n these eggs the young are
formed, and they leave the egg in a condition which has no sort of
resemblance to the perfect animal. It is, in fact, a minute oval body,
many hundred times smaller than the full-grown creature, and it
swims about with great activity by the help of multitudes of little
hair-like filaments, called cilia, with which its body is covered.
These cilia all lash the water in one direction, and so drive the
little body along as if it were propelled by thousands of extremely
minute paddles. After enjoying its freedom for a longer or shorter
time, and being carried either by the force of its own cilia, or by
currents which bear it along, the embryo coral settles down to the
bottom, loses its cilia, and becomes fixed to the rock, gradually
assuming the polype form and growing up to the size of its parent.
As the infant polypes of the coral may retain this free and active
condition for many hours, or even days, and as a tidal or other
current in the sea may easily flow at the speed of two or even
more miles in an hour, it is clear that the embryo must often be
transported to very considerable distances from the parent. 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 it 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 sma