In the Ascidians the form of the heart is oblong, and thin at the two ends, or more or less tubular. Its substance is contractile, but extremely thin and transparent, so that it is scarcely distinguishable in the cavity of its highly pellucid pericardium. Cuvier ob served that, in the species in which the branchial sac is bent upwards, he was not able clearly to discern a dilatation sufficiently marked to deserve the name of a heart, and was inclined to think that possibly in this case the heart's function was performed by the artery. Here, however, we may notice that specimens preserved in spirit generally afford but very indistinct traces of this organ ; and Cuvier does not appear to have had the opportunity of studying transparent specimens of the living animal, in which the heart can be detected by its pulsating movements.
In Ascidia intestinalis the heart, which is very long, and extended under the ventral border of the respiratory sac, communicates with the great thoracic sinus by a longitudinal slit situated at a little distance from its an terior extremity ; and when the peristaltic movements of the heart advance from behind forwards nearly all the blood contained in its cavity passes into this sinus, penetrating the vascular network of the branchial sac, and passing into the dorsal sinus, whence it is spread amongst the viscera, and returns to the posterior extremity of the heart not far from the anus. During this time the heart consequently performs the functions of a branchial ventricle, and the great thoracic sinus is a kind of pulmonary artery. But when this state of things has lasted sonic minutes, the direction of the peristaltic move ment of the heart is inverted, and the blood, instead of traversing the branchial network from below upwards as previously, moves from above downwards, and passes from the great thoracic sinus into the heart. The latter is then an aortic ventricle, and the sinus a branchial vein or aortic auricle. In a very fresh and uninjured individual, of Cynthia am pulla, Van Beneden counted 45 contractions in one direction ; and then, after a rest during the space of two pulsations, he counted 160 to 170 in the other, the pulsations being about 70 per minute.
In injecting the vascular system of the simple Ascidians, M. Delle Chiaje thought he found certain valvules so disposed as to hinder the return of the blood from the aorta into the cavity of the heart, or from passing again from the heart into the vessels through which it had arrived there. But by careful observa tions on living specimens, both Milne Edwards and Van Beneden have established the fact, that, as in the Botryllidce, the Salpidce, and the other Ascidian families, the blood of the Ascidiadce, after having flowed for some time in one direction, traverses the same circle in an opposite direction; a condition that would he impossible were any valvular hindrances to return currents of the blood present.
In Chelyosoma#, the heart is very distinctly seen in the animal when dissected ; it lies near the oesophagus, and has two distinct chambers. The aorta rising from its anterior part is a stoutish vessel, and at first lies close to the in testine : it afterwards runs in the space within the intestinal loop, ultimately breaking up into largish branches, distributed on every side. The ramifications divide some 4 or 5 times, and terminate somewhat abruptly, the extre mities appearing as if closed. Throughout the surrounding generative organs there is a very fine network of vessels, but whether they are arteries, veins, or gland-ducts is un decided. A largish vessel running along the left side of the stomach and duodenum ap pears to return the blood to the branchial sac.
Mr. MacLeay describes the heart of Cys tingia as being large, ovoidal, and of a lobular appearance ; and having four vertical, lateral openings, capable of considerable dilatation.
In Cynthia ampulla the heart is placed a little within the great intestinal loop, and near the middle of the body : it is fixed on an oblong vesicle, enclosing calcareous concre tions. This vesicle is situated exterior to and above the first or principal bend of the intestine. Its colour is a greenish yellow, and it has apparently no aperture, or commu nication with other organs. The heart itself is a slightly bent tube, with very elastic walls : it has two openings ; a single large aperture on one part, and opposite to it three ves sels that carry off the blood in different di rections.* " The circulation of the Ascidians," says Van Beneden, "differs but little from that of the Bryozoa; and is transitional between that of the Polypes and of the Molluscs. If we re move the heart of the Ascidian, the disposi tion of parts is very similar, and the simpli fication of an apparatus cannot more visibly take place. The Ascidia is but a digestive canal suspended in the midst of external mem branes, with a liquid moving in the peri intestinal space. A colourless liquid (blood) occupies this cavity ; but it is only in the branchial network and tentacles that it can be said to be contained in vessels. All around the intestinal tube this fluid is alter nately moved from right to left, and vice versa. In the vessels composing the vascular net work, and in the respiratory tentacles, the same movement of the nutrient fluid takes place. This blood contains somewhat regular globules, white as the containing liquid, that indicate the course of the fluid. In sonic in dividuals the blood is yellowish. M. Milne Edwards has observed an Ascidia with red blood."t Mr. Lister observed that in a sessile Ascidian half an inch long, the blood-globules were about the same size as those of the minute Perophora, viz. from '00025 to '0002 inch in diameter.