148 MANUAL OF ZOOLOGY sect, v 



The alimentary canal (Fig. 79) is a simple tube distin- 

 guishable into /.esophagus with longitudinally folded walls, 

 and intestine with lateral caeca (div). It ends in an anal 

 opening (a) situated near the posterior extremity of the 

 body. 



The Nemerteans possess a system of blood-vessels with 

 well-defined walls formed of an epithelium and a layer of 

 muscle. There are three principal longitudinal trunks — a 

 median dorsal and two lateral. The blood follows no 

 regular course through the vessels, but is moved about by 

 the muscular contractions of the body. 



The excretory vessels of the Platyhelminthes are repre- 

 sented in the Nemertine worms by a pair of greatly coiled 

 and branched tubes (Fig. 80, nepli), opening on the exterior ; 

 the fine terminal branches of the system are provided with 

 ciliary flames, and cilia occur also in the course of the 

 vessels themselves. 



The nervous system is in some respects more highly 

 developed than in the Turbellaria. The brain (Fig. 80, 

 cer.g) is composed of two large ganglia with lobed surfaces, 

 connected together by two commissures, dorsal and ventral, 

 between which pass the proboscis and its sheath. From the 

 brain pass backwards a pair of thick nerves which run 

 throughout the length of the body. 



Eyes are present in the majority of Nemerteans, and in 

 the most highly organised species occur in considerable 

 numbers. 



Most species are dioecious. The ovaries (Fig. 79, ov) and 

 testes are situated in the intervals between the intestinal 

 cceca. The ovary or testis is a sac, the cells lining which 

 give rise to ova or spermatozoa ; when these are mature each 

 sac opens by means of a narrow duct leading to the dorsal 

 surface, where it opens by a pore. 



