302 



THE RISE OF ANIMAL LIFE 



midsummer they are fully mature. Ample 

 amounts of fat are stored in special organs 

 called fat bodies before the frogs go into 

 hibernation at the approach of winter. 

 Frogs are cold-blooded, as are turtles and 

 snakes, and usually spend the winter buried 

 in the mud at the bottom of ponds and 

 streams. As the temperature drops, their 

 body processes slow down simultaneously 

 until the heart is beating very slowly and 

 all metabolism is reduced to the lowest 

 possible rate necessary to maintain life. In 

 this state of inactivity the food demands are 

 very slight, so that stored food carries 

 the frog along quite adequately through 

 the winter months. As the temperature rises 

 in the spring, frogs soon become active and 

 enter at once into the breeding period. 



Each female frog lays from several hun- 

 dred to several thousand eggs depending 

 on her age. Of these eggs, only a very 

 few, perhaps none, become mature frogs. 

 Enough manage to come through to ma- 

 turity, however, to maintain the race, 

 although with the onslaught by birds, tur- 

 tles, snakes, fish, and man it is amazing that 

 this little animal does survive and one won- 

 ders if it will continue to do so. One of the 

 greatest demands for its body is by begin- 

 ning zoology classes to verify points dis- 

 cussed in this book. 



The frog body plan. Frogs range in size 

 from the tiny cricket frog (Psetidacris), 

 about an inch long, to the bullfrog which 

 may be a foot over-all. In general, their 

 features are so similar that, aside from col- 

 oration and habits, a description for one fits 

 them all. When a study is made of a living 

 frog, the moist, slippery skin is at once con- 

 spicuous even if the frog is kept away from 

 water for some time. This is due to tiny 

 mucus glands in the skin which constantly 

 pour out their fluids to keep it wet. Like the 

 earthworm, the frog receives considerable 

 amounts of oxygen through its skin and 

 must therefore have a moist skin. The slip- 

 pery skin also cuts down friction when the 

 frog is swimming through the water. 



Other distinctive features include the 

 large eyes which when touched are pulled 

 down into the head (though actually they 

 bulge into the mouth cavity). The protrud- 

 ing eyes permit the frog to come to the sur- 

 face of the water and see without exposing 

 the rest of its body, a definite protection 

 against possible enemies. Lying just back of 

 the eyes are the large eardrums which are 

 a part of the hearing mechanism. Above 

 the tip of the nose are the nostrils which 

 have valves that can be opened and closed 

 at will. These function in breathing. The 

 mouth is a very large one, and is kept shut 

 all of the time except when the frog feeds. 

 At the posterior end is the anus, which is 

 the terminal opening of the cloaca. 



The front legs are turned in, "pigeon- 

 toed," and there is a swelling ( nuptial pad ) 

 on the inside disiit of each front foot of the 

 male, already referred to in the process of 

 mating. These legs function in breaking 

 the fall after a jump, as well as in support- 

 ing the anterior portion of the body. The 

 long muscular hind legs are beautifully 

 adapted for jumping. When the frog is at 

 rest on land they are kept along side or par- 

 tially under the body in the jumping posi- 

 tion, but in the water they are customarily 

 left dangling behind. When surprised on 

 land, the frog suddenly straightens out its 

 legs, throwing the body forward several 

 feet. This process can be repeated in rapid 

 succession, so that it requires an agile pur- 

 suer to overtake the little animal. These 

 are the principal external features that are 

 noted in a cursory examination. 



Outer covering. Like all vertebrates, the 

 skin of the frog consists of an outer thin 

 epidermis and a thicker underlayer, the 

 dermis. The outer layer, which is shed 

 periodically, is made up of flat cells. The 

 dermis contains many glands which provide 

 the mucus for keeping the skin moist. 

 Some species have, in addition, smaller 

 glands in this region which secrete a sub- 

 stance that is offensive to animals that 

 might feed upon it. The dermis is also heav- 



