THE FROG AS A REPRESENTATIVE VERTEBRATE 21 



and diversification. In R. pipiens there is a slight difference in 

 the croak of the two sexes, and, in addition to its croaking, the 

 animal gives forth a grunting sound under conditions that seem 

 unusually agreeable. When seized by an enemy it may utter a cry 

 called the pain scream. The croaking is produced by forcing air 

 back and forth between lungs and mouth cavity across the vocal 

 cords, which are stretched on either side of the larynx, or laryngo- 

 tracheal chamber. The latter name is sometimes given to the 

 larynx of the frog (Fig. 23, p. 43) because it is equivalent to both the 

 larynx and trachea, or windpipe, in higher vertebrates. A frog can 

 croak even though it is completely submerged in water, because 

 there is only a sKght loss of air through the nostrils during the 

 process and the air may be driven back and forth a number of 

 times before being expelled. It has been generally assumed that 

 this sound production by means of the vocal cords functions pri- 

 marily as a sex call, by means of which the males and females find 

 one another at the breeding season. This is undoubtedly one of 

 its uses, if not the only one, although the croaking is more or less 

 in evidence at other times. Among the sounds uttered by frogs 

 are community signals, like the alarm cry, which causes other indi- 

 viduals to seek safety in the water. 



Vocal cords are, of course, present only in air-breathing verte- 

 brates. The fishes have no such organs. It is interesting to find 

 in the Amphibia, wliich are the simplest of the land vertebrates, 

 structures and activities that suggest the origin of the human 

 vocal organs (Fig. 24, p. 44). It may also be remarked that a well- 

 developed sense of hearing appears only in terrestrial vertebrates, 

 and that eardrums, Hke vocal cords, do not occur in fishes. In 

 general, it appears that sound production and the ability to perceive 

 sounds go hand in hand. The ear of a mammal, with its external 

 portion and its drum sunken into a protected position (Fig. 68, 

 p. 125), in correlation with a more delicate sense of hearing, is 

 paralleled by the greater speciaHzation of the sound-producing 

 apparatus which mammals exhibit. 



Some of the diverse activities of the frog seem to be adaptive, 

 that is to say, useful, to the extent that they tend to protect the 

 individual in time of danger. When threatened by an enemy, 

 a frog or toad often folds the limbs close against its sides and 

 inflates its body by filling the lungs to their maximum capacity. 

 In this condition the animal is almost egg-shaped, and the moist 



