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MANUAL OF ELEMENTARY ZOOLOGY 



end to the pharynx. It is contractile, and in it the 

 blood is driven forwards. This vessel supplies blood 

 to, and receives it from, the gut directly and the rest 

 of the body indirectly. It communicates by many small 

 vessels with the intestine and by two large vessels in the 

 tenth segment with the oesophagus, and ends in front by 

 breaking up into branches which supply the pharynx. In 

 each of the segments 7-1 1 it gives off a pair of large contrac- 

 tile vessels or pseudo-hearts. These encircle the oesophagus 

 and join a ventral or subintestinal vessel which hangs by a 



Ab:, 



ocs.'w h£. 



Fig. 143. — A diagram of the blood-vascular system of the earthworm. 



a/f.i.v., Afferent vessels of the intestine ; aff.n.v., afferent vessels of the nephridia; 

 b.w., body-wall ; d.b.v., dorsal blood vessel ; ht., pseudo-hearts ; int., intestine ; 

 oss., oesophagus ; ces.v., oesophageal vessel ; fiar.v., parietal vessel; J. i.v. t sub- 

 intestinal vessel ; s.n.v., subneural vessel ; v.n.c, ventral nerve cord. 



A simpler form of this diagram will be found on p. 566. 



mesentery below the gut. In the pseudo-hearts the blood 

 flows downwards from the dorsal to the ventral vessel, and in 

 the latter it flows backwards. From the ventral vessel the 

 blood passes along branch vessels partly to the nephridia 

 and partly to the skin. Purified by these organs, it is 

 returned along various paths to the dorsal vessel. Among 

 the subsidiary vessels are a subneural and two lateral neural 

 vessels, in which the blood flows backwards, and parietal 

 vessels, of which a pair in each segment of the intestinal 

 region of the body connect the subneural with the dorsal 

 vessel. The main blood vessels of the earthworm cannot 

 be distinguished into arteries and veins, but their ends are 

 joined by capillaries. 



