The buccal or branchial orifice of the Asci dians terminates, as we have already noticed, by a valvular opening in the large delicate membranous sac, which in some respects ap pears to be a kind of crop, and in others a respiratory cavity. Opposite to it, and in the lower part of this cavity, is the commence ment of the oesophagus (fig. 778. d), which leads to the stomach and thence to the in testine. The viscera are always more or less lateral, except in Chelyosoma, where they are disposed flatwise, below the branchial sac. The convolutions of the intestine are placed between the respiratory sac and the muscular envelope or mantle, and are either unattached, except by slight filamentous processes arising from the external surface of the branchial sac, or buried in the substance of the liver and ovaries.
The mouth, or (esophageal orifice, is at the base of the branchial sac ; so that when the latter extends to the bottom of the cavity of the mantle, the mouth is also at the base of the mantle. When the branchial sac stops short at the middle of the muscular sac, or when it is bent upon itself, the mouth also is near the middle. In the species that have an oblong, transverse cavity, the mouth is at the postero-inferior angle of the branchial sac. The mouth is either a simple round hole or a slit, or it is an aperture divided up by slight folds or ridges. It is always destitute of lips or tentacles, unless we except the peculiar longitudinal row of tentacles passing forward from the msophageal aperture along the wall of the branchial sac in Che/yosoma and other genera. The msophagus, sometimes obsolete, is always very short, and is more or less pli cated longitudinally.
The stomach is simple, generally merely a slight dilatation of the alimentary tube (figs. 777. j, and 778. e). In A. mammillata it is strongly plicated, and the pyloric extremity narrowed by little fleshy papilla?. It is some times bent upon the intestine and adherent to it, and is often enveloped in the liver, with which it is intimately adherent. Its walls are very unequal, internally forming various la cunae, through which the bile penetrates, as in the Bivalves ; the little bile-ducts are guarded with valvules. In Boltenia reniformis the sto mach is destitute of any internal folioles or lacun? ; but in Ascidia intestinolis, Cynthia C. campus, and others, internal plicm are present ; and in C. ampulla the sto mach is almost filled up with longish folioles, and is of a light yellow colour from numerous yellowish granules contained in its tissue.
In Dendrodoa the stomach is striated ex ternally (fig. 778. e). In Cystingia it is very large, extending almost the whole length of the body, and bears externally a deep longi tudinal depression, and is marked internally with faint transverse strim. In Cynthia poly carpa and C. poniaria, a little ccecum occurs just anterior to the pylorus.
The intestine is usually short, simple, and without cmca. In several species of Ascidia its internal surface is traversed throughout nearly its whole length by a semicylindrical, hollow, ridgelike plication, the " intestinal rib" of Savigny, having the appearance of an inva ginated intestine. In its course, the intestine generally makes one or two folds. Its walls are thickened frequently by a glandular tissue, probably supplying some fluid necessary to digestion ; and its internal surface sometimes bears biliary lacuna'. The rectum leaving the peritoneum floats unattached, and opens oppo site to the second or anal orifice of the mantle, so that the excrements falling into the cloaca' cavity are carried away by the current of water leaving the body. In the stomach and intestines is usually found a mass of finely divided matter, chiefly, if not wholly, derived from diatomaceous plants. Towards the posterior extremity of the intestines the ex crements are usually moulded into little earthy-looking filaments, as in most of the Molluscs. The fmces appear to be formed into these vermiform cords in the sulcus along the side of the longitudinal intestinal fold.
In A. mammillata the duodenum has several slight transverse stria:. In Cynthia ampulla the intestinal walls are internally hollowed into lacunm and little folds, which, like the coats of the stomach, pour out a yellowish fluid like bile. In Dendrodoa the intestine is of con siderable length. In Boltenia renifornds also the intestine is long, mounting up as high as the base of the pedicle, then descending nearly parallel with itself and terminating in an as cending conical rectum, the anus having a scalloped margin. In this species there are twelve subcubical bodies, separate from each other, adherent to the upper and inner surface of the rectum, having the free edge of a part of the ovary between them and the liver. In size they differ among themselves, the largest lying towards the anus, and the smallest in the opposite direction; they have no apparent communication either with the intestines, with each other, or externally. Under the micro scope their structure appears to consist of a very fine homogeneous colourless membrane, enclosing an infinite number of excessively minute nucleated cells, readily separable from the tissue, which have much the appearance of blood globules. In Chelyosoma the digestive apparatus is large, and, from the shape of the animal, is arranged flatwise. The oesophagus commences at the posterior left corner of the branchial sac, between the two chambers of the heart. Advancing obliquely forwards, it enters the stomach, around which are clus tered the biliary ececa. The intestine goes on to make a single bend, and, coming back to the left posterior corner, proceeds towards, and terminates abruptly at a very short dis tance from, the anal orifice.