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 Sterna spis a ventral shield formed by a thickening of the 

 cuticle in the posterior region of the body bears a number of seta- 

 round its edge. 



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

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



dors, ci 



dcrs. cirr 



FIG. 340. Polynoe extenuata. Dorsal view of anterior extremity' <?<//>-. cirr. ilcisa!>-hri : 

 i/, olytra ; ^<.riW. tint, peristcmial tentacles ; ;</-.^. prtstimiv.m. (After Clapartde.) 



number of short setas each lodged in its sac; and ciiri aiv not 

 developed. 



The first segment or prostomium, together with the second or 

 peristomium, forms in many Polychseta a very distinct head: the 

 prostornium in such a case bears eyes and tentacles and contains 

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

 mouth, and also certain tentacles, the peristcmial tentacles. A 

 \cntral pair of prustomial tentacles, somewhat thicker than the 

 rest, are sometimes to be distinguished, and are termed \he palpi. 

 "Neither prostomium nor peristomium bears parapoclia, though an 



