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. {52}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. [Illustration: 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.