28 COMPARATIVE ANATOMY 



body-wall and outer integument, the other to the nephridium. 

 In the integument the blood, being brought in close con- 

 tact with the air, is oxygenated, and is returned by two sets 

 of vessels to the main trunks. One set leads straight to 

 the sub-neural trunk, in which the blood is carried back- 

 wards to the posterior part of the body. The other _ set 

 carry the blood to a special system known as the intestino- 

 tegumentary system. The intestino-tegumentary vessels arise 

 one from each side of the supra-intestinal trunk in the tenth 

 somite, and run forwards on the sides of the oesophagus, even- 

 tually breaking up into branches on the walls of the pharynx. 

 In each of the somites 10—6 they give off on either side a 

 vessel carrying blood to the walls of the CBSophagus, and they 

 receive trunks carrying back blood from the integument and 

 nephridium respectively (fig. 6, A and £). 



Posteriorly to the eighteenth somite — that is to say, 

 throughout the region of the intestine — the arrangement of the 

 blood-vessels is different. The main trunks make the follow- 

 ing communications, and give off the following branches in 

 each somite : — The sub-intestinal trunk gives off a vessel on 

 either side, which divides into two branches, one passing to 

 the nephridium, the other to the body-wall and integument, 

 and supplying these regions with blood. The sub-neural is 

 connected with the supra-intestinal trunk by a pair of commis- 

 sural vessels running in the hind wall of the septum. Each 

 commissural vessel is joined by branches bringing back blood 

 from the body-wall and integument, and by an efferent 

 nephridial vessel bringing back blood from the nephridium. 

 There is, in addition, an intestinal system consisting of two 

 pairs of efferent intestinal vessels carrying blood from the supra- 

 intestinal trunk to the walls of the intestine in every segment. 

 The blood, after passing through capillaries in the walls of the 

 intestine, is collected by two pairs of afferent intestinal vessels 

 and carried back to a typUosolar vessel running longitudinally 

 in the fold of the typhlosole, and from this it is conducted 

 back to the supra-intestinal trunk by three or four short vertical 

 vessels in each somite. 



The circulation of the blood in the earthworm is thus a very 

 complicated affair, and much uncertainty still prevails on the 

 subject. The uncertainty is increased by the fact that the 

 arrangement of the blood-vessels, and consequently the course 



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