The liver of the Ascidiadee generally occurs in a very degraded form ; it is either absent, as in Ascidia, Dendrodoa, and some Cynthio, or its place is supplied by lacuna: and folioles on the inner coats of the intestinal canal, oc curring, in Cynthia ampulla, both in the sto mach and duodenum ; or, on the other hand, it is more or less amply developed, as in Bol tenia, where it appears as an irregularly lobu hated body, coating the stomach externally behind the right ovary, and passing from the lower extremity of the body, half-way up, in the cavity of the mantle. The lobes differ in size: the largest are placed towards the pyloric or highest end of the stomach, and are more distinctly separated from each other than the smaller ones ; they are more or less rounded and granulated, their surface being minute!) papillated and composed of minute round bodies, at first sight resembling ova. In Cyn thia the liver is greenish, granulated, foliated, or, as in C. Diane, caniculate. It is sometimes formed of clustered groups of flask-shaped cellular bodies, the individual cells being fixed by their larger ends, and having a radiate ar rangement. It is intimately adherent to the external surface of the stomach, which fre quently is totally enveloped by it. The bile enters the stomach by distinct holes at the bottom of the cavities of the little lacuna; before-mentioned. In C. microcosms, and others, the liver is divided into masses, one of which is situated on the left of the bran chial sac and quite free of the abdomen. The liver in Ascidia iniestinalis has the appearance of somewhat salient glandular bodies, situated as well on part of the intestine as on the sto mach. In Chelyosonia this organ is represented by a cluster of short ccecal tubes lying all over the external surface of the stomach. The liver cells in Bottenia are elongated caeca, thick, and sometimes deeply divided at the outer extremity, attached by their thin ends, and arranged in eccentrically radiated groups, en closed in an epithelial membrane, the whole having externally a racemiform appearance.
The digestive organs of the Tunicata are subject to congenital malposition ; of which M. Savigny has described two remarkable exam ples, in Cynthia Moms and Phallusia Turcica; and Mr. MacLeay was inclined to regard as a malformation a peculiar arrangement of the intestinal canal that was presented by the unique specimen of Cysting;a Grigithsii de scribed by him. In a unique specimen of P. Turcica, examined by M. Savigny, the intestine lay to the left instead of the right of the bran chial sac, and was found bending backwards and embracing the stomach from below, in stead of bending forwards at some distance from the pylorus, approaching the superior border of the stomach, and then terminating in the rectum. In a specimen of C. Monza:, the alimentary canal was also found on the left-hand side ; but, by a very peculiar intro version, the pharynx was placed at the pos terior instead of the anterior extremity of the branchial sac. The intestine descended as far as the bottom of the mantle, folded it self forward, and ascending parallel to itself, terminated opposite to the pharynx ; so that the anus and the oesophageal aperture both opened into the branchial orifice. The ex
ternal communication through the anal orifice existed as usual. Both of these malformed individuals had their ovaries full of eggs, but were not, apparently, in strong health, and were more than usually infested with ento mostraca. In the catalogue of the Hunterian Museum (vol.i. p1.5. fig. 2.), a dissection of an Ascidian is figured after a drawing by John Hunter (the original specimen, however, has not been found), in which there is, apparently, an abnormal elongation of the oviduct, which is accompanied by a slight granular line, both lying on a large tapering tube having much the appearance of intestine ; the anus, how ever, shows itself projecting from the side of this tubular body some way lower down, in its usual place. The oviduct, and, apparently, the accompanying elongated tube, terminate externally at a minute aperture placed in the sulcus between. the two projecting terminal orifices of the test. The oviduct, however, barely reaches this aperture ; and its accom panying granular line terminates still lower down.
Organs of circulation. — In the Ascidiadce there are two large vessels or sinuses, the dorsal and ventral, to which the branchial capillaries, on the one hand, and the heart and peri-intestinal cavity, on the other, are intermediate. The circulation is of the mixed or reptilian type ; both sinuses being in con nection with systemic and respiratory capil laries, and the blood, consequently, being sent by one impulse both to the system and to the and ultimately returning from both by the same channel. We are prevented from calling either of these sinuses arterial or venous, on account of the periodic reversal of the circulation, mentioned incidentally above, and more fully detailed hereafter, whereby they are alternately changed from vein to artery and from artery to vein. One of the branchial trunks, terminating at the heart, however, answers to the branchial veins of the Gastero pods and Bivalves ; the opposite, and often double, trunk may therefore be looked upon as the branchial artery, and is connected with the veins of the body. The Ascidia, like the rest of the Acephala, has but a left or aortic ventricle, and no ventricle at the union of the vena cava and the pulmonary artery. This aortic heart or ventricle is not always easy to be seen. When the branchial sac is simply oblong, it is situated towards its base ; and consequently, when the branchial sac is as long as the body, it is situated towards the base of the mantle ; and when the sac is shorter than the body, it is placed near the centre of the mantle. When the branchial sac is bent upon itself, the heart is situated at the curve, and then it is always near the middle of the body. In general its position, according to Cuvier, appears to be deter mined rather by that of the mouth, than that of the rectum ; but M. Milne-Edwards and Van Beneden consider that it follows in its displacement the organs of generation rather than the mouth. The heart in the Tunicata is never traversed by the rectum, as in other Acephala.