GENERAL ACCOUNT OF NEMERTEA. 



185 



GENERAL ACCOUNT OF NEMERTEA. 



In appearance most Nemertines are ribbon- 

 or thread-like, and the cross-section is gener- 

 ally a flattened cylinder. They show a greater 

 diversity of size than any other "worms," 

 from a Linens, 12 or more feet in length (25 

 metres has been recorded for an extended 

 Linens longissinnis] to the pelagic Pelagone- 

 inertes, which is under an inch. The colours 

 are often bright, and tend to resemble those 

 of the surroundings. The ectoderm is covered 

 with numerous short cilia, and many of its cells 

 are also glandular, secreting the mucus which 



FIG. 89. Diagrammatic longitudinal section of 

 a Xemertean (Amphiporus lactijloreus), dorsal 

 view. After M'Intosh. 



pp., Proboscis pore; />., brain giving off the lateral 

 nerve-cords (.)' /?> oesophageal pocket; p., pro- 

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

 ;;/., retractor muscles of proboscis ; g., gut shown in 

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

 main longitudinal blood vessels which unite both 

 anteriorly and posteriorly. 



The Nemertines are worm-like animals, un segmented and 

 generally elongate in form ; thev are almost all marine, and 

 most, if not all, are carnivorous. 



The ectoderm is ciliated. There is a remarkable retractile 

 proboscis, unconnected with the alimen- 

 tary canal, and forming a tactile organ 

 or a weapon. The nervous system con- 

 sists of a brain, a commissure round the 

 proboscis and two lateral nerve-cords ; 

 in connection with the brain there is a 

 pair of ciliated pits. The gut terminates 

 in a posterior anus, and is furnished with 

 lateral pockets. There is no body cavity 

 in the adult, but the closed vascular 

 system is probably of ccelomic origin. 

 The excretory system is apparently of 

 the Platyhelminth type. The sexes are 

 usually separate and the organs simple. 

 The development is in some cases direct, 

 while in others there is a peculiar pelagic 

 larva. 



-po 



