VASCULAR SYSTEM OF PHERETIMA 3538 
as I shall show later, prevent the flow of blood backwards 
when the vessel contracts. There are also valves (vide 
infra) at the orifices of the dorgo-intestinal and commissural 
vessels. 
[t will be seen from fig. 1 that the dorsal vessel is connected 
with the intestine by two pairs of dorgo-intestinal vessels 
(di.v.) in each segment ; these vessels serve to establish a com- 
munication between ihe internal intestinal plexus and the 
dorsal blood-vessel (fig. 2). The anterior pair of dorso-intestinals 
come off from the dorsal in the anterior third of the ge ement, 
while the posterior pair lie in the posterior third, nearing the 
hinder septum of the segment, in close association with the 
so-called ‘lymph-glands ’ which lie on each side of the dorsal 
vessel in every segment here. These dorso-intestinals are 
very short vessels, being only about 450 in length, on an 
average. They soon enter the intestinal wall, in which they 
are continued as ‘ transverse vessels’ (vide infr a). 
Again, just before piercing each septum from behind, the 
dorsal vessel receives a commissural vessel (the dorso-lateral 
or the parietal vessel), which is connected ventrally with the 
subneural (comm.v., figs. 1 and 2). This commissural vessel 
runs along the posterior face of each septum very near and 
parallel to its outer edge, i.e. the edge joining the body-wall ; 
and is connected with capillaries of the nephridia and the 
body-wall. 
As I shall show later on, both the dorso-intestinal and the 
commissural vessels bring blood into the dorsal vessel and 
replenish its supply. No blood leaves the dorsal vessel in this 
region of the body. 
2. The ventral vessel.—The ventral vessel, like the 
dorsal, is single throughout its length and extends from the 
anterior to the posterior end of the body. In the region of 
the intestine it has an average diameter of 115 w and gives 
off a pair of ventro-tegumentary branches in each 
segment. Hach of these branches leaves the ventral vessel 
just anterior to the septal wall in each segment and, after 
running alongside the anterior face of each septum for a little 
