IV. NEMERTINI 



257 



■pr 



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 ceUs, while 

 they open separately to the exterior by one or 

 several openings. 



As a rule the nemertines are dioecious, the 

 gonads forming a row of lateral sacs, alternat- 

 ing with the intestinal Ijlind sacs and opening 

 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). 

 The margins of lappets and helmet are ciliated, 

 while at the top a bundle of longer cilia pro- 

 ject from a thickened patch of ectoderm, the 



•¥=#■■ 



s> 



,Jm 



Fig. 



Fig. 236. Fig. 237. 



236. — Amphiporus ptilcher fafter Burger), a, alimentar)' canal; b, brain; 



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

 11, lateral nerve cord; 0, ovary; p, poison sac; pn, protonephros ; pr, proboscis; ps, pro- 

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



Fig. 237. — Pilidium lari'a (from Lang, after Salensky.) es, invaginations which 

 later give rise to the nemertine skin; m, oral lobes; ind, archenteron; rn, ring nen'e; sp, 

 apical plate; si, oesophagus; vtk, ciliated band. 



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

 simple c£ecal 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 



