162 
dinal vessel is distinguishable anteriorly just behind the 
first seta, and may be traced to the posterior end of the 
animal. It receives blood chiefly from branches of the 
afferent vessels which supply the nephridia and_ gills. 
There is in each chetigerous annulus a series of connec- 
tions between these vessels of the body wall, viz., (1) a 
short transverse vessel on each side connecting the nephri- 
dial and dorsal longitudinal vessels, (2) a circular vessel 
passing across the dorsal middle line, and connecting the 
right and left dorsal longitudinal vessels. (3) a vessel on 
each side connecting the nephridial and neural vessels, 
and (4+) a short vessel passing over the nerve cord connect- 
ing the right and left neural vessels (fig. 24.) The 
dorsal and nephridial longitudinal vessels supply the body 
wall in the dorsal and lateral regions, while the neural 
vessels supply the nerve cord and ventral region of the 
body wall. All these vessels, which are best seen in 
transparent young specimens, are indirectly connected 
by a network of capillaries—the parietal vessels (fie. 
36). 
The ventral vessel is large and turgid in the greater 
part of the branchial region, and bears along its course 
tufts of dark brown filaments (fig. 25). The extent to 
which these are developed depends upon the size and age 
of the specimen; they are much more numerous in old than 
in young specimens. In old examples the brown filaments 
are found in other situations also, e.g., along the gonidial 
vessel of the first nephridium (fig. 27), and upon many of 
the small vessels situated upon the inner face of the body 
wall, especially in the branchial region. dach filament 
(fig. 42) consists of a blindly-ending branch of the blood- 
vessel on which it is supported, covered with cells which 
contain large numbers of yellowish or brownish granules 
which are similar to those found in the chlorogogenous 
