954 THE MEDALS OF CREATION. Cuap. VII, 
based on the peculiar characters of the polypes; the Hydra, — 
the Actinia (Sea-Anemone, Wond. p. 622), and the Alcy- | 
onium (Dead-men’s fingers, Wond. pl. v. fig. 10), being 
respectively the type of 1. the Hydroida, or Hydraform ; 
2. the Asteroida or Alcyonian ; and 3. the Helianthoida, or 
Actiniform zoophytes. 
8 te eee: 
- 
In the Hydroida the body in the compound species is — 
implanted in a horny tubular sheath, and the polypidoms i 
form branched corallines, which are fixed by the base to | 
rocks, sea-weeds, shells, &c. 
The Asteroida have a horny or calcareous axis, surrounded — 
and inclosed by the soft parts which secrete it. 
The Helianthoids, except in the simple free species, as the 
Actinia, have a lamellated calcareous polypidom, the plates” 
of which radiate from a centre. 
The calcareous secretions of the Anthozoa, especially of — 
the Helianthoida, in a great measure constitute the mass of — 
the coral-reefs and coral-islands of tropical seas. Their 
polypidoms, whether external or internal, maintain but little 
organic connexion with the compound soft substance. These — 
zoophytes increase by gemmation or budding; some throw — 
up germs from the disk, as in Astreadze ; others laterally, — 
as in Caryophillide ; and some spirally algae the stem, as 
in Madreporide ; examples of these modes of reproduction — 
are often found in fossil corals. The increase of coral-— 
rocks is produced by the continual formation of new masses, — 
by the successive generations which spring up as it were 
from the bodies of their parents ; layer upon layer, and tier 
upon tier, of Helianthoid polypidoms, are found to compose 
many of the coralline limestones of the paleeozoic forma- 
tions. 
Fosstr ANTHOZOA.—The first group of extinct corals to 
be noticed under this head is the Graptolitide, a family 
restricted to the Silurian rocks, and whose natural affinities 
have been. much questioned ; some paleontologists referring 
