IV. NEMERTINL 291 



vessels and connecting with branches terminating in flame cells, 

 while they open separately to the exterior by one or several open- 

 ings. 



The central nervous system (in some forms still in the ectoderm) 

 consists of a supracesophageal brain of a paired ganglia, from which 

 nerves run to the proboscis and two lateral cords united on the 

 ventral side by numerous transverse commissures. Connected 

 with the brain, either directly or by means of a short nerve, are the 

 cerebral organs or ciliated grooves, pits placed on the sides of the 

 head. These, formerly regarded as respiratory, are now considered 

 sense organs. Tactile organs and simple eyes are widely distrib- 

 uted ; otocysts are very rare. 



As a rule the nemertines are dioecious, the gonads forming a row 

 of lateral sacs, alternating with the intestinal blind sacs and open- 

 ing 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 

 gelatinous helmet-shaped larva with right and left below a pair of 

 lappets (fig. 256). The margins of lappets and helmet are ciliated, 

 while at the top a bundle of longer cilia project from a thick- 

 ened patch of ectoderm, the apical plate, which apparently func- 

 tions as a central nervous organ. Inside is the simple caecal arc li- 

 en teron, the mouth ( blast opore) opening between the lappets. 

 By a complicated process of growth and infolding this mesenteron 

 becomes enclosed in its own skin, produced from four inpushings 

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

 worm thus produced escapes from the rest of the pilidium, which 

 quickly dies. 



Order I. Protonemertini. 



Nervous system outside the muscles; no stylets in the proboscis; mouth 

 behind brain. Carinella* 



Order II. Mesonemertini. 



Nervous system in the muscles; mouth behind brain; no stylets. 

 Cephalothrix.* 



Order III. Metanemertini. 



Nervous system in the parenchyma inside the muscles, mouth in 

 front of brain; proboscis as a rule with stylets. Geonemertes* and some 

 species of Tetrastemma* terrestrial. Amphiporus* (numerous eyes), 

 Nectonemertes* Malacobdella,* leech-like with posterior sucker, parasitic 

 in lamellibranchs. 



