274 ELEMENTARY ZOOLOGY 



through its mouth and which passes across the gills to be 

 expelled through the gill-openings. The blood is received 

 from the body into the first chamber of the heart, a mus- 

 cular sac called the auricle. From here it passes into the 

 ventricle, a chamber with thicker walls, the contraction 

 of which sends it to the gills, thence without return to the 

 heart it passes over the body. The circulation of blood 

 in fishes is slow, and the blood, which receives relatively 

 little oxygen, is cold, being but little warmer than the 

 water in which the individual fish lives. 



Inside the cranium or brain-case is the brain, small and 

 composed of ganglia which are smooth at the surface and 

 contain little gray matter. At the posterior end of the 

 brain is the thickened end of the spinal cord, called the 

 medulla oblongata. Next overlapping this is the cere- 

 bellum, always single. Before this lie the largest pair of 

 ganglia, the optic lobes or midbrain, round, smooth, and 

 hollow. From the under side of these, nerves run to the 

 eyes with or without a chiasma or crossing. In front of 

 the optic lobes and smaller than them is the cerebrum or 

 forebrain, usually of two ganglia but sometimes (in the 

 sharks) united into one. In front of these are the small 

 olfactory lobes which send nerves to the nostrils. 



The sense organs are well developed. The sense of 

 touch has in some fishes special organs for its better 

 effectiveness. For instance certain fin-rays in some 

 fishes, or, as in the catfish, slender, fleshy, whip-like 

 processes on the head, are developed as feelers or special 

 tactile organs. Other fishes, the sucker and loach for 

 example, have specially sensitive lips and noses with 

 which they explore their surroundings. The sense of 

 taste does not seem to be well developed in this group. 

 Taste-papillae are often present in small numbers on the 

 tongue or on the palate. The sense of smell is good. 

 The olfactory organs, one on each side of the head, are 



