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those naturalists who are examining the physiology of the Radiata. It represents the vascular system of higher animals, and regulates the form of the corallites and the symmetry of large compound assemblages of them. There is no true blood circulating in the water canals or forming in the visceral cavities, but the fluid which passes along them, thanks to ever-active ciliæ, is highly aërated sea-water, holding granules in mechanical suspension, and some of the products of digestion in solution. The outside of the coral, that is to say, the membrane covering the sides and costa and the disc with its tentacules and lips are essentially respiratory surfaces, and the ciliæ are constantly changing the particles of the fluid in contact with their supporting tissue. No nourishment can be got into the tissues in this manner, and it must be admitted that the layers of cells, and nematocysts, and ciliated epithelium remote from the visceral cavities are nourished by a water system.
Within the calice, the mouth, the short esophageal tube, the stomach, the lining of the gastric and the perigastric cavities, the mesenteric folds, and their fringes, the lower part of the disc, the inside of the tentacules, and in stony corals the membranes covering the hard parts, are all in constant contact with circulating water, holding matters more or less assimilable in solution. There is a current setting in through the mouth and stomach, then laterally, and along the mesenteric folds. On the other hand, there are currents tending to carry the water out of the gastric cavities upwards through the mouth, and in and out of the water canals about to be described. Before passing to this subject, it is necessary to observe that whilst the water system of the solid hard stony corals is of necessity limited, some of the commonest divisions of sclerodermic Zoantharia, such as the Fungidæ and the Perforate or Porose corals, generally speaking, are so formed that there must be a very free communication between the cavities and the external medium, as well as between neighbouring corallites.
The markings of the water-canals on the surface of the hard corallum of Corallium rubrum are very visible, and although they are not seen on the stems of the sclerobasic Zoantharia, still LucazeDuthiers discovered them in his careful examination of Gerardia, one of the Antipathes. He proved that the Alcyonaria are unia versally furnished with a water-canal system, and he infers its presence in the Antipatharia generally. The neighbouring and the distant polypes of the soft covering of the Gorgonidæ and Antipathariæ have a very close unison as regards their growth and nutrition, and the symmetry of these processes, and the nutrition of the
parts remote from the centres of digestion can only be produced and accounted for by admitting the presence of a water system. In Gerardia there are more or less polygonal lines separating the neighbouring polypes, and the dark colour of the hard stem is there visible. Now a series of canals exists in such spots communicating laterally with the water canals of the polypes on either side. The water canals are formed, united, and covered by the same kind of tissue as constitutes the outer covering of the tentacules, and they open into the bottom of each perigastric cavity in every polype, just as the tentacular canal opens at the top. The inner walls of the water canals are very cellular, and resemble the structures of the inner part of the tentacules. There may be two, three, or four layers of cells, according to the calibre, and the innermost series is ciliated, and is continuous with the membrane lining the gastric cavity. In the Alcyonaria, and particularly in Corallium rubrum, there are several layers of superimposed water canals, but this is not the case in the Antipatharians like Gerardia, which has only one series. The solitary opening of the water canal at the bottom of each perigastric cavity is peculiar to the division, but in the Gorgonidæ there are many openings in the outside wall for several canals in each space between the mesenteric folds. This great development has reference to the greater thickness of the soft parts in the Alcyonaria. One of the most remarkable phenomena of the growth of the stony corals is the constant likeness which pervades the external form of the species. Some branch, and always in the same manner; others are invariably in a certain bush-like shape, many are flat, and never anything else in form, whilst scores are symmetrically globular, hemispherical or fungoid.
Lucaze-Duthiers inferred the existence of a water system in such stony corals, or Madreporaria (sclerodermic Zoantharia) as Dendrophyllia ramea, because the calices are very distant, and the intermediate ramose sclerenchyma is constantly growing. He very reasonably decides that this intercalicular growth cannot be accounted for by any immediate influence of the assimilation going on within the calices, but must have reference to nutritive processes carried on in the calices primarily, and influencing in a secondary manner the hard intermediate tissues with their soft parts, by means of a water system.
MM. Michelotti et Duchassaing assert that they have discovered and demonstrated a water system between the calices of some Heliastarans and other compound Madreporaria of the West Indies. They place the canals along the course of the costæ, which reach
from the outer ends of the septa more or less over the hard tissue that binds the corallites together.
In the Madreporaria, as in the other families of the Actinozoa, the fluid circulating through the canals of the water system carries more or less oxygen for the purposes of respiration besides the usual salts of the sea, and the digested salts and other eliminated matters, for the nutrition of the contiguous soft and hard tissues : it probably carries off the products of the secondary assimilation of the tissues, and must have something to do with a power of absorbing hard sclerenchyma, which is more or less necessary for the growth of certain corals.
The quantity of assimilable matter produced by the digestive processes, and then circulated more or less by the water system, must be very small when compared with the quantity of water there is to dissolve it, if the stomach, as it is called, is the sole organ for digestion. But it is very probable that the whole of the surface of the gastric and perigastric cavities has more or less of a power of digesting, for the size of the true stomach does not increase in a like ratio with the extent of the water system, the dimensions of the folds of the perivisceral cavities or the rapidly-increasing hard parts. In an aquarium corals undergo a slow starvation, and no idea can be formed from examining any well-stocked aquarium, of the enormous amount of microscopic life which exists in the sea, under certain circumstances. It is not an exaggeration to say that very often in warm summers minute assimilable entities are so prevalent in some seas that they constitute a quarter of the bulk of the whole. Under such circumstances, the vital activity of the Actinozoa becomes very great, and all the organic processes are in full vigour.
The Plate represents the corallum of Corallium rubrum growing on the same block as Gorgonia verrucosa, and a magnified view of a polype of Corallium rubrum (after Lucaze-Duthiers) is drawn below. The usual appearance of Anthelia glauca (after Edwards and Haime) is illustrated also.
COLOUR IN THE MOON.
BY THE REV. T. W. WEBB, M.A., F.R.A.S.
THOSE who view the Moon for the first time through a telescope are frequently much impressed by the whiteness of its aspect, which has been not unnaturally compared with the effect of snow, or of an iced cake; and they would probably feel much surprise if they were told that, as viewed from the Moon, the appearance of our Earth might not be very dissimilar. And yet such would be the fact. We would not be understood to assert that the light of our globe would be of equal whiteness, or perhaps equal intensity, with that reflected by the Moon, or that there would be no difference between the distant aspect, as to colour, of its continents and oceans, its forests, cultivated lands, and deserts. Such, there is reason to believe, would not be the case. The Earth is probably a more particoloured object to the Moon, than the Moon is to the Earth. We cannot, it is true, reach a sufficient height above our own globe, even in a balloon ascent, to compare our prospect with that at the remoteness of a quarter of a million of miles; and we can only reason from more limited data ; but these are sufficient to establish the assertion, as far as local colouring is concerned. We must not, indeed, overlook a large abatement which has to be made for the profusion of watery vapour in our atmosphere : the abundance of white light reflected by the upper sides of clouds and mists must greatly modify our distant appearance; as it is believed to do that of the planet Jupiter. There, the larger part of the globe is usually covered by what, as far as we can judge, are masses of luminous cloud; and the whole effect must be very unlike what it would have been had the atmosphere allowed a free view of the body beneath it. The comparison, it may be said, is somewhat unfair, inasmuch as a far greater extent of the surface of Jupiter, there is reason to believe, is thus concealed than is usually the case with our own Earth, and the real colouring of our globe must be in proportion more apparent to a distant spectator ; still it is sufficiently correct in the main to illustrate the fact, that the light reflected by the Earth upon
the Moon, considered as a whole, is whiter on this account than it would otherwise have been ; though the cloudless openings, especially in our tropical regions, no doubt afford a fair view of a curiously variegated sphere. To some persons, indeed, the idea may appear somewhat strange and far-fetched, that the Earth should ever appear as a brightly luminous body, or that at the distance of Mars or
Jupiter it would shine like one of those planets, and possibly with equal splendour, in the midnight sky. Yet there is no doubt that such is the fact. The difficulty of conceiving this arises from the circumstance that the sunshine upon those globes is viewed by us in the absence of the Sun; but we cannot do this in the case of our own globe. We never see it enlightened, that is, in reality, shining with reflected light, except in the presence of the original source of that light, and then the native splendour of this last, aided by a strongly illuminated atmosphere, overpowers the impression that it would otherwise produce even after reflection. For we need only compare the appearance of the landscape surrounding us at noonday and midnight, to satisfy ourselves that if it were possible to conceal the Sun and darken the sky, while the Earth continued to glow as it does in sunshine, the latter would be not only decidedly but strongly luminous. This effect, impossible to us, is not only possible, but realized in the case of the planets. Owing to the concealment of the Sun and the darkening of the atmosphere by the interposition of the Earth, we actually see their globes illuminated by full sunlight, while the source of light is out of sight, and the reflection alone is visible. And what we see in them, we should doubtless see in ourselves under the same circumstances; though what may be the comparative reflective power of the Earth is of course an unanswerable question.
Admitting, then, as we must, that our globe is, at a distance, a luminous planet, and that there is an earthshine upon the Moon, corresponding with the moonshine upon the Earth, we return to our original remark as to the striking and uniform whiteness of the light of our satellite; exceeding, probably, that of the Earth; and we are led to speculate as to its cause. We shall be in a better position to do this, if we first examine with some care the more familiar case of our own reflected light. Our globe, with a surface much more diversified from the presence of water and its results, would naturally, as we have said, present to the Moon a disc more variegated with colour. As to general effect, this would be quite true ; but in detail it would be less minutely spotted and streaked than might at first be supposed. We are not now speaking of its appearance towards its outer edges, where the greater depth of atmosphere through which it would be viewed would weaken its tints by its imperfect transparency, and change them by the inequality of its absorption on different parts of the spectrum. But even as regards the central portion of the visible hemisphere, where our atmosphere, at least where unclouded, would produce but little appreciable effect,