which it forms the nucleus, and which now begin to bud in
their turn, each one surrounding itself with a numerous progeny, all
remaining, however, attached to the parent. Such a community increases
till its individuals are numbered by millions; and I have myself counted
no less than fourteen millions of individuals in a Coral mass measuring
not more than twelve feet in diameter. These are the so-called Coral
heads which form the foundation of a Coral wall, and their massive
character and regular form seem to be especially adapted to give a
strong, solid base to the whole structure. They are known in our
classifications as the Astraeans, so named on account of the star-shaped
form of the little pits that are crowded upon the surface, each one
marking the place of a single individual in such a community.

Thus firmly and strongly is the foundation of the reef laid by the
Astraeans; but we have seen that for their prosperous growth they
require a certain depth and pressure of water, and when they have
brought the wall so high that they have not more than six fathoms of
water above them, this kind of Coral ceases to grow. They have, however,
prepared a fitting surface for different kinds of Corals that could not
live in the depths from which the Astraeans have come, but find their
genial home nearer the surface; such a home being made ready for them
by their predecessors, they now establish themselves on the top of the
Coral wall and continue its growth for a certain time. These are the
Mandrinas, or the so-called Brain-Corals, and the Porites. The Mandrinas
differ from the Astraeans by their less compact and definite pits. In
the Astraeans the place occupied by the animal in the community is
marked by a little star-shaped spot, in the centre of which all the
partition-walls meet. But in the Mandrinas, although all the partitions
converge toward the central opening, as in the Astraeans, these central
openings elongate, run into each other, and form waving furrows all over
the surface, instead of the small round pits so characteristic of the
Astraeans. The Porites resemble the Astraeans, but the pits are smaller,
with fewer partitions and fewer tentacles, and their whole substance is
more porous.

But these also have their bounds within the sea: they in their turn
reach the limit beyond which they are forbidden by the laws of their
nature to pass, and there they also pause. But the Coral wall continues
its steady progress;