A Guide for the Dissection of the Dogfish (squalus Acanthias)

Part 1

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

LAWRENCE E. GRIFFIN Professor of Biology in Reed College

THIRD EDITION

Portland, Oregon 1922

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Copyright, 1922 by LAWRENCE E. GRIFFIN

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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.)

Part 2

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.

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