in the course of this article, and the
Scleractiniae (= Madreporaria) or true corals.

[Illustration: FIG. 15.--Corallum of _Caryophyllia_; semi-diagrammatic.
th, Theca; c, costae; sp, septa; p, palus; col, columella.]

All recent corals, as has already been said, conform so closely to the
anatomy of normal Actinians that they cannot be classified apart from
them, except that they are distinguished by the possession of a
calcareous skeleton. This skeleton is largely composed of a number of
radiating plates or _septa_, and it differs both in origin and structure
from the calcareous skeleton of all Alcyonaria except Heliopora. It is
formed, not from fused spicules, but as a secretion of a special layer
of cells derived from the basal ectoderm, and known as _calicoblasts_.
The skeleton or corallum of a typical solitary coral--the common
Devonshire cup-coral _Caryophyllia smithii_ (fig. 15) is a good
example--exhibits the followings parts:--(1) The _basal plate_, between
the zooid and the surface of attachment. (2) The _septa_, radial plates
of calcite reaching from the periphery nearly or quite to the centre of
the coral-cup or calicle. (3) The _theca_ or wall, which in many corals
is not an independent structure, but is formed by the conjoined
thickened peripheral ends of the septa. (4) The _columella_, a structure
which occupies the centre of the calicle, and may arise from the basal
plate, when it is called essential, or may be formed by union of
trabecular offsets of the septa, when it is called unessential. (5) The
_costae_, longitudinal ribs or rows of spines on the outer surface of
the theca. True costae always correspond to the septa, and are in fact
the peripheral edges of the latter. (6) _Epitheca_, an offset of the
basal plate which surrounds the base of the theca in a ring-like manner,
and in some corals may take the place of a true theca. (7) _Pali_,
spinous or blade-like upgrowths from the bottom of the calicle, which
project between the inner edges of certain septa and the columella. In
addition to these parts the following structures may exist in
corals:--_Dissepiments_ are oblique calcareous partitions, stretching
from septum to septum, and closing the interseptal chambers below. The
whole system of dissepiments in any given calicle is often called
_endotheca_. _Synapticulae_ are calcareous bars uniting adjacent septa.
_Tabulae_ are stout horizontal partitions traversing the centre of the
calicle and d