NEMERTEA. 



169 



Class Nemertea. Nemertines. 



The Nemertines are worm-like animals, 

 unsegmented and generally elongate in 

 form ; they are almost all marine, and 

 most, if not all, are carnivorous. 



The ectoderm is ciliated. There is a 

 remarkable retractile proboscis, uncon- 

 nected ivith the alimentary canal, and 

 forming a tactile organ or a weapon. 

 The nervous system consists of a brain, 



Fig. 75. —Diagrammatic longitudinal section of 

 a Nemertean (Amphiporus lactifioreus), dorsal 

 view. — After M'Intosh. 



pp., Proboscis pore; b., brain giving off the lateral 

 nerve-cords («.); po., oesophageal pocket; /., pro- 

 boscis lying within its sheath ; st., stilet of proboscis ; 

 jn. t retractor muscles of proboscis ; ^., gut shown in 

 outline at the sides of the proboscis; e., the three 

 main longitudinal blood vessels which unite both 

 anteriorly and posteriorly. 



ation associated with increasing parasitism, and also because of the 

 relatively great simplicity. The three classes are undoubtedly nearly 

 related, for forms like Temnocephala connect Turbellaria and Trematoda, 

 and the "monozoic" Cestodes like Archigetes, Amphilina, and Caryo- 

 phyllaus, connect Trematoda and Cestoda. 



Among the most striking of the Platyhel- 

 minth characters are the nature of the excretory 

 and reproductive organs and the condition of 

 the mesoblast. The excretory system, with its 

 longitudinal trunks, its ramifying canals, and 

 "flame cells," is eminently characteristic, 

 though it occurs in more or less modified condi- 

 tion in higher forms. The reproductive organs 

 are complex, show division of labour, and are 

 furnished with ducts of their own, unconnected 

 with the excretory system — a condition of affairs 

 not common elsewhere. The presence of shells 

 around the eggs is another point of interest. It 

 becomes of great importance to the parasitic 

 flukes and tape-worms, but occurs also in the 

 free-living Turbellaria. There is no true body 

 cavity, the space between gut and body-wall 

 being filled with a packing tissue ; the absence 

 of an anus is also important in this connection, 

 the two characters taken together being held 

 to indicate affinity with the Ctenophoia. 



