156 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 the 
Fic. 194.—Internal structure of the minnow, ribs removed, and the living 
fish. A, 7, nose-pit; ¢, eye-nerve; ¢a, ear-nerve ; g, gills; 4, heart; ¢, 
food-tube ; s, stomach; &, kidney; v, vent; ¢a@, dorsal artery; a, air- 
bladder ; 4, backbone ; uv, nerve-cord or spinal cord. B, x, nose; gc, 
gill-cover; @/, arm-fin; 77, leg-fins; » 7, single fins; ms, mucous 
scales. 
mouth, and even the tongue, not being confined to the 
jaws alone. Food passes into the food-tube (Fig. 194, 4), 
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, 4. 
Circulation.—The heart (Fig. 194, 2), as a rule, consists 
