STRUCTURE — CIRCULATION 



51 



ramify over the alimentary canal and the other viscera. These 

 visceral lacunae finally comnnmicate with a third great sinus, the 

 " branchio-visceral " vessel (b.v) which runs forward along the 

 dorsal edge of the branchial sac as the dorsal aorta (d.a), 

 externally to the dorsal lamina, and joins the dorsal ends of all 

 the transverse vessels of the branchial sac. Besides these three 

 chief systems — the branchio-cardiac, the cardio-visceral, and the 

 branchio-visceral — (see Fig. 23), there are numerous lacunae in 

 all parts of the body by means of which anastomoses are established 

 between the different currents of blood. 



When the heart contracts ventro-dorsally the course of the 



JFlG. 23. — Diagrammatic dissection of Ascidia, from left side, to show course of circula- 

 tion. Front part of branchial sac opened, bacli part covered by viscera, b.a, 

 Branchial (ventral) aorta ; b.c, branchio-cardiac vessel ; b.v, branchio-visceral vessel ; 

 c.v, cardio-visceral vessel ; d.a, dorsal aorta ; ht, heart. A, anterior ; P, posterior; 

 D, dorsal ; V. ventral. — ' 



circulation is as follows : — the blood which is flowing through 

 the vessels of the branchial sac is collected in an oxygenated 

 condition in the branchio-cardiac vessel, and after receiving a 

 stream of blood from the test enters the ventral end of the heart. 

 It is then propelled from the dorsal end into the cardio-visceral 

 vessels, and so reaches the test and the digestive and other 

 viscera ; then, after circulating in the visceral lacunae it passes 

 into the branchio-visceral vessel in an impure condition, and is 

 distributed to the branchial vessels to he purified again. When 

 the heart, on the other hand, contracts dorso-ventrally, this course 

 of the circulation is reversed, the " veins " and " arteries " exchange 

 functions, and what a minute before was a " systemic," is now a 

 " respiratory " heart. This is a phenomenon without parallel in 

 the animal kingdom. 



