the ALCYONARIA
and the ZOANTHARIA. To the first-named belong the precious red coral and
its allies, the sea-fans or Gorgoniae, to the second belong the white or
Madreporarian corals.

[Illustration: FIG. 3.--An expanded Alcyonarian zooid, showing the mouth
surrounded by eight pinnate tentacles. st, Stomodaeum in the the centre
of the transparent body; m, mesenteries; asm, asulcar mesenteries; B,
spicules, enlarged.]

Alcyonaria.--In this sub-class the zooid has very constant anatomical
characters, differing in some important respects from the Actinian
zooid, which has been taken as a type. There is only one ciliated
groove, the sulcus, in the stomodaeum. There are always eight
tentacles, which are hollow and fringed on their sides, with hollow
projections or pinnae; and always eight mesenteries, all of which are
complete, i.e. inserted on the stomodaeum. The mesenteries are
provided with well-developed longitudinal retractor muscles, supported
on longitudinal folds or plaits of the mesogloea, so that in
cross-section they have a branched appearance. These _muscle-banners_,
as they are called, have a highly characteristic arrangement; they are
all situated on those faces of the mesenteries which look towards the
sulcus. (fig. 4). Each mesentery has a filament; but two of them,
namely, the pair farthest from the sulcus, are longer than the rest,
and have a different form of filament. It has been shown that these
asulcar filaments are derived from the ectoderm, the remainder from
the endoderm. The only exceptions to this structure are found in the
arrested or modified zooids, which occur in many of the colonial
Alcyonaria. In these the tentacles are stunted or suppressed and the
mesenteries are ill-developed, but the sulcus is unusually large and
has long cilia. Such modified zooids are called siphonozooids, their
function being to drive currents of fluid through the canal-systems of
the colonies to which they belong. With very few exceptions a
calcareous skeleton is present in all Alcyonaria; it usually consists
of spicules of carbonate of lime, each spicule being formed within an
ectodermic cell (fig. 3, B). Most commonly the spicule-forming cells
pass out of the ectoderm and are imbedded in the mesogloea, where they
may remain separate from one another or may be fused together to form
a strong mass. In addition to the spicular skeleton an organic horny
skelet