42 



ACALEPHvE. 



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. \\ hen 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, 

 salpse, &c., have been found in the stomachs 

 of cihograda. 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 rhizostonm. 

 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 Cuvier* was disposed 

 to consider them as cceca ; although he 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 rhizostoinu, 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 proved \ 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. la 

 others it is surrounded by a ring of conside- 

 rable density, in which muscular fibres can be 

 distinctly seen. In medusa aurlta, 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- 



* Journ. de Phys. xlix. 438 



t Ann. des Sc. Nat. xxviii. 24!(. 



