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.
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