Introdtidion to Animal Morphology, 



157 



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 *" 

 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 

 exists as two long, ciliated, often branched, 

 separate or connected lateral tubes, reticu- 

 lated within, opening by a posterior termi- 

 nal, or by a lateral pore, rarely into the fluvSmIT''»* 

 mouth (Enterostomum Fingalianum, and HaTd" groove'"; 



1\ T i. \ ■fj spine; c, sac 



Mesostomum J . of proboscis ; 



There are two, often reddish, bi-lobed nerve fi'andf;i!ceVo- 



,. -.-„-, , phagus;z',intes- 



ganglia,one at each side or the pharynx, rarely tine ; ov, ovary. 

 far apart (Valencinia) ; united by a commissure above 

 that tube. From this two lateral nerves pass backward 

 (weak in Dendrocoela, stronger in Rhabdocoela), extend- 

 ing the whole length of the body in Nemertinea, and 

 in the larva of Prosorhochmus, each ending behind in a 



* The arches beside the pharynx form in Linens a rete mirabile, from 

 which it is only a step to the branchial organ of Balanoglossus. 



