[Illustration: FIG. 18.—Dissection of _Ascidia_, from right side, to show anatomy. _a_, Anus; _At_, atrial aperture; _Br_, branchial aperture; _br.s_, _br.s′_, branchial sac; _end_, endostyle; _g.d_, genital ducts; _gon_, ovary; _hyp_, neural gland; _hyp.d_, the duct leading to dorsal tubercle; _m_, mantle; _n.g_, ganglion; _oes_, oesophagus; _p.br.c_, peribranchial cavity; _ren_, renal vesicles; _st_, stomach; _t_, test; _tn_, tentacles; _ty_, typhlosole.] The mantle forms two well-marked siphons or short wide tubes, which lead in from the branchial and atrial apertures. These are surrounded by strong sphincter muscles,[94] and are lined by the invaginated ectoderm and test. The one leads into the branchial sac or modified pharynx, and the other into the atrial or peribranchial cavity (see Fig. 18, and Fig. 19, _p.br_). Figs. 18 and 19 show the relations of the branchial and peribranchial cavities to one another. The peribranchial cavity {44}opens to the exterior dorsally by the atrial aperture, forms the cloaca along the dorsal edge of the body, and has extensions laterally on each side of the branchial sac, with the interior of which it is placed in communication by the secondary gill-slits or "stigmata" (Fig 19, _sg_). Along the ventral edge the mantle is united to the wall of the branchial sac, and it is only this union (Fig. 19, _end_) that prevents the peribranchial cavity from completely surrounding the branchial sac. The following list of the cavities present in the body of the adult _Ascidia_ may be useful at this point:— 1. The alimentary canal, including the branchial sac. This is derived from the archenteron of the embryo, is lined throughout by endoderm, and the system of cavities of the intestinal gland is to be regarded merely as an outgrowth from the alimentary canal. 2. The peribranchial (atrial) cavity, derived from two lateral ectodermal invaginations which join dorsally to form the cloaca and open to the exterior by the atrial aperture. 3. The original embryonic segmentation cavity (blastocoele) remains, where not obliterated by the development of the mesodermal connective tissue, as the irregular system of blood spaces, with its outgrowths in test and branchial sac. The heart, which has differentiated muscular walls, becomes secondarily connected at its ends with these blood spaces. 4. The pericardium and epicardium (see p. 83) originate as outgrowths from the archenteron. They may therefore be regarded as enterocoelic spaces. The pericardium becomes completely closed off and separated from the alimentary canal. The epicardium may form paired tubes of great length, and may remain permanently connected with the branchial sac. 5. The cavities of the renal vesicles and of the gonads and ducts are spaces formed in the mesoblast. They have been variously interpreted:— (_a_) As of the same nature as the blood spaces (blastocoelic), or (_b_) As formed by a splitting of the mesoblast (coelomic).