aning. There are certain architectonic types, including edifices of
different materials, with an infinite variety of architectural details
and external ornaments; but the flat roof and the colonnade are typical
of all Grecian temples, whether built of marble or granite or wood,
whether Doric or Ionic or Corinthian, whether simple and massive or
light and ornamented; and, in like manner, the steep roof and pointed
arch are the typical characters of all Gothic cathedrals, whatever be
the material or the details. The architectural conception remains
the same in all its essential elements, however the more superficial
features vary. Such relations as these edifices bear to the
architectural idea that includes them all, do classes bear to the
primary divisions or branches of the Animal Kingdom.

The three classes of Radiates, beginning with the lowest, and naming
them in their relative order, are Polyps, Acalephs or Jelly-Fishes, and
Echinoderms or Star-Fishes and Sea-Urchins. In the Polyps the plan is
executed in the simplest manner by a sac, the sides of which are folded
inward, at regular intervals from top to bottom, so as to divide it by
vertical radiating partitions, converging from the periphery toward the
centre. These folds or partitions do not meet in the centre, but leave
an open space, which is the main cavity of the body. This open space,
however, occupies only the lower part of the body; for in the upper
there is a second sac hanging to a certain distance within the first.
This inner sac has an aperture in the bottom, through which whatever
enters it passes into the main cavity of the body. A central opening
in the top forms a kind of mouth, around which are radiating tentacles
connecting with the open chambers formed by the partitions within.
Cutting such an animal across in a transverse section, we shall see
the radiation of the partitions from the centre to the circumference,
showing still more distinctly the typical structure of the division to
which it belongs.

[Illustration: Vertical section of a Sea-Anemone of Actinia: _o_, mouth;
_t_, tentacles; _s_, inner sac or stomach; _b_, main cavity; _ff_,
reproductive organs; _g_, radiating partition; _eee_, radiating
chambers; _cc_, circular openings in the partitions; _aa_, lower floor.]

[Illustration: Transverse section of a Sea-Anemone or Actinia.]

[Illustration: Staurophera seen in profile.]

[Illustration: Hippocrene seen in profile.]

[Illustratio