238 



LECTURE XI. 





A 



In the Arenicola {Jig. 106.) there is, on each side the base of the 



oesophagus, an ovoid contractile sac (y), which 

 sends off a large and short vascular trunk 

 106 downwards and backwards to the medio-ven- 

 tral line, where, uniting with its fellow trunk, 

 a ventral vessel (e), analogous to that in the 

 Eunice and Terebella, is formed. Tliis me- 

 dian vessel furnishes a pair of transverse 

 branches to eacli ring, which at tlie seventh 

 segment begin to penetrate the ramified bran- 

 chia, attached to the sides of that and suc- 

 ceeding middle segments of the body. The 

 pulsations of the two cesophageal sinuses, or 

 ventricles, propel the blood into the ventral 

 vessel from before backward through these 

 vessels (m, m) to the gills, where it receives 

 a new impulse by the contractions of these 

 organs, and, after having been oxygenised, it 

 I i is returned, partly by cutaneous vessels, which 



form many anastomoses, and chiefly by a di- 

 rect and continuous lateral vessel (k, k), to 

 the medio-dorsal intestinal artery (g). 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 (h). The vascular 

 network thus formed around the intestine, 

 gives origin anteriorly to two lateral veins 

 (^), which terminate in the dorsal vessel im- 

 mediately behind the oesophageal ventricles. 

 The blood from the inferior intestinal vein is 

 conveyed to the same point or sinus. After 

 the communication of tliis common sinus 

 Avith the two hearts, a slender median vessel 

 (g'), 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 the lower part of which the ventral vessel 

 arises, which vessel (e), passing backwards, receives tlie great accession 

 of blood from the two contractile hearts, and thus the circulation is 

 completed. 



This has much analogy with tlie circulation of the blood in the 

 earth-worm, in which the blood travels from behind forwards in the 

 dorsal vessel, and descends in great part towards the ventral vascular 



Arenicola. 



