PHYLUM PLATYHELMINTHES 



287 



FIG. 236. Anterior portion of 

 the body of a Nemertine. 

 br. brain-lobes : . lateral 

 nerves ; p. o. external open- 

 ing through which the pro- 

 boscis Is everted ; p. s. pro- 

 boscis-sheath ; pr. proboscis. 

 (Esophagus and mouth shown 

 by dotted lines. (After 

 Hubrecht.) 



various regions, sometimes with paired diverticula. Posteriorly it- 

 opens into the intestine. The intestine, constituting by far the 

 greater part of the length of the canal, may be a simple uncon- 

 stricted tube, or may be only slightly con- 

 stricted at intervals by the paired gonads. 

 In most cases the constrictions correspond- 

 ing to the gonads are very deep, so that 

 the intestine comes to be provided with 

 two rows of lateral diverticula or cseca, 

 which may be branched. The caeca are 

 separated from one another by incomplete 

 transverse septa of dorso-ventral muscular 

 fibres the arrangement of the cseca and 

 septa with the alternately arranged gonads 

 bringing about an appearance of imperfect 

 metamerism such as is observable in some 

 of the Platyhelminthes (Gunda, species of 

 Temnocephala). 



The Nemerteans possess a system of 

 vessels usually regarded as representing a 

 blood-vascular system (Figs. 235 and 

 240), 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 

 (dors. ves. d.b., d.v.) and two lateral (lat. ves. l.v., l.b). The blood is, 

 in most cases, colourless, and contains rounded or elliptical, usually 

 colourless, corpuscles. 



The excretory system has a considerable resemblance to that 



of the Platyhelminthes. It 

 consists of a pair of longi- 

 tudinal vessels (Fig. 235, neph.) 

 which give off branches, by 

 one or several of which each 

 communicates with the ex- 

 terior. In one species of 

 Eupolia there are also ducts 

 opening into the alimentary 

 canal. The fine terminal 

 branches of the system are 

 provided with ciliary flames, 

 each situated in the midst of 

 t 2:58 .-- pr , I i )0 f SCis of a Meta i a group of cells, not in the 



nemertean, with stylet reserve-sacs and .'.-., -in n 



muscular bulb. Fig. 237 retracted, Fi.'. 2:;s interior OI a Single name-Cell RS 

 everted. (After Hubrecht.) ri -m 



in most cases m the Flat- worms. 



There are no special organs of respiration in any of the group. 

 But there is evidence that this function is carried out, in part at 



