A. Hyatt Verrill
Probably no product of nature is more universally admired and wondered at than corals. From a commercial point of view they have been of value from time immemorial. As objects of beauty and ornament they are used the world over, and when burnt and ground to powder they are utilized as lime. The form in which they are usually seen, however, (dried, cleaned and bleached), unfortunately does not give a fair idea of their real beauty and delicacy of coloring. To be fully appreciated they must be seen growing on their native reefs of the tropics, where they show forth all their prismatic colors and delicate tentacles through the crystal clear water.
The various forms of coral, to say nothing of the different species, are legion. The huge, dome-shaped brain corals,— sometimes attaining a diameter of eight or ten feet,—stand at the head of the list as regards size, while the graceful, many-branched madrepores are probably the most delicate of all. Then there are the compact, round, rose corals, with their serrated petal-like partitions; the thin, semi-transparent hat corals; the curious cup-corals; the fragile millepores; the heavy star-corals; the rosy, precious coral and the closely-growing organ-pipe coral.
All are, however, completely enveloped in a fleshy mass of polyps when alive, and each polyp represents a separate animal, each living independent yet in concert with the others for the common good,— a wonderful example of successful socialism. In the majority of species the individual animals closely resemble sea anemones, consisting of a fleshy, retractile stalk ending in a roundish disc, surrounded by tentacles and furnished with a central mouth. These animals vary in color, number and size of tentacles, etc., according to the species and environment. While the animals of the brain corals are closely packed and are usually orange or brown in color, those of the madrepores are more widely separated and are greenish, while the rose corals are most often olive, blotched and streaked with white and violet.
Among this mass of living color dart bright-hued fish and Crustacea, while various molluscs crawl among their waving tentacles. Nearly all these creatures strangely enough imitate closely the colors of the zoids, for these, being brightly colored, act as a danger signal to the more voracious creatures, it being well known that most species are poisonous or disagreeable in flavor. Thus by imitating them the smaller and weaker animals, which might otherwise fall easy victims, are avoided because of their similarity.
Coral islands may be formed in many ways. The corals dying may be cast up on a reef and by constant action of waves reduced to coral sand, which becoming cemented together by the lime forms coral limestone. Living corals on their natural beds may, by volcanic action, be raised above high water level for many feet, and, becoming fossilized, remain high in the mountains, as is the case in many of the West India islands. Many islands commonly supposed to consist mainly of coral (as for instance, the Bermudas), are really composed almost entirely of marine-shell sand, cemented together and forming a compact limestone. An examination of Bermuda sand will prove this conclusively, for although fragments of coral are numerous, the great bulk of material is small, bivalve shells.
Closely resembling the true corals, and often mistaken for them, are the gorgonias which differ mainly in possessing a flexible central stem of horny-like material, surrounded by the cells containing the polyps. The polyps themselves closely resemble those of the true corals. Another group of marine animals much like corals are Zoanthidae, a family of sea-anemones which grow in clusters from a common base, thus giving the appearance,—when in large masses,—of true corals; in fact they form a sort of connecting link between the real corals and the true actinias. They may readily be distinguished, however, by the fact that they possess no internal skeleton or framework of lime. The gorgonias can be identified not only by the horny stem, but by the spicules of silica in the skin of the polyps themselves. Another form of life often confounded with the corals are the coralines,—true vegetables, allied to the algae or seaweeds; often called nullipores (no pores) because they have not the lateral row of pores or polyp pits of the gorgonias.
Although confined mainly to the tropics a few species of corals occur as far north as Nova Scotia, the northern limit of the reef-building corals being the Bermuda Islands. Although found in abundance in all tropical seas they appear to reach their highest development in the Indian and Pacific Oceans. In these seas they not only grow in an immense variety, but also in enormous numbers and to remarkable size. The Great Barrier Reef of Australia is one of the most wonderful examples of coral reefs in the world, for here grow in countless thousands corals of every size, shape and color imaginable. During very low tides great stretches of this immense reef are laid bare, and a truly wonderful sight is presented with the dome-shaped brain corals and broadly-branching madrepores rising above the surface of the water on every hand, in groups and clusters. After severe storms great numbers of these are thrown up and for miles the shore is completely covered by them.