Since these little creatures can lay down their tentacula in the openings of their stony houses, when they are not hungry, or when they are alarmed by the approach of anything that would hurt them, they do not want the tough skin with which the Actinia is covered, but have bodies of a very soft and yielding texture.
FORMS OF THE CORAL REEFS.
As I have now introduced to you, as well as I can by means of description and pictures, the inhabitants of these wonderful submarine continents, I shall now tell you something of the form in which they arrange their habitations. Those that I saw about the Society and Friendly Islands, were mostly in three different conditions. One kind forms a nearly circular reef, sometimes with an opening in it. It is a part of the common design of the Polypes always to make this opening on the Leeward side, when the place is one in which any particular wind blows during a great part of the year. I cannot tell you why this is, but it is constantly observed wherever coral reefs are found. This peculiarity makes the enclosed spaces of water, (which are called Lagoons,) capital harbours for ships to anchor in, for they are nearly always smooth, the windward side of the reef acting as a breakwater. Here is a map of three reefs of this form, in a part of the ocean where the prevailing wind is South East, and you will see all the openings are towards the North West.
Another sort have an island of which the foundation is coral, in the middle of a ring like the above; and a third sort are merely the same kind of rings round large islands, composed of other sorts of rocks and earth. A very great number of the islands hereabouts,—perhaps most of them, are surrounded in this manner.
I well remember the first impressions which the sight of a coral reef made upon me. I always like to treasure up in my mind the first impression produced by a beautiful scene; for however wonderful the objects may be, when you have seen them many times, you become in some degree indifferent to them. This first impression that I am going to describe to you, is a continual feast to me when I think upon it, and I wish you could enjoy it with me.
The first parts of the reef we saw were black, roundish masses, standing up out of the water, having just the appearance of black men's heads, and when I asked the sailors what they were, they told me they were "negroes' heads." I was curious, as you may suppose, to know what they could be, and I afterwards found they were masses of coral, which were not covered by the sea, except at very high tides, and had become blackened by the weather.
The sun was shining brightly, and there was a smart breeze. The waves breaking over the ragged surface of the coral threw up abundance of spray, which the sun's rays, every now and then, painted with the most beautiful rainbow colours. We sailed through an opening, and when we had got into the Lagoon, the sea was perfectly smooth. The water was as clear as crystal, and we saw the bottom, and what was going on there, nearly as well as if we had been close to it, though the depth was very considerable.