326 ANIMALS 



fishes water is taken in at the mouth, and passes outward through the 

 gill openings. This water contains dissolved oxygen (cf. 52) for 

 purifying the blood that comes to the gills. 



The specific gravity of a fish is very nearly that of water. An "air 

 bladder" enables the fish to change its specific gravity slightly, and to 

 sink or rise in the water, much as a submarine boat does (cf. 43). 

 When it compresses the air in the air bladder, the fish as a whole 

 becomes a little denser, and goes downward. When the compressing 



FIG. 267. 

 Circulation of Blood in a Fish, with the Connecting Organs. 



muscles are relaxed, the air expands; thus the body as a whole becomes 

 lighter, and rises. 



The fish moves by means of the strokes of its powerful tail fin, as 

 well as by the strokes of smaller fins. The side fins aid the fish in 

 steering itself, while the fins on the upper (dorsal) and the lower 

 (ventral) sides of the body keep the fish right side up. In the lower 

 fishes, such as sharks, the upper lobe of the tail is much larger than the 

 lower; in higher fishes the tail is symmetrical. 



The scales of fishes form a hard protective covering. They lap 

 over one another, and so permit the body underneath to bend. Sharks 

 have a tough skin covering without true scales. 



342. Amphibians. The name "amphibians" (pro- 

 nounced am-fib'-i-ans) comes from amphi, meaning 

 "both/' and bios, "life"; it refers to the fact that amphib- 

 ians live both on the land and in the water. Examples 

 of amphibians are frogs, toads, and salamanders. 



