And it is
easily understood how a single polype, which may give rise to hundreds,
or perhaps thousands, of embryos, may, by this process of partly
active and partly passive migration, cover an immense surface with its
offspring.
The masses of coral which may be formed by the assemblages of polypes
which spring by budding, or by dividing, from a single polype,
occasionally attain very considerable dimensions. Such skeletons are
sometimes great plates, many feet long and several feet in thickness; or
they may form huge half globes, like the brainstone corals, or may reach
the magnitude of stout shrubs or even small trees. There is reason to
believe that such masses as these take a long time to form, and
hence that the age a polype tree, or polype turf, may attain, may be
considerable. But, sooner or later, the coral polypes, like all other
things, die; the soft flesh decays, while the skeleton is left as a
stony mass at the bottom of the sea, where it retains its integrity for
a longer or a shorter time, according as its position affords more or
less protection from the wear and tear of the waves.
The polypes which give rise to the white coral are found, as has been
said, in the seas of all parts of the world; but in the temperate and
cold oceans they are scattered and comparatively small in size, so that
the skeletons of those which die do not accumulate in any considerable
quantity.
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