h 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.

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 limest