NEMERTEA. 



197 



Class NEMERTEA. Nemertines 



The ribbon-worms or Nemertines are interesting in many 

 ways, e.g. in being the simplest animals to have an open 

 gut, a closed blood-system, and, occa- 

 sionally, haemoglobin ; in having some 

 very peculiar structures, notably a pro- 

 trusible proboscis and ciliated head 

 slits; in being in many cases extra- 

 ordinarily extensile and liable to break 

 into pieces. 



The Nemertines are 'worm-like ani- 

 mals, 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 with the alimentary canal, and 

 forming a tactile organ or a weapon. 

 The nervous system consists 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 ex- 

 cretory system is apparently of the 

 Platyhelminth type. The sexes are usu- 

 ally separate and the organs simple. The 

 development is in some cases direct, while 

 in others there is a peculiar pelagic larva. 



Fig. 103. Diagrammatic longitudinal section 

 of a Nemertean (Amphiporus lactifloreus), 

 dorsal view. After M'Intosh. 



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

 nerve-cords (n.)\ /<?., oesophageal pocket; />., pro- 

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

 m., 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. 



