ANNELIDA, 251 



the head. This animal swims with great facility 

 by rapid, undulating inflexions of its body; and by 

 practising a similar succession of movements in the 

 loose sand at the bottom of the water, it quickly 

 buries itself, and even travels to considerable dis- 

 tances through the sand ; first extending the an- 

 terior rings, and then bringing up the posterior part 

 of the body ; its progress being also much assisted 

 by the action of its numerous bristly feet.* 



Facilities for progression are also given by the 

 addition of tubercles, arranged in pairs along the 

 under side of the body, which serve the purposes 

 of feet, and are often furnished with bristles or 

 hooks. In the Amphitrite, and many other genera, 

 tufts of hair occupy the place of feet on each side, 

 and being moved by muscles specially provided 

 for that purpose, serve as levers for effecting pro- 

 gressive motion. 



We find the same object accomplished by very 

 different means in other animals of this class. The 

 leech, for instance, having the rings which encircle 

 its body very numerous and close to each other, 

 could not well have advanced by the ordinary 

 modes of vermiform progression. As a substitute, 

 accordingly, it has been furnished with an appa- 

 ratus for suction at the two extremities of the body, 

 which are formed into disks for that purpose. By 

 fixing alternately the one and the other, and con- 

 tracting or elongating the body as the occasion 

 requires, the leech can move at pleasure either for- 

 wards or backwards. Thus, while the tail is fixed, 

 the head may be advanced by lengthening the 



* Osier, Phil. Trans, fur 1826, p. 342. 



