CHAP, x ELASMOBRANCHS 431 



tively large ; a tympanic cavity 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 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, 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 caniculd), the Lesser Spotted Dogfish (S. catulus\ 

 the Piked Dogfish (Acanthias vulgaris), and the Smooth 

 Hound (Mustelus vulgaris^. The following description, 

 though referring mainly to Scy Ilium, 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~ 

 bluntly-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. 117), 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 



