270 



ZOOLOGY 



SECT.- 



transition to a layer consisting altogether of longitudinal muscular- 

 fibres. A layer of circularly arranged muscular fibres always is 

 present ; but the arrangement of the 

 layers of muscle varies in different forms. 

 The muscular layers are embedded in 

 compact connective-tissue, and a mass of" 

 the same tissue fills all the space between 

 the body- wall and the enteric cavity, 

 there being no body-cavity. Vertical 

 muscular dissepiments extend across be- 

 tween the intestinal coeca and produce an 

 appearance of internal segmentation. 



The Nemerteans possess a system of 

 blood-vessels (Fig. 220) with" well-de- 

 fined walls formed of an epithelium and 

 a layer of muscle. There are three prin- 

 cipal longitudinal trunks a median dorsal 

 (mcd. U. v.) and two lateral (lat. U. v.). The 



FIG. 216. Anterior portion of 

 the body of a Nemertine. 

 br. brain-lobes; n. lateral blood follows no regular COUrS6 through 



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.) 



the vessels, but is moved about by the 

 muscular contractions of the body. The 

 blood is colourless, and contains rounded 

 or elliptical corpuscles. 



The excretory vessels of the Platy- 



helminthes are represented by a pair of greatly coiled and 

 branched tubes (Fig. 220, neph.), opening on the exterior : the- 

 fine terminal branches of the system are provided with ciliary 



FIGS. 217 and 218. Proboscis of a Hoplonemertean, with stylet reserve-pr.cs and muscular 

 bulb. Fig. 217 retracted, Fig. 218 everted. (After Hubrecht.) 



flames, and cilia occur also in the course of the vessels them- 

 selves. 



The nervous system is in some respects more highly developed 

 than in the Turlellaria. The brain (Fig. 216, br.) is composed of" 

 two large ganglia with lobed surfaces, connected together by two* 



