CROCODILES AND SNAKES 275 



found floating in ponds during- the months of early spring. Jf wc 

 take one of these masses from the water, we find that it consists of 

 a number of small round eggs, each with a black spot in the center, 

 touch. These are the eggs of the frog, which shortly hatch, and pro- 

 which are fastened to one another, and feel slimy and slippery to the 

 duce the little creatures called tadpoles. 



No one who was not acquainted with them would ever suppose 

 jlhat these tadpoles bore any relationship to the frog at all, for they 

 are as unlike their parents as they can possibly be, having no limbs at 

 all, and being, in fact, very little more than round heads furnished with 

 rather flat, wavy tails. By means of these tails they wriggle their 

 way along through the water. As the tadpoles live entirely under 

 water, and cannot breathe air, they are furnished with gills instead of 

 lungs, which extract air from the water just as do those of a fish. 



Before very long, however, the gills begin to diminish in size, 

 and finally they disappear altogether into the chest, where they are 

 protected l)y what are called gill-covers. Meanwhile other alterations 

 are taking place in the body, and two small organs break through the 

 skin at the hind part of the body, near the tail. In a short time these 

 organs develop into legs, which, however, are not as yet employed for 

 any particular purpose. Shortly another ])air of.liml)s appears in 

 front of the first pair, and the tail falls off in pieces, gradually, one ])iece 

 after another. Lastly the gills disappear altogether, after lungs have 

 been developed, and the tadpole becomes a frog, breathing air now 

 instead of w^ater, and swimming by the aid of its legs instead of that 

 of its tail. 



The life of the perfect frog, of course, is now quite different, 

 and its mission is to keep down the numbers of the various insects, 

 instead of to purify the waters of the pond. It must, however, be able 

 to swim and dive in the water as well as to live upon drv land, and so 

 must have a structure equally suited to either mode of life. 



For swimming in the water it is very well adapted, for its long 

 webbed feet make capital oars, and it can hold its breath for a very 

 long time, so that it can remain below the surface, if need be, for prettv 

 well two hours without requiring a fresh supply of air But for a life 

 upon land it is quite as well suited. It is able to travel with some little 



