Remember, first of all, that these animals multiply in two different ways—sometimes by eggs, and sometimes by little buds, so to speak, which grow out of the body of the parent. The polyps which hatch out from eggs swim about for some little time quite freely. But after a few days they fasten themselves down to the surface of a submerged rock, and after that they never move again. Other polyps soon come and settle down by them, and before very long there will be thousands upon thousands of the little animals all growing, as it were, close together, and all gradually building up coral underneath and round the margins of their bodies.
When they reach their full size they begin to multiply by "budding." Baby polyps sprout out all over their bodies, and these, instead of swimming about for a few days like those which are hatched from eggs, remain fixed where they are for the whole of their lives. Then they, in their turn, begin to deposit coral, and as they have nowhere else to put it they place it on the bodies of their parents, which before very long are completely covered in. Now, you see, there is a second layer of coral on the top of the first. Then in due course of time a third layer is formed upon the second, and a fourth layer upon the third, each generation being built in by the one that comes after it, till at last the coral bank rises above the surface of the water. Then the work has to stop; for these little creatures cannot live unless the waves can constantly break over them. But although the bank cannot be raised higher it can still be extended on all sides; and so the little polyps go working on, year after year, till at last the results of their labor are almost too wonderful to realize.
Coral Banks
These coral banks take three different forms.
First, there are the fringing reefs. These are great banks of coral surrounding the shores of a tropical island, or running for long distances on the coasts of the mainland. The island of Mauritius, for example, is entirely surrounded by a fringing reef. These reefs often spread out for miles into the sea, and they are only broken here and there by narrow passages, where some river or stream is flowing out. For the polyps cannot live in fresh water.
Next, there are barrier reefs. These are great walls of coral at a distance from the shore, with deep water between the two. For the polyps are unable to work at a greater depth than about thirty fathoms, or one hundred and eighty feet, below the surface; and it often happens that while there is deep water close to the shores of a tropical island, there is shallow water farther out. In such a case the polyps have to build out at sea, instead of close into the land, and there is a kind of moat between the coral bank and the shore. In this case the bank is called a barrier reef, and sometimes it is of enormous size. The Great Barrier Reef, for instance, runs for no less than 1250 miles along the northeast coast of Australia.
Then, thirdly, there are coral islands, or atolls. There are thousands of these wonderful islands in the Pacific and the Indian oceans, and others are still being slowly pushed up out of the sea. They always take the form of more or less circular rings, in the center of which is a lake of sea-water called a lagoon. The coral bank of which they consist is seldom more than a few hundred feet wide, but sometimes the islands are very large indeed. The biggest of all is ninety miles long and sixty miles broad, while several others are not very much smaller. Soon after they rise to the surface of the sea a kind of soil is deposited upon them, made up partly of powdered coral, ground up by the action of the waves, and partly of decaying vegetable matter which has been flung up on them. Then sea-birds bring mud upon their feet from the mainland, or from another island at a distance, and leave some of it behind them when they settle down to rest; and in that mud are seeds of plants, which soon begin to sprout and grow. So in a very few years the island is covered with low vegetation. Then one day, perhaps, a floating cocoanut is flung up, and that, too, takes root and grows, so that in course of time there is a palm-tree. Other palm-trees, of course, follow; and the result is that the first glimpse which a traveler gets of a coral island is nearly always that of a row of palm-trees upon the horizon.
The simple corals live in almost all parts of the ocean. Some of them are occasionally dredged up off our coasts, and can live in very cold water. But the reef-builders are only found in warm seas, and are never found working far outside the boundaries of the tropics.
How wonderful it seems that tiny creatures such as these polyps, which really do not appear to be much more than little lumps of living jelly, should be able to build up these vast masses of coral from out of the depths of the sea! One cannot help wondering what the results of their work will be if the world should last for a few thousand years longer. It would really seem that by that time the tropical seas will be choked up with coral islands, and the lagoons inside them will be filled up with coral too; so that not merely islands, but continents, will have been raised from the ocean by some of the smallest and weakest and most insignificant of all living animals!