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A GUIDE FOR THE DISSECTION OF THE DOGFISH (SQUALUS ACANTHIAS)

❦

By

LAWRENCE E. GRIFFIN Professor of Biology in Reed College

THIRD EDITION

Portland, Oregon 1922

Copyright, 1922 by LAWRENCE E. GRIFFIN

A Guide for the Dissection of the Dogfish

The small sharks which abound along the coasts of the United States are commonly called “dogfish” by fishermen and others. The “dogfish” of inland waters belongs to an entirely different group. Two species of sharks are caught in numbers and used in laboratory work, the “spiny dogfish” (Squalus acanthias) and the “smooth dogfish” (Eugaleus galeus). The first is easily distinguished by the sharp spine in front of each dorsal fin. Squalus acanthias is often referred to under the synonym Acanthias vulgaris, while Eugaleus galeus is more frequently named either Mustelus canis or Galeus canis. The histories of these names and systematic descriptions of the species are contained in Samuel Garman’s Monograph on the Elasmobranchs.

Several sizes of dogfish are furnished by dealers. We consider it best to purchase large, fully developed specimens. The small specimens may be a little more convenient to handle, but the large ones have the important advantage of being sexually mature, while blood vessels and nerves are dissected better in large than in small specimens. Also, a number of structures are very different in mature animals from their condition in young ones. Dealers should be requested to furnish fish with fins and tails complete instead of trimmed. It is an advantage to issue to the class equal numbers of both sexes.

An entire specimen and an extra head may be required by each student for a thorough dissection. It would be better if the head were cut off just behind the pectoral fins, instead of in front of them as is usually done, so that the vagus and hypobranchial nerves may be dissected more completely.

The spiny dogfish, which is the particular subject of this guide, is the species most frequently supplied to laboratories. However, the spiny and smooth dogfishes are so much alike that the latter may be easily dissected with these directions. Where marked differences between the forms exist the structure of Eugaleus is described separately.

The student of anatomy should realize that dissection is for the purpose of enabling him to see for himself the structures which exist, and that no dissection is satisfactory until the anatomical arrangements mentioned in the text can be completely demonstrated in his specimen.

The importance of knowing the structure of the elasmobranch is so great in comparative anatomy and embryology that it is worth while to make a thorough dissection of one of this class. As the dogfish is frequently the first major vertebrate form to be studied in detail, these directions have been written to conform to the needs of the student who is beginning comparative anatomy. The arrangement of sections in this guide is intended to permit the omission of some which it may not be considered desirable to include in the work of a class.

EXTERNAL CHARACTERS

The spindle-shaped body tapers from near the middle toward both head and tail; the head is flattened on both the dorsal and ventral sides, while the remainder of the body is nearly round, with a lateral compression which is not pronounced except in the caudal portion.

The general color of the back and sides is gray; darkest above, where the skin is spotted with scattered, small, round, light spots. The color of the upper parts shades into the yellow white of the ventral surface.

Can head, trunk, and tail regions be distinguished? If so, what characters determine the extent of each?

A little above the middle of the side of the body is the _lateral line_, (distinguished partly by color, partly by being slightly elevated), which extends from the back of the head to the tail. Cut through the skin across the lateral line at several points along the body and notice the canal which lies in the dermis under the lateral line. This is the _lateral line canal_, which opens to the surface by numerous pores (too minute to be seen), and contains a series of special sense organs along its dorsal and inner surfaces. Near the base of the caudal fin the lateral line canal passes into a groove which continues the lateral line to within a short distance of the edge of the fin. The lateral line canal in its development begins as a groove along the side of the body which becomes closed by the fusion of its edge except in this terminal portion.

The open groove does not appear in Eugaleus.

In the midline of the body are two triangular _dorsal fins_, each attached to the body for about half its length. The basal portion of each is thick and muscular, and contains supporting cartilages embedded in the muscles. The remainder of the fin is flexible and semi-transparent, horny fin-rays being faintly visible between the layers of skin. In front of each fin is a strong spine which seems to serve both as a cutwater and a weapon of defense. (Eugaleus has no spines.)

The broad, paired, _pectoral fins_, having the general characteristics of the dorsals, spring from the ventral edges of the body just back of the head. A hard bar of cartilage connecting the bases of the pectoral fins can be felt through the skin of the ventral surface of the body. This is the ventral part of the _pectoral girdle_.

Farther back, and also on the ventral surface, are the paired _pelvic fins_. The pelvic girdle can be felt through the skin between the bases of these fins. If the specimen is a male, it will have a fingerlike process projecting backward from the base and along the inner side of each pelvic fin. These organs, which attain a considerable size in adults, are modified portions of the pelvic fin used as copulatory organs. They are named variously _claspers_, myxopterygia, or pterygopodia. A groove runs along the dorsal side of the clasper from the tip to near the base, where it opens into a long sac (_glandula pterygopodia_) extending some distance in front of the pelvic fins just within the skin of the ventral surface. By feeling a cartilaginous axis of the clasper can be discovered, which extends to the tip of the organ. On the dorsal surface and near the end of the clasper is a sharp grooved spine on the outer side, and a strongly recurved hook on the inner side, both almost hidden by a flap projecting from the inner edge of the groove. This flap is stiffened by a series of small cartilages similar to the radial cartilages in the bases of the fins.

The _caudal fin_ is asymmetrical, extending along the dorsal and ventral edges of the posterior end of the body. Observe the upward bend of the vertebral column which occurs in the tail; it is this character which marks the heterocercal type of tail.

Eugaleus has a median ventral, or _anal_, fin a short distance anterior to the caudal fin.

The _mouth_ is a broad transverse slit upon the ventral surface of the head. The cartilaginous jaws can be seen and felt just within the mouth. Both upper and lower jaws are armed with rows of flat, sharply pointed teeth. Study their arrangement and approximate number. Jaws of previously dissected specimens should be examined under a dissection microscope. The exact form and arrangement of the functional teeth can then be ascertained easily, and an examination of the inner surface of either jaw will disclose several rows of developing teeth. As the young teeth develop they move, a row at a time, into position on the edge of the jaw; the oldest teeth, occupying the outer row, are shed at about the same time.

The upper jaw is partly overhung by a lip-like fold of skin. At each side of the mouth is a pocket, directed obliquely, having no communication with the mouth. These _labial pockets_ provide places for the _labial cartilages_ (which can be felt along the medial edges) when the mouth is closed, and also afford freedom of motion to the mandible. Cut along the inner edge of the labial pocket and expose the cartilages for examination. (The labial pocket of Eugaleus is much smaller, and in front of the corner of the mouth rather than lateral to it. The two cartilages are completely separated from each other, the posterior one scarcely reaching the pocket.)

In front of the mouth are the _nostrils_, their apertures apparently divided by projecting flaps of the anterior margin. Explore the cavity of the nostril with a probe to get a good idea of its size and form.

Between the pelvic fins is the _cloaca_, a large depression into which open the alimentary canal, the excretory and genital ducts, and the abdominal pores. The opening of the alimentary canal, the _anus_, is at the anterior end of the cloaca. In preserved specimens part of the intestine is frequently everted through the anus. A large fleshy process, bearing a pore at its tip, projects from the dorsal wall of the cloaca. In the male this is the _uro-genital papilla_; in the female the _urinary papilla_. In the female a _genital pore_, the opening of the oviduct, is found on either side of the papilla. An _abdominal pore_, leading into the abdominal cavity, is found on each side of the cloaca at the posterior margin. These are frequently closed in young specimens.

The cloaca of Eugaleus has a comparatively small opening upon the ventral surface, which must be enlarged before the parts described can be seen well.

The laterally placed _eyes_ are without lids; observe the considerable difference in the amount of curvature of the dorsal and ventral margins of the eye.

In Eugaleus there is a fold of skin stretched across the lower part of the eye which serves as an eye-lid, and corresponds to the so-called “third eye-lid” or nictitating membrane of other vertebrates.

On each side of the neck are five vertical _gill-clefts_, each leading into a large _gill pouch_ which communicates with the pharynx by an internal opening. Pass a probe through a gill-cleft into the mouth.

Back of each eye is a small aperture, the _spiracle_; explore this cavity with a probe. The spiracle is to be considered a gill-cleft moved forward upon the head and largely, though not entirely, deprived of its respiratory functions.

In the center of the dorsal surface of the head, between the spiracles, are two pores, the _external openings of the endolymphatic ducts_ which communicate with the internal ear. Large numbers of smaller pores can be found on all surfaces of the head, some in groups, some arranged linearly, many scattered. Most of those arranged in lines lead into the sensory canal system which continues from the lateral line canal upon the head, while the majority of the others belong to a separate type of sense organs, the _ampullae of Lorenzini_.

Make a cut encircling the pores of the endolymphatic ducts and close to them. Do not remove this piece of skin. From it make a median incision forward to the tip of the snout and back as far as the level of the first gill slit. Starting at this incision work the skin off from the tissues beneath it. This must be a careful, close dissection. When the lateral line is reached it will be seen that the lateral line canal is continued upon the head and is joined by several others. By looking through the loosened skin toward the light the pores can be seen which lead from the canals to the surface. In the hollow of the skull in front of and above the eye is a large group of tubules which open through the pores so conspicuous at this point. At the internal end of each tubule is a slight enlargement, of denser tissue, with which a delicate nerve strand can often be seen connected. The nerve strands can be traced to a large nerve passing above the eye and distributed to the snout. These tubules are the _ampullae of Lorenzini_. This group of ampullae, which may be called the dorsal group, is quite definitely demarked. It will be noted that the inner ends of the ampullae are grouped in a much smaller area than their pores.

Between the spiracle and the first gill slit will be found a lateral group of similar organs. Notice the arrangement of their tubules and pores. Under the snout are two groups of ampullae on each side of the midline. The inner ventral group is separated from the outer ventral group by the lateral bar of the rostral cartilage. Some of the tubules of the outer ventral group will be found to extend to pores situated at the sides of and behind the mouth.

An adult fish possesses from 1200 to 1900 ampullae of Lorenzini. Their function is not well understood, but it has been suggested that they are organs responsive to stimuli of pressure, either of currents or water, or resulting from depth, or even of deep tones.

The _system of sensory canals_ consists of the following members on each side of the head:—

A _supra-orbital_, passing above the eye to the end of the snout and bending back on the ventral surface to join the infra-orbital.

An _infra-orbital_, which branches off from the supra-orbital and passes ventrad between the eye and the spiracle, then turns forward along the ventral margin of the orbit, and finally bends toward the midline and extends to the tip of the snout.

A _hyomandibular_, which leaves the infra-orbital below the eye and runs back beyond the angle of the mouth.

A short _mandibular_, on the mandible close to the angle of the mouth, which is not connected with the other canals.

The canal systems of the right and left sides are connected by a _supratemporal_ canal just behind the endolymphatic pores, and often by an anastomosis of the infra-orbital canals in front of the mouth.

On some specimens two crescentic rows of pores can be found between the bases of the pectoral fins, which represent a third type of sensory organ, the _pit organs_, closely related genetically to the sensory canals. Two longer lines of pit organs, (the _mandibular pit organs_), will be found a short distance behind the mouth. Similar pit organs are found in front of the endolymphatic pores, and above the anterior part of the lateral line.

Except for a few small areas the entire surface of the body is covered with small, sharp-pointed denticles (_placoid scales_). Each consists of a diamond-shaped basal plate embedded in the dermis, from which projects a leaf-like, backward directed spine. A piece of skin should be removed and examined under a low magnification. The dermis is so dense and pigmented that the basal plate is not easily studied without further manipulation. For this purpose boil a piece of skin in 5% caustic potash solution until it is softened, but not till it disintegrates. Then clear it in glycerine. Examine the individual denticles under a higher magnification. The denticle consists of dentine, the spine being of a much denser structure than the base. The teeth and the large spines of the fins and claspers are also composed of dentine and may be considered as modified placoid scales. Denticles, teeth, and spines are covered with a shiny, enamel-like layer which, however, does not appear to be true enamel such as covers the teeth of higher vertebrates. The shape of the scales and their closeness vary on different regions of the body, and there are certain regions entirely free from them, namely, back of the dorsal, pectoral and pelvic fins, the medio-dorsal surfaces of the claspers, inside the upper lip and the labial pockets.

DISSECTION OF THE ABDOMINAL VISCERA

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.