148 MANUAL OF ZOOLOGY SECT, v 



trunks a median dorsal and two lateral. The blood 

 follows no regular course though the vessels, but is moved 

 about by the muscular contractions of the body. 



The excretory vessels of the Platyhelminthes are repre- 

 sented by a pair of greatly coiled and branched tubes (Fig. 74, 

 neph.\ 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 de- 

 veloped than in the Turbellaria, The brain (Fig. 73, br., and 

 Fig. 74, 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. 73, ov.) and 

 testes are situated in the intervals between the intestinal 

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

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

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

 surface, where it communicates with the exterior. 



