.«i*J^ 



Gills 



Ovary 



Portad vein Stomach 



Auricle 



Ventricle 



HOW FISH BREATHE 



Water taken in by the mouth passes over the gills and out again, as indicated by 

 the arrows. The fish has one auricle and one ventricle. The heart pumps the blood 

 gathered from the body to the gills, in which gas-exchange takes place. The oxy- 

 genated blood is gathered into arteries: one main branch goes forward to the brain 

 and head, the other goes backward toward the rest of the body 



instead of being swallowed into the gullet, the water passes out through a 

 series of openings in the sides of the throat and over the gills (see illustra- 

 tion above). In the sharks the gills slits are open to the exterior; in bony 

 fish they are covered by a plate with a free edge toward the rear. The gills 

 are fine, feathery structures containing many delicate blood vessels, and are 

 arranged on arches, four on each side of the pharynx. As the water passes 

 over the gills, the oxygen in solution diffuses into the blood from the sur- 

 rounding water. 



Among the amphibians the adults swallow air into the lungs. The 

 young, however, have moist skin and gills through which gases diffuse be- 

 tween the lymph and the surrounding water. Adult frogs differ from toads 

 in having moist skins and in being able to live under water for considerable 

 periods of time. 



210 



