TUBIFEX. 



331 



mental network in this worm, such as is described by 

 Beddard as occurring in Ilyodrilus and Bothrioneuron. 

 In the anterior segments of the body the perivisceral 

 trunks lie freely in the body cavity. More posteriorly, 

 particularly in that part of the body which is waving 

 about in the water, these vessels, while not branching, 

 are always pressed closely against the body wall for a 

 good part of their length, and remain in this position 



Text-Fig. 2. Diagrammatic tranverse section through segment IX to 

 show the relative positions of the principal blood vessels, hyp. epidermis, 

 l.m. longitudinal muscles, ex. chloragogen cells, in. intestine, si.v. supra- 

 intestinal vessel, d.v. dorsal vessel, v. v. ventral vessel, n. nerve cord. 



permanently. This would suggest that aeration of the 

 blood takes place through the body wall, which is very thin 

 near the posterior end of the body. At its anterior end 

 the dorsal vessel divides into two branches, which become 

 slightly coiled and then dip down ventrally to become 

 continuous with the anterior ends of the ventral vessels. 

 In segment 2 the perivisceral vessels arc given oft' from 

 the dorsal vessel Dear ilir posterior border of the segment. 



