310 



THE EARTHWORM AND OTHER ANNULATA 



The members of the Class Chcelopoda are the most important 

 of the Annulata for our present purposes, since they exhibit most 

 clearly the segmental or metameric organization which seems to 

 be the primitive condition of the phylum. This class is divided 

 into two sub-classes: the Oligochceta, which includes the earth- 

 worms and closely related annulates, mostly fresh-water species; 

 and the Polychceta, which are for the most part marine. These 



an 



Fig. 150. — Other Annulata. 



A, Polygordius, one of the Archiannelida, from dorsal view. B, Phascolosoma, one of 

 the Sipunculoidea, from lateral view. C, Placohdella, one of the Hirudinea, from ventral 

 view showing sucker at posterior end. 



an, anus; h, head; wi, mouth; n, nephridiopore; p.s., posterior sucker; t, tentacles. 



two sub-classes of the Chaetopoda, which means " bristle-footed," 

 are distinguished structurally by the fact that the Oligochseta 

 {oligos, few; and chceta, bristles) have but few setae, or cheetse, 

 on a segment; while the Polychseta (polys, many) have a larger 

 number. The four pairs of setae on each segment of the earth- 

 worm, in contrast with the many in Nereis (Fig. 152), illustrate 

 this difference. The further differences in the structure and 

 activities of these two subdivisions of the Chaetopoda are indi- 

 cated in the descriptions which follow. 



