ies,
and new septa are formed in the exocoeles on either side of them, and so
forth.

[Illustration: FIG. 17.--Transverse section through a zooid of
_Cladocora_. The corallum shaded with dots, the mesogloea represented by
a thick line. Thirty-two septa are present, six in the entocoeles of the
primary cycle of mesenteries, I; six in the entocoeles of the secondary
cycle of mesenteries, II; four in the entocoeles of the tertiary cycle
of mesenteries, III, only four pairs of the latter being developed; and
sixteen in the entocoeles between the mesenterial pairs. D, D, Directive
mesenteries; st, stomodaeum. (After Duerden.)]

It is evident from an inspection of figs. 16 and 17 that every septum
is covered by a fold of endoderm, mesogloea, and ectoderm, and is in
fact pushed into the cavity of the zooid from without. The zooid then
is, as it were, moulded upon the corallum. When fully extended, the
upper part of the zooid projects for some distance out of the calicle,
and its wall is reflected for some distance over the lip of the latter,
forming a fold of soft tissue extending to a greater or less distance
over the theca, and containing in most cases a cavity continuous over
the lip of the calicle with the coelenteron. This fold of tissue is
known as the _edge-zone_ In some corals the septa are solid imperforate
plates of calcite, and their peripheral ends are either firmly welded
together, or are united by interstitial pieces so as to form imperforate
theca. In others the peripheral ends of the septa are united only by
bars or trabeculae, so that the theca is perforate, and in many such
perforate corals the septa themselves are pierced by numerous
perforations. In the former, which have been called aporose corals, the
only communication between the cavity of the edge-zone and the general
cavity of the zooid is by way of the lip of the calicle; in the latter,
or perforate corals, the theca is permeated by numerous branching and
anastomosing canals lined by endoderm, which place the cavity of the
edge-zone in communication with the general cavity of the zooid.

[Illustration: FIG. 18.

A, Schematic longitudinal section through a zooid and bud of _Stylophora
digitata_. In A, B, and C the thick black lines represent the soft
tissues; the corallum is dotted. s, Stomodaeum; c, c, coenosarc; col,
columella, T tabulae.

B, Similar section through a single zooid and bud of _Astroides
calicularis_.

C, Similar section throu