84 
AMPHIBIANS 
they may be seen sunning themselves on a bank, but as soon 
as ice forms on the water they remain on the bottom or be¬ 
come buried in the mud. The lungs are emptied of air, the 
heart beats decrease, and all the usual life processes take 
place more slowly. This habit of passing the winter in a 
state of inactivity is known as hibernation (hi-ber-n a/shun). 
All the Amphibia, reptiles (Chapter VI, page 99), and sev¬ 
eral of the mammals hibernate during the winter. 
Enemies. —*■ As the frog’s hind legs are considered a deli¬ 
cacy, man is the worst enemy of the frog. Next come 
the snakes, birds, and fish. The leech kills frogs by sucking 
their blood. Fish eat many of the tadpoles, and, strange 
to say, some water beetles eat tadpoles also. 
71. Food. — Frogs are greedy creatures. They will eat 
almost any animal small enough to be swallowed, such as 
insects, worms, snails, tadpoles, and small frogs. These 
are caught alive and when in motion. 
72. Respiration. — The oxygen of the air passes through 
both the skin and the lungs into the blood of the frog and the 
carbon dioxide of the blood is thrown off through these same 
two organs. The frog has large blood vessels close to the skin, 
especially along the back. These send many fine branches 
into the skin. This explains why the frog can “ breathe ” 
through its skin. When the frog remains under the 
water for a long time, as during the winter, all the oxygen 
used enters the blood through the skin. When the air is 
taken into the mouth, it is forced into the lungs by the 
muscles on the floor of the mouth. In a way, the air is 
swallowed into the lungs rather than breathed in as in the 
case of mammals. Experiments have been made which show 
that the frog can get oxygen in sufficient quantities to main¬ 
tain life, even if it has not the use of its lungs. The frog 
thus possesses two organs of respiration, the skin and lungs. 
73. Excretion. — This fundamental life process is per¬ 
formed in the frog as in all other vertebrates. Like the fish, 
