From the stomach of rhizostoma, formerly described, sixteen vessels arise, and pursue a straight course outwards to the margin of the disc, near which they all enter, at equal dis tances, a circular vessel, which passes com pletely round the circumference of the animal. Four of the radiating vessels correspond with the four fleshy pillars of the process supporting the arms, and there exists on the internal sur face of each of these pillars, a groove, which establishes a direct communication between the corresponding vessel, and one of the large vessels of the central process. The other twelve are distributed by threes in the intervals between the first four, and arise from those parts of the stomach which are closed by the plaited membranes. The space intervening between the circular vessel and the margin of the disc is occupied by an innumerable multi tude of little vessels which form a net-work like the finest lace.* In medusa aurita, there are also sixteen radiating vessels, four of which arise from each of the four sacs, into which the gastric cavity in this species is divided. Two of the four vessels in each group are simple, the other two are several times bifurcated ; both the simple main trunks and all the branches so formed, enter a circular vessel sur rounding the disc, which seems to be connected also with the tubular cavities of the numerous cilia which surround the margin like a fringe, and which are capable of elongation and con traction.t Carus remarks with regard to the circular vessel, that " it may be considered as an extremely simple rudiment of the great cir culation of superior animals, in case we view the radiating as chyliferous vessels."T V. Respiration.—It is probable that the air bladders of the physograda, the swimming organs of the diphyda, and the cilia of the ciliograda are all subservient, in a greater or less degree, to the respiratory function, as well as to locomotion. The vessels in the last men tioned class, which have been described above as appertaining to the circulating system, are regarded by some as respiratory organs ; and by Lamarck were compared to the tracheae of insects. They have been called aquiferous tracheae. Those who consider them in this light believe that they are open at two points, so as to admit the circumambient fluid to pass freely through them. The most recent and accurate observations, however, leave it doubt ful whether this really takes place in the ciliograde acalephm.
With regard to the pulmograda, several parts and organs have been pointed out by different observers as being, in all probability, the seats of the respiratory function. Cuvier thought that the delicate plaited membranes which exist between the fleshy pillars of the central process in rhizostoma, and which form in part the walls of the stomach, might be re garded as the organs of respirarion. Eisenhardt supposed that he saw them in certain tentacu lated processes attached to the membranous partitions which divide the gastric sacs of some species from one another ; while Gaede looked upon the four small sacs which overlie the gastric cavities in medusa aurita as subser vient to the same function. These sacs com municate directly with the gastric cavities by means of openings in the membranous par titions which separate them. The partitions bear on their inferior surfaces, plaited mem branes, which, under the microscope, present the appearance of being studded with vesicles containing a little watery fluid. A row of filamentary organs is also attached to these membranes, which move like external cilia, even for some time after they have been re moved from the body of the animal.
VI. Secretion.—The existence of this func tion in the acalephw is made known to us by the emission from their bodies, under certain circumstances, of a glairy mucus ; by the stinging effect which some unknown product of their organization has upon our skin ; and by the remarkable phenomenon of luminousness, which a large number of them present. The organs by which the mucus is secreted have not been satisfactorily observed. Dr. Milne Edwards saw reason to conclude with regard to the rhizostoma, that a large quantity of this fluid is secreted by a glandular structure situated along the margins of the arms. The stinging property possessed by several animals of this class has been the subject of inquiry since the time of Aristotle, but to this day we remain in doubt with regard to the nature and mode of production of the agent which causes this effect. Some men seem to be in sensible to the irritation generally produced by the contact of living acalephce. But, for the most part, a slight touch of any part of their surface, and chiefly of the pendent tentacula, is followed within a few minutes, at most, by a burning pain, redness, swelling, and some times even a vesication, of all that portion of the skin which touched the animal. Sloane
said of the physalus, (" what the seamen call caravels, or Portuguese men-of-war,") " They burn violently—they do suck themselves so close to the skin that they raise blisters, and cause sometimes St. Antony's fire." Even on our own coasts, severe cases of inflammation of the skin are occasionally seen, which have been produced by the irritation received during bathing from some of the larger pulmograda. In physalus, the stinging property seems to reside chiefly in the fluid with which the ten tacula are filled. It continues to act power fully even after the organs containing it have been detached from the body. And not only so, but it is said by some observers that its peculiar properties are so permanent, that vessels in which the animals have been placed must be washed several times in water, and carefully scoured before they can be used without inconvenience. On one occasion it was found that linen, which had been merely rinsed in soap and water, had this quality of irritation fifteen days after it had been used in making observations on the physalus.* None of the cirrigrada hitherto examined possess the stinging property.
The organs by which the luminous matter is elaborated are unknown. In some species, it is evidently mixed with the mucous fluid, which is so abundantly poured out from the margins of the arms and the disc. It has been frequently observed that the ciliograda are luminous chiefly along their rows of cilia, and that these continue to emit light for some time after their removal from the body. Perhaps the greater number of the acalephw are lumi nous. According to Dr. M'Culloch, all inhabiting the British seas are so ; and indeed it is chiefly to the emission of light by animals of this class that the beautiful phenomenon of the luminousness of the sea is owing in all situations. Spallanzani, however, whose observations and experiments on this subject were as extensive as they were careful and ingenious, came to the conclusion that " the medusa which are possessed of lumi nous properties are extremely few compared with those which are destitute of it." The same philosopher remarked, with regard to some of the pulmograda, that they emit light more strongly during the contractions of their disc than at other times ; that the intensity of their light increases when they are pressed in any way ; that the luminousness resides chiefly in a peculiar fluid secreted by glands situated around the margins of the disc, along the edges of the tentacula, and in the fringed partitions of the gastric cavities ; that this fluid being mixed with other fluids, as with fresh and salt water, and especially cow's milk, imparts its lumi nousness to them ; that when spread over solid bodies it continues to shine for several minutes; and that in it there generally exists that irri tating substance which produces the stinging effect. Spallanzani applied some of this fluid on two occasions to the tip of his tongue. It excited a burning sensation, which lasted more than a day. A similar feeling, but much more painful, followed the accidental application of a single drop of the same fluid to the conjunctiva.t In most of the acalephaa, the external cover ing is very fine, smooth, and delicate ; but sometimes it is granular, or even warty. It does not appear that these differences in its structure have been observed by any naturalist to be connected with corresponding differences in the power of emitting light. (See LUMI NOUSNESS, ANIMAL.) VII. Generation.—The organs of this func tion cannot always be satisfactorily ascertained. This may, in a great measure, be owing to their minuteness and transparency when not in action. Ovaria and oviducts, however, are dis tinctly seen in several species; but no other organs connected with the generative function have hitherto been discovered. According to Eschscholtz, the ovaria in the physograda con sist of several groups of vesicles and filaments, loosely attached to the lower surface of the air-bladder. In the diphyda, they are in the form of numerous vesicles, having thick tunics filled with an opaque white fluid, and situated within one of their swimming organs. Such parts were seen by Eschscholtz only in some individuals, and on this account he was dis posed to regard them as ovaries. But Messrs. Quoy and Gaimard seem to consider it more probable that the minute botryoidal bunches of vesicles, which surround the base of each sucker on the lengthened filaments, (before alluded to as being subservient both to nutri tion and to locomotion,) are the ovaries.* It does not appear that either Eschscholtz or Messrs. Quoy and Cai mard saw the ova.