KINDS OF MOTION. 137 
bringing its head to the tail; and, having contracted every 
part as much as possible, unbends with a sudden jerk, and 
darts forward to a surprising distance. Many crabs and 
podure bend their tail, or hairs which supply its place, un- 
der their belly, and then suddenly unbending, give to the 
body a considerable degree of progressive motion. 
4. Flying.—F lying is the continued suspension and pro- 
gress of the whole body in the air, by the action of the 
wings. In leaping, the body is equally suspended in the 
air, but the suspension is only momentary. In flying, on 
the contrary, the body remains in the air, and acquires a 
progressive motion by repeated strokes of the wings on the 
surrounding fluid. 
The centre of gravity of the bodies of flying animals, is 
always below the insertion of the wings, to prevent them 
falling on their backs, but near that pomt on which the 
body is, during flight, as it were suspended. The posi- 
tions assumed by the head and feet are frequently calcu- 
lated to accomplish these ends, and give to the wings every 
assistance In continuing the progressive motion. 
The action of flying is performed by animals belonging 
to different classes. Among the Mammalia, bats display 
this faculty, by means of wings, formed of a thin membrane 
extending between the toes, which are long and spread- 
ing, the fore and hind legs, and between the hind-legs and 
the tail, In Birds, the wings, which occupy the place of 
the anterior extremities in the mammalia, and are the or- 
gans of flight, consist of feathers, which are stronger than 
those on the body, and of greater length. Among Reptiles, 
the flying lizard may be mentioned, whose membranaceous 
wings, projecting from each side of the body, without be- 
ing connected with the legs, enable it to fly from one tree 
to another in search of food. A few Fishes are likewise ca- 
pable of sustaining themselves for a short time by means 
