RIBBON-WORMS 



231 



—po 



the space between gut and body wall being filled with a packing tissue ; 

 the absence of an anus is also important, the two characters taken 

 together being held to indicate affinity with the Ctenophora. 



Class Nemertinea. Phylum Nemertea. 



The ribbon-worms or Nemertincs are interesting in many 

 ways, e.g. in being the simplest animals ^^^^^^^—pp 



to have an open gut, a closed blood- 

 system, and, occasionally, haemoglobin ; 

 in having some very peculiar structures, 

 notably a protrusible proboscis and 

 ciliated head slits ; in being in many 

 cases extraordinarily extensile and liable 

 to break into pieces. 



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 with the alimentary canal, and 

 forming a tactile organ or a weapon. The 

 nervous system consists of a brain, a com- 

 missure 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 

 coelomic origin. The excretory system is 

 apparently of the Platyhelminth type. 

 The sexes are usually separate and the 



si 





-^ 



Fig. 126. — Diagrammatic longitudinal section 

 of a Nemertine (Amphiporus ladifloreiis), 

 dorsal view. — After M'Intosh. 



p.p., Proboscis pore ; b., brain giving off the latera^ 

 nerve-cords (n.) ; po., oesophageal pocket ; p., pro- 

 boscis Iving within its sheath ; St., stilet of proboscis ; 

 m., 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. 



