VASCULAR SYSTEM OF PHERETIMA 357 
through the septo-intestinal (si.v., figs. 1 and 2), a 
vessel which I describe below along with the commissural 
vessel. 
This vessel collects blood from the small ventral part of the 
body-wall and the nerve-cord; and as the area over which 
its branches ramify is very small and the quantity of blood 
received is also small, the vessel itself is very slender as com- 
pared with the other longitudinal trunks. 
There are no supra-intestinal vessels in this region 
in this worm: a pair of longitudinal ducts attached to the mid- 
dorsal line of the gut and described as supra-intestinal blood- 
vessels by Stephenson (14) have already been shown by me to 
be excretory ducts (7). 
There are also no lateral neural vessels as found in Lum- 
bricus. 
(b) The Intestinal Blood-plexus. 
The intestinal blood-plexus (fig. 3) consists of a close network 
of capillaries and blood-vessels in the walls of the intestine. 
In Pheretima as in Megascolex (1) there are two 
capillary networks in the alimentary canal, i.e. (1) an internal 
deep-lying network, and (2) an external more superficial one. 
The internal network lies deep in the wall of the gut inside the 
layer of circular muscle-fibres, between it and the internal 
epithelial lining ; while the capillaries belonging to the external 
network lie on the surface of the gut-wall amongst or even 
outside the yellow cells (chloragogen cells) which form the 
splanchnic layer of the peritoneal lining of the coelom. When 
a freshly-killed worm is opened in saline solution it is at once 
seen that the blood-plexus on the gut is marked out into three 
distinet regions—the first region is from the fourteenth to the 
twenty-sixth segment, where the intestinal capillaries are very 
thickly set and lie at right angles to the longitudinal axis of 
the body (transverse capillaries); the second is the longest 
portion and extends from the twenty-sixth segment to twenty- 
three to twenty-eight segments in front of the anus, the main 
Bb2 
