80 LOCOMOTION. 



equally concerned in effecting locomotion, or only some of 



its parts are, employed for the purpose. 



165. The jelly-fishes (Medusae) 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 

 g ' water, and the animal is thus propelled. 



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



&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 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. 



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 tail, 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- 



i 



Fig. 32. er and by the contraction of 



muscles running lengthwise the body is shortened, and the 

 tail, losing its hold, is brought forwards to repeat the same 

 process. Most of the bivalve mollusks, such as the clams, 



