viscera in greater volume than can possibly get out through the smaller branchio-visceral vessel in the same time, the result being that the lacunae in question soon become engorged, and by back pressure cause the stoppage, and then reversal of the beat. The absence of any valves in the heart to regulate the direction of flow obviously facilitates this alternation of the current. The larger channels through which the blood flows may be lined with a delicate endothelium, but the smaller passages are merely spaces in the connective tissue. The heart, although anatomically a "ventral vessel," runs in the main dorso-ventrally. The blood-channel leaving the ventral end of the heart is the "branchio-cardiac vessel" (Fig. 23, _b.c_). This gives off a branch which, along with a corresponding branch from the "cardio-visceral" vessel (_c.v_) at the other end of the heart, goes to the test, and then runs along the ventral edge of the branchial sac as the branchial aorta (_b.a_), external to the endostyle, communicating laterally with the ventral ends of all the transverse vessels of the branchial sac. The cardio-visceral vessel (Fig. 23, _c.v_) after giving off its branch to the test breaks up into a number of sinuses which {51}ramify over the alimentary canal and the other viscera. These visceral lacunae finally communicate 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. [Illustration: FIG. 23.—Diagrammatic dissection of _Ascidia_, from left side, to show course of circulation. Front part of branchial sac opened, back 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.]