THE FROG AS A REPRESENTATIVE VERTEBRATE 19 



vertebrates, but they have developed independently in the direc- 

 tion of more specialized uses for their two pairs of limbs than is 

 commonly found. In the familiar hopping of frogs and toads, the 

 hind legs are responsible for the strength of the spring while the 

 front legs serve mainly as guides in pointing the body in the 

 desired direction. The animals are frequently seen crawling in a 

 clumsy fashion, but only for short distances, or to shift the position 

 of the body. In water, likewise, the frog swims by means of its 

 powerful hind limbs, while the fore limbs perform the function of 

 balancers. 



In the squatting posture which it assumes on the bank, the frog 

 is in a position to leap at a moment's notice, hke an athlete ready 

 for the pistol shot. In the " floating " attitude, the animal is 

 no less able to retreat suddenly. At first glance it seems so awk- 

 wardly placed that it must be at a great disadvantage, but 

 observation shows that the floating individual actually " dives," 

 with surprising rapidity. The reason for this appears when the 

 series of movements is carefully observed. The animal hangs 

 obliquely in the water with eyes and tip of nose exposed, the fore 

 limbs projecting from the body and the hind limbs moderately 

 extended. WTien diving from this position, it first withdraws 

 from the surface by bringing the hind legs into their folded position, 

 thus carrying the body backward beneath the water. While this 

 is happening, the fore limbs give a stroke upward and backward, 

 which, together with the bending of the body, directs the head 

 downward. The hind legs are then extended with their powerful 

 stroke, and the frog shoots away, the whole process occurring so 

 rapidly that it is impossible to recognize the successive changes 

 without repeated observations. 



Like other active annuals, the frog has a well-developed " sense " 

 of its position. It " knows " when it is wrong side up. If placed 

 on its back the animal rights itself without delay. It may roll either 

 to the right or to the left, but the response is unmediate unless the 

 individual has been fatigued by repeated demonstrations. With the 

 onset of fatigue, the process occurs more slowly and is readily 

 observed. In this connection a pecuhar mode of reaction may be 

 mentioned. If a frog is seized in the hands, laid upon its back, 

 and held a few moments until it has ceased its struggles, it will 

 usually remain motionless for a time, sometimes for hours 

 (r/. Fig. 10). The exact significance of this behavior, in relation to 



