CHAPTER VII 



THE NEMERTEA ROTIFERA AND 

 GASTROTRICHA 



PHYLUM NEMERTEA 



Elongated flattened unsegmented worms with a ciliated ectoderm and 

 an eversible proboscis lying in a sheath on the dorsal side of the 

 alimentary canal, with which it is not connected; no perivisceral 

 body cavity, the spaces between the organs being filled with paren- 

 chyma; aHmentary canal with mouth and anus; excretory system 

 with flame cells; a blood vascular system; gonads simple, repeated; 

 sexes separate; sometimes a larval form {Pilidium). 



The Nemertea in their general organization resemble the Platy- 

 helminthes very strongly. In certain positive features they have 

 advanced, e.g. in the development of a proboscis independent of the 

 gut, in the presence of a vascular system, and a second opening, the 

 anus, into the alimentary canal, but in the simplicity of the gonads 

 and absence of hermaphroditism the Nemertea are less specialized than 

 the Platyhelminthes. There can be no doubt, however, that the two 

 phyla are very closely connected, although the presence of an anus 

 and a vascular system is an enormous advance. 



The proboscis (Figs. 167, 168) is the most characteristic organ of 

 the nemerteans. It lies in a cavity (rhynchocoel), completely shut off 

 from the exterior, which has muscular walls (the proboscis sheath), 

 and is attached to the posterior end of the sheath by a retractor 

 muscle which is really the solid end of the piroboscis. The proboscis 

 may be compared with the finger of a glove with a string tied to 

 the inside of the tip ; when the proboscis is at rest the string, i.e. the 

 retractor muscle, keeps it turned inside out within the sheath ; when 

 the muscles of the proboscis sheath contract and press upon the fluid 

 in the rhynchocoel the proboscis is everted, but never completely, 

 because the retractor muscle keeps it from going beyond a certain 

 point. At this point, in the Metanemertini, is a diaphragm cutting 

 off the apical part of the proboscis cavity, and mounted on this is a 

 spike or stylet with reserve stylets in pouches at the side (Fig. 168 C). 

 This part of the cavity probably contains a poisonous fluid which is 

 ejected through a canal in the diaphragm into wounds caused by the 

 stylets. The proboscis in this class of nemerteans is thus a formidable 

 weapon. In other nemerteans, though the stylet is not developed, the 

 proboscis is prehensile and can be first coiled round its prey and then 



