PHYLUM PLATYHELMINTHES 



287 



FIG. 236. Anterior portion of 

 the body of a Nemertine. 

 br. brain-lobes ; n. 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 caeca, 



which may be branched. The caeca are 



separated from one another by incomplete 



transverse septa of dorso-ventral muscular 



fibres the arrangement of the caeca 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 



Temnocephald). 

 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 (Kg. 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 



FIGS. 237 and 238. Proboscis of a Bleta- a arniTn nf rplls nnt in flip 

 nemertean, with stylet reserve-sacs arid ? r ? U P , O1 eu f> ^ 



muscular bulb. Fig. 237 retracted, Fig. 238 interior of a single name-cell as 



in most cases in 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 



