The mouth is large, the oesophagus straight and wide ; the stomach is, for the most part, of an ovate form, the .intestine passes in a straight .line, and with a uniform diameter, to its ex tremity. The anus has a prominent circular margin in cydippe. No absorbent vessels can be seen arising from the gastric cavity. In many species, the alimentary canal is so large as to occupy the greater part of the interior of the body. When there is no food within it, it remains open at both extremities, and, as the animal swims generally with its mouth fore most, there is a current of water continually passing through it. Eschscholtz observed, that when suitable aliment was carried by this cur rent against the walls of the stomach, the orifices were immediately contracted, and the digestive process begun. Minute crustacea, salpa3, &c., have been found in the stomachs of ciliograda. The diligent observer just men tioned seemed to regard the canal leading from the stomach to the dorsal surface, (which we have called the intestine,) as forming no part of the digestive organs. He termed it " the water-canal," and considered it as connected merely with the peculiar mode of locomotion, inasmuch as he observed it so patent while the animal was swimming and not digesting as to admit of a free passage for the water ; which, otherwise, in entering the open mouth, would have much impeded progressive motion.
It was generally believed, until within a very recent period, that some of the pulmograda were destitute of stomachs. Hence the term of agastric medusa which was applied to them by Peron. The researches of Dr. Milne Ed wards, however, have rendered it probable that this supposition was erroneous, and founded on inaccurate observations. We have now rea son to believe that all the pulmograda have gastric cavities ; but all have not true mouths. There are some in which the only communica tion between the stomach and the outer surface is through numerous ramified canals in the pendent arms, which open externally by ex tremely minute orifices, barely sufficient, even in large species, to admit the smaller ento mostraca. Such a structure exists in rhizostoma. By injecting milk into its gastric cavity, the canals in its arms, and their oscules can be rendered visible ; and it is then discovered that from the minute oscules, which are situ ated in indentations along the margins of the arms, small vessels proceed inwards, and, uniting in twos and threes together, open into one large canal which runs through the middle of each arm. These arms are large, fleshy, foliated organs, eight in number; each of which has a triangular shape. The eight canals above mentioned unite two and two, so as to form four great trunks, which open into a large central cavity,—the only one in the body. This cavity is situated at the base of the central process pendent from the lower surface of the disc. The base, in rising upwards, enlarges into four fleshy columns, which lose them selves in the disc. It is between these four fleshy columns that the cavity of the stomach is placed. The intervals between the columns would form so many openings into this cavity were they not closed by a fine and plaited membrane, which bulges outwards when the stomach is filled. From the circumference of the stomach, at equal distances, sixteen vessels arise, and run directly towards the margin of the disc. These vessels may be regarded as arteries,
and will be hereafter described along with other structures more nearly resembling the parts of a circulating system. But envier* was disposed to consider them as caeca; although lie ad mitted that he could discover no other vessels fitted to discharge the functions of arteries. He remarked that if we regard them as arteries, we must look upon the little vessels which lead from the appendages or arms to the central cavity, as veins, or as lymphatics ; and then we might say that the sea is as a stomach to the rhizostoma, in the same way as the earth acts as a stomach for plants. But, at all events, Cuvier was convinced by his dis sections that alimentary matter enters the body through the marginal oscules of the arms, and that it is accumulated in the internal cavity before passing into the radiating vessels. By experiments on the living animal, Dr. .Milne Edwards has recently provedt that the .circumambient fluid and its contents of mi nute size do really enter the body of the rhizostoma through the margins of the arms. He placed a living rhizostoma in sea-water, .artificially coloured red. The animal did not appear to suffer from the presence of the colouring matter. Within a very short time, the puckered membrane which borders the arms was distinctly tinged red, and, gradually, the colour ascended, until the whole body assumed the same tint. Dr. Edwards does not state, however, whether he traced the progress of the coloured fluid through the brachial canals and the vascular system. On placing the same individual again in pure sea-water, the colouring matter which had been absorbed disappeared gradually, and it seemed to Dr. E. that it was thrown out chiefly from the brachial fringes, but partly also from the margin of the disc, and from the capillary orifices situated at the extremities of the arms. Dr. Edwards satisfied himself that it is impossible for ani mals larger than small animalcules to enter the central cavity of the rhizostoma. But most of the pulmograda have large central mouths, either simple and sessile, or placed at the ex tremity of a projection from the lower surface of the disc. In some, the mouth is more or less patent, but capable of being closed by the approximation of the base of the arms. In others it is surrounded by a ring of conside rable density, in which muscular fibres can be distinctly seen. In medusa aurita, there are, just within the cavity of the mouth, four open ings, which lead, by as many short but wide canals, into four spherical sacs of considerable size. These are completely separated from one another by membranous partitions. That they are stomachs is proved by the circum stance of fishes being found in them.*. From each sac, four vessels arise, which run out wards to the ,circumference of the animal. Other species (e. g. medusa capillata) have the four gastric sacs in free communication with one another ; and, frequently, (e. g. in pelagic, chrysaora, and vgina,) in connexion with these, there are four other sacs, lined with a more dense membrane than the former. These gas tric appendages have the form of simple canals in equorea and tima ; and of branched vessels in medusa and sthenonia.