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 

 podurae 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. Flying 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 point 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 hindrlegs 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 



