on of the walls of adjacent tubes, and the walls of the
calices are formed by the walls of adjacent coenenchymal tubes. Thus
the architecture of the Helioporid colony differs entirely from such
forms as Tubipora or Favosites, in which each corallite has its own
distinct and proper wall. The cavities both of the calices and
coenenchymal tubes of Heliopora are closed below by horizontal
partitions or _tabulae_, hence the genus was formerly included in the
group Tabulata, and was supposed to belong to the madreporarian
corals, both because of its lamellar skeleton, which resembles that of
a Madrepore, and because each calicle has from twelve to fifteen
radial partitions or septa projecting into its cavity. The structure
of the zooid of Heliopora, however, is that of a typical Alcyonarian,
and the septa have only a resemblance to, but no real homology with,
the similarly named structures in madreporarian corals. _Heliopora
coerulea_ is found between tide-marks on the shore platforms of coral
islands. The order was more abundantly represented in Palaeozoic times
by the _Heliolitidae_ from the Upper and Lower Silurian and the
Devonian, and by the _Thecidae_ from the Wenlock limestone. In
_Heliolites porosus_ the colonies had the form of spheroidal masses;
the calices were furnished with twelve pseudosepta, and the
coenenchymal tubes were more or less regularly hexagonal.

[Illustration with caption: FIG. 10.

A, _Edwardsia claparedii_ (after A. Andres). Cap, capitulum; sc,
scapus; ph, physa.

B, Transverse section of the same, showing the arrangement of the
mesenteries, s, Sulcus; sl, sulculus.

C, Transverse section of _Halcampa_. d, d, Directive mesenteries; st,
stomodaeum.]

Zoantharia.--In this sub-class the arrangement of the mesenteries is
subject to a great deal of variation, but all the types hitherto
observed may be referred to a common plan, illustrated by the living
genus _Edwardsia_ (fig. 10, A, B). This is a small solitary
Zoantharian which lives embedded in sand. Its body is divisible into
three portions, an upper _capitulum_ bearing the mouth and tentacles,
a median _scapus_ covered by a friable cuticle, and a terminal physa
which is rounded. Both capitulum and physa can be retracted within the
scapus. There are from sixteen to thirty-two simple tentacles, but
only eight mesenteries, all of which are complete. The stomodaeum is
co