426 TEXTBOOK OF ZOOLOGY 



Shark liver oil is of high vitamin content and has an important 

 medicinal use. 



In many countries, particularly of the Orient, these fish are com- 

 monly used as food. It is said that small sharks and skates are 

 offered for sale right along with other fish in the markets of China. 

 They are also salted and dried. In the United States there is an 

 unfounded prejudice against eating these fish, but dogfish are now 

 being canned and sold under the name of "grayfish." The wing- 

 like fins of skates and rays make delicious steaks. Sawfish steaks 

 are quite desirable and the saws are preserved as ornaments. The 

 flesh of sharks and rays is also ground up and used extensively as 

 fertilizer. In some parts of the world the fins of sharks are used 

 in the manufacture of gelatin. A good many dogfish and bonnet- 

 head sharks are sold for purposes of study in zoology laboratories. 



THE SPINY DOGFISH 



This shark is the most commonly studied representative of the 

 Elasmobranch group. Squalus acanthias is the scientific name ap- 

 plied to the common form taken along the Atlantic coast and 

 Squalus suckleyi is the name given the similar one of the Pacific 

 coastal waters. The average length of Squalus is between two and 

 one-half and three feet. It is a strong swimmer and is frequently 

 seen as a scavenger in harbors as well as going out to sea for ex- 

 tended periods. It apparently makes a spring migration northward 

 along the coast and a return movement in the fall. Because of the 

 ventral location of the mouth, these fish find it necessary to turn 

 ventral side up to eat morsels of food from the surface of the water. 



External Features 



The body is generally spindle-shaped (fusiform) tapering at both 

 head and tail. There are two pairs of fins, the anterior pectoral 

 and the posterior pelvic, or ventral fins. In addition to the paired fins, 

 there are two unpaired, median, dorsal fins, each with a spine at its 

 anterior margin (hence spiny). Male individuals may be distin- 

 guished from females by the fingerlike extensions, or claspers on the 

 pelvic fins. The dorsal and ventral lobes of the caudal fin, or tail, 

 are unequal and based on this, the tail is described as heterocercal. 



There are six pairs of uncovered gill clefts in the walls of the 

 pharynx. The anterior one, which is dorsally located and greatly 



