MONILIGASTER GRANDIS, A. G. B. 329 
circularly round the intestinal wall. These circular vessels lie 
very close together, and are connected with one another at 
short intervals by longitudinally-running junctions, the whole 
forming a rectangular meshwork. In the dorsal region these 
circular vessels join together to form the dorso-intestinal 
vessel in each segment. They take their origin from a net- 
work which runs longitudinally along the ventral surface of the 
intestinal wall on each side of the ventral median muscular 
band. As part of this network there is a small longitudinal 
vessel on each side which runs right on from segment to seg- 
ment; from this there runs in the region of each septum a 
small circular vessel, lying on the outer wall of the intestine, 
to the dorsal region. When nearly arrived there it bifurcates, 
and one branch runs over the intestine and under the dorsal 
vessel and joins its fellow of the opposite side, while the other 
is connected with a longitudinal network which lies on either 
side of the dorsal median muscular band, and which is con- 
tinuous from segment to segment. In this way provision is 
made in every segment for communication between the net- 
works of the two sides, and by means of the networks, for com- 
munication between the blood supply of one segment and that 
of another. The ventro-intestinal vessels open into dorsal 
longitudinal networks on each side. 
Dorso-intestinal Vessels.—These are the branches of 
the dorsal vessel mentioned above which place it in connection 
with the intestinal capillary networks. They are the efferent 
vessels of these networks, and the only vessels which bring 
blood into the dorsal vessel. The most anterior pair are in 
Segment x111, and behind these there is one pair in each seg- 
ment. They are joined to the dorsal vessel immediately be- 
hind the septum, i.e. in the most anterior portion of the 
segment, and each is formed of from ten to twenty branches, 
all of which spring from the intestinal wall. At the junction 
of each with the dorsal vessel is a valve precisely similar to 
that of Megascolex ceruleus, which must effectually pre- 
vent any blood passing outwards into it from the dorsal vessel 
(fig. 39). 
