478 COLLEGE ZOOLOGY 



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. When disturbed in this 

 position, the hind legs are flexed, a movement which withdraws 

 the body, the fore legs direct the frog downward, and then the 

 hind legs are extended, completing the dive. 



Frogs croak mostly during the breeding season, but 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. 



The principal enemies of frogs are snakes, turtles, cranes, 

 herons, other Amphibia, and man. The excellence of frogs' legs 

 for the table has resulted in widespread destruction, and this 

 has been augmented by the capture of great numbers for use 

 in scientific investigations. Tadpoles fall a prey to aauatic 

 insects, fishes, and water-fowl, and very few of them reach 

 maturity. 



External Features. — The body of the frog may be divided 

 into the head and trunk. The eyes usually protrude from the 

 head, but are drawn into their orbits when the frog closes its eye- 

 lids. Behind each eye is a tympanic membrane covering the ear- 

 drum. A pair of nostrils or external nares are situated on the 

 dorsal surface near the end of the snout. Just in front of the 

 eyes in some specimens is a light area, called the brow spot, 

 which, in the embryo, was connected with the brain. The mouth 

 of the frog extends from one side of the head to the other. The 

 anus is situated at the posterior end of the body. 



The fore legs are short and serve to hold up the anterior part 

 of the body. The hands possess four digits and the rudiment 

 of a fifth, the thumb. In the male the inner digit is thicker than 

 the corresponding digit of the female, especially during the breed- 

 ing season. The hind legs are folded together when the frog is 



