SECT. IX 



PHYLUM ANNULATA 



189 



1. THE CH^ETOPODA 



The rows of bristles above referred to as disposed along the 

 segments of the body in the Earthworm and the Lobworm 

 constitute one of the distinguishing features of the class 

 Chsetopoda, or " Bristle-footed " 

 Worms of the phylum Annulata. 

 Of these a good and common ex- 

 ample is Nereis a Marine Annelid 

 of common occurrence under stones 

 and among shells and sea-weed on 

 the sea-shore in all parts of the 

 world. 



In shape (Figs. 100 and 103) 

 the body, which may be about 7 or 

 8 centimetres in length, is long and 

 narrow, approximately cylindrical, 

 somewhat narrower towards the 

 posterior end. A very distinct head, 

 bearing eyes and tentacles, is re- 

 cognisable at the anterior end ; the 

 rest is divided by a series of ring- 

 like narrow grooves into a corre- 

 sponding series of segments or 

 metameres, which are about eighty 

 in number altogether ; and each of 

 these bears laterally a pair of movable 

 muscular processes called the para- 

 podia, provided with bundles of 

 bristles or seta. The head (Fig. 103) 



consists of two parts, the prostomium (pr&st.) and the peris- 

 tomium (perist.\ The former bears on its dorsal surface 

 four large rounded eyes, in front a pair of short cylindrical 



FIG. IDG. Nereis dumerilii. 



Natural size. 

 (After Claparede.) 



