[From Sharpe's Magazine]
A FEW WORDS ON CORALS.
It is the object of the following papers to illustrate the natural history of the ocean, and to introduce to the reader a few of the forms of life which the naturalist meets with in the deep sea. The sea that bathes the globe contains as countless multitudes of living beings as does the land we tread, and each possesses an organization as interesting and as peculiar to itself, as any of the higher forms of the animal creation. But the interest does not cease here, for these marine invertebrata play an important part in the vast economy of nature, some living but to afford food for the larger kinds, others devouring all matter devoid of vitality, and so removing all putrescent materials, with which the sea would otherwise be surcharged; while others, again, living in large communities, surely and slowly, by their gradual growth, so alter the physical construction of the globe as to render seas and harbors unnavigable, and in many eases even to give rise in course of ages to those islands, apparently of spontaneous growth, which are so common in the Southern Seas.
Corals and Madrepores first claim our attention, because they occupy the lowest place, with the exception of sponges, in the animal scale! Indeed, so low is their organization, that former naturalists denied their animal character, and from superficial examination of their external appearance, placed them among the wonders of the vegetable world. And from the arborescent and plant-like form assumed by many kinds, in the Flustra and others, in which the resemblance to sea-weeds is so strong as generally to cause them to be confounded together under the same group, and being fixed to submarine rocks, or marine shells, observers might easily have been led to the mistake, had not modern research rectified the error. Corals and Madrepores, as they are known to us, consist but of the stony skeletons of the animals themselves, for in the living state, while dwelling in the ocean, each portion of the stony framework was covered with an animal coating of gelatinous matter, which, closely investing it, was the living portion of the animal. But the structure of the animal is not simply this, for attached to different portions of it in the living state are to be found a countless number of little cells, which, armed with tentacles of great prehensile and tactile powers, are the apertures through which the particles of food are conveyed for the sustenance of the animal These bodies as they may be called, are the analogues of that simple polyp, the common hydra, which, abounding in almost every pond, has been long known to naturalists. It consists of a single dilated gelatinous vesicle, which is terminated at one extremity by a sucker, and at the other by a number of contractile filaments, which serve as the tentaculæ, by which it seizes its prey. This is all that represents the animal, the dilated portion of the tube being the part in which the process of digestion is carried on, and where the food is assimilated to the wants of the little creature. These hydrae live singly, each animal being independent of another, and each possesses the power of self-reparation; so that, should it happen that a tentacle is lost, another sprouts to supply its place, or should the naturalist by way of experiment divide it in half, each portion immediately reproduces the wanting section. Such, then, is briefly the structure of the simple fresh-water hydra, a polyp of common occurrence, and from this description the reader will gain some idea of the polyps of the Coral family before us; but he must remember that in the case now under discussion, the polyps are aggregated together, a number on one common stem, each possessing independent life, but all ministering to the support of the compound animal.
The hydra, then, of the Coral and Madrepore, thus explained, would appear to be the parts through which food is absorbed for the general nourishment of the body, which, as before observed, consists simply of a gelatinous film of animal matter, possessing but little evidence of vitality. Here, then, is a community of nourishment, and with it also a community of sensation, for if one portion be irritated, contiguous portions of the animal are apt to sympathize. When the Coral polyps are not in an active state, or in other words, when they are not in want of food, these hydra-form polyps may not be visible, but being retracted into cells found as depressions in the skeletons of the Madrepores, they are lost to observation, and it is only when in quest of food and nourishment that their contractile tentacles are expanded, and distinctly prominent.
The physiology of the growth of the skeleton, both in the Madrepores and the Coral, is the same. The entire skeleton, however ramified it may be, or whatever form it may assume, is secreted by the living matter with which it is invested, the materials for its formation being derived from the element in which it lives; and as its deposition takes place at different times, the central stem of some corals is apt to assume a beautiful concentric arrangement of laminæ. But the material deposited or secreted need not necessarily be hard or calcareous, but even may partake of the character of horn or other flexible materials, as is the case with some of the coral family. In other cases there is an alternation of each material; and the necessity of this change in the character of the skeleton will now demand our attention.
The common coral of the Mediterranean, possessing a stony skeleton, is found in situations where its stunted form and its extreme hardness sufficiently preserve it from the violence of the waves; but place a coral under other circumstances, and expose it to the storms of the Indian Ocean, where the waves rage with fury, dashing on and uprooting all things within their power, and the structure of the simple coralium would fail to withstand their violence. Here, then, under such circumstances, in the case of the Gorgonia, nature has provided a horny and flexible skeleton, which, spreading majestically in the sea, shall be capable of bending beneath the weight of the superincumbent waves, and so yielding to the storms. Nature has thus adapted herself to each contingent circumstance.
The next point to which we shall advert will be coral formations, which form so interesting a study to the naturalist and geologist. When we consider that we have at hand only a soft, gelatinous covering, stretched on a hard, stony frame-work—that the material on which this animal substance exists is furnished by the sea in which it lives—we can not but be surprised at the smallness of the means which nature uses for the execution of her great designs. But time compensates for the insignificance of the means employed, and the continued activity of nature's architects, during continuous ages, accomplishes these stupendous results, which have at various times excited the wonder of the navigator, and aroused the attention of the naturalist. Many examples of these are to be found in the Pacific Archipelago. Seas and shallows, once navigable, become in the process of time so filled by these living animals, as to become impassable, their stony skeletons forming hard, massy rocks and impenetrable barriers, which, rising from the bottom of the sea and shallows, constitute solid masonry of living stones.
But besides thus aggregating in the neighborhood of land and continents, formations similarly produced are constantly met with during the circumnavigation of the globe. Not only barriers and reefs owe their origin to these humble means, but large lands, stretching for miles in the centre of the ocean, rise gradually from beneath the surface of the sea, and, becoming clothed with verdure and vegetation, at last offer a resting-place for the daring seafarer. But now occurs the interesting question, How happens it that these islands are found in situations where the sea is too deep to allow of any animal life to exist? And yet these corals must have grown upward from some resting-place. The researches of Darwin have shown that the greatest depth in which corals live, is between thirty and forty fathoms beneath the surface of the sea; hence it is absolutely certain that for every island some foundation must exist in the sea for these reef-building animals to attach themselves to. Such foundation, from the observation of Darwin, would appear to be provided by submarine mountains which have gradually subsided into the sea, having originally existed above its surface. Upon these foundations the reef-building saxigenous corals have become attached, and slowly accumulating in large numbers, and gradually depositing their carbonate of lime, during the lapse of ages, by degrees construct these large piles, which, at last emerging from the ocean's bosom, appear as newly-formed continents and islands. Once above the surface, the work of the corals is at an end; no longer exposed to the salt water, the emerged portion dies, and then new agencies are called into play, before its surface can be clothed with vegetable life. The storms of the ocean and the rising waves gradually deposit on its surface the sand and mud torn up from the bottom of the sea, and the sea-weed, too, that is cast upon its tenantless shores soon crumbles into mould, and unites with the debris of the former polyps. At last, some seeds from the neighboring lands are driven to its strand, and there finding a soil united for their growth, soon sprout, under the influence of a tropical sun, into fresh life, and clothe the ocean isle with verdure and vegetation.
Then, last, man comes, and taking possession of the land, erects him a house to dwell in, and cultivating the soil he finds, soon converts the ocean-rescued land into cultivated plains. Islands thus formed are constantly increased in circumference by the same means as those that gave them birth; the same agency is ever at work, adding particle on particle to the rising land. But is it not strange that such simple means can resist the ever-flowing and roaring ocean—that such simple animals can uprear a masonry which shall resist the violence of the waves and defy the power of the breakers? Is it not strange that a single polyp can form a structure in the bosom of the ocean, which shall stand, a victorious antagonist to the storm when works of man and other "inanimate works of nature" would have crumbled into nothing before the relentless fury of a disturbed ocean? "Let the hurricane tear up its thousand huge fragments, yet what will that tell against the accumulated labor of myriads of architects at work day and night, month after month?" for here organic force is opposed to the raging elements, and opposing, is victorious.