A STUDY OF THE FROG 5 



Whenever possible the whole class should join in collecting 

 the material in order that all may see the animal in its usual 

 haunts and study its habits there. Living specimens may be 

 kept in the laboratory for some weeks during which time many 

 interesting observations may be made. But in order to make 

 a closer study of the structure of the animal it must be killed. 

 This may be done by placing the frog in an air-tight jar or other 

 vessel with a piece of cotton or cloth that has been saturated 

 with chloroform or ether. The following description is written 

 for the frog, but it will serve also as a guide for the study of the 

 toad, as the two animals are alike in general structure. 



External Structure. The body of the frog is divided into 

 two principal regions, the head and the trunk. In most verte- 

 brates there is a distinct neck between these two regions, but 

 in the frog they are closely united. The forelegs, or arms, 

 are well developed, but the hind legs are much longer and 

 stronger, enabling the animal to leap for considerable distances. 

 On the hands are four fingers or digits and a rudiment of a 

 thumb. The five toes on the hind legs are connected by a web. 



The tough skin that covers the body is kept moist by the 

 secretions from many glands. The eyes are large, prominent 

 and protruding. When they are closed they are drawn back 

 into their orbits somewhat and covered mostly by the lower 

 eyelid, which is thin and freely movable. The upper eyelid is 

 thick and not capable of much movement. The tympanum, 

 the outer membrane of the auditory organ, is a smooth ellip- 

 tical membrane just back of each eye. When sound waves strike 

 the tympanum, causing it to vibrate, the vibrations are trans- 

 ferred to the inner ear by a minute rod, the columella, which 

 extends between the two. The nostrils are in front of the eyes 

 and above the mouth. The wide mouth extends from one 

 side of the head to the other. In the frog there are a few small 

 teeth on the upper jaw and in the roof of the mouth which serve 

 only to hold the prey. No such teeth are present in the toad. 

 The tongue is attached by its anterior end. The posterior 

 end is free and can be extended forward out of the mouth nearly 

 its full length for capturing insects. As the tongue is covered 

 with a mucous secretion the insects stick to it and are quickly 



