194 



Chapter XI. 

 THE DOG-FISH. ScyUium 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 the internal skeleton remains cartilaginous 

 throughout Hfe, ijo bone being developed in it : hence Elasmo- 

 branchs are frequently 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-Hke expan- 

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

 differences are comparatively unimportant, and a skate could 

 be readily 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 wiU be found 

 the most convenient size for dissection. 



Most dog-fish lay 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 stUl closer relation is 

 estabhshed between the mother and the embryo, similar in its 

 essential features to the placenta of mammals, whereby the 

 embryo obtains during its development a supply of nutriment 

 direct from the mother. 



