II STRUCTURE—CIRCULATION 51 
ramify over the alimentary canal and the other viscera. These 
visceral lacunae finally communicate with a third great sinus, the 
“branchio-visceral”’ vessel (bv) 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. 25), 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 
iA 
eel yaa EEE 
aE, 
—_ 4 
—> 
Fic. 23.—Diagrammatic dissection of Ascidia, from left side, to show course of circula- 
tion. Front part of branchial sac opened, back part covered by viscera. 0.a, 
Branchial (ventral) aorta ; 6.c, branchio-cardiac vessel ; 6.v, branchio-visceral vessel ; 
c.v, cardio-visceral vessel ; d.a, dorsal aorta ; At, 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 be 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,” 1s now a 
“respiratory” heart. This is a phenomenon without parallel in 
the animal kingdom. 
