Place the dogfish on its back and, commencing at the middle of the
abdomen, make an incision through the body wall a quarter of an inch to
one side of the midline. Carry this forward to the pectoral girdle and
backward through the pelvic girdle to the cloaca; not, however, cutting
the wall of the cloaca.
A large vein, the _lateral vein_, runs along the inner surface of each
lateral wall of the abdomen. After identifying these, cut through the
body wall transversely on both sides of the abdomen at the level of the
posterior attachment of the pectoral fin as far as the lateral vein.
Turn the flaps outward and fasten.
The _coelom or body-cavity_ consists of two portions, the abdominal and
pericardial cavities. The _abdominal cavity_, which has now been opened,
extends from the pectoral girdle to the cloaca and along the sides of
the latter; it communicates with the exterior through the abdominal
pores on either side of the cloaca. Pass a bristle or probe through each
abdominal pore into the cloaca.
Without dissecting, identify the following parts and observe their
relations:
The _peritoneum_, the smooth lining of the body wall, which is reflected
over the viscera.
The _liver_, a large, gray organ attached anteriorly and almost
completely divided into two lobes which extend well back along the sides
of the abdominal cavity.
The _stomach_, lying between the lobes of the liver. Its posterior end
is bent forward upon itself in the form of a U. The two limbs of the
stomach are known as the cardiac (proximal) and pyloric (distal),
respectively. With a second turn to the right and backward it enters the
intestine.
The _intestine_, a large, thin-walled tube extending from the stomach to
the cloaca.
The _spleen_, a dark, triangular mass attached to the posterior border
of the curve of the stomach.
The spleen of Eugaleus is a long, slender body extending from the middle
of the proximal limb of the stomach around the posterior end of that
organ and forward again along the distal limb for two-thirds of the
length of the latter.
The _pancreas_, a firm white mass the larger part of which lies dorsal
to the posterior end of the stomach. One extremity lies on the ventral
surface of the junction of the stomach and intestine.
The _reproductive glands_, (ovaries or testes), lying on either side of
the midline dorsal to the anterior portion of the liver; they may be
mistaken for small lobes of the liver.
The reproductive glands of Eugaleus are long bodies lying above the
stomach and intestine. They are fused to each other for almost their
entire length.
The _kidneys_, two long, slender, brownish bodies extending along the
dorsal wall of the abdominal cavity outside the peritoneum, on either
side of the midline.
The dogfish usually furnished for dissection are immature, having the
genital glands and ducts only partly developed. In mature females the
_oviducts_ are conspicuous tubes ventral to the kidneys. In young
specimens they appear as slender, white tubes extending along the inner
borders of the kidneys. Anteriorly, the oviducts pass ventrad over the
front of the liver to the ventral wall of the body; at the same time
they unite to form a funnel, the _ostium tubae_, which opens into the
coelom. Vestigial oviducts opening into the coelom are found in the same
position in males.
In males, the _vasa deferentia_ appear as slender, irregularly coiled
white tubules lying near the medial border of the kidneys; they are much
less conspicuous than the oviducts, especially in young males.
THE ALIMENTARY SYSTEM. In dissecting the following organs, care should
be taken not to break the connections of the organs with each other or
with other parts, or to cut blood vessels. Organs should not be removed
until such procedure is directed.
The mouth and pharynx can be studied to better advantage later with the
dissection of portions of the vascular system.
The _oesophagus_ can be seen above the liver, by pressing that organ
aside, as a somewhat constricted tube entering the anterior end of the
abdominal cavity. It immediately joins the stomach, which is more or
less expanded according to the amount of food contained in it.
The _stomach_ passes directly back for more than half of the length of
the abdominal cavity, then turns abruptly forward, forming a distal limb
about a third as long as the proximal. (Two-thirds to three-quarters as
long in Eugaleus.) The distal limb ends with a sharp turn to the right,
where it is constricted by the _pyloric sphincter_, which marks the end
of the stomach.
The narrow beginning of the intestine forming the turn to the right and
backward is frequently distinguished as the _duodenum_. It leads from
the stomach directly into the _large intestine_, a wide, straight tube
marked externally by a spiral line of several turns. The large intestine
narrows posteriorly, forming a region somewhat arbitrarily termed the
_rectum_, which opens into the cloaca through the anus.
Dorsal to the rectum and attached to that body is a narrow
spindle-shaped body, the _rectal_ or _digitiform gland_.
The liver is attached to the anterior wall by a broad base, the
peritoneum being reflected over the entire remaining surface. The
attaching fold of the peritoneum is frequently called the _suspensory
ligament_. The peritoneum, or coelomic epithelium, can be dissected
easily from the surface of the liver or the kidney and its extreme
thinness and delicacy noted. It consists of a single layer of cells.
Most of the abdominal organs are suspended from the dorsal wall of the
body cavity by delicate membraneous sheets, or _mesenteries_. Similar
sheets between the organs are the _omenta_. The stomach is suspended by
a _mesogaster_, which extends as a free fold along the body as far as
the anterior mesenteric and lienogastric arteries. It encloses these,
and is attached to the spleen, pancreas, stomach, and anterior end of
the intestine.
The spleen is connected with the stomach by the _gastro-splenic
omentum_, formed by an extension of the peritoneal coat of the stomach
around the spleen. The liver is connected to the loop of the stomach by
the _gastro-hepatic omentum_ in which are the hepatic duct, portal vein,
and hepatic artery. Near the stomach it is joined by a fold of the
peritoneum from the duodenum, the _duodeno-hepatic omentum_, which also
unites with the mesogaster.
The rectum and rectal gland are supported by a second median mesentery,
the _mesorectum_.
In Eugaleus the mesentery extends the entire length of the abdominal
cavity. It forms a broad sheet attached to the anterior end of the
proximal limb of the stomach (mesogaster), to the anterior end of the
intestine (mesentery proper), and to the rectum (mesorectum). There is
not the reduction of the mesentery which there is in Squalus. The gonads
are suspended from the lateral faces of the mesentery above the stomach
and intestine. The gastro-hepatic omentum forms a broad sheet between
the limbs of the stomach, joining the mesogaster dorsal to the stomach
and the mesentery above the intestine.
A small division of the right lobe of the liver stands out between the
main lobes. In this is located a long, narrow _gall-bladder_. Open the
bladder by a longitudinal ventral incision. The opening into the _bile
duct_ will be found near the anterior end of the bladder.
In Eugaleus, which does not possess such a median lobe, the gall-bladder
lies hidden in the right lobe of the liver. It can be opened and
explored, but the connection with the duct can usually be demonstrated
only by scraping. Do this later.
The bile duct passes along the dorsal side of the gall-bladder and the
edge of the gastro-hepatic and duodeno-hepatic omenta to the junction of
the duodenum and large intestine, where it opens into the alimentary
canal. Trace its oblique course through the wall of the intestine. The
bile duct and the collecting (hepatic) ducts of the liver will be traced
in the liver at a later stage of the dissection.
The _pancreas_ consists of two lobes; a slender lobe lying dorsal to and
parallel with the stomach, and a flattened oval lobe lying upon the
ventral surface of the duodenum, connected with the dorsal lobe by a
slender bar of glandular tissue.
The _pancreatic duct_ passes from the extreme right end of the duodenal
lobe obliquely through the wall of the intestine, opening into the
anterior end of the large intestine. Free the edge of the lobe from the
peritoneum and follow the duct.
Open the proximal limb of the stomach by a ventral incision which shall
not cut any large blood vessels. Wash out the interior. Observe the
three coats of the stomach; the outer peritoneal, the middle muscular,
and the inner mucous coats. In the anterior portion of the stomach the
mucous coat projects in the form of large papillae (absent in Eugaleus).
Posterior to these, observe the irregular folding of the mucous coat,
depending upon the degree of contraction of the muscular coat.
The muscular coat consists of an outer circular and an inner
longitudinal layer of muscle fibres. Separate the two layers from each
other and from the mucous coat; observe the network of blood vessels
between the longitudinal muscles and the mucosa.
Open the pyloric end of the stomach, continuing the cut through the
pylorus into the intestine. Examine the coats as before, observing
especially that an outer layer of longitudinal muscle fibres is
frequently developed, and that the pyloric valve is formed by an
increase in the thickness of the coat of circular fibres.
Cut through the wall of the large intestine along the right side from
its anterior end to the rectum. Do not cut deeper than the thickness of
the wall. Corresponding to the external markings, the mucous membrane
projects internally in a spiral fold, known as the _spiral valve_.
Separate the wall of the intestine from the edge of the spiral fold upon
both sides of the longitudinal incision, exposing a considerable surface
of the valve. Wash well, and observe the character of the valve, the
direction of the folds, and the manner of the reversal of their
direction which usually takes place in the posterior half of the valve.
Cut across the _rectal gland_ at its middle. Observe the character of
its tissues, and then insert a bristle into the central cavity of the
gland and pass it into the rectum. Open the rectum and note the point of
communication of the two organs.
URINARY AND REPRODUCTIVE ORGANS. The _kidneys_ (_mesonephri, Wolffian
bodies_), are slender bodies extending along the entire length of the
dorsal wall of the abdomen. The posterior moiety of each is thicker and
wider than the anterior, which appears to have largely lost the
functions of excretion in adult dogfish. Notice the position of the
kidneys outside the peritoneum.
THE MALE. The _testes_ are white bodies lying to the right and left of
the oesophagus, dorsal to the anterior portion of the liver. Each is
suspended by a fold of the peritoneum, the _mesorchium_. (The testes of
Galeus are long bodies attached to the sides of the mesentery.)
Showing through the peritoneum, a much convoluted, white tube can be
seen on the ventral surface of the kidney. This is the _mesonephric_ or
_Wolffian duct_. In young specimens it may be nearly straight, lying
near the medial border of the kidney. In adult specimens it can be
followed forward as far as the anterior end of the testis. While the
Wolffian duct is the duct of the kidney, and is joined by tubules of the
anterior part of the kidney, it is so modified in the male that its
principal function is to serve as the duct of the testis, a _vas
deferens_. The collecting tubules of the posterior part of the kidney
join to form a _urinary duct_ which is independent of the Wolffian duct.
The posterior end of the Wolffian duct is straight and considerably
expanded, forming a large _seminal vesicle_. The duct becomes more and
more closely convoluted as it passes forward, and the kidney tissue
overlying it diminishes. At the anterior end of the mesonephros the
Wolffian duct forms a mass of tubules, the _epididymis_. Very small
tubules, the _vasa efferentia_, pass from the anterior end of the testis
to the epididymis. These are difficult for the student to distinguish.
Cut through the peritoneum along the outer side of one kidney. Then
strip the peritoneum toward the inner border of the kidney. The urinary
duct will usually be closely attached to the peritoneum and parallel
with the Wolffian duct, but nearer the midline of the body. The urinary
duct can be separated from the peritoneum by a little careful work.
Numerous small ducts pass from the kidney into the urinary duct.
Open the uro-genital papilla near its tip and extend the incision
forward so as to open the sac connected with the base of the papilla.
The pore at the tip of the papilla leads into a space within the papilla
itself, the _uro-genital sinus_, which branches to the left and right in
pouches which extend beyond the posterior ends of the vasa deferentia.
These cornua of the uro-genital sinus are of variable length, and are
often named sperm-sacs. In a mature male they may be found to be filled
with sperm, as may also the seminal vesicles and the convoluted portion
of the Wolffian duct. The openings of the vasa deferentia into the sinus
are large and easily located. The urinary duct opens into the sinus by a
separate pore just behind the opening of the vas deferens.
Cut open the seminal vesicle and part of the convoluted vas deferens.
The space within is subdivided by transverse folds or lamellae extending
from a longitudinal ridge.
No vasa efferentia can be distinguished in Eugaleus. The anterior
extremities of the kidney and testis of each side come into close
contact with each other and here the vasa efferentia pass from the
testis to the vas deferens. The sperm-sac is a large blind pouch, one or
two inches in length, leading out of the posterior end of the vas
deferens, and directed forward along its side. The vas deferens of
Eugaleus is not convoluted.
In the young specimens usually supplied to laboratories the vas deferens
is straight and no seminal vesicle is developed. The vasa efferentia are
more difficult to see; otherwise the relations of the urinary and
genital organs are as in the adult.
The suspensory ligament of the liver is continued posteriorly along the
midline of the ventral body wall; the dorsal edge supports a funnel
which opens into the abdominal cavity by a long, narrow mouth. From the
anterior end of the funnel two narrow tubes pass to the right and left
over the anterior surface of the liver. They end blindly in the tissues
dorsal to the anterior end of the liver. These are vestiges of the
Muellerian ducts (pronephric ducts) which form the oviducts of the
females.
THE FEMALE. The ovaries are large, white bodies lying at the sides of
the stomach, dorsal to the lobes of the liver. Each is covered by the
peritoneum and suspended by a fold of the same, the _mesovarium_. Ova of
various sizes may be felt in the tissue of the ovary, which should be
exposed by dissection.
The ovaries of Eugaleus are long slender bodies lying on either side of
the mesogaster, dorsal to the stomach and intestine. Their posterior
portions are fused.
The _oviducts_ (_Muellerian ducts_) are large tubes suspended from
between the kidneys by a narrow peritoneal band. The posterior portion
of the oviduct, where development of the eggs takes place, is
considerably enlarged. Each oviduct opens separately into the cloaca by
a pore at the side of the urinary papilla. Followed forward, the
oviducts pass over the anterior surface of the liver and following a
continuation of the suspensory ligament, bend around posteriorly and
unite. At the point of union they open into the coelom by a common,
large, funnel-shaped aperture, the _ostium tubae_.
Cut through the peritoneum along the outer side of one kidney. Then
strip the peritoneum toward the inner side of the kidney. Numerous small
excretory ducts will be seen joining the main _urinary duct_ (_Wolffian
duct, mesonephric duct_), which runs along the inner margin of the
kidney. Make an incision in the side of the urinary papilla to open the
cavity within it, the _urinary sinus_. The connection of this with the
pore at the tip of the papilla should be demonstrated. Extend the
incision forward. The urinary sinus divides into right and left cornua
which are of considerable size and lie dorsal to the oviducts. Trace the
Wolffian duct to the urinary sinus and demonstrate its opening into the
cornu anterior to the point where the two cornua unite.
In young specimens the ovaries are small, and the oviducts are narrow,
white tubes lying along the medial margins of the kidneys.
_Nephrostomes_, short, segmentally arranged kidney tubules which open to
the coelom by a funicular aperture, are found by a close examination
along the medial border of each kidney. They should be observed
carefully with the aid of a good dissecting lens. Learn the significance
of these structures.
In the course of development two sets of nephridia (kidneys) are formed.
The first (pronephros) develops just back of the head of the embryo, but
does not persist in the adult. Its duct, known commonly as the
Muellerian duct, develops into the functional oviduct of the female, but
forms an apparently useless vestige in the male. The second kidney
(mesonephros) develops behind the first and is the excretory organ of
the adult. Its duct (frequently given the name of Wolffian duct) is the
urinary duct in the female, but functions in the male chiefly as a sperm
duct, and therefore is called the vas deferens. The collecting tubules
of the posterior portion of the kidney of the male unite to form a
urinary duct which opens into the Wolffian duct or the uro-genital
sinus.