THE FROG AS A REPRESENTATIVE VERTEBRATE 15 



vals, they either remain in shaded places, in which there is a max- 

 imum of moisture, or return to their " holes " as they do in the day 

 time. The Amphibia are thus dependent upon water, for two 

 obvious reasons: because their respiration is in part effected 

 through the skin; and because their skins are not adapted to pre- 

 vent evaporation. If a frog escapes in the laboratory, particularly 

 if the room is artificially heated, it will usually be found shriveled 

 and dead some hours later. On the other hand, many reptiles, 



Fig. 7,— The American toad, Biifo americanus, a male with buccal sacs 



expanded in "singing." 



(From Dickerson, "Frog Book," copyright, 1920, by Doubleday, Page & Co., reprinted 

 by permiasion.) 



including most Uzards and snakes, are adapted for a hot, dry atmos- 

 phere and react accordingly under artificial conditions. 



In this connection it may be mentioned that frogs and toads do 

 not " drink " water through the mouth as do many famihar ani- 

 mals. They absorb through their skin such water as enters the 

 body uncombined with their food. This can be demonstrated 

 by exposing a frog first to a dry and later to a moist atmosphere, 

 and weighing at proper intervals. 



Among the species of frogs common in the eastern and central 

 portions of the United States, the " bullfrog," Rana catesbiana, 

 and the " green frog," R. clamitans, have similar habits and are 



