| CORALS |
| Tree coral (Dendrophyllia nigrescens). Tuft coral (Lophophyllia prolifera). Fan coral (Euphyllia pavonia). Cup corals and skeleton (Carophyllia smithii). |
The coral polyps differ from anemones only in details of structure that we need not consider, except to note the striking difference that here the base and the radiating partitions instead of being membranous secrete a firm skeleton either of lime or of the horny material termed chitin. The flesh overflows the walls, folding down from the top, so that the skeleton becomes really internal, although naked at the broad base. Some of the tropical stony corals are like big anemones, several inches across; and it is only when they infold all their richly colored tentacles and become a dull and shapeless lump that their stony cup is revealed. These are solitary, and form loosely lying corals, like that called the "mushroom." New ones are produced by the parent throwing off buds which for a time remain attached by a stalk, but finally fall off and settle down to grow—a process that may go on for a score of years. In the case of the huge coral masses called madrepores the buds remain attached to the parent. If they spread out naturally, W. Saville Kent explains, they build up by accumulation the large rounded masses known as "brain" corals and "star" corals, which are most numerous on coastline reefs, or form the base of the outer barrier reef. On the other hand, where the budding is terminal, or oblique, branching, treelike growths result in "staghorn" and similar forms.
The coral animals do not alone construct the reefs. Stony hydroids (millepores), shells of all sorts of mollusks, limy sea mosses (Bryozoa), animalcules and diatoms and various algæ stiffened or cased with lime or flint, and blown sand, contribute to build them up, especially when they near the surface of the sea.
The distribution of reef-building corals is interesting. At present they are limited to about 35 degrees each side of the equator, but are irregularly distributed, owing mainly to differences of temperature in the water, which must not be colder than 68 degrees F. Hence they exist farther away from the equator in the path of warm ocean currents. The Gulf Stream accounts for the coral islands along the coast of Florida and in the Bermudas, which is their farthest point on the American coast; and the warmth of the water accounts for their extensive presence along the eastern coasts of Australia and Africa, when few exist on the western sides of these continents; similarly the western coasts of South and Central America are nearly free of coral banks. Other causes of limitation exist. For example, the noticeable absence of coral growth along the coast of South America is largely, if not altogether, owing to the fresh water and silt brought down by the great rivers there—both prejudicial to coral life.
Coral colonies increase and ultimately form banks wherever warm, pure sea water is constantly present, and not more than about 125 feet deep. Here, spreading and continually rising on the skeletons of dead generations, they form a long line close to the land called a "fringing" reef; and outside of this, beyond a space swept by the currents, may arise a second, still more flourishing bank, termed "barrier" reef. The great barrier reef that extends for 1,200 miles along the eastern coast of Australia—a vast chain of banks and islands—is an amazing example of what these minute animals can accomplish, given time; and geology can point to still more stupendous results of their work in the early history of the globe.
Very characteristic, in the great coral-growing region of the South Sea archipelago, is the ring-shaped island or "atoll," which incloses a quiet lagoon, usually with an open entrance. The reason for such a form has excited much discussion, one explanation being that its origin was about a small island that slowly subsided, the coral keeping pace in rising as the island sank, until finally the land disappeared; another that the circular reef arose from a submerged elevation, and when it came near the surface ceased to grow except on its outer border because it ceased to get suitable water and food, until after a time the central part died out, leaving a ring. Both explanations may be true of different situations.
When a reef comes near to the surface the branching coral is knocked to pieces by the waves, and there are added to this breakage shells and bones, calcareous seaweeds, and what not; and all this is ground into sand by the surf, washed high on the top of the ridge and manured by dead plants and animals, and by the droppings of birds, until finally a soil forms beyond the reach of the tides. Then, if it is in the far southern seas, a drifting coconut may lodge there and be rolled high enough to be left to strike its roots into the sand and begin the grove that by and by will make the islet attractive to men. The thick husk of the coconut resists harm from sea water, near which this palm prefers to grow in just such a sandy, shelly soil as the uprising reef affords. The nuts that so often fall into the surf or are carried out by rivers make long voyages without losing their vitality. Here, again, the situation of most coral islets in the course of currents is advantageous, for thus not only these nuts but other useful seeds and colonizing elements drift directly to their doors, as it were. Birds, wandering widely over the waters, espy the bit of land, and aid by their visits to increase its fertility and often add to its flora. Reefs near shore, especially in Florida and southward, become jungles of mangroves, which not only spring from floating seeds but send down from their branches sprouts that become rooted in the mud and spread the growth interminably. Such a "mangrove key" soon attracts an extensive population of plants and animals and speedily becomes a considerable island.
A great variety of corals, however, are not reef builders, and some species secrete little if any lime; these solitary relatives are found scattered all over the oceans, in deep water as well as shallow, wherever the bottom is suitable, and an immense amount of interesting information about them is to be found in books devoted to this beautiful group of animals.