ANNELIDA. 281 



Facilities for progression are also given by the 

 addition of tubercles, arranged in pairs along 

 the under side of the body, which serve the pur- 

 poses 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 spe- 

 cially 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 

 ordinary modes of vermiform progression. As a 

 substitute, accordingly, it has been furnished 

 with an apparatus for suction at the two extremi- 

 ties 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 forwards 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 cavities of the body, and especially 



