TUBIFEX. 329 



commences in segment 1, where it is paired, its two parts 

 uniting with the two branches into which the dorsal 

 vessel divides, also in segment 1. These two parts of the 

 ventral vessel pass backwards as two converging trunks 

 as far as segment 3, where they unite. It is attached to 

 the intestine throughout the greater part of its length, 

 but occasionally appears to lie freely in the body cavity. 

 It should be noticed that the nephridia of the posterior 

 segments of the body are situated close to the ventral 

 vessel with the walls of the nephridial tubes closely pressed 

 against it. In the last segment of the body the dorsal and 

 ventral vessels unite and are slightly coiled. 



These longitudinal trunks are connected with one 

 another by a series of commissural vessels. These are of 

 two kinds : (a) intestinal, (b) perivisceral or coelomic. 



(a) Intestinal Vessels. The principal intestinal vessels, 

 of which there is a pair to each segment, connect the 

 supra-intestinal with the ventral vessel and are well 

 developed in all segments behind the fifth. They have 

 their origin from the supra-intestinal vessel near the 

 middle of the segment and well in front of the peri- 

 visceral trunks (PI. VI, fig. 42, in. v.). These intestinal 

 vessels do not lie freely in the body cavity but pass 

 beneath the layer of chloragogen cells, and thus encircle 

 the intestinal wall. In the posterior segments they are 

 connected with the dorsal instead of the supra-intestinal 

 vessel. 



In addition to this principal intestinal trunk, there are 

 accessory vessels, which are, however, much less con- 

 spicuous (fig. 42, in.v. 1 ). These are best seen by treating 

 the living worm with ether on a slide. At first, this has the 

 effect of causing strong contractions of the intestine and 

 blood vessels, which rather hinder than aid the observer. 

 But if the worm is allowed to remain on the slide for 



