The first Nnl,-livinious of the A raehnoderrnata established by M. de Blairsville depends on the existence or absence of a solid piece for the sopport of the umbrella or body of tho animal, and consists of tho eirehigrada, which are provided with that support, and of the PrImograda, which are without any such support. Theis- orders, observes 'l. do Blainville, are further distinguished by the very different mann, of the appendages with which the umbrella is fundahed on the buccal surface.
Thu difficulty of examining the Ace/fp/or is, from the very nature of their texture, considerable, and that of preserving them in spirit great It is not, then, to be wondered at that a great portion of their organisation remained for a long time in obscurity, and that much relating to it still remains to be cleared up. To observe them with anything like a satisfactory result, they must be studied on the spot, and while they are alive ; and thus it is that several points relative to their organization and habits, and their generation especially, have only lately been cleared up. The possibility of fairly preserving them in spirit is shown in the museum of the Royal College of Surgeons in London, where, in the department comprehending the first division of the Preparations of Nat. II lat., in spirit, several of the Acalephans, (No, 64 to 73 A, both inclusive) are to be seen so preserved. In the following remarks the structure of the typical .1leduser will be more especially referred to.
The disk presents a uniform cellular appearance internally, and the cellular substance is very soft. In its mass me fibre has hitherto, we believe, been traced, and indeed the quantity of solid matter in the whole animal must be very etnalL Those who have taken .ifedasie out of the sea, and laid them upou it dry board or dry stone, must have observed how soon they shalt into n sort of deliquescence. Spalianzani came to the conclusion that the sea-water penetrating the organic texture constitutes the greater part of the volume in the Mcduar, some of which when newly taken out of the sea weighed 50 ounces, though their dried remains gave a weight of little more than 5 or 6 grains. A fine muscular membrane appearing, when examined with a magnifying glass, to be composed of numerous fleshy fibres disposed in small bundles, radiating at regards the axis of the Medusa, and adhering closely to the gelatinous substance of the disk, may be seen in some species stretching over a given extent of the lower surface of the umbrella, a little within its outer margin. Portions of the disk, or umbrella, have been cut from these animals whilst they were alive: those portions which had no part of this muscular membrane attached to them exhibited no signs of motion ; in those, on the other hand, whose connection with the muscular membrane was preserved, the reciprocal contraction and dilatation were continued for some time.
Those Palmogreula which have cilia around their margins have also circular vessels running along their bases, and most of the projectile and extensile tentacles and filaments have sacs and canals with con tained fluids at their roots. If those cilia may be regarded, and they doubtless may be, as ono of the causes, and a principal one, of loco motion, the pensile tentacles of the Mcduscr may be viewed no ancillary at least to that faculty, though they probably are principally employed as nutrient organs. They aro hollow and simple, and appear to Increase in their extensibility in proportion to their connexion with the appendages of the digestive cavities, or when furnished with a vesicle at their base. Suckers are found at the extremities and along the sides of these tentacles in several of the genera, so as to enable them more eocurely to catch the floating destined prey, or to assist in anchoring the Medusa when it would rest, ns we have reason to believe it occasionally does.
Nerrous System and Kcnacs.—We are not aware of any quite satis factory demonstration of a nervous system in the Acalephans. Dr. Grant indeed (' tool. Trans.,' voL i.) notices a structure hi Cydippe which in his opinion can only belong to that system ; but Endes:holt; whose labours in investigating the organization of this class were not small, failed to discover nerves in the largest which he examined. That they enjoy sight has been a question. Ehrenberg has eteleavonnol to show that Medusa aurita possesses eyes in the form of small red peiute visible on the surface of the eight brown masses which are round the circumference of the umbrella ; and be has compared these so-called eyes to those of certain Rotifera and &demos/ram pie cm Riders the glandular body at the base of the pellicle to be an optic ganglion, and notices its connection with two filaments that decussate about the middle of their course ; and he views these as constituting part of a nervous circle, situated, for the greater part of its extent, directly along the bases of the row of tentacles surrounding the umbrella, and so forming a sort of outer wall of the circular vessel or appendage of the iutestinal cavity which runs round the margin of the umbrella. Ile also describes another nervous circle, formed of four ganglion-like masses. These he states to be disposed round the mouth, and to be each connected with a conosponding group of tentacles.