nourishment of the body, which, as before observed, consists simply of a
gelatinous film of animal matter, possessing but little evidence of
vitality. Here, then, is a community of nourishment, and with it also a
community of sensation, for if one portion be irritated, contiguous
portions of the animal are apt to sympathize. When the Coral polyps are
not in an active state, or in other words, when they are not in want of
food, these hydra-form polyps may not be visible, but being retracted
into cells found as depressions in the skeletons of the Madrepores, they
are lost to observation, and it is only when in quest of food and
nourishment that their contractile tentacles are expanded, and
distinctly prominent.

The physiology of the growth of the skeleton, both in the Madrepores and
the Coral, is the same. The entire skeleton, however ramified it may be,
or whatever form it may assume, is secreted by the living matter with
which it is invested, the materials for its formation being derived from
the element in which it lives; and as its deposition takes place at
different times, the central stem of some corals is apt to assume a
beautiful concentric arrangement of laminae. But the material deposited
or secreted need not necessarily be hard or calcareous, but even may
partake of the character of horn or other flexible materials, as is the
case with some of the coral family. In other cases there is an
alternation of each material; and the necessity of this change in the
character of the skeleton will now demand our attention.

The common coral of the Mediterranean, possessing a stony skeleton, is
found in situations where its stunted form and its extreme hardness
sufficiently preserve it from the violence of the waves; but place a
coral under other circumstances, and expose it to the storms of the
Indian Ocean, where the waves rage with fury, dashing on and uprooting
all things within their power, and the structure of the simple coralium
would fail to withstand their violence. Here, then, under such
circumstances, in the case of the Gorgonia, nature has provided a horny
and flexible skeleton, which, spreading majestically in the sea, shall
be capable of bending beneath the weight of the superincumbent waves,
and so yielding to the storms. Nature has thus adapted herself to each
contingent circumstance.

The next point to which we shall advert will be coral formations, which
form so interesting a study to the naturalist