INTERNAL STRUCTURE 115 



the nights in the fall grow cool, frogs make ready to 

 spend the winter in a state of inactivity. During the 

 warmer part of the day, they may be seen sunning them- 

 selves on a bank, but as soon as ice forms on the water 

 they remain on the bottom or become buried in the mud. 

 The lungs are emptied of air, the heart beats decrease, and 

 all of the usual living processes take place more slowly. 

 This habit of passing the winter in a state of inactivity 

 is known as hibernation (hl-ber-na/shiin). All of the 

 amphibia, reptiles (Chapter XII, page 129), and several of 

 the mammals hibernate during the winter. 



116. Food. — Frogs are greedy creatures and will eat 

 almost any animal small enough to be swallowed, such as 

 insects, worms, snails, tadpoles, and small frogs. These 

 are caught alive and when in motion. 



117. Enemies. — As the frog's hind legs are considered 

 a delicacy, man is the worst enemy of the frog. Next 

 come the snakes, birds, and fish. The leech kills frogs by 

 sucking their blood. Fish eat many of the tadpoles, and 

 strange to say, some water beetles eat tadpoles also. 



118. Respiration. — Both the skin and a pair of lungs 

 serve to purify the blood of the frog. The air is forced 

 into the lungs by the contraction of muscles in the floor 

 of the mouth. Experiments have been made which show 

 that the frog can get enough oxygen even if the lungs 

 are missing. In this respect frogs resemble worms, which 

 use the skin as the only organ of respiration. 



119. Internal Structure. — A study of the parts of the frog 

 or toad should be made for two reasons: (1) To understand 

 the relative positions of the internal organs of a typical 

 vertebrate; (2) to help explain the several organs of 

 man which are discussed in the second part of this book. 



Digestive Organs. — The mouth is large. Short lips 

 cover the short teeth in the edge of the upper jaw. The 



