f peculiar texture is developed in the longitudinal partition,
and a longitudinal canal is hollowed out on either side of the axis in
the substance of the longitudinal partition, so that there are four
stem-canals in all. The prorachidial and metarachidial aspects of the
rachis are sterile, but the sides or pararachides bear numerous
daughter zooids of two kinds--(1) fully-formed autozooids, (2) small
stunted siphonozooids. The pinnae are formed by the elongated
autozooids, whose proximal portions are fused together to form a
leaf-like expansion, from the upper edge of which the distal
extremities of the zooids project. The siphonozooids are very numerous
and lie between the bases at the pinnae on the pararachides; they
extend also on the prorachidial and metarachidial surfaces. The
calcareous skeleton of the Pennatulacea consists of scattered
spicules, but in one species, _Protocaulon molle_, spicules are
absent. Although of great interest the Pennatulacea do not form an
enduring skeleton or "coral," and need not be considered in detail in
this place.

[Illustration: FIG. 9.

A, Portion of the surface of a colony of _Heliopora coerulea_
magnified, showing two calices and the surrounding coenenchymal tubes.

B, Single zooid with the adjacent soft tissues as seen after removal
of the skeleton by decalcification. Z1, the distal, and Z2, the
proximal or intracalicular portion of the zooid; ec, ectoderm; ct,
coenenchymal tubes; sp, superficial network of solenia.]

The order COENOTHECALIA is represented by a single living species,
_Heliopora coerulea_, which differs from all recent Alcyonaria in the
fact that its skeleton is not composed of spicules, but is formed as a
secretion from a layer of cells called calicoblasts, which originate
from the ectoderm. The corallum of Heliopora is of a blue colour, and
has the form of broad, upright, lobed, or digitate masses flattened
from side to side. The surfaces are pitted all over with perforations
of two kinds, viz. larger star-shaped cavities, called _calices_, in
which the zooids are lodged, and very numerous smaller round or
polygonal apertures, which in life contain as many short unbranched
tubes, known as the _coenenchymal tubes_ (fig. 9, A). The walls of the
calices and coenenchymal tubes are formed of flat plates of calcite,
which are so disposed that the walls of one tube enter into the
compositi