LOCOMOTIVE APPENDAGES OF ANNELIDA. 



299 



motion and respiration, there are frequently two pairs in each 

 segment ; one belonging to its upper or dorsal portion, and the 

 other to its lower or ventral (Fig. 594) : but in other in- 

 stances the two appendages on the same side are united (Fig. 

 f)27). We usually then find, in the marine Annelida, that 

 the ventral portion is chiefly devoted to 

 locomotion ; and the dorsal to respiration. 

 Thus, in the Eunice, we find, at the under 

 part, a fleshy tubercle (), furnished with a 

 tuft of bristles, and below it a rudimentary 

 drrhus, or tendril-like organ (i) ; whilst 

 the upper part of the appendage is formed 

 by a branchial tuft (?/), and by a long 

 slender cirrhus (c). This last sometimes 

 exhibits a trace of articulation, as in the 

 Syllis (Fig. 522, a). In other cases, however, these appen- 



FIG. 521. GILL-TUFT OK 

 EUNICE. 



FIG. 522. SYLLIS MONILARIS, with one of its locomotive organs and setigerous appendage 

 attached thereto. 



iges are only represented by a few short stiff hairs, as in the 

 "thworm ; and in other instances, as the Leech, there is no 

 trace of any members or appendages to the body. The bristly 

 tufts of the Nereidans and their allies are useful to them in 

 various ways ; they serve them in part as instruments of attack 

 and defence, the bristles being usually sharp, and sometimes 

 barbed at their extremities, so as to attach themselves with force 

 to soft substances ; they assist, also, in their movements over 

 solid surfaces, taking hold, as it were, of the rock on which the 

 animal is crawling, so that the hinder part of the body is pre- 

 vented from slipping back, when the anterior part is pushed 

 forwards ; and they also aid in its movements through the 



