SELACHII (SHAKES AND SKATES). 



These forms, of which the dogfish is an example, are 

 almost all marine. They are sharply marked off from the 

 Teleosts (p. 24) by several important characters. The 

 body is covered with placoid scales, the mouth and nostrils 

 are always on the ventral side of the body, the caudal fin is 

 heterocercal, the gill-slits (usually five in number) open 

 separately to the exterior, the skeleton is cartilaginous, the 

 heart has an arterial cone, and the intestine is provided 

 with a spiral valve. An air-bladder is lacking. There is 

 usually also a spiracle (p. 17). There are two orders of 

 Selachians. 



ORDER I. SQUALI (Sharks). 



In the sharks the body is more or less cylindrical, and 

 the gill-slits open upon the sides of the neck. About 150 

 species are known, some, like the dogfish, being small, 

 others reaching an enormous size. Those forms which feed 

 on fish and the like have sharp cutting teeth, and these are 

 arranged in rows, one behind another, so that only one row 

 is in use at a time, the other serving as a reserve supply if 

 one of the front row be lost. In other sharks, which feed 

 on shell-fish, the teeth are flattened plates, the whole form- 

 ing a mill for crushing the shells. Most of the species are 

 much like the dogfish in their general appearance, but 

 there are strange forms. Thus in the hammer-head sharks 

 the sides of the front of the head are drawn out like a 



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