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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," is 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 (PI. 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 amoeboid, 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- 

 hit^ 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 



