ividing it into as many superimposed chambers. The septa in
recent corals always bear a definite relation to the mesenteries, being
found either in every entocoele or in every entocoele and exocoele.
Hence in corals in which there is only a single cycle of mesenteries the
septa are correspondingly few in number; where several cycles of
mesenteries are present the septa are correspondingly numerous. In some
cases--e.g. in some species of _Madrepora_--only two septa are fully
developed, the remainder being very feebly represented.

[Illustration: FIG. 16.--Tangential section of a larva of _Astroides
calicularis_ which has fixed itself on a piece of cork. ec, Ectoderm;
en, endoderm; mg, mesogloea; m, m, mesenteries; s, septum; b, basal
plate formed of ellipsoids of carbonate of lime secreted by the basal
ectoderm; ep, epitheca. (After von Koch.)]

Though the corallum appears to live within the zooid, it is
morphologically external to it, as is best shown by its developmental
history. The larvae of corals are free swimming ciliated forms known as
planulae, and they do not acquire a corallum until they fix themselves.
A ring-shaped plate of calcite, secreted by the ectoderm, is then
formed, lying between the embryo and the surface of attachment. As the
mesenteries are formed, the endoderm of the basal disk lying above the
basal plate is raised up in the form of radiating folds. There may be
six of these folds, one in each entocoele of the primary cycle of
mesenteries, or there may be twelve, one in each exocoele and entocoele.
The ectoderm beneath each fold becomes detached from the surface of the
basal plate, and both it and the mesogloea are folded conformably with
the endoderm. The cells forming the limbs of the ectodermic folds
secrete nodules of calcite, and these, fusing together, give rise to six
(or twelve) vertical radial plates or septa. As growth proceeds new
septa are formed simultaneously with the new couples of secondary
mesenteries. In some corals, in which all the septa are entocoelic, each
new system is embraced by a mesenteric couple; in others, in which the
septa are both entocoelic and exocoelic, three septa are formed in every
chamber between two primary mesenterial couples, one in the entocoele of
the newly formed mesenterial couple of the secondary cycle, and one in
each exocoele between a primary and a secondary couple. These latter are
in turn embraced by the couples of the tertiary cycle of mesenter