CLASS V. POLYPI: ORDER 1. HELI ANTHOID A. 637 
ORDER 1. HELIANTHOIDA. 
Of the Helianthoid polypes, deriving their name from kdios, anthos, and eidos, sun, flower, and 
ix'semblance, the common Sea- Anemones, found on nearly all coasts, may serve as an example. 
They all have a stomach, consisting of a sac quite distinct from the walls of the body ; the space 
between the stomach and the outer integuments is divided into cells by membraneous and mus- 
cular partitions, upon which the ova are produced. The mouth is surrounded by a variable num- 
ber of tubular tentacles, wdiich are generally very numerous, and arranged in multiples either of 
five or six. Wo shall treat them under the following heads : Maclreporidce, CyathojohyllidcB, As- 
trceidce, Fungidce, Zoanthidce, Actiniadce, and Lucernaridce. 
THE MADREPOEIDiE. 
These animals are generally called Tree-Corals, on account of the forms of the polypidoms 
which they build and in which they live. They are very small, and possess twelve short tenta- 
cles placed in a circle round the mouth; they occupy the cells in the polypidora, which is 
of a porous nature, the openings of tlie cells being placed at the summits of tubercular promi- 
nences of greater or less elevation. The animals are of various species and their polypidoms are 
of various forms, though generally more or less branched and tree or plant shaped. 
THE CYATHOPHYLLIDJE. 
These, which are called Cujj- Corals, form polypidoms of a more or less cup-like shape, with 
the cells occupied by the polypes, at the upper extremity. The species are large, and furnished 
with many tentacles, and the rays of the cells are also numerous. Species of this and the preced- 
ing group are found in deep water off the British coasts. 
ASTR^A VIRIDIS.* ASTR^A ROTDLOSA. 
THE ASTR^IDJE. 
It is to this family more especially that the formation of the coral reefs is to be attributed.f In 
this the corals usually form thick stony masses ; the stony rays of the cells are exceedingly numerous, 
and the cells themselves penetrate deeply into the mass of coral, although they are generally par- 
*aa, expanded polypes; i b, polypes withdrawn into their cells; c c, coral uncovered by flesh, showing the cells. 
t Humble as these creatures are, their operations occupy au important place in the history of the globe. Islands 
—some of them of considerable size, and affording a habitation to an entire race of human beings— owe their elevation 
from the bottom of the ocean, and the solidity which enables them to resist the continual action of the tremendous 
breakers of the tropical seas, to the labors of these apparently contemptible agents ; and in the geological periods 
of the world's history, they appear to have played even a still more important part. 
Three kinds of coral-reef are distinguished. Nearly all the shores of the seas inhabited by the reef-building corals, 
which occupy a broad zone extending between twenty and thirty degrees of latitude on each side of the equator, are 
more or less fringed with their living walls ; these are called skirUng reefs. Other reefs are sometimes met with at a 
much greater distance from the shore, although still, to a certain extent, running parallel to its outlines. To these the 
name of larrier-reefs has been given ; the most remarkable of them is the great reef which runs along the coast of 
Australia. The third form of reef is presented by a great number of the Polynesian islands. Many of these are of a 
