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.

It is well known that all animals and plants have the power of appropriating to themselves and assimilating the materials they need, each selecting from the surrounding elements whatever contributes to its well-being.  Now, corals possess in an extraordinary degree, the power of assimilating to themselves the lime contained in the salt water around them; and as soon as our little coral is established on a firm foundation, a lime deposit begins to form in all the walls of its body, so that its base, its partitions, and its outer wall, which in the sea-anemone remain always soft, become perfectly solid in the polyp coral, and form a frame as hard as bone.

It may naturally be asked where the lime comes from in the sea which the corals absorb in such quantities.  As far as the living corals are concerned the answer is easy, for an immense deal of lime is brought down to the ocean by rivers that wear away the lime deposits through which they pass.  The Mississippi, whose course lies through extensive lime regions, brings down yearly lime enough to supply all the animals living in the Gulf of Mexico.  But behind this lies a question, not so easily settled, as to the origin of the extensive deposits of limestone found at the very beginning of life upon earth.  This problem brings us to the threshold of astronomy; for the base of limestone is metallic in character, susceptible therefore of fusion, and may have formed a part of the materials of our earth, even in an incandescent state, when the worlds were forming.  But though this investigation as to the origin of lime does not belong either to the naturalist or the geologist, its suggestion reminds us that the time has come when all the sciences and their results are so intimately connected that no one can be carried on independently of the others.  Since the study of the rocks has revealed a crowded life whose records are hoarded within them, the work of the geologist and the naturalist has become one and the same; and at that border-land where the first crust of the earth was condensed out of the igneous mass of materials which formed its earliest condition, their investigation mingles with that of the astronomer, and we cannot trace the limestone in a little coral without going back to the creation of our solar system, when the worlds that compose it were thrown off from a central mass in a gaseous condition.