316 BACKBONED ANIMALS. 
port a thin, leathery membrane, that commences at the side 
of the neck and extends to the hind-legs, partly or wholly 
encompassing the tail, and is used with all the freedom of 
a bird’s wing. 
The sternum is 
slightly keeled, 
as in the birds, 
and the .teeth 
resemble in gen- 
eral those of 
the Jnsectivora, 
They are noc- 
turnal, and hang 
by their hind- 
legs or arms in 
Fic. 341.—Skeleton of a bat (lettered to compare 
with bird’s skeleton, p. 226). (fa, fore-arm; wu, . 
wrist; ¢, thumb; 4a, hand; 4, heel; /, foot. resting. The 
thumb is free, 
and forms a hook for holding and walking; the first 
finger is also generally provided with a claw, and in mov- 
ing on the ground the bat uses its thumb, while its other 
fingers point backward. Some species have 
peculiar disk-like clinging organs (Fig. 
342). The eyes are extremely minute, es- 
pecially in the long-eared bats, yet, even 
when deprived of these organs, they show 
marvelous skill in avoiding obstructions. 
NoTE.—At the approach of cold weather the bats Fig, 342,—Suc- 
are deprived of food, and, being unable to migrate torial disk on 
as the birds, retire to caves and secluded spots and the thumb of 
sleep away the cold months in a state of hibernation. Thyroptera 
So perfect is this sleep, that the animals have been tricolor. 
placed in illuminating gas without perceptible effect ; 
and in other experiments the air about them, upon being analyzed, 
did not show evidences of having been breathed. They and other 
hibernators are supposed during this period to obtain nourishment by 
absorbing the fat that has accumulated on the under side of the neck, 
in the so-called hibernation-glands, 
