60 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
When the heart is contracting ventro-dorsally it receives 
oxygenated blood from the branchial sac by the branchio- 
cardiac vessel (now a vein), and propels it by the cardio- 
visceral trunk (now an artery) to both sides of the viscera 
and body-wall. This blood, after circulating through the 
system, is collected as impure blood by the branchio- 
visceral vessel and conveyed to the dorsal sinus of the 
branchial sac to be re-oxygenated. The heart is then a 
systemic heart and contains pure blood. But after the 
reversal, when the heart contracts dorso-ventrally the 
veins and arteries exchange functions, the oxygenated 
blood passes from the branchial sac to the viscera, the 
heart receives impure blood from the system and propels 
it to the ventral edge of the branchial sac, and so what 
was a minute before a ‘systemic,’ 1s now a “ respira- 
tory’? heart. This is a phenomenon without parallel in 
the animal kingdom. 
The blood of Ascidians is in the main transparent, but 
contains usually certain pigmented corpuscles in addition to 
many ordinary leucocytes or colourless amoeboid nucleated 
cells (Pl. IV., fig. 6). The pigment in the coloured cells 
may be red, yellow, brown, or in some cases blue or opaque 
white, and these are the result of deposition of pigment 
granules in the older leucocytes. In Ascidia mentula a 
large number of blood corpuscles are usually brown. ‘The 
unaltered leucocytes may be actively amceboid, and can 
proliferate. As we have seen, the blood may reach the 
branchial sac either from the dorsal or from the ventral 
median sinus, according to the direction in which the 
heart is beating at the moment; and it is a most interest- 
ing and beautiful sight to watch the alternating circulation 
of the variously coloured corpuscles through the transparent 
vessels, and the lashing of the cilia along the edges of the 
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