VASCULAR SYSTEM OF PHERETIMA 885 
flow of blood from eut ends, I think Bourne’s view that blood 
in the commissural vessel comes out of the dorsal and 
flows towards the subneural is untenable even on theoretical 
grounds. He is agreed on the fact that branches joining the 
commissural vessel are veins bringing blood to it from the 
body-wall and the nephridia, and shows them as such in his 
diagrams (Pl. 26, fig. 34, 2); but he believes that all the 
blood is collected in the subneural and passes forwards along 
the lateral longitudinals (lateral oesophageals) to enter the 
posterior pair of ‘hearts’. Assuming for a moment that 
Bourne’s view is correct (although I do not agree with it) 
and that the blood from the subneural goes all the way to the 
hearts, why should any part of this blood come from the 
dorsal in each segment via the commissurals ? If the commis- 
sural is a collecting channel for all the blood from the body- 
wall and the nephridia, why should it get any blood at all 
from the dorsal vessel? There is no meaning in the blood 
coming from the dorsal into the subneural in each segment 
and then entermg the ‘ hearts’, while it could do so by going 
into the ‘hearts’ straight along the dorsal vessel. It is to 
obviate this difficulty that Bourne takes the view that the 
commissurals have practically no flow in them in one direction 
or the other and that they regulate the pressure in the peripheral 
capillaries—a supposition which is easily disproved by cutting 
the commissurals and seeing that blood does flow in them 
towards the dorsal vessel. 
As a matter of fact, so much blood leaves the dorsal vessel 
anteriorly through the ‘hearts’, of which there are four in 
Pheretima connected with the ventral vessel and others 
supplying the organs directly, that it is difficult to conceive 
on a priori grounds that any blood leaves the dorsal vessel 
at all behind the thirteenth segment. 
Having decided that the dorsal vessel all along the body 
behind the thirteenth segment is only a channel for collection 
and propulsion forwards of the blood which enters it from the 
intestinal network and the commissural vessels, the rest of the 
circulation in the worm becomes easy to follow. 
