852 KARM NARAYAN BAHL 
hlood-vessels in the first fourteen cephalized segments, and 
finally to diseuss the course of the circulation in the system. 
9. Tur Typtcan ARRANGEMENT OF THE BLOOD-SYSTEM IN 
THE INTESTINAL REGION oF THE BoDY BEHIND THE 
FOURTEENTH SEGMENT. 
The blood-system in this system in this region of the body 
consists of (a) three longitudinal trunks running parallel to 
one another, namely, the dorsal, the ventral, and the sub- 
neural vessels; (b) the intestinal blood-plexus, situated in the 
wall of the gut, is directly connected with ‘the dorsal and 
ventral vessels, and indirectly with the subneural; and 
(c) the commissural, integumentary, and nephridial vessels. 
(a) The Longitudinal Trunks. 
1. The dorsal vessel.—The dorsal vessel is the most 
prominent of all the blood-vessels in the worm and is rhythmi- 
cally contractile. It runs along the mid-dorsal line immediately 
beneath the body-wall, between the latter and the intestine, 
and is at once seen lying on the gut, when the worm is opened 
by a mid-dorsal incision. In Lumbricus the dorsal vessel 
is heavily covered over with ‘ yellow cells’, which must be 
removed before the vessel is seen; but in Pheretima 
the ‘ yellow cells’ do not cover the dorsal vessel, so that the 
latter is at once prominent on dissection. Although lying close 
upon the gut, the dorsal vessel is not actually attached to the 
wall of the former in any portion of its course. It is single 
throughout its length and has thick muscular walls which are 
responsible for its contractility. The average diameter of this 
vessel is about 220; it is narrowest at places where it pierces 
the intersegmental septa. On opening a narcotized worm, 
we can easily see the wave of contraction in this vessel travelling 
from behind forwards and consequently driving the blood 
in that direction. During its course through the body, the 
dorsal vessel, on piercing each septum, has a pair of forwardly- 
directed valves (figs. 7 and 10) in its lumen. These valves, 
