_Nervous system. —This system consists, as in other Tunicates, of a ganglion situated in the substance of the mantle, between the oral and anal orifices. It is globular, and sends off nervous twigs, firstly, to the respiratory orifice of the mantle ; secondly, to the respira tory sac, where it begins to exhibit the trans verse plates; and thirdly, to the anal orifice of the mantle.
Generative system. — The generative organs consist of two elongated tubes, closed at one end, open at the other, and having a great number of close-set parallel cmca arranged at right angles, and opening into them along each side. These tubes are attached to the internal surface of the mantle ; their mouths are free for a short distance, and prominent the rest of their extent, and the attached caeca adherent. The orifices of these organs are situate at the junction of the first with the second quarter of the animal, and one third of the other end of each turns in towards its neighbour, and then proceeds forward parallel to itself. The branchial vein runs midway between the generative tubes above, and the branchial artery in a corresponding course below, so that the threads of the attachment of the plaits on the external surface of the sac are fixed into the tubes in a series on each side.
From the details of the structure above given, it is evident that the Pelonaia is a true Tunicate. Its anatomy is important, and assists in the elucidation of the nature of the parts and organs in other members of the group. They present the positive anatomical character of a union of the mantle with the test ; so that there can belittle question of their right to be regarded as members of a distinct family of Icinicata ; and it is worthy of notice, says Prof. Forbes, that Mr. Mac Leay, in his valuable remarks on the arrangement of the Tunicates (Lineman Transact. vol. xiv.) had hypothetically indicated such a group as this now constituted. Whilst in many of their characters they approach the true As cidians, especially the unattached species of the genus Cynthia, in others they indicate a relationship with the cirrhograde Echinoder mate. They differ from their Tunicate allies
chiefly by their not being fixed, and by their form, which reminds one more of that of a Siphunculus than of an Ascidia ; indeed, they may be regarded as analogous to certain Siphunculiclw ; and in that point of view the details of their form and structure are of much interest to the naturalist. They differ also, add Messrs. Forbes and Goodsir, from the Ascidians, more particularly in being bila teral. The generative organs are symmetrical, and open one on each side of the anus, which is directed towards the ventral surface of the animal, in a line with the mouth and the nervous ganglion. The latter is thus proved to be an abdominal or subcesophageal ganglion, corresponding to or forming one of the chains of ganglia on the abdominal surface of the Articulate. In the same manner, the branchial artery or heart is proved to be the pulsating dorsal vessel, and the branchial vein the abdominal vessel (when that vessel exists) in the Annulosa. It is interesting also to perceive, that, co-existing with this decided approach to the annular type of form, we have the transverse plaits of the respiratory sac corresponding to the rings of an articu lated animal. The disappearance of a separate test is also a departure from the plan of formation in the Ascidiadce and their allied groups, and an approach to other types of form, and more particularly to the cirrhograde Echinoderms, with certain of which Pelonaia has at least an analogical relation, in the water-filled body, and in the external form.
Pelonaia, in fine, is one of those connecting genera so valuable as filling up gaps in the system, and supplying links in the chain of structures which runs through the series of organised bodies.