CLASS V. POLYPI: ORDER 1. IIELI ANTIIOID A. 



637 



OPtDEI 



HELIANTHOIDA. 



Of the Helianthoid polypes, deriving their name from hcitos, anthos, and cidos, sun, flower, and 

 resemblance, 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, which are generally very numerous, and arranged in multiples either of 

 five or six. We shall treat them under the following heads : Madrejoorklce, Cyalhojjhijllidoe, As- 

 trceidce, Fungidce, Zoanthidce, Actiniadce, and LucernaridcB, 



THE MADREPORID^. 



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 cii-cle round the mouth; they occupy the cells in the polypidom, which is 

 of a porous nature, the openings of the 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 CYATHOPHYLLID^. 



These, which are called Cu2)-Coral!<, 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.EA VIEIDIS. 



ASTR^A ROTULOSA. 



THE ASTR^IDuE. 



It is to this family more especially that the formation of the coral reefs is to be attributed.! 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*aa, expanded polypes; b b, polj'pes withdrawn into their cells; c c, coral uncovered by flesh, showing the cells. 



t Humble as these creatures are, their operations occupy an 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-budding 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 skirting reefs. Other reefs are sometimes met with at a 

 much greater distance from the shore, alUiough still, to a certain extent, running parallel to its oiulines. To these the 

 name of harrier-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 



