Astraeans: tentacles not definitely limited in number, though usually
not exceeding one hundred, and generally much below this number;
transverse floors. Maeandrines, generally referred to Astraeans, are
higher than the true Astraeans, on account of their compound Polyps.

Fungidae: indefinite multiplication of tentacles; imperfect transverse
beams.

Actiniae: indefinite multiplication of tentacles; soft bodies and no
transverse beams.

If, now, we compare this structural gradation among Polyps with their
geological succession, we shall find that they correspond exactly. The
following table gives the geological order in which they have been
introduced upon the surface of the earth.

GEOLOGICAL SUCCESSION.

Present, Halcyonoids.
Pliocene, \
Miocene, } Madrepores.
Eocene, /
Cretaceous, \ Porites
Jurassic, } and
Triassic, } Astraeans.
Permian, /
Carboniferous, \
Devonian, } Fungidae
Silurian, /

With regard to the geological position of the Actiniae we can say
nothing, because, if their soft, gelatinous bodies have left any
impressions in the rocks, none such have ever been found; but their
absence is no proof that they did not exist, since it is exceedingly
improbable that animals destitute of any hard parts could be preserved.

The position of the Corals on a Reef accords with these series of
structural gradation and geological succession. It is true that we do
not find the Actiniae in the Reef any more than in the crust of the
earth, for the absence of hard parts in their bodies makes them quite
unfit to serve as Reef-Builders. Neither do we find the Fungidae, for
they, like all low forms, are single, and not confined to one level,
having a wider range in depth and extent than other stony Polyps. But
the true Reef-Building Polyps follow each other on the Reef in the same
order as prevails in their structural gradation and their geological
succession; and whether we classify them according to their position on
the Reef, or their introduction upon the earth in the course of time, or
their relative rank, the result is the same.

[Illustration: SUCCESSION ON THE REEF.]

It would require an amount of details that would be tedious to many
of my readers, were I to add here the evidence to prove that the
embryological development of these animals, so far as it is known, and
their geographical distribution over the