A REPRESENTATIVE VERTEBRATE. FROG 



327 



movement which withdraws the body, and 

 the forehmbs direct the animal downward; 

 then the hindHmbs are extended again, com- 

 pleting the dive. 



Frogs croak mostly during the breeding 

 season, but also at other times of the year, 

 especially in the evening or when the at- 

 mosphere 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 amphibians, 

 and man. The excellence of frog legs for the 

 table has resulted in their widespread de- 

 struction, and this has been augmented by 

 the capture of great numbers for use in 



Figure 209. Feet of a male frog, Rana pipiens. A, forefoot showing enlarged first digit (thumb), 

 which is most developed during the breeding season. B, hindfoot showing web. 



scientific investigations. Tadpoles fall prey 

 to aquatic insects, fish, and waterfowl, and 

 relatively few of them reach maturity. 



External anatomy 



The body of the frog may be divided into 

 the head and trunk (Fig. 217); there is no 

 neck region. The eyes usually protrude from 

 the head, but are drawn into their orbits 

 when the frog closes its eyelids. Behind each 

 eye there is a flat eardrum ( tympanic mem- 

 brane). A pair of nostrils (external nates) 

 is situated on the dorsal surface near the end 

 of the snout. In the dorsal midline, 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 cloacal open- 

 ing, or what is sometimes called the anus, is 

 situated at the posterior end of the body. 



The forelimbs are short and serve to hold 

 up the anterior part of the body. The hands 

 possess 4 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 breeding sea- 

 son (Fig. 209). The hindlimbs are folded 

 together when the frog is at rest. The 5 toes 

 are connected by a web, making the foot 

 an efficient swimming organ. 



Body covering 



The skin is smooth and loosely attached 

 to the body. Along either side of the body, 

 behind the eyes, is a ridge formed by a 

 thickening of the skin; this is called the 

 dorsolateral fold (dermal plica). The skin 

 is colored by scattered pigment granules in 

 the epidermis and pigment cells known as 

 chromatophores in the dermis. The chroma- 

 tophores are of several different kinds, the 

 most important being those which contain 

 black or yellow pigment. There are also in- 

 terference cells that contain whitish cr}'stals. 

 There is no green pigment in the frog's skin. 

 Frogs are usually protectively colored by 

 resembling their surroundings. The color of 



