1 6 THE FROG 



The method of taking food may be demonstrated by the 

 following experiment, which will succeed only in case the 

 frog is hungry, and accustomed to its surroundings so that 

 the feeding reaction is not inhibited by fear. By means of 

 a cord dangle a small piece of red cloth in front of a frog 

 confined in a large glass cage. In favorable cases the frog 

 will snap at the cloth; observe the action of the tongue. 



Using a frog confined in a glass dish, rotate the animal in 

 a horizontal plane. Observe that the head turns in a direc- 

 tion opposite to that of the movement of the plate on which 

 the frog is resting; often the body also turns in the same way. 

 Tilt the frog back and forth about a horizontal axis and ob- 

 serve that the head is moved up and down contrary to the 

 direction of rotation. These movements are called com- 

 pensatory movements, and they serve to maintain a fixed 

 orientation in space. 



II. EXTERNAL CHARACTERS. 



The following study should be made chiefly on the living 

 frog. Preserved specimens in addition are desirable. 



Notice the divison of the entire animal into head, trunk 

 and limbs. Is there a distinct neck? Is the frog charac- 

 terized by bilateral symmetry? antero-posterior differentia- 

 tion? dorso- ventral differentiation? Observe the 'hump' 

 on the frog's back ; examine a skeleton and discover how the 

 hump is produced. Compare the form of the body with 

 that of the more primitive tailed amphibia (caudata or uro- 

 dela, represented by newts and salamanders). In what ways 

 is the frog specialized for leaping? 



Observe carefully the color and color pattern of the dorsal 

 and ventral sides respectively. Is the coloration such that it 



