246 



PRACTICAL ZOOLOGY 



body through the air. Likewise in swimming the hind legs are 

 alternately folded up and extended, and during their backward 

 stroke the toes are spread apart so as to offer more resistance to 

 the water. Frequently frogs float on the surface with just the 

 tip of the nose exposed and with the hind legs hanging down. If 

 the frog is disturbed in this position, the hind legs are flexed, a 

 movement which withdraws the body, the fore legs direct the frog 

 downward, and the hind legs are extended, to complete the dive. 

 Croaking. — Frogs croak most during the breeding season, but 

 they are heard also at other times of the year, especially in the 

 evening or when the atmosphere becomes 

 damp. Croaking may take place either in 

 air or under water. In the latter case the 

 air is forced from the lungs, past the vocal 

 cords, into the mouth cavity, and back 

 again. 



Physiological Processes. — The physio- 

 logical processes of the frog will be consid- 

 ered in the following order: (1) digestion, 

 (2) absorption, (3) assimilation, (4) circu- 

 lation, (5) respiration, (6) excretion, (7) se- 

 cretion, (8) the skeleton and its functions, 

 (9) muscular activity, (10) nervous activ- 

 ity, (11) sense organs, (12) reproduction. 



Digestion. — The worms and insects 

 used as food by the frog are captured by 

 its sticky tongue (Fig. 141) and drawn into 

 the mouth by this organ or pushed in with 

 the forefeet. The conical teeth that are 

 present in the upper jaw do not masticate the food as do man's, 

 but simply hold it. In man the salivary glands add their secre- 

 tions to the food as it is being masticated and this saliva con- 

 verts the starch in the food into sugar. There are no salivary 

 glands in the frog and hence this process of salivation is entirely 

 omitted. 



Fig. 141. — Diagrams 

 showing the movements 

 of a frog's tongue when 

 an insect is captured. 

 (From Cambridge Nat- 

 ural History.) 



