corals are very common, occur in almost all seas, and form skeletons
which are sometimes extremely massive.

With a very few exceptions, both the red and the white coral polypes
are, in their adult state, firmly adherent to the sea-bottom; nor do
their buds naturally become detached and locomotive. But, in addition
to budding and division, these creatures possess the more ordinary
methods of multiplication; and, at particular seasons, they give
rise to numerous eggs of minute size. Within 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