The Alepons, like the Aleponide, are repro duced by ova, which are formed in membra nous ovaria of precisely similar construction, and also by gemmm, which in the Alcyon are developed around the pre-existent polypes, and thus augment indefinitely the number of individuals united upon one stock. There is, however, a very important difference observ able between these two genera of zoophytes, in other respects so similar. In the Alcyons the abdominal cavity of the young polypes is not directly continuous with the abdominal cavity of their parent, and it is only by the intermedium of the vascular system described above that they are placed in communication with each other ; a modification which de pends upon another difference in the mode of formation of the reproductive gemmae. When an Alcyon stock is about to put forth a new branch, the spongy part of the polypary (that portion which is formed by the external tunic of the polypes and permeated by the vascular 'network) begins to increase in size at some determinate point of its periphery, and soon produces a tubercle of greater or smaller size, into which the vessels spoken of above are continued, and form numerous anastomoses with each other. At this early period of de velopment the new branch presents no trace of polypes, but its vascular tissue is neverthe less already studded with calcareous crystals, and exactly resembles that situated in other parts of the common mass between the abdo minal cavities of the adult polypes. It must, therefore, necessmily be traversed by the cur rents which circulate in the general vascular system. On dissecting one of these newly formed branches the vestiges of young polypes rnay be distinguished; and if the_s_prouts ex amined are still further advanced, it is easy to distinguish the young animals within, already possessing the form they will afterwards ex hibit, but which have not yet established a communication with the exterior. At length, however, this communication is established, and the newly formed polype only differs from the pre-existing ones in its small size, and as it grows its increase causes the enlargement of the polypary of which it forms a part. In this case it is very evident that the part which gives birth to the reproductive gemmm is no portion of the individual polypes of the Alcyon, but is common to them all. The generative tissue surrounds these little beings with a sort of living sheath, and produces in the interior of its own substance new polypes, quite independently of those previously in existence. These polyparies might therefore be compared to a sort of common ovary, the products of which are never completely indi vidualised, but remain permanently lodaed in its substance, and minister to the support of its existence and the aggrandizement of its tissue.
This singular mode of reproduction, M. Milne Edwards observes, seems at first sight to be very different from that observed in the Akyonidium; but, on reflection, a considerable analogy may be traced between them. In Alcyonidiunz the internal tunic of the abdo minal cavity fulfils the functions of an ovary, and produces at determinate points both gemmze and ova; whilst in Alcyon, on the contrary, while the internal membranous layer gives birth to ova, the gemmm are developed elsewhere, from the canals which permeate the common mass. But the membrane which forms these canals, and which is the seat of this kind of vegetative reproduction, is merely a continuation of the internal tunic ; and hence it is easy to understand how it may fulfil analogous functions.
CORALLIDX.—The cortical polypes, as they have been named by authors, mainly differ from the Alcyonidm from the circumstance that the fleshy cortex which constitutes the common polypary, instead of being merely indurated by the deposition of earthy spicula in its interior, secretes for itself a solid central axis, upon the ramifications of which it is spread out, and thus enabled to form itself into arborescent expansions, of dimensions such as would be quite unattainable without this arrangernent. The composition of this central axis varies in different genera; some times it is dense and stony (Lithophyta) ; sometimes flexible, and composed of a horny substance (Keratoplzyta); this difference is, however, of no physiological importance, for very frequently the two substances are se creted in the same individual in different por tions of its substance. The solid element in the Lithophyta is carbonate of lime ; in the Keratophyta it is concrete albumen.
" A species of Gorgonia, of a black colour and high polish, like black sealing-wax (Anti pathes?), examined by Mr. Hatchett*, was found, by immersion in dilute nitric acid du• ring 28 days, gradually to become transparent and of a bright brownish yellow. In this softened state it was steeped two days in water, and was then opened longitudinally ; by this the whole structure became apparent, and consisted of thin coats or tubes of a beautiful transparent membrane, which be ginning from a central point gradually be came larger, according to the order in which they receded from the centre. These mem branes were so delicate that the fibrous tex ture could scarcely be discerned.
The acid in which these had been steeped was tinged of a very pale yellow. Ammonia being added changed it to a deep yellow or orange colour; but the transparency of' the liquor was not disturbed by this or any other precipitants which had been employed in the former experiments.
When this Gorgonia was exposed to a red heat, it crackled and emitted a thick smoke, with the smell of burnt horn. The shape was soon destroyed, and a compact coal re mained. By continuing the red heat, a very small portion of white matter was obtained, which, as far as the quantity would allow, proved to be muriate of soda with some carbonate of the same.
The results of the experiments on certain Gorgonim, such as Ceratophyla, Flabellum suberosa, pectinata, and setosa, were not a little remarkable; for when the two portions which compose these Gorgonix, viz. the horny stem and the cortical substance with which it is coated, were examined separately, it was proved, — 1st, That the stems of these Gorgonim con sist of a substance analogous to horn, and that the horny matter contains a quantity of the phosphate of lime, but scarcely any of the carbonate.
2d, That the cortical part consists prin cipally of the carbonate of lime, with very little or none of the phosphate; and the car bonate is deposited in and upon a soft flexible membranaceous substance, which seems much to approach the nature of cuticle.
The coral of commerce, Corallium rubrum, is, perhaps, one of the most interesting examples of this division of Polypiferous zoophytes. In its living state this animal resembles a short stunted tree fixed to the surface of the rock by a broadly expanded base, from which it rises, at first with a single stem of varying magnitude, which soon di vides into branches so as to resemble a leaf less shrub rising to the height of about 18 inches. The central axis of the coral is of stony hardness, insomuch, indeed, that to this circumstance it owes its principal value in commerce, on account of the high polish of which it is susceptible. In the growing coral this stony centre is entirely invested with a fleshy cortex that constitutes the living por tion of the zoophyte whereby the central stem is deposited, and the whole external surface is studded at intervals with polypes, in structure exactly resembling those of the Alcyonidx, both in the number of their arms and general structure. During the au tumnal months gemmules are formed in the ovaria of these polypes, which are described as being at first white, but afterwards of a bright red colour ; these detach themselves separately from the little white groups with which they were originally connected by fila ments or umbilical cords. They escape thus into the body of the polype, behind its sto mach, where they are seen to be perfectly free, and change their position by means of their vibratile cilia. When the polypes are expanded, the sea-water has a free passage through the stomach to the gemmules, which receive new vigour from its influence, and they advance towards the open posterior part of the stomach, become entangled in its aper ture, pass through the stomach, and escape through the mouth. They then move about, by means of their cilia, in search of a place where to fix and develope themselves.
The stony branches of the Coral are suffi ciently short and strong to resist the violence of the sea, which otherwise wobld break so fragile a substance, but in the Gorgoniie and Antipathes the ramifications are so long and slender, that they would inevitably be broken by the movements of the surrounding water, were it not that the nature of their central axis is materially modified. This part of their structure is therefore very considerably modified in its texture, and being composed of flexible materials is enabled to bend beneath the passing current and rise again uninjured, while in the Isis Hippuris (fig. 37) a similar result is ob tained by combining the horny and calcareous matter in alternate joints. In these latter polyparies, however, although their central axes 'are principally composed of corneous substance disposed in concentric layers, the living cortex itself is full of granules of a cal careous nature mixed with colouring matter that varies in different genera, and as this cortex dries in a thick layer upon the central stem when the Gorgonia is removed out of the water, the varieties of colour exhibited by these zoophytes is conspicuous even in our cabinets. Cavolini# prosecuted for two successive years, 1784 and 1785, his researches on the structure of Gorgonia verrucosa (Lam.), and found the anatomy of the polypes dis persed over the surface of each branch to be similar to that described above as common to the Aleyonidm and Coral; he detected the position of the ovaria at the base of each polype, and observed that the ova were dis charged through eight small oviducts that open between the bases of the eight tenta cula. These ova lie describes as ciliated gemmules which, on their escape, swim to and fro in the surrounding water, and asserts that he saw a portion of Gorgonia, only eight inches high, discharge ninety of these in the space of an hour from the different polypes studding its surface,