THE FROG 159 
place between the carbon-dioxide in the blood in the capil- 
laries lining the walls of the lungs, and the oxygen con- 
tained in the inspired air. When the air has thus been 
driven in and out of the lungs a few times, the nostrils are 
opened, and the impure air is expelled. A constant repeti- 
tion of this process enables the animal to obtain the greater 
part of the oxygen required. The Frog’s breathing, how- 
ever, is not confined to the lungs, a considerable part of it 
being carried on through the skin. The skin is thin and 
moist, with numerous blood-vessels in its inner surface. 
All the essentials of a breathing organ are, in fact, present 
-—viz., plenty of free oxygen separated from blood-vessels by 
a thin, moist membrane, so that a supply of oxygen can be 
and is obtained through the skin. It is for this reason that 
the Frog is able to live so long under water ; indeed, it is 
practically impossible to drown a frog in ordinary water, 
where the dissolved air supplies the animal with the 
oxygen needed. 
The foregoing will show how necessary it is that the skin 
be kept moist, and’explain why the Frog, when in the sun, 
does not remain long out of water, and why, moreover, it 
invariably selects as its abode a cool shady spot. 
A Cold-blooded Animal.—The Frog is a cold-blooded 
animal like the fish, and for very.much the same reasons ; 
the supply of oxygen it requires and hence absorbs is not 
sufficient to raise the temperature of its body, which always 
feels cold to the touch. It is because the animal has but 
little bodily heat to keep in, that it is not provided with a 
warm covering, or, inversely, because it is not provided with 
a warm covering it can have but little bodily heat to keep 
in. A naked animal, if warm-blooded, would not be able 
to live in the water for months together, as the Frog does, 
unless, like the whale, it were specially protected against cold. 
The Frog’s Food.—tThe Frog lives largely on insects, 
which it catches by means of its tongue. The prey, however, 
