CHAPTER LII 

 CLASS ELASMOBRANCHII 



The elasmobranchs are very different from the cyclostomes and 

 in many respects resemble the true fishes. They have paired fins, 

 jaws, fishhke gill arches, and gills. There is a well-developed carti- 

 laginous skeleton. They differ, however, from the true 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 peculiar type of scale known 



Fig. 227. — Dogfish shark, Sgualus acanthias Linnaeus. X }io- (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 slits are exposed; and 

 (5) they have a spiral valve. A dogfish shark is usually selected as 

 the type of Elasmobranchii. 



362. 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. 227); 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 w^hich 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 

 functional as such. By means of the spiracle, however, water may be 

 taken in for respiration when the mouth is full of food. 



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