ALIMENTARY CANAL.—The mouth and pharynx (branchial sac) have already been described. The remainder of the alimentary canal is a bent tube, which in _A. mentula_ and most other Ascidians lies imbedded in the mantle on the left side of the body, and projects into the peribranchial cavity (see Figs. 18 and 19). The oesophagus leaves the branchial sac in the dorsal middle line, near the posterior end of the dorsal lamina. It is a short curved tube which leads ventrally to the large fusiform thick-walled stomach, ridged internally. The intestine emerges from the ventral end of the stomach and soon turns anteriorly, then dorsally, and then posteriorly, so as to form a curve, the intestinal loop, in which the ovary lies, open posteriorly. The intestine now curves anteriorly again, and from this point runs nearly straight forward as the rectum, thus completing a second curve, the rectal loop, in which the renal vesicles lie, open anteriorly. The wall of the intestine is thickened internally to form the typhlosole (Fig. 18, _ty_), a pad which runs along its entire length, so as to reduce the lumen of the tube to a crescentic slit. The anus opens into the dorsal or cloacal part of the peribranchial cavity near the atrial aperture. The walls of the stomach are glandular, and most of the endoderm cells lining the tube are ciliated. A system of delicate, microscopic, branched tubules with dilated ends (the "refringent organ"), which ramifies over the outer wall of the intestine, and communicates with the cavity of the stomach at the pyloric end by means of a duct is probably a digestive gland. There is in _Ascidia_ no separate large gland to which the name "liver" can be applied, as in some other Tunicata. RENAL ORGAN.—A mass of large clear-walled vesicles which occupies the rectal loop (Figs. 18 and 19, _ren_), and may extend over the adjacent walls of the intestine, is a renal organ without a duct. Each vesicle is the modified remains of a part of the primitive coelom or body-cavity, and is formed of cells which eliminate nitrogenous waste matters from the blood circulating in the neighbouring blood-lacunae, and deposit them in the cavity of {55}the vesicle, where they form one or more concentrically laminated concretions of a yellowish or brownish colour, sometimes coated with a chalky deposit. These concretions contain uric acid, and in a large Ascidian are very numerous. The nitrogenous waste products are thus deposited and stored up in the renal vesicles in place of being excreted from the body. In other Ascidians the renal organs may differ from the above in position and structure; but in no case have they any excretory duct, unless the neural gland is to be regarded as one of the renal organs—which has not yet been proved.