60 ALFRED GIBBS BOURNE. 



course. Its walls are muscular throughout its length, but 

 most so in segments vii to xiii, in which segments the vessel 

 is much dilated. In the intersegmental (i. e. septal) regions, 

 where the vessel is narrowest, there are in this portion of the 

 vessel, and in all the portion posterior to it, valves which can 

 doubtless entirely, or almost entirely, shut off the lumen of the 

 vessel in each segment from that in the other segments. These 

 valves are thick, semicircular ridges of connective tissue 

 attached to the wall of the vessel across its ventral half, and 

 presenting an irregular free edge (fig. 12). 



I have never seen the dorsal vessel double in any portion of 

 its course. I am surprised at this, as Beddard (1) distinctly 

 states that in his Pleurochseta Moseleyi " it bifurcates no 

 less than five times in the first eight segments, the bifurcations 

 always coalescing again directly .^^ ^ 



Anteriorly the dorsal vessel bifurcates — that is to say, it 

 gives off a pair of branches which behave in a manner similar 

 to the other dorso-tegumentary vessels (see p. 71), and 

 are to be considered as the most anterior pair of such vessels 

 (fig. 8). 



Posteriorly the dorsal vessel ends abruptly, as shown in 

 fig. 11. 



Ventral Vessel (figs. 4, 6 to 11, v.v.). — This is also 

 known as the subintestinal or supra-neural vessel. Its walls 

 chiefly consist of connective tissue, doubtless elastic, with 

 circularly disposed bands of muscle in the regions of the septa. 

 These bands would serve to secure a specially great blood-flow 

 in the branches of the ventral vessel in any particular region 

 of the body. It is of very uniform calibre throughout its 

 entire length. At its anterior and posterior extremities it 

 ^ To prevent any confusion upon this point I may note that I am well 

 acquainted with the "double" condition of the dorsal vessel which has now 

 been several times recorded ; it obtains in a perichsete worm which attains 

 nearly the same size as Megascolex coeruleus, and which I discovered on 

 the 24th May, 18S7, on the Nilgiri Hills in South India. In this worm the 

 dorsal vessel bifurcates in each segment from vii to xvi. I have noticed a 

 sort of longitudinal folding in of the dorsal wall in certain segments of M. 

 cccruleus, but the vessel is not double. 



