r side of the reef are in immediate contact
with the pure ocean-water, while by their growth they partially exclude
the inner ones from the same influence,--the rapid growth of the latter
being also impeded by any impurity or foreign material washed away from
the neighboring shore and mingling with the water that fills the channel
between the main-land and the reef. Thus the Coral Reefs, whether built
around an island, or concentric to a rounding shore, or along a straight
line of coast, are always shelving toward the land, while they
are comparatively abrupt and steep toward the sea. This should be
remembered, for, as we shall see hereafter, it has an important bearing
on the question of time as illustrated by Coral Reefs.

I have spoken of the budding of Corals, by which each one becomes the
centre of a cluster; but this is not the only way in which they multiply
their kind. They give birth to eggs also, which are carried on the inner
edge of their partition-walls, till they drop into the sea, where they
float about, little, soft, transparent, pear-shaped bodies, as unlike as
possible to the rigid stony structure they are to assume hereafter. In
this condition they are covered with vibratile cilia or fringes, that
are always in rapid, uninterrupted motion, and keep them swimming about
in the water. It is by means of these little germs of the Corals,
swimming freely about during their earliest stages of growth, that the
reef is continued, at the various heights where special kinds die
out, by those that prosper at shallower depths; otherwise it would be
impossible to understand how this variety of building material, as it
were, is introduced wherever it is needed. This point, formerly a puzzle
to naturalists, has become quite clear since it has been found that
myriads of these little germs are poured into the water surrounding a
reef. There they swim about till they find a genial spot on which to
establish themselves, when they become attached to the ground by one
end, while a depression takes place at the opposite end, which gradually
deepens to form the mouth and inner cavity, while the edges expand to
form the tentacles, and the productive life of the little Coral begins:
it buds from every side, and becomes the foundation of a new community.

I should add, that, beside the Polyps and the Acalephs, Mollusks also
have their representatives among the Corals. There is a group of small
Mollusks called Bryozoa, allied to