IV. NEMERTINI 



257 



widely distributed; statocysts are very rare. The excretory system 

 consists of two tubes lying beside the lateral blood-vessels and connecting 



with branches terminating in flame cells, while 

 they open separately to the exterior by one or 

 several openings. 



4 - l> As a rule the nemertines are dioecious, the 



gonads forming a row of lateral sacs, alternat- 

 ing with the intestinal blind sacs and opening 

 p n dorsally. The development is sometimes 

 direct, but usually a metamorphosis occurs in 

 which a larva, the pilidium (or a reduced 

 / form of it, Desor's larva), appears. The 

 . pilidium is a helmet-shaped larva with, right 

 and left below, a pair of lappets (fig. 237). 



P s The margins of lappets and helmet are ciliated, 



while at the top a bundle of longer cilia pro- 



- d ject from a thickened patch of ectoderm, the 





cs 



--- d 



ffi/C 



m 



FIG. 236. FIG. 237. 



FIG. 236. Amphiporus pidcher (after Burger), a, alimentary canal; h, brain; 

 c, ciliated groove; d, dorsal blood-vessel; /, lateral blood-vessel; m, retractor of proboscis; 

 n, lateral nerve cord; o, ovary; p, poison sac; pn, protonephros; pr, proboscis; /.?, pro- 

 boscis sheath; r, rectum; 5, stylet of proboscis. 



FIG. 237. Pilidium larva (from Lang, after Salensky.) es, invaginations which 

 later give rise to the nemerrine skin; m, oral lobes; md, archenteron; rn, ring nerve; sp, 

 apical plate; st, oesophagus; ink, ciliated band. 



apical plate, which functions as a central nervous organ. Inside is the 

 simple oecal archenteron, the mouth (blastopore) opening between 

 the lappets. By a complicated process of growth and infolding this 

 mesenteron becomes enclosed in a separate skin, produced from four 

 inpushings (es) ; an anus is formed, and at the time of metamorphosis 



17 



