VASCULAR SYSTEM OF PHERETIMA 387 
he blood finally passes into the dorsal vessel through the two 
pairs of dorso-intestinals in each segment. In the posterior 
region of the gut—the post-typhlosolar or the rectal region, 
however, the blood brought to the external plexus passes 
directly into the dorsal vessel through a single pair of dorso- 
intestinals in each segment, which, as already mentioned, 
communicate with the external plexus, the internal plexus 
being absent in this region. The course of blood in the intestinal 
region can be shown diagrammatically as follows :— 
Dorsal vesse/ 
Hearts 
Ventral vesse/ 
Ventro -tegumentaries Ventro-intestinals 
supplying supplying 
Body-wall ¢ Nephridia, Intestine, 
Branches trom the Branches from the 
ventral body -wa// body-wall g¢ nephridia to 
to the subneura/ the commissural vessel. 
Subneural —————» Commissural External intestinal 
Plexus, 
Commissural Shirok 
vesse/. 
Internal intestinal 
plexus 
Dorso-intestinals 
Dorsal vesse/ 
It will be seen that the ventral vessel and its branches, the 
ventro-tegumentaries and ventro-intestinals, form the arterial 
vessels, while the subneural, the commissurals, the dorso- 
intestinals, and the dorsal vessel itself are the chief veins 
(using the word in an anatomical sense) in the worm. ‘The 
blood in the dorsal vessel in a certain segment must go to 
the * hearts ’, and return by the ventral vessel into that segment 
NO, 259 pd 
