ANELLATA. 



139 



73 



^My 



U 



the vaseulai' system, this is chiefly remarkable for the size and com- 

 plexity of the branchial plexuses. 



In the Arenicola {fig- 73.) there is, on each side the base of the 

 oesophagus, an ovoid contractile sac (/), which 

 sends off a large and short vascular trunk down- 

 wards and backwards to the medio-ventral line, 

 where, uniting with its fellow trunk, a ventral 

 vessel (e), analogous to that in the Eunice and 

 Terebella, is formed. This median vessel fur- 

 nishes a pair of transverse branches to each 

 ring, which at the seventh segment begin to 

 penetrate the ramified branchia, attached to 

 the sides of that and succeeding middle seg- 

 ments of the body. The jiulsations of the 

 two oesophageal sinuses, or ventricles, propel 

 the blood into the ventral vessel from before 

 backward through these vessels (m, w) to the 

 gills, where it receives a new impulse by the 

 contractions of these organs, and, after having 

 been oxygenised, it is returned, partly by cu- 

 taneous vessels, which form many anastomoses, 

 and chiefly by a direct and continuous lateral 

 vessel {k, k} to the medio-dorsal intestinal 

 artery (ff). This artery extends from one end 

 of the body to the other. At its middle part 

 it receives many transverse branches from the 

 digestive tube, and through them anastomoses 

 with the inferior intestinal vein (/*). The vas- 

 cular network thus formed around the in- 

 testine, gives origin anteriorly to two lateral 

 veins (i), which terminate in the dorsal vessel 

 immediately behind the oesophageal ventricles. 

 The blood from the inferior intestinal vein is 

 conveyed to the same point or sinus. After 



Arenicoia. •' '■ 



the communication of this common sinus with 

 the two hearts, a slender median vessel (^'), a continuation of the 

 dorsal one, extends forwards towards the head, and terminates 

 by forming two vascular rings around the base of the proboscis, 

 from tiie lower part of which the ventral vessel arises, whicli 

 vessel (e), passing backwards, receives the great accession of blood 

 from the two contractile hearts, and thus the circulation is com- 

 pleted. 



