IT is well known that the test or outer tunic in most Simple Ascidians is penetrated by a system of tubes containing blood. These “vessels” were shown in 1872 by Oskar Hertwig1 to be developed as ectodermal evaginations containing prolongations from one of the blood-sinuses of the underlying mantle. Each vessel is divided longitudinally into two distinct tubes by a septum of connective tissue, and after ramifying through the test may be found to terminate, generally close to the outer surface, in one or more rounded enlargements or bulbs which are usually known as the “terminal knobs” (Fig. 5, B). The two blood-tubes join in the terminal knob where the septum ends, and this allows the blood which flows outwards through the one tube to turn in the bulb and flow back along the other tube. Thus temporarily the one tube acts as an artery and the other as a vein, but of course they exchange functions at each reversal of the heart's action.