ANATOMY OF THE ASCIDIADIE.—We have already referred to the external envelope of the Tunicates as being analogous to the cal ciferous shells or tests of the other Acephalans; and the muscular sac enclosed within it, as the analogue of the mantle; and the membranous sac lining the mantle as analogous to the branchim of the Acephalans.t The rest of the viscera of the Ascidians are enveloped in a peritoneumI, and the heart has, besides, its own membranous sac or pericar dium. Thus the body, properly so called, appears as if divided into three cavities ; that of the branchim, communicating with the exterior by the superior opening of the sac, and in the base of which opens the mouth; that of the peritoneum, which does not com municate with the exterior by itself, but is traversed by the intestinal tube, which, arising in the branchial cavity, communicates with the exterior by the rectum and the anal aper ture of the test ; and, lastly, that of the peri cardium, which has no direct communication with the exterior.
The position of the animal is always such, that of the two orifices the branchial is always the highest ; the entrance into the branchial sac being generally placed at or near the superior extremity of the body, and the oeso phageal opening, at the base of the branchial sac, having an upward direction. In Boltenia and Cystingia the flexible peduncle, which is attached at the summit of the body, above the branchial orifice, allows the body to droop, thus giving the animal its normal position. In the Clavellinidee, which, have rigid peduncles continuous with the base of the test, the same normal relation of the internal parts is pre served ; and although the intestinal loop in Boltcnia and Clavellina is always directed to wards the pedicle, yet this arises from the loop having, in the former an ascending, and in the latter a descending, direction.
The muscular sac or mantle, enclosed in the external envelope, is attached to the in• ternal surface of the latter by an adhesion of the external surface of the extremities of its two tubular processes, which correspond to the two external orifices of the test, to the inner borders of these outer orifices (fig. 780.). In Ascidia there is often no in timate cohesion at these points, merely an adaptation easily disturbed after death. In
Cynthia, Boltenia, &c., on the contrary, the test and the tubular prolongations are strongly adherent. Beside this mode of attachment, there is sometimes a general loose adhesion formed by the epithelial tissue between the surfaces of the test and mantle ; and in Cynthia papillate there is a partial transmission of mus cular fibres from the latter to the former ; and, lastly, there is the very slight attachment arising from the passage of bloodvessels from the body to the test.
This free condition of the mantle within the external sac occurs throughout the Asci dia&E, Clevellinidce, .Botryllidce, and Pyroso midce. In the Salpidce and the Pelonaiadce, on the other hand, the mantle and the test are connected at very many points, or even throughout the extent of their contiguous surfaces.
The mantle in Chelyosonza is more closely attached to the internal surface of the test than in other Ascidians ; its muscular tissue forming intimate connections between the various plates of the external envelope, and not only supplying the valvular pieces of the apertures, but also edging each of the larger plates with interlacing connecting fibres. Ex cept at these muscular spaces the mantle ap pears as a thin serous membrane.
The interval between these two sacs is, during life, filled with some fluid; possibly a secretion, or transudation through the one or the other of these envelopes; for, except in the case of those Ascidians whose mantle tubes are not intimately connected with the test, the sea-water cannot be directly admitted into this cavity.
The mantle is reflected upon the body, properly so called, and covers it externally, just as the peritoneum, after having coated the walls of the abdomen, is reflected upon the intestines ; with this difference, however, that it has no mesentery, and that the con nection is only at the two orifices. It has an external serous layer, continuous with the internal membrane of the test, a muscular tissue more or less extensive, and an internal serous layer. It has also numerous ramifica tions of nerves and bloodvessels. In Boltenia, Cystingia, and probably Bipapillaria, a tubu lar prolongation of the mantle traverses the peduncle.