CROCODILES AND SNAKES 
275 
found floating in ponds during the months of early spring. If we 
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 
that 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 by 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 pair of limbs appears in 
front of the first pair, and the tail falls off in pieces, gradually, one piece 
after another. Lastly the gills disappear altogether, after lungs have 
been developed, and the tadpole becomes a frog, breathing air now 
instead of water, 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 dry 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 pretty 
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 
