ANATOMY OF PYROSOMA. - The common tegumentary mass of Pyrosoma is semitrans parent, subcartilaginous, toughish, and some what extensile. The exterior of the hollow, conical, or cylindrical body, formed by the ag gregation of the individual Pyrosontata, is co vered with numerous elongated tubercles, of a rather firmer consistence than the rest of the mass. Each of these constitutes one extremity of an individual member of the living group. The opposite extremity opens into the cavity of the cylinder, and is not free, but, like the trunk of the individual's body, is closely connected by the common mass with the similar parts of other individuals lying above, below, and on either side of it. In some species the animals are arranged much more regularly than in others, and appear to form piled-up rings or circles of individuals, more or less analogous to the otherwise disposed circular systems of some of the Botryllidce. In Pyrosonta atlan ileum the tubercles are simply conical, and are perforated terminally. In P. elegans, also, the external orifice of the individual opens at the extremity of the tubercle, and through it the water contained in the great cylinder has been seen to escape freely in little jets, when the Pyrosome has been taken out of the sea. In P. giganteum the tubercles are of various sizes, some being short and indistinct, and others, on the contrary, very much deve loped. The largest are conico-cylindrical flat, and lanceolate at the extremity, with the minute branchial orifice on the inferior aspect. This lanceolate extremity is crenulated on its sharp edges, and presents on its inferior aspect, between its point and the aperture, a slightly prominent keel. The branchial orifice is sometimes surrounded by a slight, free, crenu lated membrane.
The interior of the great cavity is generally smooth. Its walls are perforated by the numerous minute anal orifices of the com ponent individuals ; and, at a slight depth, its surface is studded with a great number of yellowish, rose-coloured, or carmine spots, which are the hepatic and other visceral organs of the numerous animals. The ter minal aperture of the large, conical, compound body of the Pyrosoma has, according to Le sueur and Savigny, a membranous border, which can be sometimes drawn together so as to close the cavity ; and Mr. F.11. Bennett observed that, when first removed from the sea, the broader extremity of the cylinder pre sented a wide and circular orifice, forming nearly a continuous surface with the central tube ; but when the animal was kept in a vessel of sea-water, or much handled, this orifice was closed by the contraction of a smooth, dense membrane at its margin, and which either obliterated the aperture, or left but a minute central orifice; water at the same time being contained in the barrel or tube of the body.
Besides the common envelope or test, each individual animal has an inner tunic or man tle. This is a very thin, delicate membrane, attached apparently at four points only, two of which are at the extremities; that is, at the branchial and anal orifices; and the other two are at two rounded, compressed bodies, one on either side, just beyond the anterior mar gin of the branchim, and regarded by Savigny as ovaries.
The branchica line the inner surface of this inner tunic. They are oval in form, and their dorsal borders meet each other, and are at tached along the dorsal aspect of the mantle ; but they are separated, at their anterior and ventral borders, by a considerable space, which is partly occupied by the ventral sinus (fig. 786. i, i). The branchial tissue is traversed by numerous vessels anastomosing with each other at right angles. The transverse vessels, varying from 18 to 25 in number, are the largest and most distinct, and are folded back on themselves at the free edges of the tissue. The longitudinal vessels are from 11 to 17 in number. " Nothing is more curious," says Milne-Edwards*, " than the respiratory ap paratus of these little animals, when the vi bratile cilia, with which each of the branchial stigmata is furnished, are simultaneously ef fecting their vorticiform movements with ra pidity and perfect harmony." The oesophagus is curved, and is of a bright red colour. The stomach is subglo bular, yellowish, and opaque. The intestine is short, and strongly bent on itself ; the anus is directed backwards towards the posterior orifice. The liver is a globular body, which is but slightly developed in young individuals ; its postero-inferior portion is formed of several sections united by a centre, around which they converge, presenting the appearance of a flower with many petals, or a calyx with, most usually, 7, 8, or 10 divisions. The sec tions are not always equal. Their centre is occupied by a somewhat solid, granular sub stance, which they more or less perfectly enclose. Its colour is generally whitish, or of a light pink. It lies free in a cavity hol lowed out of the test, and is attached by a membranous peduncle to the stomach, or rather to the intestinal loop ( fig. 786. A, a, a, B and c e, e). These viscera are si tuated posteriorly to the branchial sac. By their disposition they leave a free passage to the water which traverses the branchial ca vity. The nerve-ganglion is present at the anterior extremity of the dorsal border of the branchial sac. From it there proceed filiform branches towards the neck of the external tubercle and in other directions. The vessel or sinus that runs between the two free edges of the branchim is of considerable length, and has a slight general curve. It is divided, as it were, into four, towards its largest and an terior portion, or rather seems to be com posed of two contiguous vessels bent upon themselves. These diminish in calibre as they run backwards, and, passing into a delicate filiform vessel, are lost near the stomach.