OF LOCOMOTION. 109 



SECTION II. 



OF LOCOMOTION. 



228. ONE of the most curious and important applications of 

 this apparatus of bones and muscles is for LOCOMOTION. By 

 this is understood the movement which an animal makes in pass- 

 ing from place to place, in the pursuit of pleasure, sustenance, 

 or safety, in distinction from those motions which are performed 

 equally well while stationary, such as the acts of respiration, 

 mastication, &c. 



229. The means which nature has brought into action to 

 effect locomotion, under all the various circumstances in which 

 animals are placed, are very diversified ; and the study of their 

 adaptation to the necessities of animals is highly interesting in 

 a mechanical, as well as in a zoological point of view. Two 

 general plans may be noticed, under which these varieties 

 may be arranged. Either the whole body is equally concerned 

 in effecting locomotion, or only some of its parts are employed 

 for that purpose. 



230. The medusae (fig. 173) swim by contracting their 

 umbrella-shaped bodies upon the water below, and its resist- 

 ance urges them forwards. Other animals are provided with 

 a sac or syphon, which they may fill with water, and suddenly 

 force out, producing a jet, which is resisted by the surround- 

 ing water, and the animal is thus propelled. The Holothitria 

 (fig. 232), the cuttle-fishes, the salpse, &c. move in this way. 



23 1 . Others contract small portions of their body in suc- 

 cession, which being thereby rendered firmer, serve as points 

 of resistance, against which the animal may strive in urging 

 the body onwards. The earth-worm, whose body is composed 

 of a series of rings united by muscles, and shutting more 

 or less into each other, has only to close up the rings, at one 

 or more points, to form a sort of fulcrum, against which the 

 rest of the body exerts itself in extending forwards. 



232. Some have, at the extremities of trie body, a disc, or 

 some other organ, for maintaining a firm hold, each extremity 

 acting in turn as a fixed point. Thus the leech (fig. 178) has 

 a disc, or sucker, at its tail (o), by which it fixes itself; the 

 bodv is then elongated by the contraction of the muscular 



V V 



fibres which encircle the animal ; the mouth (a) is next fixed by 

 a similar sucker, and by the contraction of muscles running 

 lengthwise the body is shortened, and the tail, losing its hold, if 



