ANATOMY OF PELONAIA.*- Muscular sys tem.—The mantle is similar to that of other Tunicates, possessing longitudinal and circular fibres ; the former in P. corrucata forming a thick bundle at their origin round the respira tory opening. A strong band of transverse fibres passes round it, in one species, imme diately below the anal orifice, encroaching on the cavity principally on that side. The chief peculiarity of the mantle is its firm adhesion to the test.
Digestive and respiratory systems. — The respiratory opening is of small size, and ex hibits no folds or tentacular fringes. The respiratory sac is elongated, cylindrical, con tracting rather suddenly towards one side to become continuous with the oesophagus. On the external surface of the sac there are about thirty parallel transverse ridges, which give it the appearance of a plaited frill. These plaits are less apparent along the course of the branchial artery and vein, but midway between them on each side they are very prominent, and are tied, each by a minute cord, to the inner surface of the test. The internal sur face of the sac exhibits along one side the ser pentine double cord which contains the bran chial vein ; along the other side the branchial artery ; and from these primary and secondary perpendicular branches proceed, as in other Tunicata. The transverse plaits on the ex ternal surface of the sac correspond to the primary or transverse branches of the vessels on the internal surface. During the life of the animal, without doubt, cilia exist in great abundance on the edges of the lozenge-shaped spaces of the sac.
The cesophagus commences by a white plicated opening at the lower end, and on one side of the sac (fig.789.). It is curved in a sigmoid form, and exhibits longitudinal rugm through its coats. Near the lower end of the mantle-cavity it terminates by suddenly dilating into the stomach (h), which is pear-shaped, and directed obliquely upwards towards the side opposite to the cesophagus. The internal surface of the stomach presents longitudinal plicae. It is succeeded by the intestine, which at first curves upward, and then down to the bottom of the branchial cavity, up along the oesophageal side of that cavity and between its walls and the branchial artery, terminating about the anterior third of the animal in a funnel-shaped anus (i), which is cut into ten or eleven processes, like the petals of a flower.
The first part of the intestine is white, and longitudinally plicated ; the rectum is dilated, and its coats are attenuated.
Vascular vascular system re sembles that of the Ascicliadee, except that there is no heart. It consists of two sets of vessels, with four sets of capillaries ; a circle in fact twice interrupted, once in the respira tory sac, and again throughont the body. The branchial veins run along the transverse plaits of the sac, receiving secondary and ternary twigs at right angles. The primary branchial venous branches empty themselves on each side into the branchial trunk, which runs in the substance of the double cord which coasts the superior aspect of the sac. This double cord terminates in an abrupt manner anteri orly near the oral orifice, and in a similar manner, but often becoming smaller near the orifice leading to the cesophagus. At this point the vein becomes an artery, and pro bably sends back vessels to nourish the sac. It now runs along the cesophagus, supplying the stomach and intestine, and giving off in its course branches to the mantle. The veins arising from the arterial capillaries of the body meet near the commencement of the ()esophagus in one trunk, which, passing along the inferior wall of the respiratory sac, oppo site to the branchial vein, performs the tune tions of a branchial artery. It is interesting to observe here the differences between the modes in which the branches enter the branchial vein, and strike off from the bran chial artery. In the former, just before the branches enter the trunk, they give off a number of vessels, which enter a trunk along side of the parent trunk, the combination forming a sort of delta ; in the latter, they leave the trunk singly, and send off their branches in a radiating direction. At a little distance from the trunks of both artery and vein, the secondary branches become parallel to one another and perpendicular to their primary branches, the more minute divisions following the same mode of ramification.