IV 



POLYCHAETA 



145 



At the base of each of these lobes there projects from the surface of the 

 body a tentacle-like projection or cirrus — known respectively as the 

 notopodial cirrus (d.c) and the neuropodial cirrus (v.c). Further each 

 lobe has embedded in it a bundle of numerous chaetae, larger and 

 projecting further beyond the surface than those of Lumbricus. One 

 of the chaetae near the centre of each bunch is thicker and stouter 

 than the rest. It is known as the aciculum (Fig. 69, ac) and its main 

 function appears to be to act as a support to the parapodium. 



At the front end of the Nereis there is a more pronounced development 

 of head than is the case in Lumbricus. The prestomium is larger 

 and more highly developed (Fig. 68, B, p). A pair of prestomial 

 tentacles {t) project from it in front while embedded in its dorsal wall 



Isolated somite of JVereii. a.c, Alimentary canal; ac, acicula; c/(, chaetae; £^.c, notopodial cirrus ; 

 m, longitudinal muscle ; «, nerve-cord ; s, coelomic septum ; v.c, neuropodial cirrus. 



are two pairs of eyes («). On each side there projects a palp (p), a 

 tentacular structure with a large base into which the terminal portion 

 can be retracted telescope-fashion. Immediately behind the prestomium 

 is the peristomium (Fig. 68, A, pe), representing two somites fused 

 together, in correlation with which it bears four cirri on each side instead 

 of only two ; it has no projecting parapodia. 



There are various features of interest to be noted in connexion with 

 the reproductive phenomena of the Polychaetes. In Nereis itself the 

 adult worm in some species assumes when sexually mature a 

 peculiar change of form which before the life-history was understood 

 caused it to be regarded as a separate genus to which the name Hetero- 

 nereis was given. In the heteronereid condition the hinder part of the 

 body, in which alone are gametes developed, becomes modified, its 

 parapodia becoming enlarged and flattened, while chaetae of a curious 



