. Animal biology. Zoology; Biology. CHAPTER LII CLASS ELASMOBRANCHII The elasmobranchs are very different from the cyclostomes and in many respects resemble the bony fishes. They have paired fins, jaws, fishlike gill arches, and gills. There is a well-developed carti- laginous skeleton. They differ, however, from the bony fishes in the following respects: (1) There is no bone in the skeleton; (2) the paired fins do not have fin rays; (3) they possess a pecuhar type of scale known. Fig. 240.—Dogfish shark, Squalus acanthias Linnaeus. X Ho- {From Jordan, "Guide to the Study of Fishes,"
. Animal biology. Zoology; Biology. CHAPTER LII CLASS ELASMOBRANCHII The elasmobranchs are very different from the cyclostomes and in many respects resemble the bony fishes. They have paired fins, jaws, fishlike gill arches, and gills. There is a well-developed carti- laginous skeleton. They differ, however, from the bony fishes in the following respects: (1) There is no bone in the skeleton; (2) the paired fins do not have fin rays; (3) they possess a pecuhar type of scale known. Fig. 240.—Dogfish shark, Squalus acanthias Linnaeus. X Ho- {From Jordan, "Guide to the Study of Fishes," by the courtesy of D. Appleton & Company.) as the placoid scale; (4) the openings of the gill sHts are exposed; (5) they have a spiral valve; and (6) there is no swim bladder. A dogfish shark is usually selected as the type of Elasmobranchii. 364. Dogfish Sharks.—The dogfish sharks belonging to the genera Squalus and Acanthias are abundant in both the north Atlantic and north Pacific oceans. Their bodies are fusiform, or spindle-shaped (Fig. 240); they do not reach a large size, being not more than four or five feet in length. They possess two dorsal fins, each of which in Squalus has a spine in front of it, a caudal fin, and pectoral and pelvic fins. The caudal fin is of a type known as heterocercal—that is, its dorsal lobe is larger than the ventral lobe. A portion of the pelvic fins is modified in the male sex forming organs known as claspers, which are used in copulation. The mouth is a transverse slit on the ventral side of the head, and the jaws are armed with very sharp teeth the points of which are directed backward. Behind the teeth in use are numerous rows of other teeth, lying against the inner surface of the jaw, which are ready to be brought forward and replace teeth which may be lost. On each side are five gill slits. Behind each eye is an opening known as a spiracle, which develops like a gill slit but which in the adult is modified and is no longer fu
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