CLASS PISCES 359 



The teeth of fishes are not used in mastication but in holding the prey. 

 They are, however, sometimes modified as crushing organs, especially 

 in those forms which eat large numbers of mollusks. Teeth that are 

 lost are generally soon replaced. 



384, Respiration. — The mechanism of respiration in the fish involves 

 the use of the mouth, the opening of which is guarded by fleshy valves; 

 the passages through the gill slits; and chambers outside the gill slits 

 and under the opercula, which open by the slits behind the opercula. 

 The chambers of the two sides communicate below and the exit from 

 them is closed externally by a mucous membrane called a hranchiostegal 

 meynhrane. While the mouth is held open (Fig. 253) the walls of these 

 cavities are dilated by the action of certain muscles and water rushes in 

 through the mouth opening, being prevented from entering through the 

 gills by the closing of the hranchiostegal membrane. Then, pressure 

 being applied to the water in these cavities by other muscles which 

 contract the walls, the oral valves are forced shut, the hranchiostegal 

 membrane opens, and the water escapes between the gills. While the 

 gills are thus bathed with the water which passes them, respiration takes 

 place. This whole operation is continually repeated. The mouth does 

 not need to close in breathing, though its opening is usually seen to 

 become alternately larger and smaller as breathing continues. Thus 

 the apparatus acts something like a force pump the chamber of which 

 is guarded by valves in such a manner as to permit water to pass only 

 in one direction. A fish can be smothered by preventing the closure 

 of either the oral valves or the hranchiostegal membrane. If a stringer 

 is passed through the mouth and the gill slits, breathing is interfered 

 with and the fish is soon killed. 



385. Senses of Fish. — A fish possesses two olfactory sacs, one on 

 each side of the head and each opening to the outside through two 

 external apertures. Taste organs occur in the mucous membrane of 

 the mouth. The senses of smell and taste are, at best, not well-developed. 

 The sense of touch is better developed, however, especially around the 

 mouth and on barbels on the head. 



The lateral line system consists usually of a continuous tube on each 

 side of the body just below the surface, lodging on its inner wall the 

 lateral line organs and opening to the outside by pores passing through 

 the tips of the scales lying in this line. Sometimes there are two or 

 three lateral lines. 



The eyes possess no eyelids, the cornea is flattened, and the lens is 

 almost spherical. The pupil is very large to allow free admission of 

 light, because the water absorbs so much of the light that even at a 

 moderate depth it is greatly reduced. When at rest the eye is focused 

 at a distance of about 15 inches and adjustment to more distant vision 

 is afforded by the movement of the lens. In other words fishes are near- 



