28 THE BIOLOGY OF THE FROG chap. 



spread apart, and as the web between them affords a con- 

 siderable resistance to passing through the water, this mo- 

 tion gives the body a forward impulse. The fore limbs are 

 held back against the body, after the stroke, and if the frog 

 does not make several strokes in quick succession, the hind 

 limbs are held extended behind the body, so that the animal 

 affords as little resistance as possible to gliding through the 

 water. The fore limbs are also used in swimming, taking 

 strokes sometimes together and sometimes alternately. To 

 a certain extent they aid in propelling the animal forward, 

 but they are also employed, as in locomotion on land, to 

 guide the direction of movement. When the animal starts 

 to swim downward, the fore legs beat backward and upward, 

 the hand being twisted so as to press its broad surface 

 against the water. This naturally pushes the anterior part 

 of the body down. In starting to swim upward, the fore 

 legs beat downward, elevating the anterior part of the body, 

 w r hich is then pushed upward by the strokes of the hind legs. 

 The fore legs are also used in causing the body to move 

 from side to side, and unequal movements of the hind legs 

 are employed for the same purpose. Bendings of the body 

 are also used to help steer the course of the animal. The 

 hind legs usually make a stroke at the same instant, but the 

 frog not infrequently uses them alternately, especially when 

 struggling near an obstacle, 



Attitude when Floating on the Surface. — When frogs 

 are kept in water beyond their depth, they spend a consider- 

 able portion of their time at the surface with just the tip of 

 the nose exposed, for the purpose of breathing air. The 

 distance which the head projects from the water may be 

 varied at will, as it depends upon the amount of air taken 

 into the lungs. The more the lungs are inflated, the less the 

 specific gravity of the animal becomes, and the higher, there- 



