V. CIRRIGRADA. Esch.) Body oval or circular, gelatinous, supported by an internal, solid, subcartilaginous body, and provided with very extensile tentacule-like cirri pendent from the whole of the lower sur face.
Gen. Velella. Porpita. Of the genera above enumerated, Eschscholtz has described about two hundred species. Messrs. Quoy and Gaimard have made us ac • quainted with several others ; but of all these a comparatively small number only have been described in detail : so that, although in the account which we are now to give of the anatomy and physiology of the acalephm, we shall, for the sake of brevity, make use of the sectional designations, it must be understood that the descriptions apply only to a few species, and that, with regard to the others grouped along with these, we can only say it is probable that they are similarly con structed.
I. Locomotion. The principal organ of locomotion in the physograda is the air-filled vesicle or bladder, which exists, of various sizes, in all the species. In physalus, ( Fig. 6.) it is a large organ, forming a great portion of the general mass of the animal. It is placed superiorly, and, for the most part, rises above the surface of the water. It has an elongated form ; the longest diameter being the hori zontal. It is somewhat pointed at one end, at the other truncated ; and at either there is a small opening, the place of which is marked by a superficial dimple, surrounded by delicate muscular fibres, acting as sphincters. When the bladder is squeezed by the hand, so as to force the contained air towards one of these openings, the air makes its escape through it; but whenever the pressure is taken off, the opening again closes. M. De Blainville states that he has satisfied himself that this air-blad der is really a dilatation of the intestinal canal; and that he regards the two openings mentioned above as the mouth and the anus. We are ignorant of the data upon which M. De Blain ville grounds his conclusion. It does not ap pear that any observer has found alimentary matter lodged within the air-sac. But whether
or not it be an organ of digestion, it is cer tainly an organ of locomotion, although only a passive one; for it is by its contained air that the animal floats on the surface of the water, so as to expose a large superficies of its crest and bladder to the wind, by which it is driven to and fro frequently with great velocity. The walls of this sac are muscular, so that by their contraction its cavity can be considerably dimi nished. And thus, partly by the escape of air forced out through the openings, and partly by the compression of what remains, the specific gravity is so much altered as to admit of the animal's sinking into the deep when danger threatens. In the other physograda, the _ _ _ • Fi 7.
slele is so small in pro g.
poltlon to the general mass of the animal that it is not sufficient to raise it above the sur face of the water.. It is generally an ovate sac, with an opening at its upper end, closed by a sphincter muscle. It is probable that its walls are muscular, and that by pressing out a portion of the contained air, and by secreting more, alternately the animal can sink and rise at pleasure. The nature of the air con ;1, LcaiLIGLI. III LIIIZJG v..ms has not yet been ascer tained. In rhizophysa .
Fig. 7.) there are, pen dent from one part of the body, certain peculiar organs, arranged very re gularly in pairs, of a mus cular structure, hollow, and furnished each with a round orifice. They differ from the tentacula in structure,and are, pro bably, organs of natation.
Similar tnlIps but only two in number, exist in diphysa ; and, anterior to them, in the same animal, there is a two-lobed organ, :he use of which is doubt Ill. In agalnia, (fig. 8.) and ;ome of the genera allied to Lt, there are certain cartila ;inous plates disposed in an imbricated manner along the sides of the body. These, Eschscholtz regards as loco motive organs. The mus cles by which they are set in motion must be extremely delicate, as a slight touch is sufficient to separate the plates from one another.