For a long time it was supposed that the reef-builders inhabited very deep waters; for they were sometimes brought up upon sounding-lines from a depth of many hundreds or even thousands of feet, and it was taken for granted that they must have had their home where they were found: but the facts recently ascertained respecting the subsidence of ocean-bottoms have shown that the foundation of a coral-wall may have sunk far below the place where it was laid. And it is now proved, beyond a doubt, that no reef-building coral can thrive at a depth of more than fifteen fathoms, though corals of other kinds occur far lower, and that the dead reef-corals, sometimes brought to the surface from much greater depths, are only broken fragments of some reef that has subsided with the bottom on which it was growing. But though fifteen fathoms is the maximum depth at which any reef-builder can prosper, there are many which will not sustain even that degree of pressure; and this fact has, as we shall see, an important influence on the structure of the reef. Imagine now a sloping shore on some tropical coast descending gradually below the surface of the sea. Upon that slope, at a depth of from ten to twelve or fifteen fathoms, and two or three or more miles from the mainland, according to the shelving of the shore, we will suppose that one of those little coral animals, to whom a home in such deep waters is congenial, has established itself. How it happens that such a being, which we know is immovably attached to the ground, and forms the foundation of a solid wall, was ever able to swim freely about in the water till it found a suitable resting-place, I shall explain hereafter, when I say something of the mode of reproduction of these animals. Accept, for the moment, my unsustained assertion, and plant our little coral on this sloping shore, some twelve or fifteen fathoms below the surface of the sea. The internal structure of such a coral corresponds to that of the sea-anemone. The body is divided by vertical partitions from top to bottom, leaving open chambers between; while in the centre hangs the digestive cavity, connected by an opening in the bottom with all these chambers. At the top is an aperture serving as a mouth, surrounded by a wreath of hollow tentacles, each one of which connects at its base with one of the chambers, so that all parts of the animal communicate freely with each other. But though the structure of the coral is identical in all its parts with the sea-anemone, it nevertheless presents one important difference. The body of the sea-anemone is soft, while that of the coral is hard.