460 



ZOOLOGY 



SECT. 



cirri are usually filamentous, sometimes jointed ; sometimes they 

 are laterally compressed and leaf -like. In Polynoe (Figs. 371 and 

 376) and its allies certain of the parapodia bear, instead of dorsal 

 cirri, 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 



FIG. 377. Heads of various Polychaeta (diagrammatic). A, Polynoid ; B, Syllid ; C, 



D, Eunice ; E, Phylloilocc ; F, Trophonia. a, prostomium ; c, cirri of body-segments ; c 1 , 

 peristomial cirri (tentacles) : c 2 , cirrus of first body-segment ; c 3 , cirrus of second body- 

 segment ; el', point of attachment of elytron ; p, palp ; s, nuchal organ ; t, tentacle ; /, peri- 

 stomium : //, ///, IV, segments. (From the Cambridge Natural History.) 



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

 round its edge. 



In the Oligochseta (Fig. 378) the parapodia are absent as pro- 

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

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

 In certain Oligochaeta setae are absent. 



The first segment or prostomium, together with the second or 

 peristomium, forms in many Polychaeta a very distinct head ; the 

 prostomium in such a case bears eyes and tentacles and contains 

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



