58 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
lying underneath the endostyle (Pl. IIL. fig. 10, br. ao.) 
to the walls of the branchial sac, where, in passing along 
the transverse and interstigmatic vessels, it is purified, 
and receives a supply of oxygen from the water passing 
through the stigmata. It 1s then conveyed, by the great 
dorsal vessel and the branchio-visceral vessel and its 
branches, to different parts of the viscera and body-wall, 
so that all the organs may receive food and oxygen, and 
have their waste materials carried away: Some of these 
branchio-visceral vessels from the branchial sac lead to 
the walls of the stomach and intestine (see Pl. IIL., fig. 10), 
and thus bring us back to the point from which we started. 
The great dorsal and ventral vessels of the branchial 
sac are connected (Pl. III., fig. 10), not only by the trans- 
verse vessels, which run like hoops round the walls, but 
also at their anterior extremities, by a circular vessel 
which surrounds the front of the branchial sac, underneath 
the peripharyngeal bands. A short branch runs from near 
the front of the dorsal vessel to the sinuses which sur- 
round the nerve ganglion. 
From each end of the heart ‘‘ vessels” are also given 
off to supply the body-wall and test. Moreover, ‘‘ connec- 
tives’? run from the transverse vessels of the branchial 
sac directly outwards, on each side, to the body-wall and 
viscera. On the left side there are three especially large 
blood tubes amongst the connectives, which cross to the 
alimentary canal and ovary, and branch through these 
viscera. 
But the course of this circulation of the blood is not 
always the same; sometimes it is exactly reversed, the 
blood flowing from the branchial sac to the heart, and 
from that organ to the viscera, and then back to the 
branchial sac again. This curious state of affairs is caused 
by the remarkable manner in which the Ascidian heart 
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