90 INVEETEBEATE ANIMALS. 



of Madagascar, in the Eed Sea and Persian Gulf, throughout the Indian Ocean 

 and the whole of the Pacific Archipelago, around the West Indian Islands, and 

 on the coast of Florida. Tlie headquarters, however, of the reef-building 

 corals may be said to be around the islands and continents of the Pacific 

 Ocean, where they often form masses of coral many hundreds of miles in 

 length. According to Darwin, coral-reefs may be distinguished into three 

 principal forms— viz., Fringing-reefs, Barrier-reefs, and Atolls, distinguished 

 by the following characters : — 



1. Fringing-reefs (fig. 52, 1). — These are reefs, usually of a moderate size, 

 which may either surround islands or skirt the shores of continents. The^e 

 shore-reefs are not separated from the land by any very deep channel, and 

 the sea on their outward margins is not of any great depth. 



2. Barrier-reefs (fig. 52, 2). — These, like the preceding, may either encircle 

 islands or skirt continents. They are distinguished from fringing-reefs by the 

 fact that they usually occur at much greater distances from the land, that there 

 intervenes a channel of deep water between them and the shore, and soundings 

 taken close to their seaward margin indicate considerable depths. 



As an example of this class of reefs may be talten the great barrier-reef 

 on the N.E. coast of Australia, the structure of which is on a gigantic scale. 

 This reef runs, with a few trifling interruptions, for a distance of more than 

 a thousand miles, with an average breadth of thirty miles, and an area of 

 thirty-three thousand square miles. Its average distance from the shore is 

 between twenty and thirty miles, the depth of the inner channel is from ten 

 to sixty fathoms, and the sea outside is '^ profoundly deep " (in some places 

 over eighteen hiindred feet). 



3. Atolls (tig. 52, 3). — These are oval or circular rffefs of coral enclosing fl 

 central expanse of water or lagoon. They seldom form complete rings, tbe 

 reef being usually breached by one or more openings. They agree in all essen- 

 tial particulars with those barrier -reefs which surround islands, except that there 

 is no central island in the lagoon which they enclose. 



The last group of the Zoantharia comprises composite organisms 

 in which tlie coenosarc is supported upon a central axis or sclero- 

 basic skeleton. These Zoantharia sderobasica require no notice, 

 except simply to remark that they are distinguished from other 

 sclerobasic Corals (such as the Gorgonidce) by the fact that each 

 polype possesses tentacles "which are a multiple of six in number. 



Order II. Alcyonaria. 



The second great order of living Actinozoa is distinguished by the 

 fact that the polypes are furnished vith. fringed tentacles, and that 

 these, as well as the mesenteries and somatic chambers, are always 

 eight in number. With few exceptions, the Alcyonaria are com- 

 posite, their polypes being connected together by a coenosarc. The 

 body-cavities of the polypes are connected with a system of com- 

 municating canals which are excavated in the coenosarc, so that a 

 free circulation of nutrient fluids is thus kept up. The struotui-e of 



