160 
Small vessels collect the blood from these parts and 
unite to form the ventral vessel which, soon after its origin, 
gives off a median vessel to the first diaphragm and 
its pouches, then a pair of vessels to the nerve cord and 
the body wall in the neighbourhood of the second setal 
sacs. <A little further back it gives off « median vessel 
running on the second diaphragm to the nerve cord, and 
a similar vessel on the third diaphragm to the nerve cord 
and first nephridium. From this point the ventral vessel, 
as it proceeds backwards, supplies the chetigerous sacs, 
body wall, nephridia and gills (in those segments in which 
the two latter are present) by segmentally arranged 
branches. As it runs along the intestine the ventral 
vessel is connected with the dorsal vessel by the intestinal 
vessels (see above), and it finally breaks up into capillaries 
near the anus. 
On the walls of the stomach and intestine are 
numerous intersecting blood-streams. In young specimens 
the blood is contained in a plexus of small vessels, but as 
the animal grows these vessels become converted into a 
system of sinuses, and in old specimens the stomach 
practically les in a gastric blood sinus, which is situated 
between the gut epithelium and the ceelomic epithelium 
which covers it. The dorsal vessel is, however, more or 
less distinct. 
It is convenient to refer here to two portions of the 
plexus or sinus which are somewhat differentiated. (1) 
There are two vessels (or sinuses) ventrally situated, and 
known as the sub-intestinal vessels (figs. 25, 36). They 
commence just behind the heart, and may be traced back- 
wards to the level of the twelfth setz, behind which point 
they gradually disappear. They receive efferent vessels 
from the first six pairs of gills. (2) On each side of the 
anterior part of the stomach there is a lateral gastric 
ss 
