s of Edwardsian mesenteries were
present and septa were formed in the intervals between them.

[Illustration: FIG. 19.--Diagram of the arrangement of the septa in a
Zaphrentid coral. m, Main septum; c, counter septum; t, t, alar septa.]

Space forbids a discussion of the proposals to classify corals after the
minute structure of their coralla, but it will suffice to say that it
has been shown that the septa of all corals are built up of a number of
curved bars called trabeculae, each of which is composed of a number of
nodes. In many secondary corals (_Cyclolites, Thamnastraea_) the
trabeculae are so far separate that the individual bars are easily
recognizable, and each looks something like a bamboo owing to the
thickening of the two ends of each node. The trabeculae are united
together by these thickened internodes, and the result is a fenestrated
septum, which in older septa may become solid and aporose by continual
deposit of calcite in the fenestrae. Each node of a trabecula may be
simple, i.e. have only one centre of calcification, or may be compound.
The septa of modern perforate corals are shown to have a structure
nearly identical with that of the secondary forms, but the trabeculae
and their nodes are only apparent on microscopical examination. The
aporose corals, too, have a practically identical structure, their
compactness being due to the union of the trabeculae throughout their
entire lengths instead of at intervals, as in the Perforata. Further,
the trabeculae may be evenly spaced throughout the septum, or may be
grouped together, and this feature is probably of value in estimating
the affinities of corals. (For an account of coral formations see
CORAL-REEFS.)

In the present state of our knowledge the Zoantharia in which a primary
cycle of six couples of mesenteries is (or may be inferred to be)
completed by the addition of two pairs to the eight Edwardsian
mesenteries, and succeeding cycles are formed in the exocoeles of the
pre-existing mesenterial cycles, may be classed in an order ACTINIIDEA,
and this may be divided into the suborders _Malacactiniae_, comprising
the soft-bodied Actinians, such as _Actinia, Sagartia, Bunodes_, &c.,
and the _Scleractiniae_, comprising the corals. The Scleractiniae may
best be divided into groups of families which appear to be most closely
related to one another, but it should not be forgotten that there is
great reason to believe that many if not most of the extinct