TV. NEilERriNI. 201 



vessels and connecting with branches terminating in flame cells, 

 while they open sej^arately to the exterior by one or several open- 

 ings. 



The central nervous S3'stem (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 nnmerous 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 simjjle 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 ])ilidium (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 cscal arch- 

 enteron, the mouth (blastopore) opening between the lappets. 

 By a complicated process of growth and infolding this mesenteron 

 becomes enclosed in its own skin, produced from four injoushings 

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

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

 rpiickly dies. 



Order I. Protonemertini. 



Nervous system outside the muscles; no stylets in the jiroboscis; moutli 

 buhiud brain. Carinella* 



Order II. Mesonemertini. 



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

 CephalijtJirix.'* 



Order III. Metanemertini. 



Nervous system in the parenchyma inside tlie muscles, mouth in 

 front of brain; proboscis as a rule with stylets. Omneitiertes * axuX s,o\\k 

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

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

 in lamellibranehs. 



