154 ZOOLOGY 
lumen of the alimentary canal is usually distended with sand, 
which is eaten in large quantities by the worm for the sake 
of the small amount of vegetable debris which may be mixed 
with it. At the commencement of the tail the intestine passes 
into the rectum, which is supported by the numerous septa of 
this region, and ends in the terminal anus. 
The blood-vessels consist of (i.) a dorsal vessel (Fig. 97), 
which at the anterior end anastomoses with the ventral vessel 
—the blood flows forward in this; (ii.) a ventral vessel under- 
neath the alimentary canal, in which the blood flows backward ; 
(ii1.) a subintestinal vessel which les in the wall of the 
intestine parallel and dorsal to (i1.). In the first six of the 
gill-bearing segments this vessel receives the efferent vessels 
from the gills. There are also a pair of small lateral vessels 
which end anteriorly in the heart. 
The heart consists of a pair of enlarged, muscular, con- 
tractile transverse vessels, which le in the sixth segment. 
They receive blood from the dorsal, subintestinal, and lateral 
vessels, and by their contraction force it into the ventral 
vessel. There are numerous capillaries given off from the 
chief vessels to supply the various organs of the body; the 
blood is red. 
The blood in the ventral vessel is mainly venous, in each 
of the thirteen segments which carry gills this vessel gives off 
a pair of afferent branchial vessels, one of which passes to 
each gill. The gill consists of a number of branching fila- 
ments, into each of which the body-cavity is prolonged. Up 
one side of the filament runs the afferent vessel; down the 
other side courses the efferent vessel to open in the seventh to 
the twelfth segments into the subintestinal vessel, and in the 
thirteenth to the nineteenth segments into the dorsal vessel. 
The nephridia are twelve in number, a pair being found 
in each of the last four segments of the neck and the first two of 
the gill-bearing region. They consist of the usual funnel-shaped 
opening into the body-cavity, of a large vesicle which opens to 
the exterior, and of a glandular swelling which opens into the 
vesicle, and is probably the secreting portion of the apparatus. 
In the breeding season the whole organ is crowded with ova 
or spermatozoa. 
