210 



Chapter XI. 



THE DOG-FISH. Scyllium canicula. 



The dog-fish, together with the sharks and skates, belongs to 

 the group of fish known as Elasmobranchs, the great majority 

 of which are powerful swimmers, of carnivorous habits, and 

 exceedingly voracious. One of the chief characters of the 

 group is that almost the whole of the internal skeleton remains 

 cartilaginous throughout life : hence Elasmobranchs are fre- 

 quently spoken of as cartilaginous fish. 



There are several kinds of dog-fish found off our own coasts. 

 The following description is taken from one of the most abun- 

 dant species, but will apply with but slight changes to any of 

 the common forms. Skates differ markedly from dog-fish in 

 external appearance, owing to the enormous size of their 

 pectoral fins, which form the great lateral wing-like expan- 

 sions of the body : as regards internal structure, however, the 

 differences are comparatively unimportant, and a skate might 

 easily be dissected by following the directions given in this 

 chapter. All the measurements here given are taken from 

 a dog-fish of about two feet in length, which will be found 

 the most convenient size for dissection. 



Scyllium canicula, like most other species of dog-fish, lays 

 eggs, which are of large size, and enclosed in horny capsules 

 produced at their angles into long spirally twisted strings for 

 attachment. In many cases the eggs are retained within 'the 

 oviduct during part or the whole of their development; 

 and in some few species a still closer relation is established 

 between the mother and the embryo, similar in many respects 

 to the placenta of mammals, whereby the embryo obtains 



