432 



ZOOLOGY 



SECT, 



flattened scales, the elytra (el.), richly supplied with nerves : 

 these are sometimes looked upon as modified dorsal cirri ; but in 

 some members of the group cirri and elytra occur together on 

 the same segment. 



In Sternasrjis a ventral shield formed by a thickening of the 

 cuticle in the posterior region of the body bears a number of setae 

 round its edge. 



In the Oligochaeta (Fig. 341) the parapodia are absent as pro- 

 cesses of the body-wall, and are merely represented by a small 



rlst len 



dors.ci 



FIG. 340. Polynoe extenuata. Dorsal view of anterior extremity 1 dors. cirr. dois-al 



t/, elytra ; ptrift. tent, peristcmial tentacles ; prast. prostcmii;m. (After Clapartde.) 



number of short setae each lodged in its sac ; and cirri are not 

 developed. 



The first segment or prostcmium, together -with the second 

 peristcmium, forms in many Polychseta a very distinct head ; the 

 prostomium in such a case bears eyes and tentacles and contaii 

 the cerebral ganglion ; on the peristomium is the opening of th< 

 mouth, and also certain tentacles, the peristcmial tentacles. 

 ventral pair of prostomial tentacles, somewhat thicker than th( 

 rest, are sometimes to be distinguished, and are termed the palpi. 

 Neither prostomium nor peristomium bears parapodia, though an 



