CHAP. XI ELASMOBRANCHS 415 



The cerebellum is relatively large ; internal nostrils and a 

 tympanic cavity and membrane are not present. There is 

 only one auricle in the heart and no post-caval 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 Tekostomi. The Elasmobranchs 

 are all marine forms, and include the dogfishes, sharks, 

 rays, and skates : their endoskeleton is composed almost 

 entirely of cartilage, like that of the tadpole. The Teleo- 

 stomi, 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, and include a number 

 of genera and species. They are all powerful swimmers, 

 and feed voraciously on other fishes, crustaceans, etc. 



The commonest British forms are the Rough Hound 

 {Scyllium canicula), the Lesser Spotted Dogfish {S. cafulus), 

 the Piked Dogfish {Acanthias vulgaris), and the Smooth 

 Hound {Musielus vulgaris). The following description, 

 though referring mainly to Scyllium, will apply, in essential 

 respects, to any of these. 



External Characters and General Structure. — The 

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

 blundy-pointed snout and behind tapering off into an up- 

 turned tail. On the ventral surface of the head is the large, 

 transversely elongated mouth (Fig. 108), supported 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 skull, having nothing to 



