CHAP, x ELASMOBRANCHS 431 



and membrane are not present, and except in the Dipnoi, 

 there is only one auricle in the heart and no postcaval 

 vein. A urinary bladder developed as an outgrowth of 

 the enteric canal (p. 210) is wanting. There is never 

 such a marked metamorphosis as in the case of the frog. 

 The two most important sub-classes of the Pisces are 

 the Elasmobranchii and the Teleostomi. The Elasmo- 

 branchs are all marine forms, and include the dogfishes, 

 sharks, rays, and skates : their endoskeleton is com- 

 posed of cartilage, like that of the tadpole. The 

 Teleostomi, in which the skeleton is mainly or to a 

 large extent bony, include by far the greater number 

 of fishes both marine and fresh-water forms such as 

 the Salmon, Cod, Herring, Perch, as well as the Sturgeon 

 and its allies. 



The dogfishes are small sharks, of which there are a 

 number of genera and species. They are all powerful 

 swimmers, and feed voraciously on other fishes, 

 crustaceans, &c. 



The commonest British forms are the Rough Hound 

 (Scyllium canicula], the Lesser Spotted Dogfish (S. 

 catidns), the Piked Dogfish (Acanthias vulgaris), and the 

 Smooth Hound (Mustelns vulgaris). The following 

 description, though referring mainly to Scyllium, will 

 apply, in essential respects, to any of these. 



External Characters and General Structure. The dog- 

 fish has a spindle-shaped body, ending in front in a 

 bluntly-pointed snout and behind tapering off into an 

 upturned tail. On the ventral surface of the head is 

 the large, transversely elongated mouth (Fig. 117), sup- 

 ported by a pair of jaws which work in a vertical, and 

 not, like those of the crayfish, in a transverse plane : 

 they are, in fact, like those of the frog, portions of the 



