XII 



PHYLUM CHORDATA 



331 



between the lizard and the rabbit consists in the presence 

 of two pairs of jointed limbs, anterior and posterior, the 

 principal divisions of which correspond in their general 

 arrangement. In the dogfish these are found to be rep- 

 resented by very different-looking structures, the paired fins. 

 At this point all external resemblance ceases, and we see 

 nothing but differences. 



The skin of the dogfish, though almost smooth, is harsh to 

 the touch, and, when we examine it with a lens, this is found 

 to be due to the presence of innumerable minute hard gran- 



FIG. 208. LepUS cuniculus. Lateral view of skeleton with outline of body. 



ules, set closely together so as to give the surface the charac- 

 ter of a fine file. The general shape of the body is adapted 

 to cleaving the water rapidly, long and narrow, nearly 

 fusiform, pointed at the ends, and the fins are obviously 

 swimming organs. The fins are all of the same general 

 character, so far as their superficial appearance is con- 

 cerned ; they are all of the nature of flap-like outgrowths, 

 thick at the base, where they are obviously supported by 

 hard parts, thinner towards the margins, where their sole 



