ANNELIDA. 201 



Tentacula of various kinds arc also met witli in several of 

 the more active and vivacious kinds of Annelida, such as the 

 Nereis (Fig. 129,) proceeding from the margin of the mouth 

 and other parts of the head. This animal swims with great 

 facility by rapid, undulating inflections 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 distances through the sand, first ex- 

 tending the anterior rings, and then bringing up the poste- 

 rior 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 Jimphilrite^ 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 progressive 

 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 or- 

 dinary modes of vermiform progression. As a substitute, 

 accordingly, it has been furnished with an apparatus for suc- 

 tion at the two extremities of the body, which are formed 

 into disks for that purpose. By fixing alternately the one 

 and the other, and contracting 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 whole body, and when 

 the head is fixed, the hinder sucker can be brought forwards 

 by the contraction of the body, and applied to the ground 

 near to the head, and preparation may thus be made for 

 taking another step. 



Most of the parasitic animals which inhabit the interior 



• Osier, Phil. Trans. 1826, p. 342. 

 Vol. I. 26 



