CHAPTER XXVII 

 THE FROG, A TYPICAL VERTEBRATE 



Frogs live in or near fresh-water lakes, ponds, and streams, 

 and are distributed over the entire North American continent 

 as well as in other parts of the world. Frogs have many 

 enemies, being preyed upon by snakes, turtles, cranes, herons, 

 other frogs, and man. They have no means of protection and 

 must therefore remain concealed from their enemies or escape 

 by rapid locomotion. Those who have looked for frogs amid the 

 water plants in a pond or in the grass and rushes along the banks 

 of streams will readily admit that their colors effectively conceal 

 them from view. If they are approached too closely, they have 

 an excellent refuge, the water, into which their hind legs quickly 

 propel them. As is the case with the snails, the skin of the frog 

 is naked and covered with mucus which impedes evaporation. 

 Because of this naked skin frogs must live in very damp places 

 or near enough to water so that they can take a plunge occasion- 

 ally or else their skins will become dry and they will perish. 



Movements. — The ordinary movements of the frog are 

 those employed in leaping, diving, crawling, burrowing, and 

 maintaining an upright position. Some of these movements 

 are due to internal causes such as hunger, but many of them are 

 the responses to external stimuli. Frogs are sensitive to light, 

 and tend to congregate in shady places. They also seem to be 

 stimulated by contact, as shown by their tendency to crawl 

 under stones and into crevices. 



The frog leaps on land and swims in the water. The hind 

 legs are large and powerful. When the frog is on land, the 

 hind legs are folded up, and a sudden extension propels the 



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