ANIMAL LIFE 359 



back keeps the fish right side up, while the side fins are 

 used in steering. 



The specific gravity of a fish is nearly that of water. 

 By means of an " air bladder " the fish may increase or 

 decrease its volume enough to cause it to sink or rise in 

 the water. The cartesian diver illustrates this principle 

 admirably. Fishes breathe by means of gills which take 

 oxygen from the water. The fish has a well-developed 

 circulatory system, including a heart with two chambers. 



Amphibians. Amphibians are animals that live both 

 on the land and in the water, as the name indicates. 

 " Amphi " means both and " bia " means life. Common 

 amphibians are frogs, toads, mud puppies, and salaman- 

 ders. In the primary stages of their development these 

 animals breathe in oxygen by means of gills Most of 

 them undergo a complete change during life. Some of 

 them develop lungs in their second stage. The circulation 

 is of slightly higher development than in the fish, the heart 

 having three chambers, two auricles and one ventricle. 



The frog is an amphibian which may be easily studied. 

 In the early spring the frogs lay their eggs in water. In 



\ 2 3 4~ | 5 



FIG. 321. The Development of the Frog. 



a short time these eggs hatch into tadpoles. Tadpoles 

 resemble fish and breathe with gills as fish do. As the 

 tadpole develops, legs appear on the body, and its organs 

 of breathing change from gills to lungs. The toad tadpole 

 develops in a month or two after hatching, but the frog 

 tadpole does not make the complete change to a frog until 

 the second summer (Figure 321). 



