328 ANIMALS 



The young of frogs and toads undergo a metamorphosis (cf. 339). 

 As hatched from the egg they are "polywogs," or tadpoles. This is 

 the "larva" stage. Tadpoles are water animals, provided with gills 

 for breathing, and with a long tail for swimming. As they develop, 

 the tail becomes shorter, and the hind legs appear. The fore legs are 

 at first hidden by a fold of the skin that grows out to cover the gills. 

 The outer gills disappear, and inner gills and lungs are formed. 



The tadpole lives upon vegetable food, but in changing into an 

 adult frog it changes its diet to animal food, chiefly insects. The toad, 

 although it is first a tadpole, leaves the water entirely when it grows up. 

 In the winter time, frogs burrow into the mud at the bottom of ponds, 

 and live there (hibernate) through the cold weather without feeding. 

 To hibernate means to "pass the winter." 



343. Reptiles. Reptile is from a Greek word meaning 

 "to creep," or "to crawl"; the Latin serpo, from which 

 "serpent" is derived, means the same thing. Reptiles 

 include lizards, chameleons, alligators, crocodiles, turtles, 

 and snakes. 



Reptiles, like amphibians, are cold-blooded; but they 

 differ from amphibians in that they breathe, from the 

 first, with lungs. The heart of the crocodiles is higher in 

 development than that of other reptiles, for it has two 

 auricles (receiving chambers) and two separate ventricles 

 (chambers from which blood is pumped to the body and 

 the lungs). In this respect crocodiles are like birds and 

 mammals, including man (cf. 375). Other reptiles are 

 like amphibians. When there are two ventricles, the 

 pure blood from the lungs and the impure blood from the 

 body are not mixed in the heart. 



Reptiles are covered with horny or with bony plates, or 

 both. The horny plates come from the outer skin, and 

 are usually "shed" and renewed, as in the case of snakes. 





