FROGS, TOADS, AND SALAMANDERS 231 



Air is taken into the mouth through the nostrUs which 

 are then shut by means of the valves. At this time the 

 floor of the mouth is bulged downward, showing that the 

 mouth is full of air. By an action similar to that of swal- 

 lowing, in which the floor of the mouth is raised, the air 

 is now forced into the lungs. The air is expelled from the 

 lungs by the contractions of their walls aided by the mus- 

 cular contractions of the waUs of the abdomen. 



The skin is furnished with many minute blood vessels 

 and aids in the process o( respiration. 



The mouth and tongue. — The mouth of the frog is large, 

 and the jaws can be opened very wide. There is a row of 

 small teeth on the upper jaw and a few on the roof of the 

 mouth but none on the lower jaw. These serve merely to 

 hold the prey, not to masticate the food. 



The tongue is attached to the mouth by its anterior end, 

 thus leaving the posterior end free. This arrangement 

 enables the frog to extend its tongue outside of the mouth 

 nearly its whole length, since it is attached at the front part 

 of the mouth. The tongue is covered with a sticky, mucous 

 secretion. 



Legs and locomotion. — The frog has four well-developed 

 legs, the hind pair being much the longer and stronger. 

 The thighs of the hind legs are furnished with strong m-uscles 

 and the hind feet are long, broad, and webbed. 



The frog has three methods of locomotion — walking, 

 swimming, and leaping. Under certain conditions — for 

 example, when climbing up a sharply inclined surface — this 

 animal moves in a slow, awkward walk. Its main method 

 of locomotion on land is by long and powerful leaps with 

 the hind legs. In water the frog is a model swimmer. 

 The front legs take no active part in this method of loco- 



