5 6 



BACKBONED ANIMALS. 



distinct types : heterocercal, in which the lobes are un 

 equal, as in the shark (Fig. 198), and homocercal, where 

 they are equal, as in the perch (Fig. 193). 



Digestion. The teeth that rudely prepare the food 

 for digestion are loosely attached to the bones of th& 



FIG. 194. Internal structure of the minnow, ribs removed, and the living 

 fish. A, n, nose-pit ; e, eye-nerve ; ea, ear-nerve ; g; gills ; /z, heart ; ^, 

 food-tube ; .?, stomach ; &, kidney ; v, vent ; d a, dorsal artery ; a, air- 

 bladder ; , backbone ; n v, nerve-cord or spinal cord. B, n, nose ; gc, 

 gill-cover ; a/ t arm-fin ; //, leg-fins ; -s /, single fins ; m s, mucous 



mouth, and even the tongue, not being confined to the 

 jaws alone. Food passes into the food-tube (Fig. 194, /), 

 and thence into the stomach, s, the rejected portions being 

 expelled at v. The liver is generally large. The position 

 of the kidneys is shown in Fig. 194, k. 



Circulation. The heart (Fig, 194,^), as a rule, consists 



