Thus firmly and strongly is the foundation of the reef laid by the Astraeans; but we have seen that for their prosperous growth they require a certain depth and pressure of water, and when they have brought the wall so high that they have not more than six fathoms of water above them, this kind of Coral ceases to grow. They have, however, prepared a fitting surface for different kinds of Corals that could not live in the depths from which the Astraeans have come, but find their genial home nearer the surface; such a home being made ready for them by their predecessors, they now establish themselves on the top of the Coral wall and continue its growth for a certain time. These are the Mandrinas, or the so-called Brain-Corals, and the Porites. The Mandrinas differ from the Astraeans by their less compact and definite pits. In the Astraeans the place occupied by the animal in the community is marked by a little star-shaped spot, in the centre of which all the partition-walls meet. But in the Mandrinas, although all the partitions converge toward the central opening, as in the Astraeans, these central openings elongate, run into each other, and form waving furrows all over the surface, instead of the small round pits so characteristic of the Astraeans. The Porites resemble the Astraeans, but the pits are smaller, with fewer partitions and fewer tentacles, and their whole substance is more porous.
But these also have their bounds within the sea: they in their turn reach the limit beyond which they are forbidden by the laws of their nature to pass, and there they also pause. But the Coral wall continues its steady progress; for here the lighter kinds set in,—the Madrepores, the Millepores, and a great variety of Sea-Fans and Corallines, and the reef is crowned at last with a many-colored shrubbery of low feathery growth. These are all branching in form, and many of them are simple calciferous plants, though most of them are true animals, resembling, however, delicate Algae more than any marine animals; but, on examination of the latter, one finds them to be covered with myriads of minute dots, each representing one of the little beings out of which the whole is built.
I would add here one word on the true nature of the Millepores, long misunderstood by naturalists, because it throws light not only on some interesting facts respecting Coral Reefs, especially the ancient ones, but also because it tells us something of the early inhabitants of the globe, and shows us that a class of Radiates supposed to be missing in that primitive creation had its representatives then as now. In the diagram of the geological periods introduced in a previous article, I have represented all the three classes of Radiates, Polyps, Acalephs, and Echinoderms, as present on the first floor of our globe that was inhabited at all. But it is only recently that positive proofs have been found of the existence of Acalephs or Jelly-Fishes, as they are called, at that early period. Their very name indicates their delicate structure; and were there no remains preserved in the rocks of these soft, transparent creatures, it would yet be no evidence that they did not exist. Fragile as they are, however, they have left here and there some faint record of themselves, and in the Museum at Carlsruhe, on a slab from Solenhofen, I have seen a very perfect outline of one which remains undescribed to this day. This, however, does not carry them farther back than the Jurassic period, and it is only lately that I have satisfied myself that they not only existed, but were among the most numerous animals in the first representation of organic life.
The earliest Corals correspond in certain features of their structure to the Millepores. They differ from them as all early animals differ from the succeeding ones, every geological period having its special set of representatives. But still they are always true to their class, and have a certain general correspondence with animals of like kind that follow them in later periods. In this sense the Millepores are in our epoch the representatives of those early Corals called by naturalists Tabulata and Rugosa,—distinguished from the Polyp Corals by the horizontal floors, waving in some, straight in others, which divide the body transversely at successive heights through its whole length, and also by the absence of the vertical partitions, extending from top to bottom of each animal, so characteristic of the true Polyps. As I have said, they were for a long time supposed, notwithstanding these differences, to be Polyps, and I had shared in this opinion, till, during the winter of 1857, while pursuing my investigations on the Coral Reefs of Florida, one of these Millepores revealed itself to me in its true character of Acaleph. It is by its soft parts alone—those parts which are seen only in its living state, and when the animal is fully open—that its Acalephian character can be perceived, and this accounts for its being so long accepted as a Polyp, when studied in the dry Coral stock. Nothing could exceed my astonishment when for the first time I saw such an animal fully expanded, and found it to be a true Acaleph. It is exceedingly difficult to obtain a view of them in this state, for, at any approach, they draw themselves in, and remain closed to all investigation. Only once, for a short hour, I had this opportunity; during that time one of these little creatures revealed to me its whole structure, as if to tell me, once for all, the story of its existence through all the successive epochs from the dawn of Creation till now, and then withdrew. With my most patient watching, I have never been able to see one of them open again. But to establish the fact that one of the Corals represented from the earliest period till now, and indeed far more numerous in the beginning than any other, was in truth no Polyp, but an Acaleph, the glimpse I had was all-sufficient. It came out as if to bear witness of its class,—as if to say, "We, too, were among the hosts of living beings with which God first peopled His earth."
With these branching Corals the reef reaches the level of high-water, beyond which, as I have said, there can be no further growth, for want of the action of the fresh sea-water. This dependence upon the vivifying influence of the sea accounts for one unfailing feature in the Coral walls. They are always abrupt and steep on the seaward side, but have a gentle slope towards the land. This is accounted for by the circumstance that the Corals on the outer side of the reef are in immediate contact with the pure ocean-water, while by their growth they partially exclude the inner ones from the same influence,—the rapid growth of the latter being also impeded by any impurity or foreign material washed away from the neighboring shore and mingling with the water that fills the channel between the main-land and the reef. Thus the Coral Reefs, whether built around an island, or concentric to a rounding shore, or along a straight line of coast, are always shelving toward the land, while they are comparatively abrupt and steep toward the sea. This should be remembered, for, as we shall see hereafter, it has an important bearing on the question of time as illustrated by Coral Reefs.
I have spoken of the budding of Corals, by which each one becomes the centre of a cluster; but this is not the only way in which they multiply their kind. They give birth to eggs also, which are carried on the inner edge of their partition-walls, till they drop into the sea, where they float about, little, soft, transparent, pear-shaped bodies, as unlike as possible to the rigid stony structure they are to assume hereafter. In this condition they are covered with vibratile cilia or fringes, that are always in rapid, uninterrupted motion, and keep them swimming about in the water. It is by means of these little germs of the Corals, swimming freely about during their earliest stages of growth, that the reef is continued, at the various heights where special kinds die out, by those that prosper at shallower depths; otherwise it would be impossible to understand how this variety of building material, as it were, is introduced wherever it is needed. This point, formerly a puzzle to naturalists, has become quite clear since it has been found that myriads of these little germs are poured into the water surrounding a reef. There they swim about till they find a genial spot on which to establish themselves, when they become attached to the ground by one end, while a depression takes place at the opposite end, which gradually deepens to form the mouth and inner cavity, while the edges expand to form the tentacles, and the productive life of the little Coral begins: it buds from every side, and becomes the foundation of a new community.
I should add, that, beside the Polyps and the Acalephs, Mollusks also have their representatives among the Corals. There is a group of small Mollusks called Bryozoa, allied to the Clams by their structure, but excessively minute when compared to the other members of their class, which, like the other Corals, harden in consequence of an absorption of solid materials, and contribute to the formation of the reef. Besides these, there are certain plants, limestone Algae,—Corallines, as they are called,—which have their share also in the work.
I had intended to give some account of the Coral Reefs of Florida, and to show what bearing they have upon the question of time and the permanence of Species; but this cursory sketch of Coral Reefs in general has grown to such dimensions that I must reserve a more particular account of the Florida Reefs and Keys for a future article.
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