56 LOCOMOTION. 



under which these varieties may be arranged. Either the 

 whole body is concerned in effecting locomotion, or only 

 some of its parts are employed for the purpose. 



165. The jelly-fishes (Medusaa) 

 swim by contracting their umbrella- 

 shaped bodies upon the water con- 

 tained within, and its resistance 

 urges them forwards. Many others 

 are provided with a sac or siphon, 

 which they fill with water. By fore- 

 Fig. 31. ing out the water suddenly, a jet is 

 produced, which is resisted by the surrounding water, and 

 the animal is thus propelled. The Biche-le-mar, (Holo- 

 thuria), the cuttle-fishes, the Salpas, &c., employ this method. 



166. Others contract small portions of the 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 extend- 

 ing forwards. 



O 



167. Some have, at the extremities of the body, a cup or 

 some other organ for maintaining a firm hold, each one 

 acting in turn as a fixed point. Thus the Leech has a cup 

 or sucker at its tail, by which it fixes itself ; the body is 

 then elongated by the contrac- 

 tion of the muscular fibres! 



which encircle the animal ; the 



other end is fixed by a similar | 



sucker, then by the contraction Fig. 32. 



of muscles running lengthwise, the body is shortened, and 



the tail is brought forwards to perform the same process 



