In Ascidia (Fig. 23) the heart is an elongated fusiform tube placed on the ventral and posterior edge of the stomach, projecting into a space (the pericardium) which is a part of the original coelom, the remainder of which is represented in the adult by the reproductive and renal cavities. The wall of the heart is continuous along one edge with that of the pericardium, and the heart is to be regarded as a tubular invagination of the pericardial wall, shutting in a portion of the surrounding space (the blastocoel of the embryo), and having open ends which communicate with the large blood sinuses leading to the branchial sac, to the viscera, and to the body-wall and test. The cavity of the heart is not divided and there are no valves. Its wall is formed of a single layer of epithelio-muscular cells, the inner, muscular, ends of which are cross-striated fibres running round the heart—the only striated muscular tissue found in the body. Waves of contraction pass along the heart from end to end, first for a certain number of beats in one direction, and then, after an interval, in the other. If a small or young Ascidia be placed alive, left side uppermost, in a watch-glass or small trough of sea-water, and examined with a low power of the microscope, the heart will be readily seen near the posterior end of the transparent body. It will be noticed that the "beating" looks like successive waves of blood pressed through the tubular heart from one end to the other by its contractions. After watching the waves passing, let us say, from the right hand end of the heart to the left for about a minute and a half (perhaps 60 or 80 to 100 beats), it will be seen that they gradually become slower and then stop altogether. But after seven or eight seconds a faint wave of contraction will start from the left end of the heart and pass over it to the right; and this will be followed by larger ones for a minute and a half, and then again a pause will occur and the direction change. It has been suggested that the cause of this remarkable reversal may possibly be that the heart being on the ventral vessel, which is wider than the corresponding dorsal trunk, pumps the blood into either the lacunae of the branchial sac or those of the 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 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.
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.
When the heart contracts ventro-dorsally the course of the 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," is now a "respiratory" heart. This is a phenomenon without parallel in the animal kingdom.
All the blood-spaces and lacunae are probably derived, like the cavity of the heart, from the blastocoel of the embryo, and are not, like the cavity of the pericardium, a part of the coelom (of endodermal origin).
Neural Gland and Dorsal Tubercle.—In the dorsal median line near the anterior end of the body, and imbedded in the mantle on the ventral[[95]] surface of the nerve-ganglion, there lies a small glandular mass—the neural gland—which, as Julin first showed, there is some reason to regard as the homologue of the hypophysis cerebri of the Vertebrate brain. Metcalf has recently shown that the neural gland may be a double structure—partly cerebral and partly stomodaeal—as in Vertebrates.
Fig. 24.—Antero-dorsal part of Ascidia showing the relations of the layers of the body, and of the nervous system. A, in sagittal section; B, in transverse section. d.bl.s, Dorsal blood-sinus; d.l, dorsal lamina; d.n, dorsal nerve; d.t, dorsal tubercle; ect, ectoderm; en, endoderm; e.p.br, epithelium of peribranchial cavity; gl.d, duct of subneural gland; l.v points to the ciliated epithelium covering a longitudinal vessel of branchial sac; m, mantle; n, nerve; n.g, ganglion; n.gl, neural gland; p.br, peribranchial cavity; pp.b, peripharyngeal bands; sph, branchial sphincter; t, t′, test; tn, tentacle.
The function of this gland is still somewhat mysterious. It may merely form the viscid secretion which is carried along the peripharyngeal bands and down the dorsal lamina. On the other hand, it has been suggested that the function of the organ may possibly be renal, for the removal of nitrogenous waste matters in the neighbourhood of the nervous system. Finally, it may be a lymph gland.