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Full text of "The skate, Raja erinacea Mitchill, a laboratory manual"

THE SKATE 

A LABORATORY MANUAL 



jn^>cb^o 



THE MACMILLAN COMPANY 

NBW YORK • BOSTON • CHICAGO • DALLAS 
ATLANTA • SAN FRANCISCO 

MACMILLAN & CO., Limited 

LONDON • BOMBAY • CALCUTTA 
MELBOURNE 

THE MACMILLAN CO. OP CANADA. Lto. 

TORONTO 



THE SKATE ^^ 



Raja erinacea Mitchill 

A LABORATORY MANUAL 



BY 



CHARLES W. CREASER, Ph.D. 

ASSISTANT PROFESSOR OF ZOOLOGY IN THE 
COLLEGE OF THE CITY OF DETROIT 



THE MACMILLAN COMPANY 
1927 

All rights reserved 



Copyright, 1927, 
By the MACMILLAN COMPANY. 



Set up and electrotyped. 
Published March, 1927. 



Printed in the United States of America by 

J. J. LITTLE AND IVES COMPANY, NEW YORK 



PREFACE "^ 

This manual is written for beginning students and pre- 
supposes no previous work in dissection. It is not intended 
as a treatise on the skate but rather as an introduction to 
vertebrate anatomy for inexperienced students. It is im- 
possible for the student of comparative anatomy to dissect 
specimens in as much detail as the student of human 
anatomy, but he should be so drilled in details as to be 
capable of working out any vertebrate animal which is 
employed in further study. It was with such an ideal in 
mind that this manual was prepared. 

Numerous practical reasons may be cited in favor of the 
dissection of the skate as the introductory study in a course 
in vertebrate zoology. Students in most beginning courses 
receive little or no training in the dissection of any verte- 
brate animal. Experience with many classes in vertebrate 
zoology at this College and at the University of Kansas 
has shown that each student needs for his first study an 
easily dissected adult animal. The skate, particularly Baja 
erinacea Mitchill, fits this need in a surprisingly satis- 
factory way. This skate when fully mature is only six- 
teen to twenty inches in total length. It is therefore pos- 
sible to provide the student with a mature specimen of a 
size convenient for dissection, easy to care for and to work 
with in the laboratory. 

The skate has a very generalized internal structure 
which is in every way comparable to the shark. It is 

V 



vi PREFACE 

superior for laboratory purposes in some ways since the 
specimens are usually mature, which allows a full dissec- 
tion of the urogenital system. The body form of the 
skate is highly specialized, which makes necessary the in- 
troduction of demonstration material to illustrate the gen- 
eralized form. This body shape, however, makes the skate 
much easier to dissect, since it naturally rests in a suitable 
position and does not need to be fastened to the dissection 
board. 

The laboratory dissection of the skate conforms with the 
current textbooks of vertebrate zoology, since it deals with 
a specimen not discussed or figured in any detail yet serves 
as an excellent example for comparison with the textbook 
material on related forms, making possible the legitimate 
and desirable use of such books in the laboratory. 

Besides these fundamental points in its favor the skate 
is easier to obtain and to store. The cost of skates, accord- 
ing to the prices current with supply companies, is almost 
one-half that of immature sharks. Only one singly in- 
jected specimen is needed for this manual. 

Figures of the venous system and the skeleton are in- 
cluded since these systems are most difficult to dissect. 
The figure of the venous system is unlabeled and is in- 
tended to aid in the student's organization of this system. 
The skeleton of the average specimen is badly injured dur- 
ing dissection but much may be determined with the aid of 
the figure. 

Numerous laboratory guides, outlines and textbooks have 
been consulted and their suggestions reworked into the 
present manual during the dissection of many skates in 
our laboratory. Considerable material has been utilized 
from ** Zootomy" by T. J. Parker, ''Outlines of Zoology," 
by J. A. Thompson; *'A Laboratory Manual for Compara- 



PREFACE vii 

tive Vertebrate Anatomy/' by L. H. Hyman, and ** Out- 
lines for Comparative Anatomy," by Dr. Peter Okkelberg 

(unpublished). 

Charles W. Creaser. 

College of the City of Detroit, 
Detroit, Michigan, 
January, 1927. 



7 



CONTENTS 

PAGE 

Preface . . . i. v 

CHAPTER 

I. General Directions 1 

II. External Characteristics 4 

Key for Determination of Species 4 

Animal Form 6 

Imaginary Planes and Axes 6 

Symmetry 7 

III. The Eespiratory System 11 

The Mouth and Pharyngeal Cavity 11 

Visceral Arches 12 

IV. The Coelom, Digestive System, and Mesenteries . . 14 

The Coelom and Its Walls 14 

The Pleuroperitoneal Viscera 15 

The Digestive Tract and Derivatives 15 

The Spleen 18 

The Mesenteries .............. 18 

V. The Urogenital System 19 

The Female • . 20 

The Male 22 

VI. The Circulatory System 24 

The Pericardial Cavity and the Heart 24 

The Venous System 26 

The Arterial System and the Heart 30 

ix 



^//^' ?/ 



X CONTENTS 

CHAPTER PAQB 

VII. The Neevous System and Sense Oegans .... 37 

Integumentary Sense Organs 37 

Organs of Special Sense 38 

The Central Nervous System . . . 42 

The Cranial Nerves ..... . .. :.. „; . . 44 

VIII. The Muscular and Skeletal System 48 

Index , . .; . , . i.. . . .. . ..: i.. ..: i.. i.. i.. 53 



THE SKATE 

A LABORATORY MANUAL 



THE SKATE 

(Raja erinacea Mitcliill) 

CHAPTER I 

GENERAL DIRECTIONS 

1. Supplies. 

These dissecting instruments and drawing materials are 
necessary for the course: scalpel, fine scissors, stout and 
slender probes, forceps with straight points, drawing paper, 
eraser, hard drawing pencil, ruler, cheese cloth and a towel. 
A laboratory coat or rubber apron is desirable for the pro- 
tection of the clothing. 

2. Drawings. 

Make all drawings on drawing paper with a hard pencil. 
Draw from the actual material unless the directions state 
otherwise. Do not make rough sketches to be completed 
later but do all the drawing work in the laboratory. 

Drawings should be large and should show all structures 
pointed out in the outline. They must be accurate, in cor- 
rect proportion (use a ruler), and resemble the actual speci- 
men as nearly as possible. They should be dated and have 
the student's name in the upper right-hand corner. 

By following the procedure given below anyone can make 

the simple line drawings which are desired in this course. 

Good results in this mechanical line work can be secured 

by inexperienced students. 

1 



2 THE SKATE 

Draw on one side of the paper. First determine how large the 
drawing is to be, then select a suitable space on the page. Rule 
this space with very light vertical and horizontal guide lines of a 
proper size so that the drawing may be made symmetrical. With 
the ruler reduce or enlarge the length and width of the object so 
that it will fit the space selected. With very light lines make an 
outline of the object; measure with the ruler to determine the loca- 
tion, using the proportion established above. When the light lines 
are in good proportions and the details are as nearly like the object 
as you can get them, erase the light lines until you can barely see 
them; then go over them making the final lines, firm, continuous and 
clear. Every line on the drawing should represent a structure present 
on the object. 

Label the drawings fully. Number each plate and figure. In 
labelling the figures draw a straight line parallel to the top of the 
sheet, out from the structure to be labelled and then write or print 
the label on this line. Label all structures, even those labelled in pre- 
vious drawings. 

3. Dissecting. 

Dissection consists in separating the parts of an animal 
in such a way as to make them visible. It does not refer 
to the cutting of animals into pieces. The parts should be 
left as intact as possible. Follow the directions and do not 
cut anything or remove anything unless specifically di- 
rected to do so. Clean away the connective tissue binding 
and concealing the parts by use of blunt instruments such 
as the probe or forceps, or by the fingers. Avoid use of the 
scalpel and scissors; when using these instruments, make 
short cuts and remove small portions at a time. Do not 
cut the animal unnecessarily, but separate the parts. Im- 
proper initial dissection will render the study of the later 
parts very difficult. 

4. Care of Specimens. 

Each student will be furnished with the necessary speci- 
mens. Do not discard any animal until directed to do so. 
The specimens will be kept in a common container. A num- 



GENERAL DIRECTIONS 3 

bered metal tag will be given to you for identification. 
When ready to leave the laboratory wrap the animal in 
cheese cloth and tie it with a cord. Have the cord long 
enough so that one end can be left to which a tag bearing 
your name may be tied. When first starting work on a 
specimen wash it off in water. Always keep the specimen 
moist because drying ruins the animal for dissection. 
Specimens which are not properly cared for become diffi- 
cult to dissect. 

5. Corrections to Drawings. 

When you have finished a plate before leaving that sec- 
tion of the manual refer it to the instructor at a convenient 
or stated time and have it checked for errors. Much of the 
drawing work is checked in the laboratory as dissection 
proceeds. 



CHAPTER II 
EXTERNAL CHARACTERISTICS 

I. GENERAL CONSIDERATIONS. 

1. The Skate. 

The skates are elasmobranch fishes which are highly modi- 
fied in external form and proportions. They demonstrate, 
however, the characteristics of the vertebrates in most of 
their morphological details. The little skate, Eaja erinacea 
Mitchill, matures at a smaller size than the other common 
Atlantic skate, the big skate. Raja diaphanes Mitchill, and 
is the one upon which these directions are based specifically. 
Most skates have a very similar anatomy and may be dis- 
sected by the same procedure as is given here. 

The little skate is found along the coastal waters off the 
Atlantic seaboard of America from Nova Scotia to Vir- 
ginia. For further information on skates read the section 
on elasmobranchs in ** Fishes'^ by David S. Jordan and 
*' Fishes of the Gulf of Maine" by H. B. Bigelow and Wil- 
liam W. Welch. The following key is compiled from the 
latter book. 

Determine what species your specimen is from the key. 
Work out the identification of other specimens on demon- 
stration. 

2. A Key to the Skates and Rays Common to the 

American Atlantic Coast. 

a-1. The tail without a long dorsal spine. 
b-1. No distinct caudal fin but two small dorsal fins on the 
tail. The skates, Family Eajid(Es 

4 



EXTERNAL CHARACTERISTICS 5 

c-1. A row of large thorns on the midline of the back either 
on the rear part of the disk, or on the tail, or on both. 
d-1. The anterior angle of the disk is much greater than 
a right angle. The entire dorsal surface more or 
less covered with spines, the midline having large 
spines behind the shoulders. The Prickly Skate, 
Eaja scabrata Garman. 
d-2. The anterior angle of the disk roughly a right angle, 
the snout hardly projecting, relatively blunt. Dor- 
sal surface with few, but large thorns, the midline 
having large spines on the disk and the tail. The 
outer corners of the disk are bluntly angular. Brier 
Skate, Eaja eglanteria Bosc. 
d-3. The anterior angle of the disk roughly a right 
angle. The thorns are few and small; the midline 
having thorns on the disk as well as on the tail. 
Tip of snout sharp-pointed. Outer corners of disk 
rounded. Rare. Smooth Skate, Eaja senta Gar- 
man. 
d-4. The anterior angle rf the disk more acute than a 
right angle. The long, blunt tipped, snout project- 
ing. Midline with thorns only on the tail; all 
thorns small. Barn-door Skate, Eaja stabiiUforis 
Garman. 
c-2. Without a row of large thorns directly on the midline 
of the dorsal surface. Several rows may be present 
on either side. 

e-1. About 50 rows of teeth, the dorsal surface brown 
with dark spots. Little Skate, Eaja erinacea 
Mitchill. 
e-2. About 90 rows of teeth, the dorsal surface usually 
with two large, whitish, round spots near the pos- 
terior angles of the disk. Spotted Skate, Eaja 
diaphanes Mitchill. 
b-2. A large trinagular caudal fin and two dorsal fins on the 
tail. The torpedo. Family Torpedinidae. Electric 
Skate Narcacion nobilianus Bonaparte. 
a-2. The tail with a long dorsal spine (the sting-ray). 

f-1. A dorsal fin on the tail in front of the long 
spine. Family Myliobatidae. Cow-nosed Ray. 
Ehinoptera qiiadriloda Le Sueur. 



6 THE SKATE 

f-2. No dorsal fins. 

g-1. Tail rounded above, without keel. Sting ray, 
Dasyhatus marinus Klein. 

g-2. Upper side of the tail with a distinct keel 
posterior to the long spine. Sting ray, Dasy- 
hatus hastatus De Kay. 

3. Animal Form. 

Vertebrate animals with few exceptions carry themselves 
in an horizontal plane, their surfaces always retaining the 
same relative position. These surfaces are indicated with 
reference to this position. Locate the following regions of 
the skate: dorsal, the back or upper side (posterior in hu- 
man anatomy) ; ventral, the under side (anterior in human 
anatomy) ; lateral, the sides right and left (throughout this 
manual right and left refer to the skate's and not to the 
student's right or left) ; anterior, cephalic, or cranial, the 
head end (superior in human anatomy) ; posterior, or 
caudal, the tail end (inferior in human anatomy) ; and 
median, the middle. 

Adverbs may be formed by substituting a *^d" for the 
last letter of these terms with the meaning ' ' in the direction 
of" as caudad meaning towards the tail. Anterior and 
posterior may be employed to indicate the relative positions 
of structures with reference to the head or tail, for exam- 
ple the pelvic fins are posterior to the pectoral fins. 

The head, trunk, tail, and neck when present, make up 
the axial part of the animal while the fins or limbs make 
up the appendicular portion of the animal. 

Drawing 1. Draw the outline of the skate from the dorsal view- 
point indicating the above named surfaces and regions. 

4. Imaginary Planes and Axes. 

Determine these imaginary planes and axes of the skate, 
a. The sagittal plane or section is any vertical plane 



EXTERNAL CHARACTERISTICS 7 

passing through the body longitudinally. The median 
plane is that sagittal plane which passes through the mid- 
line of the body and divides it into two mirrored or iden- 
tical halves. 

b. The horizontal or frontal plane or section is any hori- 
zontal plane passing longitudinally through the body. It 
is at right angles to the median plane and parallel to the 
dorsal and ventral surfaces. 

c. The transverse or cross section or plane is that plane 
which passes through the body at right angles to the sagittal 
and the horizontal planes. 

d. The longitudinal or anteroposterior axis is any axis 
which is in the median plane extending from the head to 
the end of the tail. 

e. A sagittal or dorsoventral axis is any axis in a sagit- 
tal plane and passes from the dorsal to the ventral surface. 

Drawings 2, 3, 4. Diagram the planes and axes of a vertebrate 
and demonstrate them using plastic clay. 

5. Symmetry. 

An animal which has symmetry may be divided by a line 
or plane in such a way that one of the halves will be a 
mirrored image of the other. Asymmetrical animals cannot 
be so divided. The opposite like parts of a symmetrical 
animal are called antimeres. There are three well marked 
types of symmetry in animals, universal, radial and 
bilateral. The embryo of many vertebrates passes through 
all stages of symmetry; universal symmetry in which no 
opposite sides are different (blastula) ; radial symmetry in 
which opposite ends are different (gastrula) ; and bilateral 
in which opposite ends and two opposite sides are differ- 
ent, which is the symmetry of adult vertebrates. 

There is but one plane of symmetry in a bilateral animal. 
The median sagittal plane is the one and only plane that 



8 THE SKATE 

will divide the animal into two mirrored halves. The 
structures of a vertebrate therefore are either paired, in 
which case they are located symmetrically on each side, or 
are unpaired, in which case they are located in* the median 
plane. 

II. EXTERNAL STRUCTURES. 

1. Parts of the Body. 

The body is divided into a greatly flattened anterior 
portion consisting of the head and the trunk, and a slender 
posterior portion, the tail. This broad, flat form is char- 
acteristic of bottom-feeding fishes and is obtained by a 
shortening of the dorsoventral axis. For a generalized 
body form see the shark which is on demonstration. 

2. Fins. 

The skate has median or unpaired fins and paired fins. 
Locate the two small dorsal fins on the tail; a pair of 
pectoral fins which are enormously enlarged forming the 
lateral expansions of the trunk; and a pair of pelvic fins 
which are posterior to the pectoral fins and continuous with 
them in some species. The pelvic fin will be found to con- 
sist of two lobes, one of which in the males bears a long 
clasper which is grooved on its posterior lateral margin 
and is used in mating. Only a trace of the caudal fin will 
be found at the end of the tail. For the normal fin develop- 
ment see the shark on demonstration. 

3. Head. 

The head is very greatly flattened and its margins are 
continuous with the pectoral fins. The head terminates in 
a rostrum. A pair of eyes without lids is located on the 
dorsal surface, and behind each eye is a spiracle or first 
gill-slit. On the anterior face of the spiracle is a valve 



EXTERNAL CHARACTERISTICS 9 

which is marked by parallel ridges representing a rudi- 
mentary gill. The mouth is on the ventral side and is 
limited by the tooth-bearing upper and lower jaws. An- 
teriorly from the corners of the mouth running to the 
nostrils is a flap, the nasofrontal process, under which is 
the oronasal groove. This groove foreshadows the develop- 
ment of the closed passage found in the more specialized 
vertebrates. There are five pairs of gill-slits posterior to 
the mouth. These may be designated as the second, third, 
fourth, fifth, and sixth gill-slits. 

4. Tail. 

The cloacaJ aperture or anus (the term anus should be 
restricted to the opening of the digestive tract) is located 
between the bases of the pelvic fins. Abdominal pores 
which lead into the body cavity are located just posterior 
to and on each side of the aperture. Their use is obscure, 
but they may represent the remains of segmental ducts. 

5. The Skin. 

A. The Placaid Scale. 
The skin is quite tough. It has scattered placoid scales 
whose projecting spines are very evident. The placoid 
scale occurs in the elasmobranch fishes and is regarded as 
the ancestral structure from which true vertebrate teeth 
were developed. Remove a piece of skin containing one 
placoid scale. Clean away the skin and expose the com- 
plete scale, which consists of a basal plate embedded in the 
skin, and a projecting curved spine. Within the spine is a 
pulp cavity, which may be located by probing with the 
point of a needle at the center of the under side of the 
basal plate. The coating of the spine is composed of 
enamel. The basal plate and the interior of the spine are 
composed of dentine. 



10 THE SKATE 

B. Homology and Analogy. 

The structure and mode of origin of placoid scales and 
the teeth of all vertebrates are exactly the same, although 
their function and superficial appearance is very different. 
Teeth and placoid scales then are said to be homologous 
structures. Analogous structures are those structures 
which resemble each other in superficial appearance or in 
function but which have different origins. The lining of 
the mouth cavity is composed of skin which has become 
turned in, consequently the placoid scales which were car- 
ried with it into the mouth cavity have been adapted to 
new purposes and the homologous parts have begun a 
divergence. Analogous structures result from a con- 
vergence which is often the result of the action of similar 
conditions upon structures of different origin. Identify 
the following parts on the longitudinal section of a verte- 
brate tooth; the crown, homologous with the spine; the 
root homologous for the most part with the basal plate; 
the pulp cavity, a central space, filled in the living tooth 
with the dermal papilla of connective tissue, blood vessels, 
nerves, etc.; the dentine, a bone-like substance composing 
most of the tooth and the enamel the shiny outer coating 
of dentine on the crown. Compare part for part the placoid 
scale and the vertebrate tooth. 

The theory of evolution accounts for homologous struc- 
tures by its central concept that all vertebrates have been 
derived from a common ancestor. Consequently homolo- 
gous structures would be the result of the inheritance of 
modifications of a common primitive structure. 

Drawing 5. Draw the skate from the ventral surface. 
Drawing 6. Draw a section of a placoid scale and beside it 
diagram the tooth of a vertebrate. 



CHAPTER III 
THE RESPIRATORY SYSTEM 

The skate obtains its oxygen for respiration from the 
water. A muscular expansion of the mouth and pharynx 
causes water to enter the oral cavity through the spiracle 
and the mouth. The spiracle is located dorsally in the skate 
and so is free from the sand and mud of the bottom. Con- 
traction of the muscular expansion of the mouth and 
pharynx causes the water to be expelled through the gill 
slits. In so doing the water passes over the gill filaments 
where the oxygen is taken on by the blood. 

1. The Mouth and Pharyngeal Cavity. 

Make the following incisions very carefully as success 
in much of the future work depends upon this initial dis- 
section. Work a blade from a pair of heavy scissors or a 
knife into the left (skate's left) corner of the mouth and cut 
the jaw in the direction of the gill-slits; continue cutting 
the entire tissue between the mouth cavity and the exterior 
towards the gill-slits; extend the cut along the median 
margin of the gill-slits and continue it posteriorly to the 
heavy cartilaginous bar of the pectoral girdle ; cut through 
this bar with the scalpel; then turn the 7-shaped flap 
formed over to the side. This exposes as a continuous 
cavity the mouth and pharynx. In very rare cases the 
stomach is accidentally everted into the cavity and care 
should be taken not to injure it. 

The anterior part of this cavity, which is between the 

11 



12 THE SKATE 

moutli and gill arches, is the oral or mouth cavity. It is 
limited in the front by the upper and lower jaws which 
bear the teeth. The tongue is absent in the skate. The 
lower and upper jaws are the two halves of the first or 
mandibular gill-arch. 

The posterior part of the cavity is the pharynx. It con- 
verges at its posterior end into the esophagus, which con- 
nects directly with the stomach. Probe into the esophagus. 
Six internal gill-sUts break the lateral wall of the pharynx. 
The first, which is much modified, is the spiracle, which is 
an opening in the roof of the mouth just posterior to the 
mandibular arch. Locate the other five gill-slits. 

The internal gill-slits communicate with large visceral 
pouches which open to the exterior through the external 
gill-slits. Kun a probe through the internal gill-slit of the 
right side, pushing it into the visceral pouch and out the 
external gill-slit. This is the course which the water takes 
during respiration. The visceral arch is composed of the 
tissue between two successive visceral pouches. 

Drawing 7. Draw the mouth cavity and the pharynx. 

2. Visceral Arches. 

Examine the parts of the visceral arches where they have 
been cut. The interbranchial septum is the central portion 
of the arch which extends to the outer surface. The spaces 
between successive septa on the outer surface of the body 
form the external gill-slits, and those on the inner surface 
of the body the internal gill-slits. The septum bears plates 
and folds on its sides, the branchial or gill filaments. 
These extend into the visceral pouches. The filaments on 
one face of the septum form a half -gill or demibranch. The 
two demibranchs of a septum make up an entire gill or 
branchia. Count the demibranchs and determine whether 



THE RESPIRATORY SYSTEM 13 

any are missing. On each visceral arch cross-section note 
the cartilaginous gill-arch and in the septum towards the 
outer surface from the gill-arch are the cartilaginous gill 
rays. The cut end of the blood vessel, the afferent branch- 
ial vessel, is located just external to the center of the gill- 
arch. This vessel brings the non-aerated blood to the gill. 
On either side of the gill-arch is another blood vessel, usu- 
ally injected with a yellow colored paste. These are the 
efferent branchial vessels. Small branches of these will 
be found in the gill filament. All of these efferent branch- 
ial vessels collect the aerated blood and, as will be deter- 
mined later, they unite to form the dorsal aorta. 

Drawing 8. Draw a cross-section of the entire visceral arch. 



CHAPTER IV 

THE COELOM, DIGESTIVE SYSTEM, AND MESEN- 

TERIES 

1. The Coelom and Its Walls. 

Make an incision through the body wall of the skate just 
anterior and to the left of the cloaca; run the incision 
towards one side until the lateral wall of the cavity is 
reached and then continue it forward along this margin 
to the pectoral girdle. Do the same for the other side, 
starting at the original incision. Leave the floor of the 
body wall attached to the girdle. Be careful not to injure 
the organs within the cavity. 

The body cavity of the vertebrates is the coelom, which 
in its early stages is a continuous cavity extending the 
entire length of the trunk region. In the adults of all 
vertebrates this coelom is divided into at least two parts by 
the formation of a transverse septum at the anterior end. 
This septum separates the heart from the rest of the body 
organs and will be located and studied later. The greater 
part of the coelom which has just been opened is designated 
as the pleuroperitoneal cavity. This cavity is partially 
subdivided by certain mesenteries. It communicates with 
the exterior through the abdominal pores. Probe into these 
pores. 

The wall of the coelom is made up of these parts; the 
skin, muscle, and a smooth shining inner membrane, the 
pleuroperitoneal membrane. This membrane is divisible 
into three categories ; the parietal peritoneum, which is that 

14 



THE DIGESTIVE SYSTEM 15 

part lining the inner surface of the body wall ; the mesen- 
teries or ligaments, the double-walled continuations of the 
peritoneum from the mid-dorsal or mid-ventral line run- 
ning to the organs of the body cavity; the visceral peri- 
toneum or serosa of the organs, formed from the two layers 
of the mesenteries which separate and surround the organs. 

2. The Pleuroperitoneal Viscera. 

The viscera or organs are exposed by the opening of the 
pleuroperitoneal cavity. The organs cannot be said to be 
in the cavity since they are completely separated from it 
by their peritoneum and its extensions the mesenteries. 
They appear as if they had been let down into the cavity 
from the dorsal wall, by the peritoneum, while still sur- 
rounded by it. They are thus suspended in the cavity. 

Drawing 9. Make a line diagram of an imaginary section through 
the anterior end of the pleuroperitoneal cavity showing body wall, 
parietal peritoneum, visceral peritoneum of the intestine, and the 
mesentery suspending it. 

3. The Digestive Tract. 
A. External Aspects. 

The digestive tract consists of a thick-walled tube ex- 
tending from the anterior end to the posterior end of the 
body. It is differentiated into various regions having dis- 
similar functions. Part of the digestive tract has already 
been encountered, the mouth, the pharynx and the 
esophagus. 

In the following study do not disturb the position of the 
internal organs more than is absolutely necessary and do 
not break them apart or separate them from the body wall. 
Occasionally the stomach is accidentally everted into the 
mouth cavity. In such cases it should be pulled back into 
position. 



16 THE SKATE 

At the anterior end of the cavity is the liver, which is 
composed usually of three lobes of about the same size. 
In the angle between the median and the right lobes of 
the liver is the gall bladder. Dorsal to the liver and on the 
skate's left side is the j -shaped stomach. It is continuous 
with the esophagus and is partly concealed by the lobes of 
the liver. The anterior part of the stomach is the cardiac 
portion while the smaller bent portion is the pyloric region. 
The latter terminates the stomach in a sharp constriction, 
the pylorus. The remainder of the digestive tract to the 
anus is the intestine. The first section of the intestine just 
beyond the pylorus is the duodenum. Beyond the duode- 
num the intestine becomes enlarged and the lines of inser- 
tion of the spiral valve, which is within the intestine, are 
visible. 

Near the posterior end of the intestine is the small 
cylindrical rectal gland which is attached to the intestine 
by a short duct. The intestine is divisible into two parts, 
the anterior intestine (small intestine) and the posterior 
intestine (large intestine). The place of separation is at 
the rectal gland. The terms small and large intestine may 
be given to these parts out of consideration for their 
homology, but in the skate they are just the reverse in 
actual size. The posterior intestine is very short and opens 
by the anus into the terminal chamber, the cloaca, which in 
turn opens to the exterior by means of the cloacal aperture. 

B. The Derivatives of the Digestive Tract. 

Along the digestive tract are numerous organs and glands 
which have been derived from the tract. Some of these 
will be considered here. A series of derivatives have al- 
ready been encountered, including the visceral pouches and 
the gills. 

The thyroid gland is an outgrowth from the ventral wall 



THE DIGESTIVE SYSTEM 17 

of the pharynx and will be located later when it is exposed 
during the study of the circulatory system. 

The liver is one of the largest of the glands and is an out- 
growth of the wall of the intestine. It is composed of 
three lobes. The gall bladder is a greenish or yellowish 
transparent sac located between its right and middle lobes. 
The common bile duct should be located in the mesentery 
just below the gall-bladder and followed forward. It will 
be found to originate at the gall-bladder. A short cystic 
duct leads from the gall-bladder and a right and left 
hepatic duct join it as it passes into the common bile-duct. 
Posteriorly it passes to the dorsal side of the duodenal 
wall and runs a short distance caudad imbedded in the wall 
before it penetrates the cavity of the duodenum. 

The pancreas arises from several outgrowths of the intes- 
tine proper. The ventral lobe is a white body in the curve 
of the duodenum and is continuous with the dorsal lobe 
which is dorsal to the duodenum. The pancreatic duct 
empties into the intestine on the opposite side from the 
entrance of the bile-duct. It will be found imbedded in 
the posterior margin of the ventral lobe of the pancreas. 
Split open the intestine near this point and try to locate 
the entrance of the pancreatic and bile ducts. 

C. The Internal Aspects of the Digestive Tract. 

Cut open the stomach and place the contents in a dish. 
The Little Skate is omnivorous. Hermit and other crabs, 
shrimps, worms, amphipods, ascidians, bivalve moUusks, 
squids, small fishes, and other animals may be present in 
the stomach. 

Cut open the anterior intestine along one side just be- 
tween the large longitudinal blood vessels found on its wall. 
The spiral valve is its chief structure. It consists of a fold 
of the intestinal wall spirally coiled so as to form a series 



18 THE SKATE 

of incomplete overlapping cones. This formation greatly 
increases the digestive and absorptive surface of the intes- 
tine and makes a long intestine unnecessary. 

D. The Spleen. 

The spleen is a dark-colored organ located on the dorsal 
side of the stomach at its bend. It is not a part of the 
digestive system, but belongs to the category of lymph 
glands of the lymphatic system. Its function appears to be 
the destruction of foreign particles, bacteria and the like, 
and the addition of white blood corpuscles to the blood 
circulation. 

Drawing 10. Make an outline drawing of the digestive tract, 
its derivatives and the spleen. 

E. The Mesenteries. 

The viscera are suspended and held in place by mem- 
branes, the mesenteries or ligaments which are the exten- 
sions of the pleuroperitoneum. The dorsal mesentery is 
incomplete in the skate. It extends from the dorsal wall 
to the anterior part of the stomach and the posterior part 
of the esophagus as the mesogaster. It incloses the spleen 
in an extension from the stomach, the gastrosplenic. 

A remnant, the lesser omentum, is all that is left of the 
ventral mesentery, which typically is an outfolding from 
the median ventral wall. It extends from the right side 
of the stomach to the liver and is roughly divisible into two 
parts: the hepatoduodenal, which extends from the duo- 
denum and which contains the bile duct and blood vessels, 
and the gastrohepatic, which extends from the stomach to 
the liver. The falciform ligament is another remnant of 
the ventral mesentery and is located at the anterior end of 
the liver, extending from the midventral surface of the 
liver to the midventral line of the body wall. 



CHAPTER V 

THE UROGENITAL SYSTEM 

1. The Urogenital System. 

The excretory system of vertebrates consists of certain 
excretory organs with their ducts. The reproductive sys- 
tem consists of a pair of gonads or sexual glands and their 
ducts. The reproductive glands use the ducts of the ex- 
cretory system to transport their product of germ cells 
to the exterior, hence these two systems are generally quite 
closely united and are considered together as the urogenital 
(urinogenital) system. Work out the urogenital system of 
your specimen, then study the dissection of a specimen of 
the other sex which is being prepared by another student. 

Males may be distinguished from the females by several 
characters. Claspers are present on the male. The greater 
part of the sides lacks scales in the male. There are also 
fewer rows of spines along the middle of the back and tail. 
The scales on the margin of the head are much enlarged. 
Two rows of erectile spines are present on the lateral ex- 
pansion of the trunk (about one inch from the margin). 
These can be erected and lowered into depressions in the 
skin. In the females, scales are present over the sides of 
the body, several rows are present on the dorsal surface of 
the tail and the dorsal median part of the body. The female 
has no erectile spines. 

During the development of vertebrates there are three 
more or less distinct excretory organs or kidneys. (1) The 
pronephros is present in the embryonic stages of fishes and 

19 



20 THE SKATE 

amphibians when it is probably functional as an excretory 
organ. They persist in adult lampreys and a few fishes, 
while in reptiles, birds and mammals they appear during 
embryonic stages as transient structures without function. 
(2) As the pronephroi degenerate a second excretory organ 
is developed, the mesonephros (pi. mesonephroi) or Wolf- 
fian body. The mesonephros is the kidney of adult fishes 
and amphibians and appears in the embryos of reptiles, 
birds and mammals when it is functional until the forma- 
tion of the (3) metanephros or true kidneys of the adult 
reptiles, birds and mammals. 

A. The Female, 

The main portion of the kidney of the female skate con- 
sists of rounded lobes (masses about 30 x10 mm.) which 
are located against the dorsal wall of the body cavity at 
the extreme posterior end and on either side of the median 
axis. Carefully remove the pleuroperitoneum which covers 
the ventral surface of the kidney on the left side 
starting at its lateral margin and proceed towards the 
midline. This reveals the caudal mesonephros. An an- 
terior extension, the cranial mesonephros is nearly degen- 
erate in the female, but may be noticed as a brownish diffuse 
tissue extending from the anterior end of the caudal 
mesonephros. 

In the male skate it is much better developed and extends 
forward as a light-colored flat body on either side of the 
median axis. 

On the median surface of the caudal mesonephros are 
several ducts, the accessory mesonephrio ducts (to avoid 
later difficulty the term ureter should be applied only to the 
excretory duct of the metanephros). These ducts may best 
be located by dissecting away, with blunt end of probe or 
scalpel, the connective tissue which extends from the 



THE UROGENITAL SYSTEM 21 

mesonephros to the dorsal anterior wall of the cloaca. 
The ducts are those white tubes running in this tissue 
which do not break or cut as readily as the connective tissue. 
They extend medially and forward and enter a chamber, 
the urinary sinus, situated on the dorsal surface of the 
anterior end of the cloaca. Remove the connective tissue in 
this region. The two sinuses unite to form a common cham- 
ber, the urinary vesicle (it is not a true bladder or homolo- 
gous with that organ since it is the enlarged termination 
of the mesonephric ducts rather than an evagination of the 
ventral wall of the cloaca). Make a small incision in the 
sinus and run the probe into its entrance to the vesicle. 
Probe through the vesicle and into the opening in the mid- 
dorsal wall leading into the cloaca. The Wolffian ducts 
(degenerate and functionless for the most part in the 
female) of the cranial mesonephros are slender tubes which 
extend anteriorly from the urinary vesicle lying on the 
dorsal surface of the strong white portion of the mesen- 
teries. The remainder of the excretory system (cloaca and 
cloacal aperture) is in common with the digestive system. 
Cut open the extended cloaca along the median ventral line 
and also open the intestine a short distance. Note the open- 
ing of the intestine, the anus, into the ventral part of the 
cloaca and the horizontal fold which separates this from 
the dorsal or urogenital portion. Cut into the latter by cut- 
ting forward through this fold. The urogenital opening m 
the median dorsal wall is half way between the openings 
of the oviducts which are on either side. A probe may now 
be run from the opening made in the urinary sinus through 
the urinary vesicle and into the cloaca by way of this 
urogenital opening. 

The ovaries are two soft bodies (masses about 25 mm. x 
40 mm ) which contain large ova or eggs and are situated 
in the back portion of the anterior half of the pleuroperi- 



22 THE SKATE 

toneal cavity. Each ovary is attached to the structures 
dorsal to it by a mesentery, the mesovarium. Dorsal to the 
ovary is the oviduct, an enlarged tube which is connected 
with it only by mesentery. Dissect away this mesentery. 
Trace one of the oviducts forward and notice in your speci- 
men or in a demonstration specimen that its narrow an- 
terior portion passes along the dorsal coelomic wall, curves 
around the anterior dorsal margin of the liver and unites 
with its fellow from the other side, where is located the 
ostium, a wide funnel-shaped opening for receiving the 
eggs after they break out from the ovary into the coelom. 
Trace the oviduct caudad and notice an enlargement of 
the tube, the uterus. At its beginning is a bilobed mass, the 
oviducal gland, which secretes the horny case (see demon- 
stration) in which the eggs are contained. Egg cases will 
often be found in the uterus of the skates being dissected. 

Drawing 11. Draw the female urogenital system. 

B. The Male. 

The testes in which the sperm cells are developed are 
oval flat bodies (about 60 mm. x 20 mm.) attached to the 
more dorsal structures by mesenteries, the mesorchia. The 
kidney or excretory organ is like that of the female. Dis- 
sect it out after the manner outlined above for the female. 
The mesonephric or "Wolffian ducts are well developed tubes 
running posteriorly along the ventral faces of the cranial 
mesonephros. They serve as a passageway for the sperm 
cells. They are greatly convoluted tubes located near the 
median line. 

The testis is connected with the cranial mesonephros by 
means of delicate ducts, the vasa efferentia, which run in 
the mesorchium. These vasa efferentia connect with tubules 
of the mesonephros. The sperm tubules of the mesonephros 
connect with the Wolffian duct. Trace the duct and notice 



THE UROGENITAL SYSTEM 23 

that it enlarges on the surface of the caudal mesonephros, 
forming the seminal vesicle (best developed in adult speci- 
mens). Remove the pleuroperitoneum from the ventral 
surface of the caudal mesonephros and find that at the 
posterior end and at the sides of the cloaca that the seminal 
vesicle opens into the lower end of the sperm sac. These 
lead into the urogenital sinus by a common opening. 

Split open the cloaca (in the male it is much smaller 
than in the female) ventrally and notice on the dorsal 
wall the projecting urogenital papilla. The urogenital 
sinus leads into the cloaca at the tip of the urinary papilla. 
Run a fine probe into the opening of the sinus and trace 
the entrance of the sac and seminal vesicle. The sperm 
cells pass out through the cloacal aperture and along the 
grooved lateral margins of the claspers. 

The accessory mesonephric ducts are similar to those 
of the female described above and are located by following 
the same procedure. In the male skate several of these 
accessory ducts will be found coming from the median side 
of the mesonephros and going into the sperm sac. 

Drawing 12. Draw the male urogenital system. 



CHAPTER VI 
THE CIRCULATORY SYSTEM 

In vertebrates the circulatory system is composed of two 
series of tubes, the lymphatic and the blood-vascular sys- 
tems, which inclose fluids that circulate. The blood-vascu- 
lar system is commonly termed the circulatory system. It 
consists of a set of branching continuous tubes which form 
a closed system and are unconnected with any other system. 

1. The Pericardial Cavity. 

Remove the skin from the ventral surface of the region 
posterior to the mouth. Start with an incision at the pec- 
toral girdle and work forward between the gill-slits of the 
two sides. Remove with the forceps the layers of visceral 
muscle found below the skin until a membrane, the parietal 
pericardium, in the posterior median region is uncovered. 
Remove this membrane and expose the pericardial cavity, 
within which is the heart. The cavity may be better re- 
vealed if the tissue along the anterior face of the girdle 
is removed. This should be cut away where necessary, care 
being taken not to injure the heart. 

The pericardial cavity is a part of the coelom. The other 
part, the pleuroperitoneal cavity has been studied. The 
two parts are separated by a membrane, the transverse 
septum, which forms the posterior wall of the pericardial 
cavity. 

The septum is not complete since an opening, the 
pericardio-peritoneal canal, which will be pointed out later, 

24 



THE CIRCULATORY SYSTEM 25 

connects the cavities. The septum is formed as a bridge, 
enabling the dorsal blood vessels to pass to the heart, which 
is ventral in position. 

2. External Aspects of the Heart. 

The heart is attached at its posterior and anterior ends 
by deflections of the parietal pericardium, which may be 
located by lifting up the heart. The visceral pericardium 
is an extension of this, which covers the heart and is indis- 
tinguishably fused with its wall. 

The wings of the sinus venosus may be located on either 
lower corner of the main mass of the heart as thin-walled 
wide tubes. Make a very small incision in the right wing 
and insert the probe and run it towards and into the main 
mass of the heart. The wings of the sinus venosus are in 
the transverse septum. Non-aerated blood from the various 
parts of the body is returned to the heart through the sinus 
venosus. 

During development the heart is at first a straight tube, 
but later it becomes bent upon itself dorso-ventrally in 
the form of a letter S. The walls of the tube are differen- 
tiated into chambers which should be located on the ex- 
terior. The first chamber is the sinus venosus already 
located. The thin-walled second chamber just anterior to 
the sinus venosus and on the dorsal side of the heart is 
the auricle or atrium. It expands on each side of the an- 
terior portion of the heart so that a portion of it may be 
seen from a ventral viewpoint. The third chamber is the 
thick-walled, heart-shaped, ventricle. The posterior lateral 
end of the ventricle is the apex, the anterior side is the base. 
The fourth chamber is the conus arteriosus, a heavy-walled 
tube which originates at the base of the ventricle and pene- 
trates the anterior pericardial wall. The blood, all of 
which is non-aerated, circulates through the chambers of 



26 THE SKATE 

the heart in the order mentioned. The internal structures 
of the heart will be considered later. 

Drawing 13. Sketch the heart from the ventral viewpoint. 

3. The Venous System. 

Veins are those blood vessels through which blood is re- 
turned to the heart. There are three kinds of veins in the 
vertebrates: (1) The systemic veins which empty directly 
into the heart; (2) the portal veins which carry the blood 
to a system of capillaries (e.g., in the liver), where it is 
brought together in systemic veins and delivered to the 
heart; (3) the pulmonary veins in vertebrates with lungs 
which bring the aerated blood back to the heart from the 
lungs. 

A. The Systemic Veins. 

The dissection of the veins is the most difficult part of 
the entire study of the skate. Follow the directions very 
closely and most of the system will be clearly unfolded. 
Refer to the figure to aid in your organization of this 
system. Many of the veins of the skate are not sharply 
defined vessels, but are large spaces in the tissue, without 
very definite walls and are more correctly called sinuses. 

The systemic veins enter from both posterior corners of 
the dorsal side of the heart by way of the sinus venosus, 
which has already been pointed out. The central portion 
of the sinus venosus, which receives the two tubes from 
either corner, is attached to the transverse septum by a 
sheet of connective tissue which should be broken away 
carefully. Trace out the right side of the sinus venosus, 
which is buried in the transverse septum, until it disappears 
dorsal to the pectoral girdle cartilage. Follow the sinus 
laterally by carefully shaving away the cartilage and the 
surrounding tissue. With fine scissors cut open the ventral 



THE CIRCULATORY SYSTEM 



27 




Fig 1 The ventral aspect of the venous system of Baja namita, 
(Eediawn from Parker, Zootomy, Macmillan and Co.) 

wall of the sinus in a crosswise direction. Leading directly 
into the sinus is the chamber or tube, the common cardinal 
or duct of Cuvier, which bends towards the dorsal surface. 



28 THE SKATE 

All of the systemic veins carry blood to this common car- 
dinal vein. 

There is a slight fold at the juncture of the sinus and the 
common cardinal vein near which is the rather small en- 
trance of the inferior jugxdar, a vein which collects blood 
from the walls of the pericardial cavity and the floor of 
the mouth and pharyngeal cavities. Use a bristle to probe 
into it. The hepatic sinus enters through the posterior wall 
of the common cardinal. Its main chamber is a large space 
situated between the anterior end of the liver and the 
transverse septum. Locate this chamber and cut it open. 
It has an outlet towards both right and left common car- 
dinal veins. Several small hepatic veins enter it from the 
liver. 

The pericardio-peritoneal canal runs through the center 
of this sinus. It will be remembered that this canal con- 
nects the two otherwise separated parts of the coelom. 
Kefer back to Chapter IV. 

The posterior cardinal vein enters the common cardinal 
through the dorsal part of the posterior wall. Probe into 
this part of the common cardinal and towards the midline 
of the dorsal wall of the pleuroperitoneal cavity. The 
posterior cardinal continues as a large space located on 
the dorsal side of the testes in the male and on the dorsal 
side of the oviduct in the female. Posteriorly it will be 
found to unite with the one from the other side, forming 
a median sinus which is directly connected through its 
ventral walls with the genital sinus. The latter originates 
as large spaces about the gonads. The median sinus sepa- 
rates again posteriorly into two veins proceeding on the 
medial side of the kidneys, where they are again united. 

The anterior cardinal sinuses return the blood from the 
anterior part of the body and head. Eun the probe from 
the common cardinal towards the dorsal side of the skate, 



THE CIRCULATORY SYSTEM 29 

turn the skate over and locate the end of the probe. Make 
a longitudinal incision at the end of the probe and run it 
forward to the eye. This exposes the anterior cardinal 
sinus, which is a smooth-walled elongated cavity situated 
between the dorsal ends of the visceral pouches. 

A branch of the brachial vein may be located on the dor- 
sal surface. Make an incision along the medial face of the 
anterior cartilage of the pectoral fin and locate this vein. 
It runs into the common cardinal vein. 

Within the pleuroperitoneal cavity a few other veins may 
be located. On the lateral wall is the lateral abdominal 
vein which receives a parietal branch from each myosep- 
tum. It originates near the cloaca. It passes along the 
internal surface of the pectoral fin cartilage and extends 
to the common cardinal vein. Brachial veins will be found 
joining the lateral abdominal vein. One may be located 
on the posterior side of the pectoral girdle cartilage. 
There are others. The largest of the iliac veins will be 
found along the posterior side of the cartilage of the 
pelvic girdle. The iliac veins come from the pelvic fin. 

Deawinq 14. Outline the systemic veins. 

B. The Portal Systems, 

In the skate there are two portal systems, the hepatic 
portal and the renal portal. The hepatic portal system 
collects the venous blood from the various parts of the 
digestive tract and spleen and carries it into the liver, 
where it breaks up into a network of capillaries which are 
collected later and enter the hepatic sinus. 

a. The Renal Portal System. Study the renal portal 
system from the figure. It consists of a caudal vein which 
collects the blood from the caudal region and runs forward 
to the cloacal aperture, where it forks into the two renal 
portal veins extending to the ventral side of the kidney 



30 THE SKATE 

and there giving off branches. These capillaries unite to 
form veins which connect with the posterior cardinal vein. 

b. The Hepatic Portal System. In many of the speci- 
mens the hepatic portal system shows up as dark-blue 
tubes, the color being due to the presence of blood in the 
veins. If your specimen does not show the veins clearly in 
the region of the stomach and the intestine another speci- 
men will be loaned to you from the general supply. The 
veins are often near the yellow-colored arteries. 

The main hepatic vein is located in the hepatoduodenal 
ligament near the bile duct. It receives branches from the 
digestive tract and its derivatives. There are three main 
tributaries. (1) The g^astric vein, which enters from the 
left side, coming from the right margin of the stomach 
and having a ventral and a dorsal vein from the spleen. 
(2) The lienomesenteric vein enters the hepatic vein as the 
middle vein. It has two main tributaries; a posterior 
mesenteric from the left side of the intestine arising at 
the end of the rectal gland and receiving branches from 
the spiral valve attachments and the pancreas; and a 
splenic from the right or medial end of the spleen. (3) 
The pancreatico-mesenteric joins the hepatic vein from the 
right side, coming mainly from the pancreas. Its main 
tributaries are the anterior mesenteric from the region of 
the duodenum, and a posterior gastric from between the 
pylorus and the bend of the stomach. 

Drawing 15. Outline the hepatic portal system including the 
digestive system with dotted lines. 

4. The Arterial System. 

A. The Ventral Aortcb and the Afferent Branchial 
Vessels. 

These arteries are located in the tissue of the floor of 
the mouth. They are filled with blood, which gives them a 



THE CIRCULATORY SYSTEM 31 

dark color. Remove the skin and pick away the tissue and 
muscle from the region between the anterior end of the 
pericardial cavity and the lower jaw. Dissect away and 
expose the vessels, starting just anterior to the heart. 

a. The ventral aorta is a continuation of the conus 
arteriosus and is on the median ventral line. Trace it out 
on the left side and expose the afferent branchial arteries. 
The first pair of these is given off at the point where the 
conus arteriosus passes into the ventral aorta. It divides 
into three branches, which supply the fourth, fifth, and 
sixth visceral arches and extend into the interbranchial 
septa, giving off branches to each of the demibranchs. 

It is necessary for dissectional reasons to point out here 
the coronary arteries which supply the heart walls with 
aerated blood. They are yellow in color and should not be 
injured more than is necessary in tlie following work. 
Their connections will be pointed out later. 

The second pair of afferent branchial vessels is formed 
by the bifurcation of the ventral aorta at its anterior end. 
Follow the aorta forward almost to the lower jaw where 
the second pair originates. 

Trace the left one. It divides into two vessels which 
penetrate the interbranchial septa of the second and third 
visceral arches. The embryonic first afferent artery of the 
first arch, the spiracle, is not persistent in the adult skate 
probably because there is no functional gill on this 
arch. 

Drawing 16. Draw the ventral aorta and the afferent branchial 
vessels. 

5. The Internal Structures of the Heart. 

Review the drawing of the heart, number 13. The heart 
is a modified tube bent dorsoventrally into the various 
chambers which have been mentioned before. Because of 



32 THE SKATE 

this bending, which results in an s-shaped structure, the 
ventricle is brought into contact with and overlies the 
sinus venosus, and the auricle is brought into contact with 
and is beneath the conus arteriosus. 

First note the distribution of the posterior coronary 
artery coming from the posterior corners of the pericardial 
cavity. Their origin will be indicated later. 

Kemove the heart from the pericardial cavity by cutting 
it at the base of the ventral aorta and freeing the sinus 
venosus from the transverse septum. Split open the sinus 
venosus which leads into the auricle by a sino-auriculax 
aperture which is supplied with sino-auricular valves. 
Open the auricle and wash out the blood clot. It opens 
into the ventricle through the auriculo-ventricular aperture 
which is supplied with auriculo-ventricular valves. Open 
the ventricle to locate these valves. Within the ventricle 
note the numerous cavities and crevices in the spongy inner 
wall where the blood is held, and the muscular band, the 
columnae oarnae. Open the conus arteriosus which is 
provided with many pocket-like semi-lunar valves. There 
are three longitudinal rows of valves and five valves in each 
of the rows. To locate and open these pockets run the 
point of a probe from the anterior end towards the pos- 
terior end. 

In the skate, as in all fishes, there is a single circulation 
through the heart. The heart contains only venous blood, 
which enters the sinus venosus from the systemic veins and 
passes through the auricle, ventricle, and conus arteriosus 
which connects with the ventral aorta, from which the blood 
is distributed to the gills by means of the afferent branchial 
vessels. The blood passes through the gill filaments and 
is aerated. Since the heart contains no aerated blood its 
walls are supplied by the coronary arteries from the arterial 
system. 



THE CIRCULATORY SYSTEM 33 

Drawing 17. Make a drawing of the heart showing its internal 
structure with arrows to indicate the course of the blood. 

6. The Thyroid Gland and Pericardio-peritoneal Canal. 

The thyroid gland, the secretion of which seems to regu- 
late growth, may now be located as a brownish tissue just 
behind the lower jaw and immediately ahead of and in the 
angle of the bifurcation at the anterior end of the ventral 
aorta. 

The pericardio-peritoneal canals may also be located 
now. These canals, it will be remembered, connect the two 
parts of the coelom, the pericardial and the pleuroperi- 
toneal cavities. In the center of the posterior wall of the 
sinus venosus there is an opening of moderate size. Probe 
into this. It leads into a canal, the pericardio-peritoneal 
canal, which passes through the center of the hepatic sinus. 
It forks into two canals in the ventral wall of the esophagus 
and opens into the pleuro-peritoneal cavity by small open- 
ings. Refer back to Chapter IV. 

7. The Dorsal Aorta. 

The dorsal aorta and the efferent branchial arteries will 
be found on the roof of the mouth and pharyngeal cavity. 
With the forceps pick off the mucous membrane and expose 
the arteries which are filled with yellow starch paste in- 
jected from the tail. There are three main pairs of efferent 
branchial arteries. The blood comes from the gills and is 
collected by the union of the efferent arteries to form a 
dorsal aorta which extends backward just ventral to the 
vertebral column. 

Trace the efferent branchial arteries towards the gills, 
removing the cartilage where necessary. Each efferent 
artery is formed at the dorsal angle of the internal gill- 
slits by the union of two arteries. A large post-trematic 



34 THE SKATE 

from the demibranch on the posterior side of the visceral 
pouch (the anterior face of a gill-arch) and a small pre- 
trematic from the demibranch on the anterior face of the 
visceral pouch (posterior face of the gill-arch). The post- 
trematic and the pre-trematic completely surround the 
pouch, forming a loop around the gill cleft, meeting on the 
ventral side. At the angle of the ventral and dorsal in- 
ternal surfaces of the gill-arch a cross-branch is located 
which unites the post-trematic of a gill-arch with the pre- 
trematic which is parallel and just posterior to it in the 
same arch. 

The anterior coronary arteries, the distribution of which 
has been pointed out, carry aerated blood to the walls of 
the heart. They arise from the extension around the fourth 
and fifth gill-slits of certain vessels from the ventral ends 
of the loops. The posterior coronary arteries are distri- 
buted along the sinus venosus and originate from a branch 
of the subclavian which will be encountered later. 

The common carotid originates at the junction of the 
post-trematic and the pre-trematic from the most anterior 
gill pouch as it forms the most anterior efferent artery. 
Follow it forward. It bends slightly towards the median 
line and just lateral to the anterior margin of the spiracle 
it bifurcates, forming the external carotid which continues 
towards the outside, and the internal carotid which con- 
tinues towards the median line, where it meets with its fel- 
low from the other side and passes through the cartilage 
to the brain. 

There is an hyoidean artery which arises at about the 
middle of the pre-trematic of the most anterior efferent 
branchial artery. Follow it forward and laterally by cut- 
ting away the cartilage behind which it passes. It goes to 
the rudimentary gill of the spiracle and the adjacent mus- 
cles. The branches to the spiracle reunite, forming the 



THE CIRCULATORY SYSTEM 35 

ventral carotid which passes laterally and over (ventral) 
to the external carotid and enters the cartilage, later unit- 
ing with the internal carotid within the brain cavity. 

The two most anterior efferent branchial arteries fuse 
and just posterior to their union the vertebral artery arises 
which enters the skull and is distributed to the brain and 
the spinal cord. 

The efferent branchial arteries unite in pairs along the 
median line and form the dorsal aorta which proceeds back- 
wards into the pleuroperitoneal cavity. 

Deawing 18. Show the efferent branchial arteries and their vari- 
ous branches. 

8. The Distribution of the Dorsal Aorta. 

Trace the dorsal aorta posteriorly by separating the 
esophagus from the body wall on the left side. A sub- 
clavian artery is given off to each side between the points 
where the third and fourth pairs of efferent branchial 
arteries enter the dorsal aorta. They go to the pectoral 
fins. Trace the left subclavian. It gives off these arteries : 
a posterior coronary artery, the distribution of which has 
already been noted ; lateral arteries along the lateral body 
wall; a^d a ventral abdominal artery which runs pos- 
teriorly as a conspicuous colored artery on the ventral 
wall; continuing as the brachial it is distributed to the 
pectoral fin. 

A little posterior to the origin of the subclavian arteries 
the dorsal aorta is completed. It then passes posteriorly 
along the mid-dorsal line of the pleuroperitoneal cavity. 
Find the following branches, doing the work on the left 
side after turning the viscera over to the left side of the 
body. 

a. Unpaired Visceral Branches. The coeliac is the first 



36 THE SKATE 

to be given off to the viscera. Its chief branches are : the 
hepatic artery to the liver ; and the anterior gastric divisi- 
ble into a dorsal gastric and a ventral gastric of the 
stomach; the splenic; a gastroduodenal with a posterior 
branch to the posterior part of the stomach, the posterior 
gastric, pancreatic branch; and a duodenal to that part 
of the digestive tract. 

The superior mesenteric artery has its origin just pos- 
terior to the origin of the coeliac and sends branches to the 
pancreas, and spleen, while its main branch continues to 
the intestine and the spiral valve. 

The inferior mesenteric artery arises from the dorsal 
aorta posterior to the superior mesenteric and passes to 
the rectal gland giving rise to a genital artery which passes 
to the gonads and their ducts. 

b. Lateral Visceral Branches. Renal arteries are lo- 
cated on the dorsal side of the kidney. Loosen the kidneys 
from the body wall and expose these arteries. The genital 
arteries already located belong to this category. 

c. The Somatic Branches. Several paired parietal ar- 
teries branch from the dorsal aorta and pass to the body 
wall, enlarged examples of which are the subclavian to 
the pectoral fin and the paired iliac arteries to the pelvic 
fins. The latter arise from the dorsal aorta just anterior 
to the cloaca and follow a course along the body wall, giv- 
ing off many branches, some of which unite with the pos- 
terior end of the ventral abdominal artery before entering 
the pelvic fin. 

The dorsal aorta continues posteriorly into the tail, 
v/here it is known as the caudal artery. The injection of 
yellow paste was made through this artery. 

Drawing 19. Draw the dorsal aorta and its chief branches. 



CHAPTER VII 

THE NERVOUS SYSTEM AND THE SENSE ORGANS 

I. THE SENSE ORGANS. 

The sense organs are structurally suited to receive sen- 
sory impressions from the outside world. They are all 
connected with the brain. Two rather distinct types of 
sense organs may be recognized, the much specialized sense 
organs having many associated structures such as those 
connected with sight and hearing, and the simpler integu- 
mentary sense organs concerned with touch and perception 
of temperature. Dissection of the brain often ruins the 
ear and eye. Therefore, it is advisable to study the sense 
organs first. 

1. Integumentary Sense Organs. 

A. Several integumentary sense organs are present in 
the skate. The ampullae of Lorenzini occur in five groups 
on the head. Remove a piece of skin from the ventral side 
of the head. The pores lead into the canals of Lorenzini 
which end in a very small bulb, the ampullae of Lorenzini 
supplied with white nerve fibers. These organs seem to 
function in the perception of vibrations and pressure 
stimuli from the water. 

B. The lateral line system consists of several canals, 
chief of which are: the supraorbital canal above the eye 
extending forward at the side of the rostrum ; an infra- 
orbital canal which separates from the supraorbital back 
of the spiracle ; the lateral line canal extending backwards 

37 



38 THE SKATE 

from the junction of the supraorbital and the infraorbital 
along the side of the vertebral column to the tip of the 
tail; the hyomandibular canal with various branches. 
Tubules lead to the surface from all of these canals and 
may be easily seen in many specimens. 

C. The pit organs are on the dorsal surface and may 
be seen with the unaided eye. They are in three groups : 
a row of five or less under the eye ; a row along the back 
on each side medial to the lateral line canal; and a group 
medial to the spiracle on each side. 

2. The Organs of Special Sense. 

A. The Olfactory Organ. 

There are a pair of olfactory sacs on the ventral side of 
the rostrum which open to the nostrils, but which have no 
communication with the oral cavity. Expose the olfactory 
sac by cutting away the skin just in front of the eye. 
Then make a cross-section of the organ. Internally there 
are numerous plates arranged in rows which are covered 
with an epithelium sensitive to odors. 

Drawing 20. Draw the olfactory organ. 

B. The Eye. 

Expose the eye or eyeball, a spherical body, by carefully 
dissecting away the skin and connective tissue from around 
the eye. First cut away the skin over the eye, then care- 
fully cut away a little of the cartilage between the eye and 
the brain and a part in front of the eye. The eyeball is in 
a cavity of the skull, the orbit, to which it is attached by 
six eye muscles. Four of these muscles may be identified 
from the dorsal side. Each muscle is attached at its two 
ends and is more or less free in between. The attached 
end which is fixed and immovable is called the origin while 



THE NERVOUS SYSTEM 39 

the attached end which is moved when the muscle con- 
tracts is called the insertion. The superior oblique muscle 
has its origin at the anterior wall of the orbit. The other 
three are attached to the lateral posterior wall ; the most 
anterior of these being the internal rectus with its insertion 
on the eyeball covered by that of the superior oblique; 
most dorsally the superior rectus ; and the one on the pos- 
terior side, the external rectus. Determine what would be 
the result of the contraction of each of these muscles. 

Raise the eyeball and cut through the conjunctiva, a 
thin layer adhering to the external surface of the eyeball 
and continuous with the lining of the lower lid. Lift the 
eyeball and notice the inferior oblique muscle originating 
from the anteriomedial corner of the orbit and the inferior 
rectus from the posteriomedial angle of the orbit. They 
are inserted together in the middle of the ventral face of 
the eyeball. 

Drawing 21. Draw the eyeball showing muscles. 

Remove the eyeball by cutting through the eye muscles 
at their insertions. The outermost coat covering the front 
of the eyeball is the conjunctiva which is deflected onto the 
inner surface of the eyelids. It is a part of the epidermis 
of the skin and not a true coat of the eye. The outermost 
coat of the eye ball is the sclera, a tough membrane of con- 
nective tissue. It is transparent in front, forming the 
cornea to which the conjunctiva is inseparably fused. Cut 
off the dorsal side of the eyeball and study under water. 
Note the crystalline lens which aids in focusing the light, 
the black choroid coat internal to the sclera which darkens 
the interior of the eyeball, and internal to the choroid coat 
is the greenish layer, the retina which is often collapsed. 
The retina is an outpushing of the brain containing the rods 
and cones which can be stimulated by the light. Follow 



40 THE SKATE 

the choroid coat to the front of the eye where it separates 
from the cornea, forming a black curtain, the iris, which 
may be seen through the transparent cornea and in the 
center of which may be seen the opening or pupil. 

The iris divides the cavity of the eyeball into two cavities, 
an external cavity the anterior chamber between the iris 
and the cornea and the vitreous humor cavity between the 
lens and the retina. The anterior chamber is filled with 
the aqueous humor while the vitreous humor cavity con- 
tains in life a gelatinous vitreous humor. In life the lens 
is attached to the margins of the pupil and the margins 
of the retina, leaving a small space between these two points 
of attachment called the posterior chamber. 

Drawing 22. Make a diagram of the cross-section of the eye. 

After the eyeball is removed from the orbit note (1) the 
origin of the six eye muscles, (2) the optic pedicle for 
support of the eyeball which is a cartilaginous stalk, 
situated among the rectus muscles, and (3) the optic 
nerve, a stout white nerve in front of the rectus muscles. 
On the floor of the orbit is the infraorbital nerve. 

C. The Ear. 

The internal ear of the skate is located between the spira- 
cle and the mid-dorsal line where there is an elevation of 
the chondocranium. A pair of small holes is present in the 
median line between these elevations. If the internal ear 
has been destroyed on one side during the dissection of 
the eye use the other side, being careful not to injure the 
brain. 

Remove the skin from the region of these holes and trace 
the entrance of the endolymphatic ducts which pass 
through the fossae in the chondocranium. These ducts con- 
nect the cavity of the internal ear with the exterior. 



THE NERVOUS SYSTEM 41 

Carefully shave off the cartilage of the elevations until 
the canal within the cartilage is exposed. It contains a 
curved, colorless tube, the anterior vertical semicircular 
duct. Keep this duct intact and continue the dissection 
until a posterior vertical semicircular duct is reached. Be- 
low this is the thin-walled vestibule from which the semi- 
circular ducts spring. A third duct, the horizontal semi- 
circular duct, is below these and may be exposed by further 
cutting away the cartilage, leaving the ducts and vestibule 
in place. 

With the structures just exposed still in place identify 
the parts of the internal ear. The ducts are semicircular 
and each terminates in a sac, the ampulla. These are in 
communication with the large dorsal utriculus which is 
a part of the vestibule. The other ventral portion, the 
saoculus, is located in a pit in the cartilage. The ampullae 
are in connection with each other. Each is supplied with a 
sensory area, the white crista, bearing the nerve. A white 
mass of crystalline material or sand grains, the otolith, is 
present in the utriculus. The endolymphatic duct also 
enters the vestibule, but this entrance is difficult to locate 
in the skate. 

The functions of the ear are those of hearing and equili- 
bration. The canals in the cartilage are filled with peri- 
lymph, and the ducts and vestibule with a fluid, endo- 
lymph. Changes in pressure, either from sound waves or 
the change in the position of the head, appear to excite 
the sensory cells in the ampullae and vestibule. The am- 
pullae and the vestibule control equilibration. The capacity 
to detect sound waves seems to be limited to the vestibule. 

The above organs represent the inner ear of the higher 
vertebrates. The middle and outer ears are absent in the 
skate. 

Drawing 23. Make a drawing of the internal ear. 



42 THE SKATE 

II. THE NERVOUS SYSTEM. 

The function of the nervous system is that of conduction, 
coordination and correlation of stimuli. 

The nervous system may be divided into three parts: 
(1) the central nervous system, composed of the brain 
within the skull and the spinal cord within the neural canal 
of the vertebrae; (2) the peripheral nervous system, com- 
posed of the cranial nerves from the brain and the spinal 
nerves from the spinal cord; (3) the sympathetic system 
which controls and regulates the involuntary activities and 
the organs involved, such as the heart, digestive tract, 
respiratory and reproductive systems, and the smooth mus- 
culature in general. It is connected with outgrowths of 
the peripheral nervous system. 

The brain and spinal cord are composed of nerve-cell 
bodies (gray matter) and nerve-cell processes (white mat- 
ter). The cranial and spinal nerves are made up of proc- 
esses only. 

1. The Brain. 

The central nervous system is developed from the tube 
formed originally by an infolding of the ectodermal medul- 
lary plate of the embryo. During the development of the 
brain there are at first three vesicles, which have been 
named the forebrain or prosencephalon, the midbrain or 
mesencephalon, and the hindbrain or rhombencephalon. 
The first and third subsequently divide into two. Each 
of the five divisions is further changed by evaginations, 
foldings and thickenings. The adult brain has, however, 
the ^Ye main divisions : the telencephalon ; the diencepha-j 
Ion or thalamencephalon ; the midbrain or mesencephalon ; 
the metencephalon or cerebellum; and the mylencephalon 
or medulla oblongata. 

Remove the skin from the top of the head back to a 



THE NERVOUS SYSTEM 43 

point two inches behind the eyes. Cut away the cartilage 
and expose the brain. The white strands are cranial nerves, 
which pass through the cartilage and should not be injured. 
The brain is situated in a cavity in the chondocranium 
and is covered by the primitive meninx, the membrane in 
which the blood vessels of the brain run. This membrane 
is connected here and there to the walls of the cranial cavity 
by strands of tissue. The space between is filled with fluid 
in life. 

a. The Telencephalon. At the anterior corner of the 
main brain mass is the elongated olfactory bulb. Cut away 
the cartilage and expose the bulb. It gives off a very small 
olfactory nerve forward to the olfactory sacs. The bulb 
is connected by the stock, the olfactory tract, to the en- 
larged olfactory lobes which are a part of the main mass 
of the brain. Medial to these lobes and separated only by 
a slight groove are the cerebral hemispheres. 

b. The Diencephalon or Thalamencephalon. Posterior 
to the hemispheres is the depressed diencephalon. The thin 
roof of this consists of the choroid plexus, blood vessels 
of the third ventricle. The diencephalon is the center for 
the coordination of sensations and is the chief controlling 
portion of the brain. The optic nerve passes from the orbit 
towards its ventral surface. 

c. The Mesencephalon or Midbrain. The mesencephalon 
is posterior to the diencephalon and consists largely of the 
optic lobes (corpora bigemina), which are centers for the 
auditory, visual and skin sensations. The trochlear (fourth 
cranial) arises from the posterior edges of the optic lobes 
and runs forward to the muscles of the eye. The oculomo- 
tor (third cranial) passes to the orbit from the ventral side 
of the mesencephalon. 

d. The Metencephalon or Cerebellum. Posterior to the 
optic lobes and overhanging them is a large cerebellum. 



44 THE SKATE 

It is marked by longitudinal and transverse grooves. It 
is the center of motor coordination. 

e. The Myelencephalon or Medulla Oblongata. Posterior 
to the cerebellum is the elongated myelencephalon. Its an- 
terior part consists of the choroid plexus of the fourth ven- 
tricle, below which is the cavity of the fourth ventricle. 
There are two earlike projections (auricles) on the anterior 
end of the medulla. 

The posterior end of the medulla marks the end of the 
brain which grades indefinitely into the spinal cord. 

Drawing 24. Draw the brain from the dorsal side. 

2. The Cranial Nerves. 

The dissection of the cranial nerves in the skate is easier 
than in any other vertebrate animal. 

There are ten cranial nerves. These nerves show a defi- 
nite relation to the original segmentation of the head and 
to the gill. Throughout the evolution of the vertebrates 
each nerve continues to be distributed to homologous parts 
regardless of the animal. So close is this homology that 
structures of unknown history having the same innervation 
can be known as homologous regardless of great differ- 
ence in function and position. 

The cranial nerves are indicated either by number which 
corresponds to the order of their appearance or by name. 
Since the roots of some of these as the fifth, seventh, and 
eighth are intermingled as they leave the brain they should 
be located some distance from their origin as indicated in 
the directions. 

a. The first or olfactory tract leaves the olfactory lobe 
extending to the nostrils and expanding into olfactory 
bulbs. 

b. The second or optic tract or nerve is a stout, white 
nerve piercing the orbit and running from the retina to the 



THE NERVOUS SYSTEM 45 

ventral side of the diencephalon. Since the retina is an 
outpushing of the brain wall, the connection it retains with 
the brain is not a nerve but a tract of the brain. 

c. The third or the oculomotor arises from the floor of 
the mid-brain and ascends to the orbit near the trochlear 
nerve. It supplies four of the six muscles of the eye. There 
is a ciliary ganglion belonging to the sympathetic system 
which appears as a small brown mass on the oculomotor 
nerve. 

d. The fourth or trochlear is a small nerve arising 
dorsally from the midbrain and emerging at the groove 
between the optic lobes and cerebellum. It supplies the 
superior oblique muscles of the eye. 

e. The fifth or trigeminus is a very large nerve with 
four branches. It arises from the medulla oblongata. Its 
roots are located just behind the auricles of the medulla 
and are mingled with those of the seventh and eighth 
nerves. The three branches of the trigeminus are: the 
maxillary and the mandibular to the muscles of the jaw, 
and the superficial ophthalmic which runs over the eye 
to the snout closely united with a similar branch of the 
seventh. 

f . The sixth or abducens is a slender nerve hidden be- 
neath the fifth arising near the midventral line. It will 
be seen as a white ridge on the ventral surface of the ex- 
ternal rectus muscle of the eye. 

g. The seventh or facial is the nerve of the spiracular 
cleft. It arises in common with the trigeminus from the 
anterior end of the medulla and divides into three main 
branches. Two of these pass to the orbit. It supplies all 
the ampullae of the head and has numerous branches which 
go to the lateral line organs. 

h. The eighth or auditory nerve extends from the in- 
ternal ear to the brain. It enters the anterior end of the 



46 THE SKATE 

medulla J its roots are mingled with those of the fifth and 
seventh nerves. 

i. The ninth or glossopharyngeal nerve passes through 
the floor of the auditory capsule. It is attached to the 
medulla posterior to the auditory. It has several branches 
to the first functional branchial arch, to the pharynx and 
to a few sense organs on the mid-dorsal line of the head. 

j. The tenth or vagus nerve is apparently made up of 
several cranial nerves. It has numerous roots and divides 
into six main ganglionated portions which supply the four 
posterior clefts and arches, and the heart and stomach. 

Drawing 25. Add the cranial nerves to the drawing of the brain. 

3. The Spinal Cord. 

Expose about an inch of the spinal cord by shaving away 
the neural arches through which it runs. It is divided by 
deep dorsal and ventral fissures. The dorsal half of the 
system is sensory and is concerned with the bringing into 
the system of stimuli. The ventral half is concerned with 
the initiation and conduction of impulses, exciting activity 
of muscles, etc. 

4. The Ventral Aspect of the Brain. 

Free the brain from the chondocranium by cutting 
through the olfactory tracts and lifting up the anterior 
end of the brain. Cut through the two optic nerves and 
pare away the wall of the orbit on one side. Certain of 
the structures attached to the ventral surface of the dien- 
cephalon extend ventrally into a pit in the floor of the 
cranial cavity. Cut through the cranial nerves and across 
the spinal cord and lift out the brain intact. Study the 
ventral surface in light of the previous study of the brain. 
Note the distribution of the internal carotid artery. 

Several additional structures are present on the dien- 



THE NERVOUS SYSTEM 47 

cephalon. The optic chiasma is formed by the crossing of 
the optic nerves. Posterior to the optic chiasma the floor 
of the diencephalon bulges ventrally and posteriorly form- 
ing the infundibulum made up of two inferior lobes. From 
between the two inferior lobes a sac, the hypophysis or 
pituitary body projects caudad. It is a gland of internal 
secretion in mammals. Parts not identifiable on the brain 
will often be found adhering to the floor of the brain case. 

5. The Ventricles of the Brain. 

The brain retains the hollow spaces of its early tube con- 
struction as the ventricles or cavities of the brain. 

The fourth ventricle is the most posterior of these. It is 
the cavity within the medulla oblongata. Its ventral por- 
tion is the fossa rhomboidea. Make a ventral sagittal cut 
through the brain and examine under water. 

The aqueduct of the brain extends anteriorly from the 
fourth ventricle. It communicates with the cavity, the 
cerebellar ventricle of the cerebellum and the optic ven- 
tricles of the optic lobes. The third ventricle is in the 
diencephalon, the roof of which consists of the vascular 
choroid plexus which folds into the cavity. 

Just in front of the optic lobe is a small thickened por- 
tion of the diencephalon, the habenula, from which a slen- 
der process, the pineal body, extends dorsally. The third 
ventricle connects by the ventricle-of-Monroe or inter- 
ventricular foramen with the very small lateral ventricles 
which represent the first and second ventricles. 



CHAPTER VIII 
THE MUSCULAR AND SKELETAL SYSTEMS 

1. The Muscular and Skeletal Structures of the Tail. 

Cut a cross-section through the tail just anterior to the 
cut made for the injection of the arteries. Shave away a 
section until the end of a vertebra is reached at which point 
four rays, white areas of calcification which form a cross, 
will be seen. Examine, using a hand lens. 

The outer layer is the integument bearing the placoid 
scales. Next is a series of voluntary muscles made up of 
myotomes or muscle segments which appear as whorls in 
the section. The layers are separated from each other by 
myosepta or myooommae composed of connective tissue. 
The myotomes are separated by an horizontal skeletogen- 
ous septum into a dorsal portion, the epaxial muscles, 
and a ventral portion, the h3rpaxial muscles. The muscles 
will not be further studied in the skate; in general they 
show segmentation. 

The vertebra shows these structures : a concave centrum 
with a dorsal neural arch forming the neural canal for the 
spinal cord and a ventral haemal arch for the caudal ar- 
tery. The arch ends in a haemal spine corresponding to 
the neural spine of the neural arch. Continuous with each 
is a skeletogenous septum running to the median dorsal 
and ventral lines. 

Drawing 26. Draw the cross-section of the tail. 

48 



MUSCLES AND ENDOSKELETON 49 

From the cross-section made above run a median sagittal 
section for a short distance. A vertebra is of the shape of 
an hourglass; diamond-shaped spaces are therefore found 
between the vertebrae. These contain the gelatinous 
notochord which is thus mostly restricted to the ends of 
each vertebra. 

Drawing 27. Draw a vertebra in sagittal section. 

Cut a section of the vertebral column from the middle 
part of the body. Clean one side down to the cartilage. 
Haemal arches are not present in the body section, but 
they are represented by a pair of small cartilages, trans- 
verse processes, on the sides of the ventral part of the 
centrum. Within the portion of the transverse cartilagi- 
nous septum a cartilage, the rib, will be found. Notice 
that between each neural arch there is an intercalary arch. 
Find the exit of the spinal nerves. 

Drawing 28. Draw the cross-section of a body vertebra and a 
side view showing the intercalary arch. 

2. The Skeleton. 

The skeleton of the skate is mostly cartilaginous except 
for a few ossifications in the vertebrae, teeth, scales, and 
elsewhere. Ossification is a replacement process, not a 
calcification or stiffening of the cartilage. 

In the study of the skeleton use the demonstration skele- 
ton, the figure, and your specimen. 

A. The Girdles. 

The pelvic girdle is the ischiopubic bar of cartilage 
across the ventral side of the trunk between the pelvic 
fins. Iliac processes project from its main body dorsally, 
prepubic processes anteriorly. This girdle bears two ar- 
ticulating facets. The posterior facet bears the metaptery- 



50 



THE SKATE 



Boatifat eafttta^ 



^SilifHtal tartilage 



PttrygO'4iuadrate_ 
cartilage or 
upp rjato 

Spiraeular 
cartilage 



Byomandibular 



Propterygium 



Metopttrygxwn 




bial cariilagt 

Labial cartilage 



.Labial cartilage 

Meckel's cartilage 
— or lower jato 
—Hypo-hyal 
■Ceratohyal 
~Epi-kyal 

•Hypo-branchial of the 
third branchial arch 
^Third cerato-branchialt 
—Third epi-branchial 
.Hypo-branchial of the 

fifth branchial arch 
-Fifth oerato-branchiale 
.interior glenoid facet 
■Soapulo-coracoid 
fontanelle 
'iddle glenoid facet 



Coraeoid fontanelle 
<FoeUrior glenoid facet 



AnUrior^ ^ - 
acetabular facet 



Hiac proeaee 
'oeterior acetabular faeet 

Commencement of hasmat eanal 



FIG 2. The ventral aspect of the skeleton of Raja namta. (Eedia™ 
from Parker, Zootomy, MacmiUan and Co.) 

gium, the strong basal piece of the hind limb. From the 
metapterygium and the anterior facet the jointed radials 



MUSCLES AND ENDOSKELETON 51 

proceed from the support of the fin. The claspers of the 
males are closely connected with the posterior part of the 
hind-fin and have a complex cartilaginous skeleton. 

The pectoral girdle forms an almost complete hoop of 
cartilage which is attached dorsally to the vertebral plate. 
The ventral region is the cora^oid bar and is separated 
from the dorsal or scapular processes by three facets to 
which the three pieces of the pectoral fin are attached. 
The anterior piece is the propterygium ; the very small 
median piece, the mesopterygium ; and the posterior piece, 
the metapterygium. All of these and a part of the girdle 
bear jointed radials, parts of the endoskeleton. 

B. The Skull and Visceral Skeleton. 

The chondooranium is the cartilaginous case inclosing 
the brain. At the anterior end is the rostrum and the orbit 
which hold the eye in at one side. On the dorsal surface 
are two large anterior fontanelles and on either side of 
the rostrum is an olfactory capsule. The chondocranium 
also has a foramen magnum for the entrance of the spinal 
cord, auditory capsules inclosing the ears, olfactory cap- 
sules, nasal capsules, and two occipital condyles, articulat- 
ing with the first vertebra. 

The splanchnocranium or visceral skeleton supports the 
gills. It consists of several gill-arches. Two are much 
modified in connection with the jaws. The first gill-arch, 
the mandibular arch of the upper jaw, is a strong trans- 
verse bar formed by the union of two pterygoquadrate 
cartilages. The lower jaw is formed by the union of two 
Meckel's cartilages. The second or hyoid is a slender arch 
with a ventral median basihyal, a ceratohyal on each side 
of the basihyal, and dorsal to these the hyomandibular. 
This articulates with the otic region of the skull and acts 
as a suspensor of the lower jaw. 



52 THE SKATE 

Following these modified arches there are five branchial 
arches each typically of five pieces which form the frame- 
work of the gill region. 

There are other cartilages, such as the labial cartilages 
about the nasal capsule, the antorbital, uniting the nasal 
capsule with the end of the pectoral fin and a cartilage 
supporting the rudimentary gill of the spiracle. 



INDEX 



Abdominal pores, 9, 14 

Ampulla, 41 

Ampullae of Lorenzini, 37 

Analogy, 10 

Anterior, 6 

Anterior chamber of eye, 40 

Antimeres, 7 

Anus, 9, 16, 21 

Appendicular, 6 

Aqueous humor, 40 

Arterial system, 30 

Arteries, afferent branchial, 31 

anterior gastric, 36 

brachial, 35 

caudal, 36 

coeliac, 35 

common carotid, 34 

coronary, 31, 34 

dorsal aorta, 33, 35, 36 

dorsal gastric, 36 

duodenal, 36 

efferent branchial, 33 

external carotid, 34 

gastroduodenal, 36 

genital, 36 

hepatic, 36 

hyoidean, 34 

iliac, 36 

inferior mesenteric, 36 

internal carotid, 34 

lateral, 35 

pancreatic, 36 

posterior gastric, 36 

post-trematic, 33 

pre-trematic, 34 

renal, 36 

splenic, 36 

subclavian, 35 

superior mesenteric, 36 

ventral abdominal, 35 

ventral aorta, 30 



ventral carotid, 35 

ventral gastric, 36 

vertebral, 35 
Atrium, 25 
Auditory capsules, 51 
Auricle, 25 
Axes, 6 
Axial, 6 

Basal plate, 9, 10 
Bile duct, 17 
Bladder, 21 
Brain, 42 

case, 51 

dorsal aspect, 42 

ventral aspect, 46 

ventricles, 47 
Branchia, 12 
Branchial arches, 52 
Branchial filaments, 12 

Canal, haemal, 48 

lateral line, 37 

pericardio-peritoneal, 24, 28, 
33 

semicircular, 41 
Capsule, auditory 51 

nasal, 51 

olfactory, 51 

otic, 51 
Cartilage, 48, 49, 50, 51, 52 
Caudal, 6 
Cavity, pericardial, 24 

pleuroperitoneal, 14 
Centrum, 48 
Cephalic, 6 
Cerebellum, 43 
Cerebral hemispheres, 43 
Chamber, anterior, 40 

aqueous, 40 

posterior, 40 

vitreous, 40 



53 



54 



INDEX 



Chondocranium, 43, 51 
Choroid coat, 39 
Choroid plexus, 43 
Circulatory system, 24 
Clasper, 8 
Cloaca, 16 
Cloacal aperture, 9 
Coelom, 14 
Columnae carnae, 32 
Conjunctiva, 39 
Conus arteriosus, 25 
Cornea, 39 
Corocoid bar, 51 
Cranial, 6 
Cranial nerves, 44 
Crista, 41 
Crown, 10 
Cystic duct, 17 

Demibranch, 12 
Dentine, 9, 10 
Diencephalon, 43 
Digestive system, 15 

derivatives, 16, 17 

external aspect, 15, 16 

internal aspect, 17 
Dissection, 2 
Dorsal, 6 
Drawings, 1 
Duct, bile, 17 

mesonephric, 20, 23 

pancreatic, 17 

Wolffian, 21 
Duodenum, 16 

Ear, inner, 40 

Egg case, 22 

Enamel, 9, 10 

Endolymph, 41 

Endolymphatic ducts, 40 

Esophagus, 12, 15 

Evolution, 10 

External characteristics, 4 

Eye, 8, 38 

Eyeball, 38 

Eye muscles, external rectus, 39 

inferior rectus, 39 

inferior oblique, 39 

internal rectus, 39 

superior rectus, 39 



superior oblique, 39 

Falciform ligament, 18 
Fins, 8 

Foramen magnum, 51 
Forebrain, 42 
Form, 6 

Gall bladder, 16, 17 
Gastrosplenic mesentery, 18 
GiU, 12 

Gill-arch, 13, 51 
Gill-filaments, 12 
Gill rays, 13 
Gill-slits, 8, 9, 12 
Girdle, pectoral, 51 

pelvic, 49 
Gland, oviducal, 22 

rectal, 16 

thyroid, 16 

Habenula, 47 
Head, 8 
Heart, 24 

external aspect, 25 

internal aspect, 31 

valves, 32 
Hepatic duct, 17 
Hepatoduodenal mesentery, 18 
Hindbrain, 42 
Homology, 10 
Humor, aqueous, 40 

vitreous, 40 
Hyoid, 51 
Hyomandibular, 51 
Hypophysis, 47 

Iliac processes, 49 
Infraorbital nerve, 40 
Infundibulum, 47 
Integumentary sense organs, 37 
Intercalary arch, 49 
Intestine, 16 
Iris, 40 
Ischiopubic bar, 49 

Jaws, 9, 12 

Key to the skates, 4 
Kidney, 19 



INDEX 



55 



Labial cartilages, 52 
Lateral line, 37 
Lens, crystalline, 39 
Ligament, 18 
Liver, IG, 17 

Mandibular arch, 12, 51 
Meckel's cartilage, 51 
Medulla oblongata, 44 
Meninx primitive, 43 
Mesencephalon, 43 
Mesenteries, 15, 18 

dorsal, 18 

ventral, 18 
Mesogaster, 18 
Mesonephric ducts, 20, 23 
Mesonephros, 20, 22 
Mesopterygium, 51 
Alesorchia, 22 
Mesovarium, 22 
Metapterygium, 51 
Metencephalon, 43 
Midbrain, 43 
Mouth, 9, 15 
Mouth cavity, 11 
Muscles, epaxial, 48 

hypaxial, 48 
Muscular system, 48 
Mylencephalon, 44 
Myliobatidae, 5 
Myocommae, 48 
Myosepta, 48 
Myotomes, 48 

Nasal capsules, 51 
Nasofrontal process, 9 
Nerves, 44 

abducens, 45 

auditory, 45 

facial, 45 

glossopharyngeal, 46 

oculomotor, 45 

olfactory, 44 

optic, 44 

trigeminus, 45 

trochlear, 45 

vagus, 46 
Nervous system, 42 
Nostrils, 9 
Notochord, 49 



Occipital condyles, 51 
Olfactory organs, 38 

bulb, 43 

capsule, 51 

lobes, 43 

nerve, 43 

sac, 38 

tract, 43 
Omentum, 18 
Optic, chiasma, 47 

lobes, 43 

nerve, 40 

pedicle, 40 
Oral cavity, 12 
Orbit, 38, 51 
Organs, 15 
Oronasal groove, 9 
Ostium, 22 
Otolith, 41 
Ova, 21 
Ovary, 21 
Oviduct, 22 

Pancreas, 17 

Pancreatic duct, 17 

Pectoral girdle, 51 

Pelvic girdle, 49 

Pericardial cavity, 24 

Pericardio-peritoneal canal, 24, 

28, 33 
Pericardium, parietal, 24 

visceral, 25 
Perilymph, 41 
Peritoneum, parietal, 14 

visceral, 15 
Pineal body, 47 
Pit organs, 38 
Pituitary body, 47 
Pharyngeal cavity, 11 
Pharynx, 12, 15 
Placoid scale, 9 
Planes, 6 

Pleuroperitoneal cavity, 14 
Pores, abdominal, 9, 14 
Posterior, 6 
Primitive meninx, 43 
Pronephros, 19 
Propterygium, 51 
Prosencephalon, 42 
Pterygoquadrate, 51 



56 



INDEX 



Pulp cavity, 9, 10 
Pupil, 40 
Pylorus, 16 

Rajidae, 4 

Rectal gland, 16 

Reproductive system, female, 20 

male, 22 
Respiratory system, 11 
Retina, 39 

Rhombencephalon, 42 
Rib, 49 
Rostrum, 8, 51 

Sacculus, 41 

Scale, 9 

Scapular processes, 51 

Sclera, 39 

Semicircular ducts, 41 

Semi-lunar valves, 32 

Seminal vesicle, 23 

Sense organs, 37 

Sex differences, 19 

Sino-auricular, aperture, 32 

valves, 32 
Sinus venosus, 25 
Sinuses, 26, 27, 28 
Skates, 4 
Skeletal system, 48, 49, 50 

girdles, 49, 50 

skull, 51 

visceral skeleton, 51 
Skin, 9 
Skull, 51 

Species of skates, 4 
Sperm sac, 23 
Spinal cord, 46 
Spinal nerves, 49 
Spines, 9 
Spiracle, 12 
Spiral valve, 16, 17 
Spleen, 18 
Stomach, 16 
Stomach contents, 17 
Supplies, 1 
Surfaces of body, 6 

dorsal, 6 

lateral, 6 

ventral, 6 
Symmetry, 7 



Tail, 9 
Teeth, 12 
Telencephalon, 43 
Testes, 22 

Thalamencephalon, 43 
Thyroid, 16, 33 
Torpedinidae, 5 
Torpedo, 5 

Transverse processes, 49 
Transverse septum, 24 
Trunk, 8 

Urinary, sinus, 21 

vesicle, 21 
Urinogenital system, 19 
Urogenital, opening, 21 

papilla, 23 
Urogenital system, 19 

female, 20 

male, 22, 23 
Uterus, 22 
Utriculus, 41 

Valve, auriculo-ventricular, 32 

semi-lunar, 32 

sino-auricular, 32 

spiral, 16, 17 
Vasa efferentia, 22 
Veins, anterior cardinal, 28 

anterior mesenteric, 30 

brachial, 29 

caudal, 29 

common cardinal, 27 

duct of Cuvier, 27 

gastric, 30 

genital sinus, 28 

hepatic portal, 29 

hepatic sinus, 28 

iliac, 29 

inferior jugular, 28 

lateral abdominal, 29 

lienomesenteric, 30. 

median sinus, 28 

pancreatico-mesenteric, 30 

parietal, 20 

posterior cardinal. 28 

posterior gastric, 30 

posterior mesenteric, 30 

renal portal, 29 

splenic, 30 



INDEX 



57 



Venous system, 26 
portal veins, 29 
systemic veins, 26 

Ventral, 6 

Ventricle, 25 

Ventricles of brain, 47 

Vertebra, abdominal, 49 
caudal, 48 



Viscera, 15 
Visceral, arches, 12 

pouches, 12 

skeleton, 51 
Vitreous humor, 40 

Wolffian, body, 20 
duct, 21