The Common Bullfrog 



three and one-fourth to three and one-half times in the total 

 length, whereas the usual proportion is one to three. The eyes are 

 surprisingly small and little elevated, giving the flat head an 

 expression greatly lacking in intelligence and alertness. The 

 longitudinal diameter of the eye in a frog 6J inches long is a 

 short I inch, while it is fully | inch in an Eastern Bullfrog meas- 

 uring 5| inches. The smaller size of the eye results in a greater 

 space between the eye and the nostril and a longer fold of skin 

 above the ear. In this frog the vocal sacs of the throat extend 

 backward over the arm so that when the frog croaks, not only 

 does the throat swell, but pouches appear between the ear and 

 the arm as in the case of the Leopard Frog. The increase in vocal 

 powers in this frog is shown also in the fact that the female croaks 

 in a manner similar to that of the male, but without quite the 

 same force and without the appearance of pouches at the sides 

 of the head. 



The Common Bullfrog is a powerful swimmer, with great 

 strength and length of hind legs (which may measure from 

 seven to ten inches long) and with very large webs. It is inter- 

 esting to see one dive under the water. The legs are straight- 

 ened powerfully, and then slowly drawn forward into position 

 for the second stroke. The huge web is alternately extended 

 into a flat resisting membrane and folded again as the leg is 

 drawn forward. The eyes are shut; that is, they are flattened 

 until they are level with the head, by being lowered in their 

 sockets, which project downward from the roof of the mouth. 

 (Fig. 279.) This takes the eyes out of danger during the swift 

 motion through the water, but also makes it necessary that the 

 frog shall swim rapidly but a short distance at a time. He must 

 stop, or slacken speed and open his eyes to see where he is, 

 and then, perhaps, may make a new plunge with eyes lowered, if 

 an enemy is near. The frog, like the toad, has no outer ear to 

 hinder him in swimming. The ear-drum is at the surface of the 

 head, covered and protected merely by the moist skin. 



Something else is seen to take place, also, as the frog dives 

 under water. Large bubbles of air are given oflf from the nostrils, 

 which are then closed tightly. The frog does not use the lungs 

 in breathing under water; the nostrils are kept closed, and the 

 throat shows none of the swallowing movements so conspicuous 

 when the frog is breathing air. The frog's moist skin is like a 



231 



