PHYLUM PLATYHELMINTHES 



291 



(Metanemertiiii) project for- 

 wards below the cesophagus 

 as a ventral ccccum, which 

 may give off paired lateral 

 diverticula. The intestine, 

 constituting by far the 

 greater part of the length of 

 the canal, may be a simjjle 

 unconstricted tube, or may 

 be only slightly constricted 

 at intervals by the paired 

 gonads. In most cases the 

 constrictions corresponding 

 to the gonads are very deep, 

 so that the intestine comes 

 to be provided with two rows 

 of lateral diverticula or caeca, 

 which may be branched. The 

 ca3ca are separated from one 

 another by incomplete trans- 

 verse septa of dorso-ventral 

 muscular fibres — the ar- 

 rangement of the cseca and 

 septa with the alternately 

 arranged gonads bringing 

 about an appearance of im- 

 perfect metamerism such as 

 is observable in some of 

 the Platyhelminthes {Gunda, 

 species of TewMOcephala). 



The Nemerteans possess 

 a system of vessels usually 

 regarded as representing a 

 blood - vascular system 

 (Figs. 230 and 235), with 

 well-defined walls consisting 

 of a layer of epithelium 

 surrounded by a thin layer 

 of muscular fibres arranged 

 circularly. There are three 

 principal longitudinal trunks 

 — a median dorsal {dois. 

 ves.) and two lateral (lat. 

 ves.). The blood is, in 

 most cases, colourless, and 

 contains rounded or ellipti- 

 cal, usually colourless, cor- 

 puscles. 



/>roi ap 



lat. ne 

 lat-ves 



dors.ves 



op.neph 



retr.mvs 



an 



Fic. 230.— Tetrastemma. General view of the 

 internal organs, an. anus ; «c. st. aceessoiy 

 stylet ; cer. g. brain ; cU. <ir. ciliated groove of 

 cerebral organ ; dors. ves. dorsal vessel ; lat. ne. 

 lateral nerve; lat. ves. latei*al vessel; neph. 

 nephridiuin ; op. nepli. nephridial aperture ; 

 jn-ob^. eversible part of proboscis ; prob^. non- 

 eversiblc part of proboscis ; prob. ap. aperture 

 for the protrusion of the proboscis ; rett: mus. 

 retractor muscle of the proboscis ; st. stylet. 

 (From Hatschek's Lehrbv.ch.) 



u 2 



