GROUPS OF ANTMAL8 263 



The development of many of the amphibians is inter- 

 esting. The eggs of the frog, for example, are laid in 

 shallow water in the early spring. After these eggs 

 are fertilized they very soon hatch into "polywogs" 

 or tadpoles. These are fitted for living in the water. 

 They have gills for breathing and a tail for swimming. 

 The tadpole grows larger and gradually the outer gills 

 are replaced by gills which grow out under a fold of 

 the skin. The legs soon appear, the hind ones first. 

 By late summer lungs have developed; the legs are 

 Avell grown; and the tail gradually disappears, being 

 absorbed into the other parts of the body. About this 

 time the young frog leaves the water. Its food is 

 changed from a vegetable to an animal diet, chiefly 

 insects. In some kinds of frogs it takes a longer time 

 for the adult frog to be developed. 



When marked changes, such as those in the frog, take 

 place in the development of animals, the process is called 

 metamorphosis (meta, beyond; morphe, form). 



Fishes. Fishes, like birds, are easily recognized. To 

 this group belong probably as many as 13,000 different 

 species. It is the largest class of vertebrates. Fishes 

 resemble each other in the following characteristics: 

 gills are used in breathing, the body is often covered with 

 scales, the appendages are fin-like, and the blood is cold. 



Fishes are well fitted for their life in water. The 

 shape of the body is such that the water is easily ' ' cut, ' ' 

 the fins and large tail are used in locomotion, and gills 

 are present for breathing. The air sac is an interesting 



