44 CLASSIFICATION AND CREATION. 
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 re- 
mains the same in all its essential elements, how- 
ever the more superficial features vary. Such 
relations as these edifices bear to the architec- 
tural idea that includes them all, do classes bear 
to the primary divisions or branches of the Ani- 
mal Kingdom. 
The three classes of Radiates, beginning with 
the lowest, and naming them in their relative 
order, are Polyps or Sea-anemones and corals, 
Acalephs or Jelly-Fishes, and Echinoderms or 
Star-Fishes, Sea-Urchins and the like. In the 
Polyps the plan is executed in the simplest 
manner; the body consists of 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 periph- 
ery toward the centre. These folds 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 
