenzini of the Selachii. Zool. Bull., A^ol. 1, pp. 
163-178, 9 text figs. 

1898. Retzius, G., Zur Kenntnis der Lorenzini'schen Ampullen der Selachier. Biol. Unter- 
sueh. (N. F.), Bd. 8, pp. 75-82, Taf. 18. 

1880. Solger, B., Neue Untersuchungen zur Anatomie der Seitenorgane der Fische. II. Die 
Seitenorgane der Selachier. Arch, raikr. Anat., Bd. 17, pp. 458—479, pi. 39. 

ORGANS OF TASTE AND TOUCH 

1915. Fahrneholz, Curt, tiber die Verbreitung von Zahnbildungen und Sinnesorganen im 

Vorderdarm der Selachier und ihre phylogenetische Beurteilung. Jena. Zeitschr. 

Naturwiss., Bd. 53, pp. 389-444, Taf. 6-7, 7 text figs. 
1890. Purvis, G. C, Note on Certain Terminal Organs resembling Touch-Corpuscles or 

End-Bulbs in Intra-Muscular Connective-Tissue of the Skate. Quart. Jour. Micr. Sci., 

Vol. 30 (n. s.), pp. 51.5-518, pi. 33 (figs. 1-4). 



XI 

UROGENITAL SYSTEM 

UROGENITAL SYSTEM OP HEPTANCHUS MACULATUS 

Urinary System 

The mesonephrotic kidneys in Heptanchus (kd., fig. 251) appear as right and 
left longitudinal bands lying along the entire roof of the body cavity and at 
the sides of the spinal column. Each kidney extends as a narrow ribbon of 
tissue from the pericardio-peritoneal septum posteriorly one-half the length 
of the body cavity; back of this it broadens out and becomes much thicker so 
that the main mass of the tissue lies posterior to the region of the superior 
mesenteric artery. 

In general it may be said that the kidney is made up of multitudes of small 
lobules which in ventral view give little evidence of segmentation. A close 
study, however, reveals the fact that a division into segments is present. This 
may be further verified by the segmental arrangement of the collecting tubules 
which extend from the kidney tissue to the Wolffian duct (iv.d.), or in the 
posterior part to the ureter (u.). 

From the upper part of the kidney the collecting tubules enter the Wolffian 
duct and in the lower part they join the enlarged ureter. In an immature fe- 
male ten of these collecting tubules may be seen to join the Wolffian duct on its 
median side and twelve to join the ureter laterally, twenty-two in all being 
present. A similar condition is present for the upper part of the kidney of the 
adult female as is seen in figure 252 (facing p. 290) . But in that figure the ure- 
ter has not been thro\\ai to the side and so not all the ducts entering it appear. 

In the immature female (fig. 251a, 70.3 em. in length) the Wolffian duct 
passes the entire length of the kidney apparently as a straight tube, increas- 
ing from 0.5 to 1 mm. in size. In its anterior segment it lies just ventral to the 
kidney, and in the posterior division it passes along the ventral and median 
margin of the ureter. In the lower part of its course it does not receive col- 
lecting tubules. 

The Wolffian duct of the adult female (fig. 252) presents a most interesting 
condition in its upper segment where it is singularly coiled like that of the 
male. This distinct torsion, however, extends only a little over one-half of the 
anterior segment or to the posterior part of the sex gland (ov.). The remain- 
ing part of this segment is straight and the posterior segment, like that in the 
immature female, passes ventral to the enlarged ureter. 

The ureter in Heptanchus is a thin walled, elongated blind sac which in the 
immature specimen above described reaches a diameter of 7 mm. Anteriorly, 
and essentially at the segment where the kidney begins to increase in size, the 

[287] 



288 



THE ELASMOBRANCH FISHES 





Fig. 251. Urogenital system of immature Heptanchiis maculattis. (Frances Torrey, orig.) 
A. Female. B. Male. 

c.c, central canal of testis; cl., cloaca; els., clasper; ct., collecting tube; fl., funnel; 
l-d., kidney; od., oviduct; ov., ovary; s.g., shell gland; t., testis; u., ureter; ug., urogenital 
sinus; u.p., urinary papilla; u.s., urinary sinus; v.d., vas deferens; v.e., vas efferens; 
w.d., Wolffian duet. 



THE ELASMOBRANCII FISHES 289 

ureter ends blindly. In its median segment it increases somewhat in diameter 
and then taiHM-s gradually to its posterior part where as a nsnal thing it joins 
the Wolffian duet. The collecting tul)nles which enter the ureter are arranged 
with regularity thronghont the greater part of its course, but the anterior two 
or three join as a common duct and enter a kind of pocket in the median side 
of the ureter (see fig. 252) . In the short connnon segment formed by the union 
of ureter and Wolffian duct, near the cloaca, considerable variation prevails. 
In one immature female the left combined Wolffian duct and ureter entered 
slightly posterior to the right and in another these relations were reversed. 
In figure 252 it will be observed that the Wolffian duct and the ureter enter 
the urinary sinus ( u.s.) separately and that in this specimen a singular condi- 
tion obtains in which there are two urinary sinuses, one on the right, the other 
on the left side. 

The urinary sinus terminates in a urinary papilla (two papillae in fig. 252) 
which is perforated at its end. Nitrogenous waste matter collected from the 
kidney is ejected through this into the cloaca (cl.) and thence to the exterior. 

It would appear from general proportions that the posterior part of the 
kidney is by far the more effective part of the organ in the removal of nitrog- 
enous waste matter. If this is so the greater part of the waste passes through 
the ureter. 

The urinary organs in the male have much the same appearance in general 
as those in the female. In the male, however, some of the tissue and ducts have 
undergone great change correlated with the fact that in the adult they entered 
secondarily into the service of the genital system. 

The kidney of an immature male of 75 cm. in length is shown in figure 251b. 
It is 18.8 cm. long and has 12 segments in the anterior part and 14 in the poste- 
rior. In general it extends farther forward and is somewhat better developed 
anteriorly than is that of the female. But this is due, as we shall see presently, 
to its relation to the genital organs. 

The collecting tubules (ct., fig. 251a) entering the Wolffian duct are ex- 
ceedingly small and could be made out with care in the most anterior part of 
the kidney. Posterior to this region they are distinctly arranged segmentally, 
each tubule leaving the segment at about its middle part. At the proximal end 
of the kidney there are certain other tubules {v.e., fig. 251b) which are a part 
of another system which will be described presently. 

The Wolffian duct, in the ui)per part of its course, is thrown into numerous 
coils like those of the adult female, but here they are more pronounced and 
continue to the beginning of the enlarged part of the kidney. From this place 
posteriorly the tube increases in diameter and passes ventral to the enlarged 
ureter as it does in the female. 

The ureter (u., fig. 251b) is much like that of the female and receives prac- 
tically the same number of collecting tubules. After it unites with the vas def- 
erens (modified Wolffian duct) the two empty into the urogenital sinus (ug.). 



290 THE ELASMOBRANCH FISHES 



Genital System 

The genital system in Heptanchus is of an interesting type. Each ovary {ov., 
fig. 252) of the adnlt female is large and is located in the anterior part of the 
body cavity where it is suspended by a mesentery, the mesovarium. It con- 
tains numerous ova which can be seen through the thin wall. From the main 
mass of the ovary there is a posterior continuation of tissue which is devoid 
of ova. This is possibly the rudiment of an epigonal organ (epg.) found in 
some of the other Elasmobranchs. 

A singular condition is found in Heptanchus, similar to that described by 
Semper (1875) for Hexanchns, in which a rudimentary testis is associated 
with the ovary. In Heptanchus maculatus this testis {t., fig. 252) lies in the 
mesovariimi at the base of the ovary and runs parallel with it. It consists of an 
anterior larger part and a marked ridge or swelling which extends posteriorly 
practically the entire length of the ovary. It will be noted that the posterior 
extent of this rudimentary testis is about the same as that of the coil in the 
"Wolffian duct previously described. 

Unconnected with the ovaries are the tubes or oviducts {ocl.) through which 
the ova reach the exterior. Right and left oviducts are reached from the body 
cavity by a common opening or wide funnel (^., figs. 251a and 252) located 
just ventral to the base of the liver. The oviducts pass outward from the fun- 
nel and then inward to the anterior margin of the mesovarium where they en- 
large to form the shell gland (s.g.). Posterior to the shell gland the oviduct 
passes ventral to the Wolffian duct as a tube of considerable size, but it is not 
so greatly enlarged in Heptanchus as in many other Elasmobranchs in which 
it forms the conspicuous uterus (see fig. 253a, Squalus sucMii). At their 
termini the two oviducts enter the cloaca separately and not in common with 
ureters or Wolffian dvicts, that is, not through the urinaiy papilla. 

The genital glands of the male are the paired testes which like the ovaries of 
the female occupy an anterior position in the body cavity. In figure 251b of 
the immature specimen the testis (t.) appears as a long mass of tissue. Only 
the anterior part of this, however, is functional. The posterior part represents 
the rudimentary epigonal organ like that in the female. The testis is swung 
from the body wall by a mesentery, the mesorchium, which is comparable to 
the mesovarium suspending the ovary. 

Running along the median and anterior part of the testis is the central canal 
(c.c.) , which is put into communication with the vas deferens (Wolffian duct) 
by vasa eflferentia (v.e.), six of which are present in Heptanchus. The vasa 
efferentia are derived from funnels, present on the mesorchium. We shall de- 
scribe this system more completely in the general part, but here attention may 
be directed to it briefly. The general plan of these tubes may be made out in 
figure 254a {nph.) where several of them open on the mesorectum which sus- 
pends the rectal gland. These openings are the nephrostomes, the tubes of 
which pass out toward the kidney tissue. In the region of the testis the mouths 




I* 



THE ELASMOBRANCH FTSIIES 291 

of the most aiitoi-ior of these are united with the central canal of the testis, 
and the tubes pass down over the uiesorchium and join the upper part of the 
coiled vas deferens. 

The vas deferens in Heptanehus is very simple in that it is a single con- 
voluted tube which increases only slightly in size l)ack to the anterior tip of 
the ureter. From here it passes backward in the immature specimen more or 
less as a straight tube. After having received the ureter, as we have seen, it 
enters the urinary sinus. Since in the adult this urinary sinus also receives sex 
cells from the vas deferens it becomes a urogenital sinus in the male. The 
spermatozoa and the fluid collected in the urogenital sinus are forced through 
the papilla and out through the cloaca. As they leave the cloaca they are di- 
rected through a groove on the clasper and may be transferred to the cloaca 
of the female. 

In a number of immature males examined, right and left rudimentary ovi- 
ducts were present. These were united at the midventral line and had a com- 
mon funnel as in the female. In all the specimens examined, however, they 
were of short extent, all of them ending blindly posterior and being attached 
to the body wall anterior to the segment of the shell gland. 



292 THE ELASMOBRANCH FISHES 



UROGENITAL SYSTEM OF ELASMOBRANCHS IN GENERAL 

The two systems, urinary and genital, included nnder this head, although 
differing in function, are so closely associated anatomically that they are usu- 
ally considered together. First, however, we shall consider them separately 
and then discuss their secondary relationship. 

Urixary System 

KIDNEY (MESONEPHROS) 

Upon opening the body cavity by a ventral incision and removing the viscera, 
the urinary organs of an Elasmobranch appear as dorsally placed structures 
on each side of the spinal column. In the sharks they may extend as ribbon- 
like bands, narrow at the base of the liver anteriorly, and only slightly wider 
at the cloacal region posteriorly. In the rays they are characteristically en- 
larged posteriorly where most of the tissue is confined. These statements are 
true only in general, for great variation is present in different species of 
sharks and rays; differences are further to be noted in the different sexes of 
the different species. 

The long ribbon-like type of mesonephros or kidney characteristic of Squa- 
lus (fig. 253a) and Galeus loses something of this shape in ScylUum where its 
anterior part is narrower and, in the female, ends short of the base of the liver. 
Again, the type of kidney characteristic of the shark although extending far 
anteriorly may be at the same time like the kidneys of the rays, broader poste- 
riorly (Sqiiatina, fig. 254a) . In the rays, excepting Torpedo, the kidney rarely 
extends far forward, but sometimes a large part of it is located back of the 
cloaca {Raja clavata, fig. 254b, Trygon). 

Sexual differences which are marked in the kidney of the Elasmobranchs 
are produced by two factors : one is the reduction of the anterior part of the 
kidney in the female; the other is the hypertrophy of this part in the male. 
This hypertrophy results from the fact that the anterior part in the male 
comes into the service of the genital system and takes on a secondary function. 
In Torpedo sexual differences are slight, for a ray. In the female a band of 
tissue is continuous forward, and this is only slightly less developed than 
in the male. Sexual differences are evident in a type like Squatina in which 
the kidney of the adult female does not extend anteriorly to the liver. In 
Scyllium, where the difference in the two sexes is marked, the anterior part of 
the kidney of the female falls short of the base of the liver; and in R. clavata 
the kidney is limited to the posterior segment, while the anterior part in the 
male (fig. 254b) represents a great mass of tissue. 

Furthermore, the kidney varies greatly in its degree of complexity in the 
different Elasmobranchs. In some of the sharks it retains in part a simple 
metameric arrangement by which in dorsal view it agrees with the segmenta- 
tion of the body, characteristic of the embryo {Squatina, fig. 254a). In most 
other sharks, however, this simple arrangement is lost at least in the posterior 



THE ELASMOBRANCII PISHES 



29: 







y 



-*~\ 



■■\ 




V 

X 



A B 

Fig. 253. Urogenital system, Squalus suckJii. (Duncan Dunning, orig.) A. Female. B. Male. 
cl, cloaca; els., clasper; d.a., dorsal aorta; M., kidney; od., oviduct; ov., ovary; so., in 
male, sperm sac; s.g., shell gland; t., testis; u., ureter; up., urinary papilla; u.s., urinary 
sinus; ut.. uterus; ug.. urogenital sinus; v.s., vesieula seminalis; v.d., vas deferens; ted., 
Wolffian duet. 



294 



THE ELASMOBRANCH FISHES 



part and the only way that the number of segments can be determined is by 
the number of collecting tubules leaving the kidney. In the rays the kidneys 
may be divided into numerous asymmetrical lobules, which show little tend- 
ency toward orderly arrangement ; or the kidneys of the two sides may be en- 
tirely dissimilar. This is seen sometimes in Baja clavata where the left kidney, 





Fig. 254. Urogenital system of male. A. Sqnatina. B. L'aja. (From Borcea. ) 

C.C., central canal of testis; ct., collecting tube; led., kidney; m.v., median vesicle; nph., 
nephrostome; sc, sperm sac; s.d., segmental duct; s.v., vesicula seminalis; t., testis; «., 
ureter; ug., urogenital sinus; v.d., vas deferens; v.e., vas efferens. 

probably because of pressure from the digestive organs, becomes divided into 
widely separated parts. 

DUCTS OF KIDNEY 



In a type like Sqimiina (see male, fig. 254a) the collecting tubules {ct.) which 
drain the anterior part of the kidney empty directly into the Wolffian duct, 
and those of the posterior part join a ureter (?<.). This is essentially the con- 
dition in Heptanchus, except that in Heptanchus the ureter is of immense 
size. In Scyllium (see p. 189, fig. 177a) the Wolffian duct is terminated by an 
enlarged portion, the urinary vesicle (u.v.), and the collecting tubules in the 
posterior part of the kidney are dispersed at their termini, several of them 
joining to form a diminutive ureter only at the place where they empty into 
the urinary sinus. A modification of this plan is met with in the rays in 



THE ELASMOBRANCH FISHES 



295 



wliicli the part of the kidney lying posterior to tlie urinary sinus is drained by 
collecting tubules (fig. 254b) some of which unite anteriorly into one or two 
groups (ureters), while others of them enter enlarged horns of the urinary 
sinus (female of R. clavata). In the female of the sting ray, Trygon, all the 








w.d. 



A 



B 



Fig. 255. Renal corpuscles, Acanthias. (From Borcea.) A. Active. B. Atrophied and in the 
service of the male sex system. 

cil., ciliated cells; cj)., Bowman's capsule; c.t., collecting tube; gJ., glomerulus; r.t., renal 
tubule; tv.d., Wolffian duct. 

collecting tubules enter the large horns of the urinary sinus independently, 
excepting the single anterior one which is a direct continuation of the small 
Wolffian duct. 

The lower part of the Wolffian duct in a type in which this does not receive 
tubules may be enlarged as the so-called urinary vesicle. In Sqimtina the duct 
is swollen and in Scyllium it is of large size (p. 189, fig. 177a, u.v.). 

From this it is seen that the Wolffian duct decreases in importance in the 
female as we approach the rays. In a type like Squalns sucklii, however, a plan 
is given in which the Wolffian duct assumes a more important role. Here in 
the female (fig. 253a) the ducts receive the collecting tubules from practically 
the whole of the kidney, so that a ureter may be said to be absent, or if present 
to receive only a few tubules. 

The urinary sinus (u.s., fig. 253) into which the Wolffian ducts empty varies 
greatly in size and shape. In the sharks it is simpler in its external form than 
in the rays and may be described as a delta-shaped sac which empties posteri- 



296 THE ELASMOBRANCH FISHES 

orly by a conical urinary papilla. Its complexity in the rays results, from the 
anterior extension of the horns of the sinus, which take the shape of the 
arms of a tuning fork, the narrower urinary sinus being the base (Trygon). 

An incision through the papilla (Squalus, fig. 253a) gives a view of the 
inner walls of the urinary sinus. Emptying into it on each side of the female 
is the Wolffian duct (w.d.), and as evaginations of its walls are certain wdde 
pockets (sc.) comparable to the sperm sacs of the male. 

The relation of the ureter to the Wolffian duct (vas deferens) in the male is 
markedly different from that of the female. Only the ureter wall be described 
at this place since a description of the WoMan duct will be given in a study 
of the genital system. The ureter in the male is confined to the posterior part 
of the kidney where it receives collecting tubules from ten to fourteen seg- 
ments. These may enter regularly' as short tubules along its course {Squalus 
and Squatina) or they may join it in two groups, one at its anterior end and 
the other near its entrance to the urinary sinus {Scyllium, Raja) . In Scyllium 
the ureter of the male is an enlarged sinus, as in Heptanchus. 

FINER ANATOMY OF KIDNEY 

In a section through the Elasmobranch kidney multitudes of structures are 
met which are the effective organs for the removal of nitrogenous waste; these 
are the renal corpuscles. A renal corpuscle, in simple terms (fig. 255a), is like 
a hollow rubber ball (Bowman's capsule) , one side of which has been pushed in 
to form a double wall and the opposite side pulled out over a small area into 
a long neck (renal tubule, r.f.). Into the cavity of this invagination a blood 
vessel enters, coiling up as the glomerulus (gl.) or knot of vessels. Nitrog- 
enous waste matter collected by the blood is brought by the glomerulus into 
the capsule through the w^alls of which it passes into the renal tubule. The 
renal tubule (r.t.) carries it into the collecting tuble (c.f.) which joins the 
Wolffian duct (w.d.) (or the ureter). Thus it passes through the urinary 
papilla and out at the cloaca. 

NEPHROSTOME AND SEGMENTAL DUCT 

In order to understand the origin of a Bowman's capsule a second series of 
correlated structures may first be considered. Each one of these when com- 
plete, consists of a nephrostome {nph.) or funnel (fig. 258) opening from the 
body cavity, a terminal part, the median vesicle {m.v.), and between the two a 
segmental duct {s.d., fig. 258b). 

The nephrostome may best be studied in the Elasmobranchs h\ treating 
them first with Flemming's fluid. To get the best results the digestive tract 
should be removed from a fresh specimen and a little of the fluid allowed to 
remain for a short time in the dorsal part of the body cavity. Under such a 
procedure the nephrostomes are colored as dark patches of the dorsal peri- 
toneal lining of the body cavity near the middle line and on the mesentery. 
These are the funnels which may be relatively large as in Squatina {nph., 



THE ELASMOBRANCH FISHES 297 

fig. 254a), where some of them are two to three and a lialf millimeters in 
diameter; or they may be small as in Scyllium. Frequently when funnels are 
present in the embryo they become rudimentarj^ or wholly absent in the adult 
(Raja). When the ne])hrostonies are present they may be traced along the 
mesorectal mesentery and the mesenteries proper, the most anterior of which 
reach the mesenteries of the sex glands. The number of nephrostomes varies 
according to the species, the individual, the sex, and the age. In a type like 
Squat ina (fig. 254a) there are usually nineteen or twenty pairs present, but 
these may be reduced in number. In the female the number is always smaller 
than in the male. Twenty-three or twenty-four pairs are present in the adult 
Acanthias; while in the embryo there may be as many as thirty-five pairs. 

The segmental ducts lead from the funnels (nephrostomes) outward toward 
the kidney tissue. These may be clearly marked as in Squat ina or they may 



Fig. 256. Diagram of development of segmental ducts and their relation to the Wolffian 
and Miilleriaii duets in the embryo of Scyllium. (From Balfour.) 

fl., funnel; )i2)h., nephrostome; o.d., Miillerian duet (oviduct); s.d., segmental duct; 
w.d., Wolliian duct. 

partly degenerate so that the nephrostome is sessile (ScijlliuDi). In some 
other types no trace either of the ducts or of the funnels remains in the adult. 
In those types in which they are developed, the ducts may run more or less 
directly outward in the midbody region (Squatiua), or they may be V-shaped 
with the apex pointing forward {Squalus sucklii) . In the region of the rectal 
gland they may be more difficult to see by reason of the thickness of the meso- 
rectum. Here in general they extend obliquely backward and may be long 
drawn out (Squatina) or short as in Scyllium. As the segmental ducts pass 
outward they pass above the AVolffian ducts {Acanthias) . 

It was formerly supposed that the segmental ducts were directly continued 
into collecting tubules of the urinary system. If this were so it would be 
possible for waste substances to be collected from the body cavity and passed 
out through the Wolffian duct or ureter to the exterior (see fig. 256, Scyllium). 
Such a connection between the two systems, however, probably does not exist 
for the adult of any forms since in the adult the segmental duct ends blindly as 
the median vesicle ventral to the kidney tissue (see Squatina, fig. 254a, m.v.) . 

Proof that no connection exists between segmental duct and collecting 
tubule has been beautifully shown by the experiments of Schneider (1897) 
who injected India ink mixed with carmine into the body cavity of the living 
Squatina. If a connection exist between the nephrostomes of the body cavity 
and the kidney tissue the ink should be eliminated to the exterior through the 
Wolffian duct or through the ureters. Upon killing the fish a few days after 
the experiment Schneider found that the ink and grains of carmine had col- 



298 



THE ELASMOBRANCH FISHES 



leeted in tlie median vesicles at the end of the segmental duct (see fig. 254a, 

m.v.) and there appeared as large colored patches. In no specimen did the ink 

or carmine which entered the nephrostome gain access to the tissue of the 

kidney. 

DEVELOPMENT OE NEPHKOSTOME AND 

SEGMENTAL DUCT 



A section through the body cavity of an embryo of Heterodontus francisci 
(fig. 257) cuts through the nephrostome or funnel (nph.). If traced farther 

back, it would be found that this funnel 
by means of the segmental duct joins the 
pronephrotic duct (pr.d.). At first the 
most anterior of such funnels may pro- 
vide a passageway from the body cavity 
to the pronephrotic duct, but later the 
most anterior of these fuse into a single 
enlarged funnel. Those segmental funnels 
arising back of this have segmental ducts 
(s.d., fig. 258) which join the pronephrotic 
duct only secondarily. 

In the segments farther back, however, 
each segmental duct of Acanthias grows 
from its funnel laterally, enlarging into a 
median vesicle (m.v., fig. 258a) . From this 
it may even continue inward to join an 
outgrowth from the pronephrotic duct 
(pr.d.). The following structures are 
found in order from the funnel or nephro- 
stome to the pronephrotic duct: (1) the 
nephrostome (nph.); (2) its segmental 
duct (s.d.); (3) a median vesicle (m.v.) 
which supplies the tissue for Bowman's 
capsules; (4) the tube leaving the capsule which becomes the renal tubule 
(r.t.); and (5) the primitive collecting tubule (c.t.) which according to 
Borcea (1906) buds off from the pronephrotic duct and later lengthens out 
into the collecting tubule of the adult. 

At this point in its development, connection is made from the nephrostome 
to the pronephrotic duct (ScylUnm, fig. 256), and it is not impossible that 
while this temporary connection lasts nitrogenous waste may pass from the 
body cavity through the pronephrotic (now the Wolffian) duct and out 
through the cloaca. But this connection is early lost even in Scyllium. Such 
connection between the coelom and the kidney tissue is never actually present 
in Acanthias, for in it the tube early fragments at the median vesicle before 
its terminal part, the renal tubule, has reached the pronephrotic (Wolffian) 
duct. It is from the fragmenting tissue of the median mass that Bowman's 
capsules are formed. 




Fig. 257. Development of nephrostome, 
Heterodontus francisci. (H. M. William- 
son, orig.) 

np/i., nephrostome; of7., oviduct ; pr.d., 
pronephrotic duct. 



THE ELASMOBRANCH FISHES 



299 



BOWMAN'S CAPSULES 

According to Borcea (1906) a Bowman's capsule is i)roduced from a part of 
the median vesicle in the following manner (Acanthias). The median vesicle 
(fig. 258b-c) divides into two parts, a median part connected with the seg- 
mental duct (s.d.) and an outer (deeper) part continued by the renal tubule 
(r.f.). The outer part is the first to give rise to a Bowman's capsule (fig. 258c) 
and this is accomplished by the loose cells from the lining forming over the 
aperture caused by the separation of the median vesicle. Into this the knot of 




u 




kj 




nph 



pr.d.. 



A 



B 



C 



D 




Fig. 258. Diagrams A to E to show stages in the development of Bowman's capsules, 
Acanthias. (From Borcea.) 

c.t., collecting tubule; m.v., median vesicle; nph., nephrostome; pr.d., pronephrotic duct; 
r.t., renal tubule; s.d., segmental duct; I, II, and III, primary, secondary, and tertiary renal 
corpuscles. 

blood vessels (the glomerulus) pushes. The body segment is thus provided at 
first with a single primary (I) renal corpuscle on each side, and a true metam- 
erism obtains in the kidney tissue. As development progresses, however, 
secondary and tertiary Bowman's capsules are formed. Some of these pass 
over into the adjacent segments and soon destroy the primitive metamerism. 
The formation of the secondary Bowman's capsules and their connection 
with the pronephrotic duct takes place as follows: from that part of the 
median vesicle of the segmental canal which remains after the formation of a 
primary, that is, from the superior and inferior vesicles, other Bowman's 
capsules (secondary and tertiary) arise. The secondary capsules are formed 
from the median part of each superior and inferior vesicle and then extend in 
the shape of a gourd toward the collecting tubule (see fig. 258d, //) . It will be 
further noted that enlargements arise on each collecting tubule (c.t.) and 
that these send out processes which meet and fuse with the tips of the gourd- 
like structure, the superior processes fusing with the terminus of the sec- 
ondary Bowman's capsule derived from the inferior vesicle. Upon the fusion 
of this terminus of the gourd with the process of the collecting tubule and the 
breaking through of the connection between them, a secondary urinary tubule 
results (//, fig. 258e). 



300 



THE ELASMOBRANCH FISHES 



Tertiary Bowman's capsules result from the further fragmentation of the 
remains of the inferior and superior parts of the median vesicle. These, like 
the secondary capsules, are gourd-like and their termini unite with tertiary 
processes. The tertiary collecting tubules {III, fig. 258e) result in part as 
processes which spring from the sides of the origin of the secondary process. 
These pass to meet and fuse with the tertiary Bowman's capsules. 

At this stage, Acanthias, 8 cm., each body segment has one primary (/) , two 
secondary (//), and four tertiary Bowman's capsules (III). Four of these, 
one primary, one secondary, and two tertiary, belong to the median vesicle 
from which the primary was derived; the remainder arise from the following 

vesicle. By the time the adult stage is 
reached luimerous renal corpuscles and 
renal tul)ules are present in each seg- 
ment. In fact it is these, together watli 
connective tissue, which make up the 
mass of the kidney. 




Genital System 



Fig. 259. Early sex cells, Acanthias. 
(From Woods.) 
p.o., primitive ova. 



The genital system consists of the sex 
glands and their tubes. The adult glands 
arise as a collection of germ cells {p.o., fig. 259) , which, before passing into the 
glands are scattered more or less widely in the tissues. These cells appear very 
early and are well shown before a genital ridge is formed. They later, through 
migration, take up their position in the sex gland. At this early "indifferent" 
stage it is impossible to tell what the sex of the individual will be. The cells 
then begin to specialize and to take on the characters of the sex cell of the male 
or the female, whereupon the glands become the testes or the ovaries, respec- 
tively. We shall describe these organs in the male first. 



GENITAL ORGANS OF MALE 
TESTES 



The paired testes of the male vary considerably in size. In some of the Elasmo- 
branchs they are relatively small {Torpedo) ; while in many others, especially 
during the breeding season, they are of large size (Heterodontus; Raja clavata, 
fig. 254b) . It often happens at this time that the testes are irregular in outline, 
being made up of numerous lobes {t., fig. 253b, Squalus siichlii). In certain 
forms the testes may be connected posteriorly with the rectal gland by a heavy 
mass of tissue, the epigonal organ {Heterodontus, Scyllium), a vestige of 
which we saw in Heptanchus. An epigonal organ, however, is only slightly 
developed or entirely wanting in many types {Acanthias, fig. 253; Squatina, 
fig. 254a ; and Torpedo). 



THE ELASMOBRANCII FISHES 



301 



FINER ANATOMY OF TESTIS 



The testis is divided into columns, each of which consists of connective tissue 
and follicular cells. The sex cells appear in various stages of development 
which may be followed from the indifferent stage, previously mentioned, to 
the mature sperm cell or spermatozoon. Through division, the primitive ova 




/III' 'i V 



Fig. 260 




Fig. 261 

Fig. 260. Horizontal section cutting testis and anterior part of kidney of Squatina. 
(From Borcea.) 
CO., central canal of testis; I.e., longitudinal canal of epididymus; v.e., vas efferens. 

Fig. 261. Section through ovary of Spinax to show corpus luteum (X), which fills the 
place occupied by the ovum. (From Wallace.) 

of the male form multitudes of spermatogonia, which after a period of rest 
form spermatids. The spermatids next undergo an interesting series of 
changes in shape and form, becoming the adult spermatozoa. 



VASA EFFEEENTIA 
O 

The sex cells in the male instead of passing into the body cavity and out at the 
abdominal pores, as they do in the Cyclostome fishes, pass out through the 
Wolffian duct, now called the vas deferens, which they reach through the vasa 
eff'erentia. The vasa efferentia arise as metamorphosed tubules of the anterior 
segmental ducts. Those ducts in the region of the sex glands of the male have 
some of their funnels opening into the tissues of the testis or into the central 
canal at its base and afford a passageway for the sex cells. 

A horizontal section through the testis and anterior part of the kidney of a 
young Squatina (fig. 260) shows that the segmental ducts themselves thus 
become the efferent ducts or vasa efferentia within the tissues of the testis; 
the parts of the nephrostomes representing the funnels fuse to form a central 
canal {c.c.) while the median vesicles of the segmental ducts similarly join the 
kidney to form the longitudinal canals {I.e.) of the epididymus. From this 
canal a connection is effected, through collecting tubules, with the Wolffian 



302 THE ELASMOBRANCH FISHES 

ducts. Several of the anterior segmental ducts may now become vasa efferentia, 
but others which are on the mesorchium may fail to reach the tissue of the 
testis. Those actually penetrating each testis in Squatina (v.e., fig. 254a) are 
six in number. In Squalus four such enter, while four others end on the mes- 
orchium. In Scyllium two or three vasa efferentia are present and in the rays 
(fig. 254b) a single vas efferens is present. In this form the vas efferens joins 
the vas deferens directly without the intermediation of a longitudinal canal. 

The passageway for the sex cells of the adult male, then, is from the testes 
through the vasa efferentia into the greatly coiled Wolffian ducts each of which 
is now a vas deferens. We may next notice in detail the changes which the 
Wolffian duct and its associated parts undergo in its metamorphosis into the 
vas deferens. We shall first consider the changes undergone in the anterior 
segment of the kidney. 

The anterior part of the kidney, which in the young male is in the service 
of the urinary system, undergoes profound modification in the adult male, 
characteristically coming to be of large size (Borcea, 1906). If a transverse 
section is taken of this area in the adult it will be found to be devoid of the 
Bowman's capsules which were previously present in it (see fig. 255b, Acan- 
thias) . In their places will be found numerous enlarged sacs the walls of which 
have become greatly thickened. The two types of cells which compose the walls 
are: (1) ciliated cells (ciZ., fig. 255b), which border on the lumen, and (2) non- 
ciliated cells with basal nuclei. These cells secrete a viscous whitish substance 
which acts as a seminal fluid. The upper end of the kidney, therefore, which at 
first functioned in the young male in the removal of the nitrogenous waste, 
has thus entirely changed its function in the adult so that it now acts as a 
gland for the secreting of a kind of spermatic fluid. 

Upon the transformation of the anterior part of the kidney, the vas deferens 
becomes a coiled tube {Squalus, fig. 253b; Raja, fig. 254b; Torpedo) which as 
it passes posteriorly receives collecting tubules. In the region of the posterior 
kidney it becomes the enlarged vesicula seminalis into which no collecting 
tubules empty. The vesicula Seminalis in many of the sharks (Squalus, fig. 
253b, v.s.) is a long tube but in the rays (fig. 254b) it is much shorter. 

When opened longitudinally the vesicula seminalis in Squalus shows a 
series of transverse semipartitions which give to the inner wall a corrugated 
appearance. Along this wall in the breeding season are found myriads of 
sperm cells or spermatozoa. 

The seminal vesicles of the right and left side empty into the enlarged 
urogenital sinus (u.s., fig. 253b) usually ventral to and laterad of the entrance 
of the ureter (u.). Passing forward on each side of the urinary sinus is the 
blind sperm sac (sc.) which appears to be an evagination of the urinary sinus 
but which is formed from the posterior remnants of an oviduct like that of the 
female. The sperm sac of Squalus or of Raja is of small size and in the rays it 
has been spoken of as a urinary bladder. In Squatina (fig. 254a) and Scyllium 
(see p. 189, fig. 177a) the sacs reach a much greater length. Since the sinus 
receives both the nitrogenous waste and spermatozoa it is properly designated 



THE ELASMOBRANCII FISHES 



303 



urogenital. The urogenital sinus, as in Heplnuchus, then emi)ties by means 
of a urogenital ])ai)illa into the cloaca. 

Leigli-Sharpe (1920-21) has described fully the sii)lions of a number of 
Elasmobranclis. These are composed of longer or shorter closed sacs which 
end po.steriorly by siphon tubes. In a type like Acanthias the sac lies under the 
skin ventral to the base of the pelvic fin, and its tube empties into the proximal 
part of the clasper tube. In a large specimen of Galiorhinus this siphon sac 
extended almost to the pectoral girdle. The walls of the sac are muscular and 
its function appears to be the forcing 
of the sperm cells through the clasper 
groove. Glands may line the whole sac 
as in Lamna or the dorsal side of its 
wall only (see p. 28, fig. 31) . The func- 
tion of the clasper gland in the latter 
condition is not definitely known. 

GENITAL ORGANS OF FEMALE 
OVARIES 




Fig. 262. Section through shell gland, Scyl- 
Hum. (From Borcea.) 
t.gh, secretory cells. 



The ovaries of the adult female usu- 
ally arise as paired structures, and 
are bound to the anterodorsal wall of 
the body cavity by a mesentery, the 

mesovarium (Squalus, fig. 253a). Not infrequently, however, the left ovary 
atrophies in the adult (Scyllium, Prisfiophorus, Carcharias, Galeus, Mustelus, 
and Zygaena) . They occupy the anterior part of a mass of tissue which, as the 
epigonal organ, may extend along the dorsal wall of the body cavity pos- 
teriorly where it joins the rectal gland. In numerous forms, however, the 
epigonal organ is wanting as, for example, in Acanthias. The ovary varies 
greatly, depending upon the stage of maturity of the ova contained. It appears 
as a sac through the walls of which the ova may be seen varying in size from 
relatively minute spots to mature ova often from 3 to 5 cm. in diameter (see 
01'., fig. 252). 

In development an indifferent sex cell divides several times forming 
oogonia. Each oogonium then undergoes a period of growth to become a pri- 
mary oocyte. By the first maturation division, this primary oocyte gives rise 
to the secondary oocyte and first polar cell. The former soon undergoes the 
second maturation division, thereby forming the ootid and second polar cell. 
The ootid without further division increases in yolk content to become the 
ovum. The ovum then breaks through the wall of the ovary. At this stage it 
contains only one-half the number of chromosomes characteristic of the body 
cells. If it be fertilized by a spermatozoan, which also bears only one-half the 
normal number of chromosomes, the number of chromosomes characteristic 
for the species is restored. 

The place where the egg was located in the ovary now becomes filled up by 
a corpus luteuni {x, fig. 261). 



304 



THE ELASMOBRANCH FISHES 



OVIDUCTS 

The oviducts in an immature female consist of a pair of slender tubes extend- 
ing the entire length of the body cavity and emptying into the cloaca. They 
take their origin by splitting off from the Wolffian duct (see fig. 257, od.) and 
therefore retain the primitive funnel (fl., fig. 256) by means of which they 
open anteriorly into the body cavity. Occasionally one of the oviducts is 
rudimentary in the adult (Trygon). This in all probability is due to the 





Fig. 263. The 
Waite.) 



A B 

shell of Heterodontus. A. E. francisci (orig.). B. H. gal eat us. (From 



crowding of the unusually large valvular intestine. In the adult the oviduct 
is divided into several functional sections which may now be discussed. 

The oviducal funnel or the opening into the body cavity is formed as a 
common aperture for the two oviducts (fl., fig. 251a). This is slit-like and is 
lined with ciliated cells, the cilia of which may have something to do with 
directing the eggs into the oviduct after they have reached the body cavity 
from the ovary. Just below the funnel, in the part comparable to the fallopian 
tube of higher forms, fertilization of the mature egg takes place. The egg then 
passes downward to the area of the shell gland, where such exists, to receive 
its shell. 

SHELL GLANDS 

The shell glands (s.g., fig. 253a) vary greatly in the different Elasmobranchs. 
In Torpedo there are present in this region of the oviduct only a few strands 
of granular tissue, which are incapable of producing a shell. In a type like 



^i 




F'tii. '2()i. Fjgg .slu'll, C('ith<ihisciiUhn\\ 



THE ELASMOBRANCH FISHES 



805 



Squalus the gland is considerably increased in size and in Raja, Scyllium, and 
Heterodontus francisci it becomes relatively of immense dimensions. If the 
gland in Scyllium be taken as a type for study we find that it is divided into 
a dorsal and a ventral half. A section through it shows that these halves are 
divided into anterior and posterior areas; the former secretes albumen, the 
latter the shell proper. The glands which are actively engaged in secreting the 
shell are seen to advantage in figure 262. Here the folds are very high and the 
secretory cells (t.gl.) at their bases are large. As the horny substance is formed 
for the shell it passes into the cavity of the 
shell gland which acts as a mold for the shell. 

TYPES OF EGG SHELLS 

Two types of shells are formed : the per- 
manent and the temporary shells. In the per- 
manent shell the young undergoes its de- 
velopment to the form of the adult, after 
w^hich it emerges {Scyllium^ Raja, Heiero- 
dontus) . In both Scyllium and Raja the shell 
is a rectangle, from the angles of which pro- 
jections extend. These projections function 
either as tendrils (Scyllium.) which coil 
around solid objects and anchor the egg, or 
they serve as spikes to fix the developing egg 
in mud or sand flats (rays). Figure 264 
shows the shell of the California swell shark, 
Cephaloscyllium, which in all essential re- 
spects is like that of the other Scyllidae ex- 
cept that in the figure its tendrils appear 
shorter. These tendrils are, however, long 

and are produced both from the upper and the lower angles. In color the ma- 
ture shell is clear amber of equal shade throughout. Such shells of Cephalo- 
scyllium, however, which are in the process of formation are whitish when first 
removed from the oviducts; but these color w^ith age. 

In Heterodontus the shell is shaped like a screw with a characteristic double 
flange extending from its apex to the large perforate end. The flange in Heter- 
odontus francisci (fig. 263a) or H. philippi is broad and is thrown into four 
or five coils. In Heterodontus galeatus (fig. 263b) the flanges are narrower, 
and tendrils are present which may reach the extreme length of more than 
seven feet (Waite, 1896). 

The attachment of the egg in egg-laying has been studied in Scyllium by 
Kopsch (1897). It is found that as the egg passes through the cloaca its 
tendrils, upon coming in contact with a solid, coil firmly around it, thus fixing 
the egg in place (fig. 265). Two eggs are usually deposited at about the same 
time and many are laid during the season. 




Fig. 265. Attachment of egg of 
ScyUium. (After Kopsch.) 



306 



THE ELASMOBRANCH FISHES 




Fig. 266. Ovidiical valve, SquaJus sucMii. 
(F. Hurni, orig.) 
od., oviduct; ut., uterus; vl., valve. 



The young, thus protected by the shell and supplied with an abundance of 
food yolk, undergo a period of development outside of the body. This period 
varies greatly depending largely upon the temperature of the surrounding 

water. Under favorable conditions the 
eggs may hatch in six or seven months, 
but the period is more likely to ap- 
proximate nine months. At the end of 
this time hatching is accomplished bj' 
the perforation of the upper end 
(Scyllium) , or the lower eiid (Baja) 
of the shell. In Heterodontus the two 
layers at the large or perforate end 
separate making a large aperture 
through which the young emerges. 
In many of the Elasmobranchs a temporary shell is formed which serves 
the young fish only through its early development. From this temporary shell 
the embryos emerge and undergo more or 
less of their development in the oviduct of 
the mother (Acanthias, Mustelus, Rhino- 
hat is). 

A temporary shell is very often a struc- 
ture of exquisite beauty. In Squalus 
SKcklii it consists of a long thin-walled 
tube of a clear amber color, each shell con- 
taining from four to six eggs. The eggs 
undergo their early development incased 
and protected until the time when the ex- 
ternal gills begin to be absorbed. The shell 
then ruptures and the young embryos 
take up their development in the uterus 
of the mother. In Rhmohatis a similar 
shell is found and after it has been dis- 
carded by the embryos it may be found 
rolled up in the uterus. 





UTERUS 



Fig. 267. A. Section of uterine lining 
to show villi and blood supply, Squahis. 
B. Transverse section through a single 
villus, Acanthias. (From Brinkmann.) 

cp., capillaries; t.a., terminal artery; 
vi., villus. 



We have said that the uterus is much more 
highly developed in those sharks which 
give birth to living young; for in these it serves as a place in which a consider- 
able part of the development is undergone. Such a uterus is that of Squalus 
sucklii {ut., fig. 253) in which it may be a greatly enlarged sac with well de- 
fined boundaries. Anteriorly the uterus of Squalus is separated from the for- 
ward part of the oviduct by a well defined oviducal valve (fig. 266) which is a 
wavy constriction with a very narrow lumen effectively closing the uterus to 
the upper part of the oviduct. 



THE ELASMOBRANCH FISHES 



307 



The lining of the uterus differs greatly in oviparous and viviparous types. 
In the former it may be practically smooth or it may be thrown into low folds 
as in Scyllium. In viviparous types it may be singularly modified as maturity 
is reached. In an immature specimen of Squalus the lining is smoother than 
that of the oviparous Scyllium, but in a specimen of Squahis which is pregnant 
the whole surface of the lining is thrown into oblique rows of flaps or villi 
(vi., fig, 267a), each of which is a leaf -like structure with exceedingly thin 
walls. In a type like Torpedo or the butterfly ray, Pterojjlatea micrura, the 
uterine wall may be thickly covered with long papillae some of which in the 
latter may reach 10 to 20 mm. in length. The terminal part of such a papilla 
is shown in figure 269b, in which it is seen that the wall is like a sponge. 




Fig. 268. Section through the uterus of MusteVus antarcticus. (From T. J. Parker.) 
m., muscular layer; mu., mucous liuiug; p., peritoneal layer. 

Viviparous females injected during pregnancy show that the blood supply 
to the uterus is exceedingly profuse. The arterial supply, in Acanthias, for 
example, consists in part from anterior and in part from posterior oviducal 
arteries which break up into branches to the oblique rows of villi. Each ter- 
minal artery courses along the free border or fold of the row of villi (t.a., fig. 
267a) supplying each individual villus with blood. If a cross-section be taken 
through a single villus of Acanthias (fig. 267b) the finer vessels of the villus 
may be made out. The large openings are for the terminal artery (t.a.) and 
the central and smaller apertures are for the central veins which drain the 
villus into a main uterine vein. Around the surface of the villus is the capillary 
net (cp.) connecting the two systems. The same arterial arrangement is pres- 
ent in Scymnus, although here a single villus is not so wide. 

KELATION OF UTEEUS TO EMBRYO 



The relation of the villi to the embryo is seen to advantage upon opening the 
uterus of Acanthias. Here the villi come in close contact with the embryo and 
multitudes of them are found clinging to the yolk sac on which the embryonic 
blood system is profusely spread out. By the close relation of the embryo to 
the maternal tissue an exchange between the two is insured. 

In Mustelus laevis the blood system of the yolk sac comes into still closer 
relation with the walls of the uterus than in Acanthias. For here, branched 



308 



THE ELASMOBRANCH FISHES 



processes from the yolk sac of the embryo form close attachment to the uterine 
wall. Through this attachment nutriment may be secured by the embryo. 

The relation of the uterine wall to the embryo in Mustelus antarcticus (fig. 
268) shows still another widely different relationship. In this form, according 
to T. J. Parker (1882), the uterus, by the ingrowth of its lining, becomes 



yi^ 



'• A ^J^ 



% 













Fig. 269. Development of the butterfly ray, PteropJatea micritra. (From Alcock.) 
A. The embryo in the uterus. B. A tip of a single villus highly magnified. 



divided up into as many rooms as there are embryos within the uterus. The 
uterine wall is composed of an outer peritoneal lining (p.), a second or thin 
muscular layer (m.) , and a third or inner mucous lining (mu.) . It is the last- 
named layer that grows out to form the partitions separating the uterus into 
rooms. These rooms are filled with fluid in which the embryos lie and by which 
they are protected. In this type of Elasmobranch we see a device for protecting 
the developing young which, in a way, is like that in higher animals. In this 
form, however, the protective sac is produced by the maternal tissue, while 
in higher forms it is formed by the embryo. 

In Pteroplatea micrura the villi or papillae on the uterine wall of the mother 
may be numerous and those which are over the spiracle of the embryo become 
long and strap-shaped (fig. 269a) . In an embryo that is far advanced the yolk 



THE ELASMOBRANCH FISHES 



309 



sac is small and its blood supply is lacking. There is therefore no passage of 
nutriment from the villi of the female through the blood system on the yolk 
sac. But the long strap-like villi (fig. 269b) enter the spiracle and supply 
nutriment direct to the digestive tract, as can be demonstrated by opening up 
the digestive tract of the embryo ( Alcock, 1892) . 

At their posterior terminus the two uteri in an immature female may be 
separated from the cloaca by a mem])rane or hymen across the oviduct. The 
relation of the hymen (in Torpedo) may be seen from figure 270 by Wida- 
kowich (1908) . The median union of the right and left uteri is prolonged back- 
ward toward the cloaca by the uterine septum {s.) and a fold on each side 
separates the oviduct from the cloaca. During 
pregnancy the uterus is filled with a fluid and the 
apertures remain closed. 

In Elasmobranchs in which the shape of the 
claspers of the male is flat, the openings into the 
uteri are slit-shaped, and in those forms in which 
the claspers of the male are provided with sharp 
hooks the lining of the terminal part of the uterus 
is thickened. 



Secondary Sexual Characters 




Fig. 270. Diagram to sliow 
the hymen between oviduct 
and cloaca. (From Widako- 
wich.) 

cZ., cloaca; 7ii/., hymen; o.d., 
oviduct; s., uterine septum. 



Barring the fact that the female may be slightly 
larger than the male, the most important second- 
ary character separating the sexes in the Elasmo- 
branchs is the presence of claspers in the male. 
These, as we have seen, are formed as modifica- 
tions of the inner lobe of the pelvic fin. In types like Heptanchus maculatus 
the claspers in immature specimens are relatively insignificant so that it is 
often difficult upon casual examination to distinguish male from female. In 
most other types, however, the claspers are well developed and in the rays 
they may be of enormous size. 

In one immature specimen of Heptanchus supposed to be a male, in addition 
to the rudimentary oviduct only one of the pelvic fins bore a clasper. The con- 
dition of gynandromorphism, in which one side of the body is male, the other 
female, has been found in insects and birds, and its occurrence in the Elasmo- 
branchs has also been previously noted ( Vayssiere and Quintaret, 1915) . 

The mucous covering of the claspers is usually devoid of placoid scales and 
is provided with a lubricant. Distally, as we have seen in a study of the skele- 
ton, the claspers are provided with one or more terminal pieces. These by 
muscular action may be erected at right angles to the main axis of the clasper. 

The claspers have long been known to function in uniting the male and 
female in copulation. This process among the Selachians was early studied 
byAgassiz (1871) who discovered that one (or both) of the claspers is inserted 



310 THE ELASMOBRANCH FISHES 

into the cloaeal opening of the female and fixed in position by the erection of 
terminal pieces. The sperm cells reach the groove or tube in the claspers and 
are forced thence into the oviduct of the female by a current produced by the 
siphon (Acanthias). The cells traverse the oviduct to the region of the fal- 
lopian tube where they remain until the mature eggs enter the oviduct and 
fertilization takes place. 



THE ELASMOBRANCH FISHES 311 

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Chapter XT 

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312 THE ELASMOBRANCH FISHES 

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THE ELASMOBRANCH FISHES 313 

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314 THE ELASMOBRANCH FISHES 

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THE ELASMOBRANCH FISHES 315 

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316 THE ELASMOBRANCH FISHES 

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THE ELASMOBRANCH FISHES 317 

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318 THE ELASMOBEANCH FISHES 

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1892. ZiEGLER, H. E., and Ziegler, F., Beitrage zur Entvfickelungsgeschichte von Torpedo. 
Arch. mikr. Anat., Bd. 39, pp. 56-102, pis. 3-4, 10 text figs. 

1894. Ziegler, H. E., Ueber das Verhalten der Kerne im Cotter der meroblastichen Wir- 

beltiere. Ber. d. natur. Gas. Freiburg, Bd. 8 (Festschr. f. Weismann), pp. 192-209. 
1898. Ziegler, H. E., Der Ursprung der mesenchymatischen Gevrebe bei den Selachiern. 

Arch. mikr. Anat., Bd. 32, pp. 378-400, pi. 13. 
1902. Ziegler, H. E., Lehrbuch der vergleichenden Entwickelungsgeschichte der niederen 

Wirbeltiere. Jena. 
1908. Ziegler, H. E., Ein Embryo von Chlamydoselachus anguineus Garm. Anat. Anz., Bd. 

33, pp. 561-574, 7 text figs. 



INDEX* 



Abdominal pores, 126, 142; function of, 
in Cyclostomes, 142 

Abducens nerve, 223-224, 236, 241; fora- 
men of, 44, 60; muscle supplied by, 
100, 223-224, 241; nucleus of, 236 

Abyssal habitat, 1 

Aca7ithias, 2, fig. 5; cf. Urolophus, 8, 10, 
11 

Acanthodes, 1 

Accessory efferent-collector arteries, 180 

Accessory lateral cutaneous vein, 217 

Accessory valves of conus, 172 

Accommodation of eye, 269-270 

Adductor mandibulae, 93, 107; artery to, 
182; function of, 93; nerves of, 241 

Adductor muscles, 93, 107; absent from 
hyoid, 93, 107; of Chlamydoselachus, 
107-108; of claspers, 95, 110-112; of 
visceral arches, 90, 103, 107 

Adductores arcus, 107 

Advehentes, 200, 208 

Afferent arteries, 149, 161, 172-173; 
branchial, 161, 172, 173; formation of, 
178; hyoidean, 161, 172, 173; in holo- 
branch, 149, 153, 157, 173 

Alisphcnoidal cartilage, 53, 55 

Alopias, 2, fig. 2 

Amphistylic, 45 

Ampullae of Lorenzini, 7, 262, 274, 279- 
280; development of, 281; distribution 
of, 262, 279-280; modified in spiracle, 
153, 280; nerves to, 224, 242, 243, 262, 
280 

Ampullae of semicircular canals, 258-259, 
273 ; nerves to, 224-225 ; 261, 273 ; struc- 
ture of, 273 

Ampullary centrum, 280 

Ampullary organs. See Ampullae of Lor- 
enzini 

Ampullary pockets, 280 

Anal fin, 6, 12, 52; absence of, 12; ar- 
teries to, 194; skeleton of, 52, 83; vein 
from, 217 

Ancient sharks, 1, 2 

Angel fish. See Squatina 

Annular arteries, 167, 188 

Annular veins, 201, 210, 211 

Anterior cardinal sinus, 198, 205, 206, 207 

Anterior cardinal system, 204-208 

*For coordination of entries see Contents. 



Anterior cardinal vein, 198, 204 

Anterior cerebral artery, 165, 183 

Anterior cerebral vein, 204, 206-207; fo- 
ramen for, 44 ; tributaries of, 206-207 

Anterior dorsolateral artery, 168, 191 

Anterior facial vein, 204 

Anterior fontanelle, 43, 57 

Anterior gastric artery, 166, 190 

Anterior gastric vein, 201 

Anterior gastro-pancreaticosplenic artery, 
167, 189-190 ; in Dasyatis, 188 

Anterior gastro-pancreaticosplenic vein, 
200, 211 

Anterior gastrosplenic artery, 188 

Anterior intestinal artery, 166, 188; 
branches of, 166 

Anterior intestinal vein, 201, 210-211; in 
rays, 210; tributaries of, 201, 210-211 

Anterior lateral artery, 192. See also Lat- 
eral abdominal artery 

Anterior lobe of hypophysis, 234 

Anterior mesenteric artery. See Superior 
mesenteric artery 

Anterior oblique semicircular canal, 258, 
261, 273 

Anterior rectus muscle. See Internal rec- 
tus muscle 

Antorbital process, 44, 60 ; function of, 60 ; 
muscle from, 93 

Aortic arches. Sec Embryonic aortic arches 

Appendicular skeleton, 49-52, 75-83 ; div- 
isions of, 49 

Aqueduct of Sylvius, 235-236 

Aqueous humor, 267 

Arcuales communes muscles, 93 

Arcus communes, 108, 109 

Argentium, 27 

Armament, 1 

Arteria spinalis, 165, 193 

Arteries, 161, 170, 172; cf. veins, 170; iu 
Elasmobranchs in general, 172-194; in 
Heptanchus, 161-169; to deeper struc- 
tures, 169, 192-193; to digestive tract, 
166-168, 186-191; to extremities, 168, 
169, 191-192; to head, 165, 180-186; to 
heart, 163-164, 179-180; to hypobran- 
chial area, 162-165, 178-180; to trunk, 
186; to tail, 169, 186, 193-194 

Ascending aorta. See Ventral aorta 



[319] 



320 



THE ELASMOBRANCH FISHES 



Asterospondyly, 75 

Atrium. See Auricle 

Auditory (otic) capsule, 43, 55, 258, 271; 
in embryo, 55 

Auditory nerve, 224-225, 243, 261, 273; 
ganglion of, 224-225, 243 

Auditory organ (ear), 258-259, 264, 271- 
273 

Auricle, 160, 170-171 

Auriculoventricular valves, 161, 171 

Autonomic nervous system. See Sympa- 
thetic system 

Axial cartilage. See Basal cartilage 

Axial skeleton, 43-49, 53-75 

Axis cylinder, 238 

Axone, 229, 238; origin of, 238 

Basal angle, 44, 55; origin of, 53—55; re- 
lation of, to orbital process, 44 

Basal (axial) cartilage, 50, 51, 81 

Basal (germinative) layer of epidermis, 
23, 26 

Base of scale, 24, 32 

Basibranchial cartilage, 46, 65, 66 

Basilar artery, 186 

Basipterygium, 50, 51, 81, 82 

Batoidei, 8 

Beaker cell. See Gland cell 

"Beta" cartilage, 50, 51, 80, 95 

Bile, 137 

Bile duct, 124, 137 

Bipolar nerve cell, 229, 238; development 
of, 229, 238 

Blood, 204 

Blood stream, 170 

Body cavity. See Coelom 

Body shape. See External form 

Bowman's capsule, 296, 298, 299-300, 302: 
origin of, 299-300; transformation of, 
in male, 302 

Brachial artery, 168, 191; in ray, 191 

Brachial vein, 202, 213, 214 

Brachioscapular artery, 168, 179 

Brachioscapular vein, 214 

Brain, 221-222; 229-236; arteries to, 165, 
183-186; development of, 230; form of, 
230-235; finer structure of, 236; veins 
of, 198, 205, 206-207 

Branchial adductors, 93, 107 

Branchial afferent arteries, 161, 172-173; 
in Chlamydose'laclius, 173 

Branchial arches, 45-46, 64-66; muscles 
of, 93, 105-108; relation of, to gill 
pouch, 147-148; segments of, 45-46, 



64-66; supernumerary rudimentary, 47, 

65-67 
Branchial basket, 46, 204 
Branchial clefts, external, see Gill clefts; 

internal, see Internal branchial aper- 
tures 
Branchial efferent arteries, 165, 182 
Branchial nerves: of glossopharyngeal, 

244; of vagus, 149, 225-226, 245 
Branchial rakers. See Gill rakers 
Branchial rays, 45, 46, 67 ; in Torpedo, 67 ; 

on hyoid arch, 45, 67; relation of, to 

gill septum, 91, 152; relation of, to 

muscles, 93, 105-106, 152 
Buccal ampullae, 262, 279; divisions of, 

262, 279; nerve to, 224, 242 
Buccal artery, 182 
Buccal cavity, 121-122, 128; lining of, 

128; stomodeal denticles in, 31, 38, 128; 

teeth in, 38, 128-131 
Buccalis nerve, 224, 242; function of, 262, 

279; ganglion of, 242 
Bursa entiana, 136 

Calcification, 53, 74-75 

Canals of head, 261-262, 274, 276-277; 
development of, 275; divisions of, 274- 
275; in rays, 277-278; nerves to, 262, 
278-279 

Capillaries, 153, 157, 161, 170, 173, 212 

Carciiarias, 2, 9, fig. 16 

Cardiac stomach, 123, 135 

Cartilage, 53 

Cartilaginous branchial rays. See Bran- 
chial rays 

Caudal (aorta) artery, 166, 169, 186 

Caudal fin, 6, 12, 14; skeleton of, 51-52. 
74 

Caudal vein, 199, 208 

Caudal vertebrae, 72, 74 

Caudate lobe of liver, 124, 137 

Central canal of testis, 290, 301 

Central nervous system, 221-222, 229- 
238; development of, 230 

Centrum, 48, 69, 71; calcification of, 70; 
development of, 70 

Cephalic canals. See Canals of head 

Cephaloptera, 3 ; locomotion in, 13 

Cephaloscyllium, 2, fig. 1; color in, 27 

Ceratobranchial cartilages, 45, 46, 64, 65, 
67; muscles to, 92, 152 

Ceratohyoid cartilage, 45 

Cerebellum, 222, 234 



INDEX 



321 



Cerebral arteries, 165, 183-18G; divisions 
of, 165, 183 

Cervical plexus, 227, 246 

Cetofliinus (Selache) maximus, 3, fig. 4; 
gill rakers of, 37, 38, 154, 155 

ChcilosciilUum, color of, 26 

Chlamydoselachus anguincus, adductor 
muscles of, 108 ; afferent arteries of, 172- 
173; cranium of, fig. 46; dorsal fin of, 
6; duct to thyroid, 134; nervous col- 
lector of, 247; plan of fin skeleton, 77; 
teeth of, 128-130 

Chorda. See Notochord 

Chorda tympani, 224, 243 

Chordae tendineae, 161, 171 

Choroid coat of eye, 269 

Chromatophores, 26 

Circular constrictor muscles. See Super- 
ficial constrictors 

Ciliary body, 267, 268 

Ciliary ganglion, 240; relation to sympa- 
thetic, 247 

Ciliary nerve, 223, 241 

Circle of Willis, 184 

Circulation of blood, in gill filament, 157 

Circulatory system, 160, 170, 198, 204; 
divisions of, 170; of Elasmobranchs in 
general, 170-194, 204-218; of Heptan- 
clms macuJatus, 160-169, 198-203 

CJadoselacJius, 1, fig. 10 ; muscle fibers of, 
1; paired fins of, 15 

Clasper, 6; as secondary sexual character- 
istic, 309; function of, 291, 309; mus- 
cles of, 95, 110-112; relation of, to 
apertures of uterus, 309-310; relation 
of, to siphon, 310; skeleton of, 51, 81- 
82 

CUmatius (?), 1, fig. 11 

Cloaca, 126, 141; ducts to, 126, 141, 291, 
298; lining of, 126, 141 

Cloacal papillae, 126, 141 

Cloaeal pits, 141 

Cloacal vein, 203, 213 

Coeliac axis, 166, 186; branches of, 166, 
167, 186-188 

Coeliacomesenteric artery, 190 

Coelom, 96 

Collecting tubules, 287, 289, 294, 295, 297, 
298, 299; development of, 299-300 

Colon, 126, 141 

Color, 6, 26-27 

Commissural arteries, 162, 178-179 

Compressor muscle, 95, 111-112 

Cones of retina, 269 



V 



/, 



("(instrictor of sac, 95 

( 'onstrictor spiraculae,^ 103 

Conus arteriosus, 160-161, 170, 172 ; valves 
of, 161, 172 

Coracoarcuales muscles, 93, 94, 108 

Coracobranchial muscles, 94, 109 

Coracohyoideus muscles, 94, 109 

Coraeoid artery, 163, 168, 179, 192 

Coracoid cartilage, 49, 79, 80, 81 

Coraeoid vein, 202, 203, 217 

Coracomandibularis muscles, 93-94, 109 

Corium, 23, 26; origin of, 96 

Cornea, 258, 267 

Coronary artery, 164, 179-180; posterior, 
164, 168 

Coronary vein, 215 

Corpora bigemina. See Optic lobes 

Corpora restiforme, 222, 235 

Corpus luteum, 301, 303 

Cranial canals. See Canals of head 

Cranial nerves, 223-227, 238-246; abdu- 
cens, 223-224, 241; auditory, 224-225, 
243; facial, 224, 241-243; glosso- 
pharyngeal, 225, 243-244; oculomotor, 
223, 239-240; olfactory, 223, 238-239; 
optic, 223, 239; trigeminal, 223, 240- 
241; trochlearis, 223, 240; vagus, 225- 
227, 244-245 

Cranium, 42-44, 53-61; development of, 
53-55; of Chlamydoselachus, 43; of 
Zygaena, 58-60 

Cross-trunks, 162, 174, 175, 176-177 

Crystalline lens. See Lens 

Cupula terminalis, 272, 273 

Cutaneous veins, 204, 216-217; nature of, 
217-218 

Cuticular plate. See Dermatome 

Cyclospondyly, 74 

Danielian sinus, 198, 199 

Basyatis dipterura, anterior gastro-pan- 

ereaticosplenic artery of, 190; sting of, 

37 
Demibranch, 148; absent behind last 

cleft, 176; kinds of, 148 
Dendrite, 229 
Dental ridge, 128 

Dentinal canals, 32, 35, 37, 130-131 
Dentinal papillae, 128 
Dentine, 32, 33, 131; of fin spine, 33-34; 

of saw tooth, 35; of sting, 37; types of, 

33-35 
Depressor hyomandibularis, 104, 105 
Depressor of lid, 102 



322 



THE ELASMOBRANCH FISHES 



Depressor mandibulae, 105 

Depressor rostri, 105 

Dermal fin-rays, 89, 94 

Dermal papilla, 30 

Dermatome, 96 

Dermis. See Corium 

Digestive tract, 121-126; 127-142; ar- 
teries to, 166-168, 186-191; develop- 
ment of, 127-128; mesenteries of, 121, 
127; veins of, 200-201, 210-212 

Diencephalon, 221, 230-233; cavity of, 
236; outgrowths from, 233 

Digitiform gland. See Eectal gland 

Dilator muscle of sac, 95, 111, 112 

Dilator spiraculae, 102-103 

Diphycercal, 74 

Diplospondyly, 51, 72, 74; function of, 
74; incomplete, 49 

Bisceus thayeri, 10, fig. 21 

Diverticula of spiracle, 148, 153 ; nerve to, 
153 

Dorsal aorta, 165-166, 186; development 
of, 186; paired, 165, 186; tributaries 
of, 166, 186; unpaired, 165, 186 

Dorsal bundles, 89, 96, 99 

Dorsal constrictor muscles, 90-91, 102- 
104 

Dorsal cutaneous vein, 202, 203, 216-217 

Dorsal fin, 6, 12, 14; in notidanids, 6; 
position of, 12 ; skeleton of, 51, 82-83 

Dorsal gastric artery, 187 

Dorsal horn of cord, 222, 237 

Dorsal intercalary plate, 47, 48, 70, 71 

Dorsal intestinal artery, 167, 190 

Dorsal intestinal vein, 200-201, 211; 
tributaries of, 211 

Dorsal marginal cartilage, 82 

Dorsal myelonal vein, 208 

Dorsal plate, 47, 48, 72; foramen of, 48; 
incomplete diplospondyly, 49 

Dorsal pterygial vein, 213 

Dorsal root ganglion, 227, 246 

Dorsal root nerve, 227, 246; foramen of, 
48, 210, 246 

Dorsal spinal vein of ray, 210 

Dorsal suspensory ligament of eye, 270 

Dorsolateral artery, 168, 191 

Dorsolateral bundle, 97 

Dorsomedian bundle, 97 

Duct of Cuvier, 171; tributaries to, 198- 
199, 200, 202, 205, 210, 212, 214 

Ductless glands, 134 

Ducts of kidney, 294-296 

Ductus choledochus, 124, 137-138 



Duodenum, 124, 136-137; arteries to, 166- 
167; blind sacs of, 137; derivatives of, 
136-137; ducts to, 124, 137, 138; mesen- 
tery of, 121 

Ear, 258-261, 264, 270-274; capsule of, 
258, 271; development of, 274; nerve 
of, 261, 273; parts of, 258-259 

Ear stones. See Otoliths 

Ectethnioidal process. See Antorbital pro- 
cess 

Efferent arteries, 165, 182; branchial, 
165, 175, 182; development of, 175; 
hyoidean, 165, 182 

Efferent branchial arterioles, 157 

Efferent-collector arteries, 161-162, 173- 
177; branches of, 163-165, 178-180; de- 
velopment of, 175-177; in holobranchs, 
149, 153, 157, 162 

Egg (ovum), 303; development of, 303; 
hatching of, 305-306 

Eggshell, 305-306; attachment of, 305; 
flanges on, 305; perforation of, 306; 
production of, 305; tendrils of, 305; 
types of, 305-306 

Elasmobranch fishes, divisions of, 8; se- 
ries of, 9-10 

Elastica externa, 69, 70 

Elastica interna, 69, 70 

Electric cone, 114 

Electric discs, 112, 113, 114; layers of, 
114 

Electric nerve, 115, 243, 244 

Electric organ, 112; anatomy of, 114-115; 
in Torpedo, 115; nerves of, 115, 243, 
244; origin of, in rays, 114 

Electric ray {Torpedo), 3 

Embryonic aortic arches, 175 ; derivatives 
of, 178 

Enamel, 30, 32; derived from, 132-133; 
in rays, 32; nature of, 32; of fin spine, 
33; of saw tooth, 35; of sting, 37; of 
teeth, 128, 130; in Carcharias, 132-133 

Enamel organ, 30, 33, 35, 128 

Endolymphatic ducts, 5, 43, 55, 259, 271, 
272, 273 

Endorachis, 237 

Endoskeleton. See Skeleton 

Ependymal cells, 229-230 

Epiblastic fold, 15 

Epibranchial cartilages, 45, 64, 65, 93, 107 

Epidermis, 23, 28, 30; derivatives of, 28- 
38; layers of, 23, 26, 30 

Epididymus, 301 



INDEX 



323 



Epigastric artery, 163, 179 

Epigonal organ, 190, 300, 303; artery to, 
190; relation of, to rectal gland and 
testes, 300 

Epiphysis, 233; development of, 233 

Erythrocytes, 170 

Ettnopterus, light organs of, 29-30 

External branchial apertures. See Gill 
clefts 

External carotid artery, 164, 180 

External filaments of embryo, 148, 151, 
152 

External flexor muscle, 95, 110-112 

External form, 5-7, 8-16; in development, 
10-12 

External mandibular (VII) nerve, 243; 
divisions of, 243; ganglion of, 243 

External (posterior) rectus muscle, 90, 
100; nerve to, 100, 223-224, 241; origin 
of, 99 

Extrabranchial cartilages, 47, 67, 69; re- 
lation to gill septum, 69, 149 

Extrahyal cartilage, 67, 69 

Extraseptalia, 69 

Extravisceral cartilages, 47, 67, 69 

Eye, 5, 258, 266-270; development of, 
269; muscles of, 89-90, 99-100; orbit 
of, 58; pupil of, 258; structure of, 267- 
269 

Eyeball, 60, 258; muscles to, 89-90, 99, 
101 

Eyelid, 5, 258, 267; muscles to, 102; mov- 
able, in ScyUium, 267 

Eye muscles, 89-90, 99-100, 258; develop- 
ment of, 99-100; nerves to, 221, 223, 
240, 241, 242 

Facial nerve, 224, 241-242; divisions, 224, 
241-242; nucleus of, 241; sensory fibers 
of, 241 

Fallopian tube, 304, 310 

Fasiculi lateroventrales, 235 

Fasiculi longitudinales mediales, 235 

Femoral artery, 169, 192 

Femoral vein, 202, 213 

Fenestrae, 43, 55, 271 

Fibrillae of nerve, 238 

Filaments. See Gill filaments 

Fins, 1; anal, Heptanchns, 6, 12; ancestral 
type of, 14-16; caudal, Meptanchus, 6, 
12; dorsal, Heptanchus, 6, 12, 13; form 
and position of, 12-16 ; function of, 12- 
14 

Fin-fold theory, 14-15, 247 



Fin spine, 33; development of, 33-34; 
parts of, 33 

Foramen of (foramina): abducens nerve, 
44; 60; anterior cerebral vein, 44; dor- 
sal plate, 48; dorsal root nerve, 48, 
246; facial, 44, 60; intercalary plate, 
48; internal carotid artery, 53; inter- 
orbital canal, 44; magnum, 57; oculo- 
motor nerve, 44, 60 ; ophthalmicus pro- 
fundus nerve, 43, 60; ophthalmicus su- 
perficialis (VII) nerve, 44, 60; orbito- 
nasal canal, 44; optic, 44, 60; pectoral 
girdle, 49, 81; pelvic girdle, 51; ramus 
anastomoticus artery, 44 ; trochlear, 44, 
60; ventral root nerve, 48, 246 

Forebrain, 230 

Formatio-reticularis, 237 

Fossa rhomboidalis, 235 

Fourth ventricle, 222, 235, 236 

Funnel (nephrostome), 296 

Funnel of oviduct, 290 

Galeus, color of, 26 

Gall bladder, 124, 137 

Ganglion: gasserian, 240, 241; geniculate, 
242, 243; habenular, 236; oculomotor, 
240; ophthalmicus profundus, 240; 
ophthalmicus superficialis (V), 240; 
sympathetic, 247-248 

Gastric artery, 166-167, 187 

Gastric juice, 135 

Gastric veins, 200, 201 

Gastroduodenal artery, 166, 167, 188 

Gastrohepatic artery, 166, 186 

Gastro-pancreaticosplenic artery, 188 

Gastrosplenic vein, 201 

General cutaneous nucleus, 236 

Genital glands, 290 

Genital organs: of female, 290, 291, 303- 
309; of male, 290, 291, 300-303 

Genital ridge, 300 

Genital system, 290-291, 300-309; rela- 
tion of, to urinary system, 292 

Germ cells, 300 

Germinative layer of epidermis, 23 

Gill. See Holobranch 

Gill-arch theory of Gegenl)aur, 14-15 

Gill clefts, 5, 11, 132, 147, 148, 150; in Hep- 
tanchus, 147; in rays, 150; in Squatina,9 

Gill filaments, 147, 148, 149, 151, 152, 154, 
173, attachment of, 149, 151; blood to, 
153, 157, 175; circulation in, 157; em- 
bryonic, 148, 152; on spiracular pocket, 
153 



324 



THE ELASMOBRANCH FISHES 



Gill pocket, 147, 150; accessory, 151; 

apertures of, 147, 151; development of, 

151; fiiaments on, 147; in Heptanchus, 

147 
Gill rakers, 37, 147, 154; of Cetorhinus, 

37, 154; of Squalus sucklii, 37, 154 
Gill septum, 147, 148-149; attachment of, 

149 
Gill supports, 148 
Gland cell, 23, 28; in buccal cavity, 28; 

in cloaca, 28; lumen of, 28; modified, in 

light organs, 29; of claspers, 28; of 

sting, 28; origin of, 28 
Glomeruli of olfactory bulb, 239, 264 
Glomerulus, 296, 299 
Glossal projection, 65 
Glossopharyngeal nerve, 225, 243-244; 

branches of, 225, 244; ganglion of, 225, 

244; nucleus of, 243 
Goblet cells. See Gland cells 
Golden cells, 27 
Grey matter of cord, 237 
Guanin, 27 
Gular line, 263 

Gustatory organs, 264, 265-266 
Gynandromorphism, 309 

Habenular ganglion, 236 

Haemal arch, 47 

Haemoglobin, 170 

Hair cell, 273, 278; of ampulla, 273; of 
neuromast, 278 

Hammerhead shark (Zygaeno), 2. 

Head somite, 99 

Heart, 160, 170-172; arteries to, 163-164, 
178-180; nerves to, 227; position of, 
160; rooms of, 160, 170; veins of, 215- 
216 

Hepatic artery, 166, 187 

Hepatic portal system, 200, 204, 210-212; 
development of, 212; parts of, 200- 
201, 210-212; relation of, to subintes- 
tinal, 208, 212 

Hepatic portal vein, 201, 211; tributaries 
of, 211 

Hepatic vein, 200, 201, 211, 213 

Heptanchus, figs. 12, 13, 14, 15 

Heptanchus maoulatus, 2, 3; antorbital 
process of, 44; arteries of, 161-169; 
digestive tract of, 121-126; endoskele- 
ton of, 43-52; external form of, 5-7; 
genital system of, 290-291; integument 
of, 23-25; musculature of, 89-95; ner- 
vous system of, 221-228; respiratory 



tract of, 147-149; special senses of, 

258-263; veins of, 198-203 
Heptranchias, 3 note 
Heterocercy, 74 
Heterodontus francisci, 2, fig. 17; egg case 

of, 305; melanophores of, 27; teeth of, 

130 
Hexanchus, 2, 3 ; hypobranehial arteries 

of, fig. 169 
Hindbrain, 230 
Holobranch, 148 
Horizontal semicircular canal, 258, 273; 

development of, 274 
Horns of cord, 222, 237; dorsal, 222, 237; 

ventral, 222, 237 
Hymen, 309 
Hyoid arch, 45, 63; attachment of, 45, 

58, 63; muscles of, 92 
Hyoidean afferent artery, 161, 172 
Hyoidean ampullae, 279-280 
Hyoidean efferent artery, 165, 182 
Hj-oidean nerve. See Eamus hyoideus 

(VII) 
Hyoidean sinus, 208 
Hyoidean somite, 99 
Hyoidean vein, 208 
Hyomandibula, 63-64 
Hyomandibular canal, 261, 274, 277; 

modification of, in rays, 277; nerves to, 

224, 242, 243, 279 
Hyomandibular nerve, 224, 242, 243; 

branches of, 224; foramen of, 44 
Hypobranehial arteries, 162-165, 178- 

180; lateral, 163, 178; median, 163, 179 
Hypobranehial cartilages, 45-46, 64, 65; 

attachment of, 46; muscles to, 94 
Hypobranehial muscles, 90, 93, 108-109; 

nerves to, 227, 246; origin of, 108 
Hypophysis, 233; divisions of, 233 

Iliac artery, 169, 192 

Iliac vein, 213 

"Indifferent stage" of sex, 300 

Inferior jugular vein, 199, 204; tributaries 

of, 199 
Inferior lobes of brain, 221, 233 
Inferior lobes of hypophysis, 234 
Inferior mesenteric artery, 167-168, 186, 

190-191 
Inferior oblique muscle, 89, 100; in Pris- 

tiophorus, 100; origin of, 99; nerve to, 

100, 223, 239-240 
Inferior rectus muscle, 90, 99; origin of, 

99; nerve to, 100, 223, 239-240 



INDEX 



325 



Infiaoil)ital canal, 242, 261, 274, 277; 

nerve to, 224, 242, 279 
Infraorbital plate, 53, 57; absence of, 57 
Infundibulum, 221, 233, 236 
Inner buccal ampullae, 262, 279 
Inner zone, 70 
Integument, 23, 26; in Elasmobranchs in 

general, 26-38; in Eeptanchm, 23-25 
Interarcuales muscles, 90, 92, 106; dorsal, 

92, 106; lateral, 92, 106-107 
Intcrbranchial muscles, 91, 105-106, 149, 

151 
Intercalary plate, 48, 72 
Intercostal arteries, 169, 193 
Intermediate lobe of hypophysis, 234 
Internal branchial aperture, 37, 147, 154; 

relation to gill rakers, 154 
Internal carotid artery, 165, 182-183; fo- 
ramen of, 53; relation to ramus auas- 

tomoticus, 165, 183 
Internal flexor muscle, 95, 112 
Internal mandibular nerve, 243 
Internal pretrematicus (IX), 225 
Internal (anterior) rectus muscles, 90, 99; 

nerve to, 100, 239; origin of, 99 
Interorbital canal, 44, 205 
Interorbital vein, 198, 205 
Intima, 170 

Intraintestinal artery, 166, 188 
Intraintestinal vein, 200, 210, 212; in 

Zygaena, 210 ; relation of, to subiutes- 

tinal vein, 210, 212 
Iris, 258, 267; of light organ, 29 

Kidney, 287, 289, 292; arteries to, 169, 
193; ducts of, 294-296; finer anatomy 
of, 296; in Elasmobranchs in general, 
292-294; in Heptanchus, 287; in rays, 
292 ; metamerism of, 292, 299 ; metamor- 
phosis in male, 292, 302; sexual differ- 
ences in, 289, 292; veins of, 200, 208, 
209, 210 

Labial cartilage, 67, 68; in Heptanchus, 
68; in Hexanchus, 68 

Laemargus, photophores of, 29 

Lagena, 261, 272; nerve to, 273 

Lamina terminalis, 239 

Lamna cornubica, 9 ; muscles in tail of, 97 

Lateral (abdominal) artery, 168, 179 

Lateral abdominal system, 202, 204, 214, 
217; history of, 214-215 

Lateral abdominal vein, 202, 213; tribu- 
taries to, 202, 213-214 



Lateral bundles, 97 

Lateral cutaneous veins, 203, 214, 217; 
relations of, 203, 217 

Lateral fin-fold, 14, 15, 214 

Lateral fin-fold theory, 14; evidence for, 
15 

Lateral hypobranchial artery, 162-163, 
178 

Lateral line, 264, 274, 275, 276; in Acan- 
thias, 276; development of, 264, 275- 
276, 278; function of, 279; in Heptan- 
chus, 7, 89, 261; nerves to, 225, 242, 243, 
278; section through, 278 

Lateral plate, 96, 109 

Lateral pterygial artery, 191 

Lateral pterygial vein, 213 

Lateral septum, 97 

Lateral ventricles, 236 

Lateralis nerve, 225, 245, 278 

Lens cell of light organ, 29 

Lens of eye, 267; development of, 269 

Leopard shark (Triakis), color of, 26 

Levator hyomandibularis, 103 

Levator labialis muscles, 93, 100-101 

Levator maxillae, 90, 101, 102; nerves of, 
241; relation of, to first dorsal constric- 
tor, 90 

Levator rostri, 104 

Lipochrome, 27 

Lipophores, 27; in Heterodontus, 27 

Littoral, 1 

Liver, 124, 137; artery to, 166, 187; ducts 
of, 124, 137; lobes of, 124, 137; oil of, 
137; veins of, 211-212 

Lobes of vagus, 236, 237 

Lobi inferiores, 233 

Locomotion: caudal, 13; pectoral, 12, 13 

Longitudinal canal of epididymus, 301 

Lymphatic vessels, 218 

Lymphocytes, 133 

Lymphoid organ, 133 

Macula neglecta of ear, 273; hair cells of, 
273; nerve to, 273 

Mandible, 63, 92, 279; muscles to, 92, 93- 
94 

Mandibular ampullae, 279 

Mandibular arch, 44, 45, 62, 63; articu- 
lation of, 45, 63; attachment of, in 
Heptanchus, 45; in embryo, 62; parts 
of, 45, 62 

Mandibular branch (V), 223, 240, 241; 
ganglion of, 240; motor fibers of, 240; 
sensory fibers of, 240 



326 



THE ELASMOBRANCH FISHES 



Mandibular canal, 275; nerves to, 243 
Mandibular groove, 262; nerve to, 224 
Mandibular somite, 99 ; muscle derivatives 

of, 99 
Marginal cartilage of clasper, 81-82 
Maxillaris branch (V), 223, 240, 241; 

ganglion of, 240, 241; origin of, 241 
Meckel's cartilage. See Mandible 
Median anterior sulcus, 231 
Median cardiac vein, 215 
Median cerebral artery, 165, 183 
Median hypobranchial artery, 163, 178, 

179 
Median hypobranchial piece, 46, 65, 67 
Median longitudinal bundles of medulla, 

235 
Median olfactory nucleus, 221, 232, 239 
Median pterygial artery, 191 
Median pterygial vein, 213 
Median vesicle, 296, 298; derivation of, 

298, 300; relation of, to segmental duct, 

296, 298; relation of, to collecting tu- 
bules, 297 
Medulla, 222, 235, 236; finer structure of, 

236; section through, 236 
Melanophore, 26; in Heierodonlut^, 26 
Meningeal lining, 237 
Mesencephalon, 221, 222, 230, 234; fibers 

terminating in, 239 ; in Heptanchus, 

221—222 
Mesenteric arteries: inferior, 167—168, 

190-191 ; superior, 167, 188-190 
Mesenteries, 121, 127-128; in Heptanchits, 

121; in Hypnos, 127 
Mesopterygium, 50, 77, 78 
Mesorchium, 290; vasa efferentia on, 302 
Mosorectum, 121, 297; nephrostomes on, 

290 
Mesovarium, 290, 303 
Metapterygium, 50, 77; development of, 

77-79 
Metencephalon, 221, 222, 230, 234; cavity 

of, 235-236; in Heptanchus, 221-222 
Midbrain. See Mesencephalon 
Mixed nerve, 243, 246 
Motor fibers, 236; origin of, 236 
Motor (ventral) root, 246 
Mouth, 5, 121, 128 
Mucous pores, 7, 262 
Multipolar nerve cell, 229 
Muscle buds, 109, 110; growth of, 238; 

of fin, 109, 110; bearing of, on origin 

of fin, 110 
Muscles of eye. See Musculature 



Muscle fibers, 96-97; in CladoselacJius, 1; 
metamorphosis of, to electric discs, 114— 
115 

Muscularis, 170 

Musculature: buccal and pharyngeal, 90- 
93, 101-108; dorsal bundles, 89; of 
claspers, 95, 110-112 ; of Elasmobranchs 
in general, 96-112; of electric organ 
(developing), 112-114; of eye, 89-90, 
99-100; of eyelid, 102-103; of fin, 89, 
95, 109-110; of Heptanchus, 89-95; of 
hypobranchial region, 93, 108-109 ; to 
lens, 269-270; ventral bundles, 89, 97 

Musculospinal artery, 192 

Mustelus californicus, 9 ; color of, 26 ; nic- 
titating membrane of, 102 

Myeleneephalon, 221, 222, 235; cavity of, 
235-236; in Heptanchus, 221; nerves 
from, 235 

Myelonal artery, 186, 192 

Myelonal veins, 208 

Myliohatis californicus, 3, fig. 8 

Myoblast, 96 

Myocoele, 96 

Myosepta, 89, 96, 97; direction of, 89, 96, 
97; in electric organ, 112 

Myotome, 96, 109; derivative of, 109 

Nasal apertures, 5; in Elasmobranchs in 
general, 264-265; in Heptanchus, 5, 44, 
258 

Nasal capsule. See Olfactory capsule 

Nasal cartilage, 43-44, 58 

Nasal pit or cup, 5; relation of, to oro- 
nasal groove, 122 

Neopterygium, 79 

Nephrostome, 296, 297, 298, 301; develop- 
ment of, 298 

Nerve cells, 229 

Nerve fiber. See Axone 

Nerves: cranial, 223-227, 238-246; occip- 
itospinales, 227, 245-246; spinal, 227- 
228, 246-247; sympathetic, 247-248 

Nervous collector, 228, 247; relation of, 
to origin of paired fins, 247 

Nervous system: central, 221-228, 229- 
248; development of, 230; in Elasmo- 
branchs in general, 229-248; in Hep- 
tanchus, 221-228; peripheral, 222, 238 

Neural arches, 47, 69; composition of, 47- 
48 

Neural canal, lining of, 237 

Neural crest, 238 

Neural fold, 230 



INDEX 



327 



Neural tube, 230, 238; cells of, 229-230, 
238 

Neurilemma, 238 

Neurocoele, 230 

Neurogleal cell, 230 

Neuromast, 262, 275, 278 

Neurone, 229 

Neuropore, 230, 239 

Nictitating membrane, 5, 102, 266; ab- 
sent in Heptanchus, 5; development of, 
266 ; muscles of, 102-103 

Nictitator muscle, 102, 103 

Nitrogenous waste matter, removal of, 289, 
296, 298 

Notidanids, 3, 6, 71, 264; auditory re- 
gion of, 58 

Notochord, 47, 70; constructions, 47-48; 
relation of, to spinal column, 47, 70; 
section through, 48, 70; sheath of, 47, 
70 

Notochordal sheath, 48, 70, 75; zones of, 
48, 70 

Xotorhynchus (Hepfanchus), 3 note 

Nutrient arteries, 180 

Nutrient vein, 205, 207 

Occipital condyle, 57; in rays, 57 

Occipital crest, 43 

Occipitospinales nerves, 71, 227, 245-246 

Oculomotor nerve, 222, 223, 234, 239; 
division to eye muscles, 239-240; fora- 
men of, 44; ganglion of, 240; muscles 
supplied by, 100; origin of, 239 

Odontoblasts, 30, 33, 34, 130 

Oesophagus, 123, 134; cells of, 134; folds 
of, 123; lining of, 123, 134 

Olfactory bulbs, 223, 239, 258, 264 

Olfactory bundle. See Olfactory nerve 

Olfactory capsule, 43, 53, 57, 238; in em- 
bryo, 53-55; in Zygaena, 55 

Olfactory cells, 264 

Olfactory fila, 239 

Olfactory lobe of brain, 223, 239, 258, 264 

Olfactory membrane, 239, 258, 265 

Olfactory nerve, 223, 238-239, 258, 264, 
265 

Olfactory organ, 258, 264-265 ; circulation 
in, 265; divisions of, 264-265; func- 
tion of, 265; nerves of, 264 

Olfactory receptors, 258 

Olfactory sac, 264 

Olfactory tract, 221, 223, 239, 258, 264 

Omentum, 121 

Omphalomesenteric vein, 212 



Oocyte, 303; primary, 303; secondary, 
303 

Oogonium, 303 

Ootid, 303 

Ophthalniica magna artery, 165, 181 

Ophthalmicus profundus (V), 223, 241; 
ganglion of, 240-241; origin of, 240; 
sensory fibers, 240 

Ophthalmicus superficialis (V), 223, 240, 
241; ganglion of, 240, 241; origin of, 
240; sensory fibers, 240 

Ophthalmicus superficialis (VII), 224, 
242 

Optic artery, 183 

Optic chiasma, 221, 223, 239 

Optic cup, 269 

Optic lobes, 221 

Optic nerve, 221, 234; origin of, 239, 269; 
relation of, to sclera, 268; termination 
of, 234, 239 

Optic organ. See Eye 

Optic pedicel, 60, 90, 268; function of, 
258 

Optic stalk, 269 

Optic thalamus, 236 

Optic vesicles, 230, 269 

Orbit, 53, 55, 60, 89; in Heptanehus. 5, 44; 
muscle attachment to, 89 

Orbital artery, 182 

Orbital fissure, 44, 60 ; in embryo, 60 ; re- 
duced, in Mustelus, 60 

Orbital process, 44, 62 ; in embryo, 62 

Orbital sinus, veins to, 198, 204, 205 

Orbitonasal canal, 44 

Organ of special sense, 258, 264 

Oronasal groove, 122 

Otolith, 272 

Outer buccal ampullae, 262, 279 

Outer (heavy) layer of retina, 268 

Ova, 290, 303 

Ovary, 290, 303; development of, 303 

Oviducal artery, 169, 193; anterior, 193; 
posterior, 193 

Oviducal funnel, 290, 304 

Oviducal valve, 306 

Oviducal vein, 209 

Oviduct, 290, 304; arteries to, 169, 191, 
193; development of, from Wolffian 
duct, 304; divisions of, 304; rudimen- 
tary, in male Heptanchus, 291, 309 

Oviparous, 307 

Paired fins, muscles of, 94, 109; origin of, 
14-16; skeleton of, 50 



328 



THE ELASMOBRANCH FISHES 



Palatine nerve, 224, 243 

Palatoquadrate cartilage, 45, 62 ; muscles 
to, 90 

Pallial eminence, 221, 232 

Pancreas, 124, 138; arteries to, 166; duct 
of, 124, 138; lobes of, 124, 138 

Pancreatic duct, 124, 138 

Papillae of oesophagus, 134 

Paracentral mass, 237 

Parachordal plates, 53 

Paraphysial arch, 233 

Paraphysis, 233 

Parietal fossa, 43, 55, 271 

Pectoral fin, 6, 12; arteries to, 168, 191; 
function of, 12, 13, 14; fusion of, to 
sides in rays, 12; muscles of, 89, 94; 
nerves of, 227, 246-247; veins of, 202, 
213 

Pectoral girdle, 49, 79-81; development 
of, 77; muscles to, 89; parts of, 49, 79- 
81 

Pectoral muscles, 89, 94; development of, 
110 

Pectoral plexus, 227, 246 

Pelagic, 1 

Pelvic fin, 6, 14; claspers of male on, 6 

Pelvic girdle, 6, 51, 82, 228; origin of, 82 

Pelvic plexus, 228, 247 

Pentanchid types, 150 

Pepsin, 135 

Peptic cells, 135 

Pericardial artery, 163, 179 

Pericardial cavity, 151, 171 

Pericardio-peritoneal septum, 287 

Perimeningeal space, 237 

Peripheral nervous system, 223-228, 238- 
248; development of, 238 

Pharyngeal nerve (IX), 225, 244; (X), 
225-227, 245 

Pharyngobranchial cartilages, 46, 64; fu- 
sion of, in last arch, 65; muscles of, 
92, 106-107 

Pharynx, 122, 131-134; lining of, 132; 
muscles of, 90-93, 101-108; perfora- 
tions of, 122, 131; stomodeal denticles 
in, 122 

Photogenic cell, 29 

Photophore, 29 

Pigment, 23, 27, 29, 33; function of, 27 

Pineal stalk, 221, 233, 236; development 
of, 233 

Pit organs, 262, 274, 281-282; nerves to, 
225, 244, 282 

Pituitary, 221, 233 



Placodes, 264 

Placoid scales, 7, 30-32; absence of, 32, 
309; base of, 24, 32; canals of, 30, 32; 
development of, 30 ; finer anatomy of, 
32; modified, 24-25, 33, 154; spines of, 
32,33 

Plasma, 170 

Plenr acanthus, 1 

Poison gland of ray, 28 

Pori abdominales. See Abdominal pores 

Portal vein. See Hepatic portal 

Portio major (V), 240 

Portio minor (V), 240 

Postbranchial body, 151 

Postcardinal sinus, 200, 204, 210; in rays, 
210; relation of, to subscapular vein, 
214 

Postcardinal vein, 200, 204, 210; tribu- 
taries of, 200, 210 

Posterior cerebral artery, 165, 184 

Posterior cerebral vein, 207 

Posterior commissure, 233 

Posterior coronary artery, 168, 179, 192 

Posterior dorsolateral artery, 168 

Posterior efferent-collector artery, 176 

Posterior gastro-pancreaticosplenic artery, 
166, 188 

Posterior gastro-pancreaticosplenic vein, 
211 

Posterior gastrosplenic artery, 166, 188 

Posterior intestinal artery, 167, 188, 190; 
in rays, 190 

Posterior intestinal vein, 200, 211 ; tribu- 
taries of, 200 

Posterior mesenteric artery. See Inferior 
mesenteric 

Posterior oblique semicircular canal, 259, 
261,273 

Posterior (outer) buccal ampullae, 262, 
279 

Posterior rectus muscle. See External rec- 
tus 

Posterior thyroid artery, 164 

Postorbital groove, 205 

Postorbital process, 43, 44, 45, 58, 60 ; in 
ChJainydoselachus, 58; in notidanids, 
58 ; in rays, 58 

Post-trematicus nerve (IX), 225, 244; 
(X),225, 245 

Postvelar arch, 233 

Premandibular somite, 99 

Preorbital process, 44, 58 ; in Zygaena, 58 

Prespiracular ligament, 63 

Prespiracular nerves, 224, 243 



INDEX 



82!) 



Pretiematicus nerve (VII), 243; (IX), 
225, 244; (X), 149, 225, 245 

Primary renal corpuscle, 299 

Primitive ova, 301 

Pristis, 3, 9, fig. 19 ; tooth of, figs. 39, 41 

Pristiunis, development of fin skeleton of, 
15-16 

Proctodeum, 128 

Pronephrotic duct, 298, 299 

Propterygium, 50, 77; development of, 77 

Prosenceplahon, 230; derivatives of, 230 

Pseudobranchial artery, 165, 182; in em- 
bryo, 182; in rays, 180 

Pterygial arteries, 191 

Pulp cavity, 32, 35 

Pupil of eye, 268 

Pyloric stomach, 123, 135 

Pyloric valve, 123, 136 

Quadrate, 45, 62 

Eadial cartilages, 50, 51, 77, 78, 79; de- 
velopment of, 77-78; of pectorals, 75- 
76; of pelvic, 81; postaxial radials, 50, 
79; preaxial radials, 50 

Eadial muscle, 94-95 

Baia erinacea, 9, fig. 20 

Eamuli to lateral line canal, 278 

Eamus anastomoticus artery, 165, 183; 
foramen of, 44; function of, 181 

Eamus dorsalis artery, 192 

Eamus dorsalis nerve (IX), 225; (X), 
225, 245, 278 

Eamus hyoideus (VII), 243 

Eamus oticus (VII), 242, 279; relation of, 
to spiracle, 153 

Eamus palatinus, 224, 243 

Eamus ventralis artery, 192 

Eamus visceralis (or intestinalis) (X), 
227, 245 

Eays : batfish, 3 ; Cephaloptera, 3 ; elec- 
tric, 3; guitar fish, 9; sawfish, 3, 9; 
small sting, 8; spiracle of, 150 

Eecessus neuroporicus, 221 

Eecessus utriculi, 259, 272; nerve to, 273 

Eectal gland, 121, 126, 141; artery of, 167, 
191; function of, unknown, 141; rela- 
tion of, to epigonal organ, 300; struc- 
ture of, 141; veins of, 200, 211 

Eectal artery, 169, 192 

Eectum, 126, 141; artery to, 169, 192; in 
Heptanchus, 126; mesentery to, 121; 
veins of, 213 

Eectus abdominalis, 97 



Ecd blood cells. See Erytiirocytos 

Eenal artery, 169, 192, 193 

Eenal coi-puscle, 296, 299, 300; develop- 
ment of, 299-300; function of, 296; 
primary, 299; secondary, 299 

Eenal portal system, 204, 208-209; rela- 
tion of, to subintestinal vein, 208 

Eenal portal vein, 208 

Eenal tubule, 297, 298, 299, 300 

Eespiration, 157 

Eespiratory current, 155; course of, 156; 
direction of, 156; reversal of, in rays, 
156; in Urolophus, 157 

Eespiratory membrane, 149 

Eespiratory tract, 147, 150; of Elasmo- 
branchs in general, 150-157; of Eep- 
tanclius, 147-149 

Eestiform bodies. See Corpora restiforme 

Eetina, 268; cells of, 268-269; develop- 
ment of, 269; nerve from, 269; struc- 
ture of, 268-269 

Eetractor palpebrae superioris, 102 

Eevehentes, 210 

Bhinohatis productus, 9, fig. 7. 

Bhinodon (BJiineodon) typiciis, 3 ; color 
of, 26, 27 

Ehombencephalon, 230 

Eibs, 48, 72 

Eods of retina, 269 

Eostrum, 43, 55, 57; in Acanthias, 57 

Sacci vasculosi, 233 

Saccular nerve, 273 

Sacculus, 259, 272 

Sawfish (Pristis), 9 

Saw tooth, 35; development of, 35; struc- 
ture of, 35 

Scales. See Placoid scales 

Scapula, 49, 79; muscles to, 94, 104 

Scapular canal, 277 

Schneiderian folds, 264; development of, 
265; primary, 265; secondary, 265 

Sclera, 258, 268; in Cetorhinus, 268 

Sclerotome, 70, 96 

Scroll valve, 139 

Scyllium, development of pectoral girdle 
of, 15 

Scymnus, epidermis of, 26 

Secondary dentine, 34; origin of, 34 

Segmental artery, 169; branches of, 169, 
192-193; caudal, 169, 193 

Segmental duct, 296, 297, 298; develop- 
ment of, 298 

Selachii, 8 



330 



THE ELASMOBRANCH FISHES 



Semicircular canal, 55, 258, 271, 273; am- 
pullae of, 259, 273; anterior oblique, 
43, 258, 273; development of, 275; 
nerves to, 261, 273; posterior oblique, 
43, 259, 273 

Seminal vesicle, 302 

Sense of smell, 265 

Sensory canal system, 261-263; canals of, 
261-262; function of, 279; nerves to, 
262; neuromasts of, 262, 278; of Elas- 
mobranchs in general, 274-279; of 
Heptanchus, 261-263; structure of, 278 

Sensory fibers, 238; origin, 238 

Sensory (dorsal) root nerve, 246 

Septum, constricting notochord, 48; of 
gill, 147, 148, 149 

Serosa, 170 

Serum, 170 

Sex cells, 301, 302, 303; passage of, to 
cloaca of female, 310; passage of, to 
clasper of male, 303; sex gland, 300 

Sex characters (secondary), 309; claspers, 
309 

Shagreen denticles, 7, 23 

Sharks: angel fish (Squatma) , 9, fig. 18; 
ancient, 1; basking, 3; blue, 3; flattened, 
3 ; generalized, 2 ; hammerhead, 2 ; hep- 
tanchid, 5-7, figs. 12, 13, 14; heterodont, 
1, fig. 17; lamnoid, 9; man-eater, 2, 9, 
fig. 16; modern or recent, 2, 3; preda- 
cious, 9; specialized, 2; swell, 2, fig. 1; 
thresher, 2, fig. 2 

Shell gland, 290, 304-305; secretory cells 
of, 305 

Sinu-auricular aperture, 171 

Sinu-auricular valve, 160, 171 

Sinus, 204 

Sinus of photophore, 29 

Sinus venosus, 160, 170, 171; tributaries 
to, 201, 211, 215 

Siphon, 28, 303; function of, 303; relation 
of, to clasper tube, 303 

Skeleton, 43-52, 53-83; development of, 
77-79; nature of, 53; in anal fin, 52, 83; 
of caudal fin, 51; of clasper, 51, 81-82; 
of dorsal fin, 51, 82-83; of pectoral fin, 
49, 75-79; of pelvic fin, 51, 81-82; of 
paired fins, 50-51, 75-82; of unpaired 
fins, 51, 82-83 

Skin. See Integument 

Skull, 43-47, 53-69; components of, 43, 
53 

Somatic layer, 96 

Somatic sensory fibers; terminus of, 236 



Somite, 70, 96; degeneration of, 97; of 
head, 99-100; parts of, 96 

Special senses, of Elasmobranchs in gen- 
eral, 264-282; of Heptanchus, 258-263. 
See also Organs of special sense 

Si^erm sac, 302 

Spermatagonia, 301 

Spermatic fluid, 302 

Spermatid, 301 

Spermatozoa, 291, 301, 302 ; course of, to 
female, 309 

Spinal artery, 186 

Spinal column, 47, 69; calcification in,74- 
75; in Elasmobranchs in general, 69- 
75; in Heptanchus, 47-48; relation of, 
to notochord, 70 

Spinal cord, 222, 237-238; arteries to, 
169, 192, 193; development of, 230, 237; 
in Elasmobranchs in general, 237—238; 
in Heptanchus, 222 

Spinal nerves, 227, 246; in Elasmobranchs 
in general, 246-247; roots of, 227, 237, 
246 

Spinalis artery, 186 

Spinax, color in, 27; photophores of, 29 

Spiracle, absence of, 150; blood supply 
to, 181-182; diverticula of, 147-148, 
153; filaments of, 147, 153; in rays, 
150; relation of, to gill clefts, 150 

Spiracular ampulla, 280 

Spiracular cartilage, 63, 153 

Spiracular diverticulum. See Diverticula 
of spiracle 

Spiracular valve, 153; muscles of, 153- 
154; support of, 153 

Spiral intestine, 139-140; mesentery to, 
121 

Spiral valve, 124, 126, 139; arteries to, 
167; development of, 140; extent of, 
126, 139-140; function of, 140; lining 
of, 126, 140 ; turns of, 124, 140 ; veins of, 
210-211 

Splanchinie layer, 96 

Spleen, arteries to, 166, 167, 188; in Hep- 
tanchus, 124, 138; position of, 138-139 

Spouting, 156-157; course of, 156-157; in 
Squat ina, 156-157 

Squalii^ sucMii, gill rakers of, 37 ; relations 
of subscapular vein of, 214 

Squatina, 9, fig. 18; clasper gland of, 28; 
spouting in, 156-157 

Stapedeal artery. See Orbital artery 

Sting, 35, 37; function of, 37; glands of, 
28; structure of, 37 



INDEX 



:5:]1 



stomach, 123, 135-136; arterios to, 166, 
167, 186, 188, 189-190; crypts of, 135; 
glands of, 135; limbs of, 123; lining of, 
123, 135; mesentery to, 121; nerves to, 
227, 245; secretion of, 135; veins of, 
200-201, 211 

Stomodeal denticles, 25, 38, 122, 266; 
function of, 38; in thyroid,Chlamydosel- 
achiis, 134 ; location of, 38, 122 ; of Hep- 
tanchus, 25, fig. 27; rudimentary, 38 

Stomodeum, 127 

Subclavian artery, 168, 191; in Heptan- 
chus, 168, 191-192 

Subclavian vein, 202, 214 

Subintestinal vein, 208, 212; derivatives 
of, 208, 210 

Subscapular sinus, 203 ; tributary to, 203 

Subscapular vein, 202, 214; in Squahis, 
214 

Subspinalis muscle, 92, 106; nerve to, 227 

Superficial constrictors, 90, 101; functiou 
of, 90 

Superficial intercostal artery, 169 

Supraophthalmic ampullae, 262, 279 

Superior commissure, 233 

Superior lobes of hypophysis, 233-234 

Superior mesenteric artery, 167, 188-190; 
in rays, 190; its branches, 167, 188-190 

Superior oblic|ue muscle, 89, 99 ; nerves to 
44, 100, 223, 240; origin of, 99 

Superior rectus muscle, 90, 99; nerve to, 
100, 223, 241; origin of, 99 

Supernumerary branchial arches, 47 

Supporting cells in ampulla, 273 

Suprachoroidea, 268 

Supracranial fontanelle, 57; in rays, 57 

Supraorbital canal, 261, 274, 276-277; in- 
nervation of, 224, 242, 274 

Supraorbital crest, 44, 57, 58, 241 

Suprapericardial body, 151 

Suprarenal body, 248 

Suprascapula, 49 

Supratemporal canal, 261, 276, 278; in 
Elasmobranchs in general, 278; in Hep- 
tanclms, 261 ; nerves to, 225, 245, 278 

Supratemporal nerve (IX), 244, 279; (X), 
245, 278 

Suspensory ligament of liver, 121 

Sympathetic nervous system, 247-248 

Tail (caudal fin); electric organ in, 112; 
of Heptanchus, 6 ; of Prist is, 9 ; of 
Eliinohatis, 9 



Taste buds, 265, 266; nerve to, 243 

Taste organ, 265—206 

Tectospondyly, 75 

Teeth, 1, 38, 122, 128; development of, 
128; finer structure of, 130; of Car- 
charodon, 38; of Heptanchus, 122; of 
Heterodontus, 130 ; of Lamna, 9 ; of 
Mustelus, 9; of MyUohatis, 38; pave- 
ment, 130; replacement of, 131 

Telencephalon, 221, 230 

Tendineae. See Chordae tendineae 

Terminal cartilage, of clasper, 81-82 

Terminal nerve of Locy, 221, 223, 239; in 
Heptanclms, 221, 223 

Tertiary renal corpuscles, 300 

Testes, 290-291, 300, 301 ; central canal of, 
290-291, 301 ; development of, 300 ; rela- 
tion of, to epigonal organ, 300 ; rudi- 
mentary, in female Heptanchus, 290; 
structure of, 300, 301 

Third ventricle, 236 

Thread cell of ear, 273 

Thymus gland, 122-123, 132-133 ; develop- 
ment of, 132; with duct in Heptanchus, 
122-123 ; function of, 133 

Thyroid artery, 164-165 

Thyroid gland, 122, 123, 133-134 ; arteries 
to, 164-165; capsule of, 133; denticles 
of, in Chlamydoselachu-s, 134; develop- 
ment of, 134; duct of, in Chlamydose- 
lachns, 134; follicles of, 133, 134; his- 
tory of, 133-134 

Thyroidean sinus, 208 

Tongue, 122, 128 

Tooth germs, 128 

Torpedo, 3, fig. 6; color of, 26, 27 ; electric 
organ of, 112, 115 

Trabecular cartilage, 53 

Tract cells, 236 

Tractus arteriosus lateralis, 192 

Trapezius muscle, 92, 104 

TriaJcis semifasciatus, integument of, 26 

Trigeminal nerve, 223, 240-241; divisions 
of, 223, 240-241; ganglion of, 240-241; 
nucleus of, 240 

Trochlear nerve, 223, 240; foramen of. 
44; muscle supplied by, 100; origin of, 
234 

Trunk myotome. See Myotome 

Tuberculum acusticum, 236 

Tympanic membrane, 272 

Tympanum, of Heptanchus, 272; of rays, 



332 



THE ELASMOBRANCH FISHES 



Unpaired arteries, 186 

Ureter, 287, 289, 291, 295; relation of, to 

collecting tubules, 294, 295; relation 

of, to male Wolffian duct, 296 
Urinary papilla, 289, 290, 291, 296, 303 
Urinary sinus, 289, 291, 294, 295; ducts 

to, 294-295; horns of, 295, 296; walls 

of, 296 
Urinary system, in Elasmobranchs in gen- 
eral, 292-300; in Heptanchus, 287-289 
Urinary vesicle, 294 
Urogenital sinus of male, 291; relation 

of, to seminal vesicle, 302; relation of, 

to vas deferens, 302 
Urogenital system, in Elasmobranchs in 

general, 292-310; in Heptanchus, 287- 

291 
Vroloplius halleri, fig. 9; cf. Acanthias, 8, 

10-12; poison gland of, 28 
Uterine septum, 309 
Uterus, 290, 306-309 ; blood supply to, 193, 

307; lining of, 307, 308; partitions of, 

308 ; relation of, to embryo, 307-309 
Utriculus of ear, 259, 272, 273 

Vagus nerve, 225-227, 244-245; divisions 
of, 225-227, 244-245; lobes of, 236; 
nucleus of, 236; visceral sensory fibers 
of, 236 

Valves, oviducal, 306; of heart, 160-161, 
171, 172; of spiral intestine, 124, 126, 
139, 140; of uterus, 306; of veins, 204 

Valvular intestine, 124, 126, 139-140; ar- 
teries to, 166, 167, 188, 190; veins of, 
201, 210-211 

Vas deferens, 290, 291; relation of, to 
Wolffian duct, 290, 296 

A'asaefferentia, 290, 301-302 ; origin of, 301 

Vascular sacs, 208, 221, 233 

Vasodentine, 35, 130 

Veins, 170, 198, 204; cf. arteries, 170, 204; 
of body wall, 202, 204; from digestive 
tract, 200-201, 204, 210-213; from 
head, Heptanchus, 198, 204; of heart, 
215-216; of kidney, 208, 210; of skin, 
202-203, 204, 216-217; of tail, Hep- 
tanchus, 199 ; walls of, 170, 204 

Velum, 233 

Vena limitans, 210 

Vena profunda, 217 

Venous system, in Elasmobranchs in gen- 
eral, 204-218; in Heptanchus, 198-203 

Ventral aorta, 161, 172, 175 

Ventral bundles, 89, 97 



Ventral constrictor muscles, 91, 104-105; 
nerves to first ventral constrictor, 243 

Ventral cutaneous veins, 203, 217; rela- 
tion of, to cloacal vein, 217 

Ventral gastric artery, 166, 187 

Ventral gastric vein, 201, 211 

Ventral horn of cord, 222, 237, 238; cells 
of, 238 

Ventral intercalary piece, 48 

Ventral intestinal artery, 167, 188; ab- 
sent in rays, 188 

Ventral intestinal vein, 201, 210; tribu- 
taries of, 201, 210-211 

Ventral lobe of pancreas, 124, 138 

Ventral longitudinal muscles. See Hypo- 
branchials 

Ventral myelonal vein, 208 

Ventral root nerve, 227; foramen of, 48 

Ventricle, 160, 161, 170, 171, 179 ; arteries to, 
163-164, 172; walls of, 160-161,170, 171 

Ventrolateral bundles in medulla, 235 

Ventrolateral muscle bundles, 89, 97 

Ventromedian muscle bundles, 97 

Vertebrae, 69, 70, 71, 72; in Alopias, 69 

Vertebral plate, in rays, 71 

Vertebromuscularis artery, 192, 193-194 

Vertebrospinalis artery, 192 

Vertebrospinal vein, 210 

Vesicula seminalis, 302 

Vesicles of Savi, 274, 281 

Vessels of Thebesius, 215-216 

Vestibular nerve, 273 

Villus, arteries of, 307; of intestine, 140; 
nutriment from, 307-308; of uterus, 
307-309; veins of, 307 

Visceral arches, 44-^7, 62-67; in embryo, 
62; muscles to, 100-108 

Visceral nerve (X), 225 

Visceral sensory fibers, 236 

Visceral skeleton, of Elasmobranchs in 
general, 62-69; of Heptanchus, 44—47 

Visceromotor nucleus, 236, 240 

Vitelline vein, 212, 213 

Vitreous body (humor), 268 

Vitrodentine, 35 

Viviparous, 307 

White blood cells, 170 

White matter of cord, 237 

Wolffian duct, 287, 289, 294, 295, 296, 297 
modified as vas deferens, 290, 296, 301 
relation of, to collecting tubules, 294 
relation of, to vas deferens, 302 

"Wonder net," 181