LOCOMOTION. 



equally concerned in efTecting locomotion, or only some oi 



its parts are employed for the purpose. 



165. Tiie jelly-fishes (Medusoe) swim 

 by contracting their umbrella-shaped 

 bodies upon the water below, and its 

 resistance urges them forwards. Other 

 animals are provided with a sac or 

 siphon, which they may fill with water 

 and suddenly force out, producing a jet^ 

 which is resisted by the surrounding 

 ^^* * water, and the animal is thus propelled. 



The Biche-le-mar, (Holothuria,) the cuttle-fishes, the Salpne, 



&c., move in this way. 



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 boJy 

 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 extendinir 

 forwards. 



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

 some other organ for maintaining a firm hold, each extremity 

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

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



elongated by the contraction 



of the muscular fibres which 



encircle the animal ; the mouth 



is next fixed by a similar suck- 



■t'lg- ^-- er and by the contraction of 



muscles running lengthwise the body is shortened, and the 



tail, losing its hold, is bro;iLi;ht forwards to repeat the same 



process. ?.Iost of the bivalve mollusks, such as the clams, 



