Introduction to A nimal Morphology. 1 5 7 



in place by thread-like, muscular mesenteries) ; but in 

 most, the digestive cavity is an excavation in the 

 body parenchyma, lined by epithelium, and without 

 or with (Geoplana), a muscular coat for its chief 

 branches. The whole digestive tract is ciliated in 

 Nemerteans. 



The simplest forms have no circulatory system. 

 Nemerteans possess two lateral, pulsating vessels, 

 which dilate as they pass backwards, and Fig. 21. 

 unite in front. Each of these sends a 



: L ^V^ i <7 



branch round the front of the nerve gan- = 

 glion, which unites with its fellow to form a 

 third medio-dorsal tube. These may give 

 off no branches (Tetrastemma), or may be 

 joined by transverse canals (indicating a 

 hidden metameric structure ?). The blood 

 is white or reddish. There are no respira- 

 tory organs.* The water-vascular system 

 :s as two long, ciliated, often branched, 

 separate or connected lateral tubes, reticu- 

 lated w r ithin, opening by a posterior termi- 

 nal, or by a lateral pore, rarely into the 

 mouth (Enterostomum Fingalianum, and ttf Uoove- 

 Mesostomum). %*%&S? ; 



There are two, often reddish, bi-lobed nerve 



i 1 i r i i l>h;itfi; 



tfanc(lia,one at each side of the pharynx, rarely tine; , 

 far apart (Valencinia) ; united by a commissure above 

 that tube. From this two lateral m-rvrs pass backward 

 : inDendroccela, stronger in Rliabdoccela), extend- 

 ing the whole length of the body in X<-mTtinea, and 

 in the larva of Prosorhochmus, each ending In-hind in a 



M arches beside the pharynx form in I. incus a rctc mirabile, from 

 which it is only a step to the br..: us. 



