204 SEGMENTED WORMS OR ANNELIDA. 



partly muscular, partitions or mesenteries which moor the 

 gullet ; in the tail region there are many such ; the median 

 part of the gut swings freely. Posteriorly there are also 

 oblique partitions which divide the segments into a median 

 and two lateral chambers. 



The Vascular System. 



The blood has a bright red colour. It flows forward in a 

 dorsal vessel, running along the mid-dorsal line of the 



b s 



,/> 



FIG. 66. Dissection of anterior region of Arenicola. 

 (After COSMOVICI.) 



M.2., m.3. Second and third mesenteric septa ; #., ventral nerve- 

 cord ; b.s., bristle sac ; z/.z/., ventral vessel ; n.6., sixth nephridium ; 

 z., intestine; l.v., lateral vessel; d.v.^ dorsal vessel; ^., heart; 

 /., one of the two larger oesophageal glands ; a*., oesophagus. 



gut, backward in a ventral vessel below the gut. Two 

 sub-intestinal vessels lie between the ventral vessel and the 

 gut, and receive tributaries from the anterior gills. (Some 

 deny that the sub-intestinal vessel is double.) On each side 

 of the digestive part of the gut there is a lateral vessel. 



Just behind the posterior pair of cesophageal glands lies a 

 very contractile heart. It consists of two lateral chambers 



