TWO AQUATIC OLIGOCHiETES. 



21 



ventral side. The fibrillar portion, which is elliptical or rhomboidal 

 in cross section, is bounded by special branches of the parieto- 

 visceral muscles. 



7. Vasculae system. 



This system consists of the dorsal vessel, the ventral vessel, 

 the supra -intestinal vessel, the commissural vessels, and the con- 

 tractile hearts. 



1). The dorsal vessel runs through the whole body length 

 on the dorsal side of the alimentary canal. It commences directly 

 under the prostomial ganglion and runs straight backwards. 



.Running directly under the 

 brain, it passes through the 

 posterior notch of the latter 

 and comes to lie on a level 

 with the upper surface of the 

 brain (fig. 10). It then con- 

 tinues its backward course, and 

 sometimes undergoes wind- 

 ings in the segments behind 

 the fifth. It is to be noted 

 especially that it shifts its 

 position to the ventral side 

 and lies near the ventral vessel 

 in segments IX-? XVII, which 

 contain the genital organs. 

 Behind these segments it re- 

 assumes its position on the 

 dorsal side. 



2). The ventral vessel runs through the whole length of the 

 body on the ventral side of the alimentary canal and mostly 

 attached to it. It is paired in its anterior portion, which com- 

 mences immediately under the pharynx at the middle of segment 

 I, and the two vessels unite at the middle of segment V, where 

 it receives a small median vessel coming from the anterior part 

 of the alimentary canal (fig. 11). 



Fig. lO. 



L. gotoi. Diagrammatic side -view of 

 head ; lateral prostomial nerve, nerve-com- 

 missure and body wall of the right side 

 omitted, br — brain, bw — body wall, b.com 

 — commissural vessels, n.com — peripharyn- 

 geal nerve-commissure, lpn — left lateral 

 prostomial nerve, mpg — prostomial gang- 

 lion, bs — blood sac, dv — dorsal vessel, en — 

 ventral nerve cord, ph — pharynx, m — 

 mouth. 



