THE STRUCTURE AND ACTIVITIES OF FISHES 275 



is large so as to allow more light to enter a necessity under 

 water where the light is not strong. Fishes probably cannot see 

 in the air. 



There is no outer or middle ear, but only the membranous 

 labyrinth is present, since the water transmits the sound waves 

 directly to the inner ear. 



Unlike the lamprey eel, there are two nostrils in the fishes, 

 each of w r hich is a sac connected with the water through a pair 

 of openings in front of each eye, and containing many sense cells 

 of smell in their walls. 



The sense of taste is not well developed. Fishes swallow their 

 food whole or in large pieces, and a few sense cells that are present 

 in the walls of the mouth are sufficient. 



The entire skin, but especially that of the lips, is provided with 

 tactile sense cells. 



Respiration. Respiration in fishes is typically aquatic, taking 

 place in the gills. In a few fish the air bladder may also serve as 

 a respiratory organ. The sharks (Fig. 161) possess rows of gill 

 slits on either side of the head, but in most fishes the gills are pro- 

 tected from injury by a gill cover, the operculum (Fig. 155, 0). 



The four pairs of gills usually present are supported by four 

 pairs of gill arches. Each gill bears a double row of branchial 

 filaments which are abundantly supplied with capillaries. The 

 afferent branchial artery brings the blood from the heart to the 

 gill filaments; here an exchange of gases takes place. The 

 carbonic acid gas with which the blood is loaded passes out of 

 the gill, and a supply of oxygen is taken in from the continuous 

 stream of water which enters the pharynx through the mouth 

 and bathes the gills on its way out through the gill slits. Be- 

 cause oxygen is taken up by the capillaries of the gill filaments, 

 a constant supply of fresh water is necessary for the life of the 

 fish. If the fish is deprived of water entirely, respiration is pre- 

 vented, and the fish dies of suffocation. 



Reproduction. As in the frog, the eggs of most fish (Fig. 

 1 60, A) are deposited in the water by the female fish and then 



