270 



ZOOLOGY 



SECT. 



transition to a layer consisting altogether of lo-/t</i/tx/inal 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 Avell-de- 

 fined walls formed of an epithelium and 

 a layer of muscle. There are three prin- 

 cipal longitudinal trunks a median dorsal 

 (med. l>l. v.) and two lateral (lat. lil. v.). The 

 blood follows no regular course through 

 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 



n 



FIG. 21T,. Anterior portion of 

 the body of .1 Xemertine. 

 &/. brain-lobes ; n. lateral 

 nerves ; p. o. external open- 

 ing through which the pro- 

 boscis is everted ; p. .. pro- 

 boscis sheath ; pr. proboscis. 

 (Esophagus and month shown 

 by dotted lines. (After 

 Hubrecht.) 



Fn;s. 217 and 21S. I'robuscis of a Hoplonemertean, with stylet reserve-sues and 

 bulb. Fig. 217 retracted, Fig. 21s everted. (After Hubruuht.) 



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 Irain (Fig. 216, l>r.) is composed of 

 two large ganglia with lobed surfaces, connected together by two 



