00 JNVEUTEIilfATE AXIMALS. 



of MadagaJicar, in tlie Red Hen and Persian Gulf, tlirougliont tlie Indian Ocean 

 and the wlioleof tlie Pacific Archii)elago, around the West Indian Islauils, and 

 on the coast of Flori(La. The head(|uarters, liowever, of the reef-huilding 

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

 Ocean, where tliey often form masses of coral luany hundreds of miles in 

 length. According to Dai'wiii, coral-reefs may he distinguished into tlire^' 

 ])rincipal forms — viz., Fringing-reefs, Barrier-reefs, and Atolls, distinguished 

 by the following charai.-ters ; — 



1. Fi'i)i<jing- reefs {iig. 52, 1). — These are reefs, nsually of a moderate size, 

 which may either surround ishands 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 dexitli. 



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

 idands or skirt continents. They are distiuguislied 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 theni and tlie shore, and soundings 

 taken close to their seaward margin indicate considerable depths. 



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

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

 'J'his reef runs, with a few trilling interrujitions, 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 tlie inner channel is from ten 

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

 over eighteen hundred feet). 



3. Atolls {hg. 52, 3). — These are oval or c'rcular reefs of coral enclosing a 

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

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

 tial particulars witli those barrter-reefs which surround islands, excei)t that there 

 is no central island in the lagoon which they enclose. 



The last group of the ZotintJctria comprises composite organisms 

 in which the CiTenosarc is supported upon a central axis or sclero- 

 basic skeleton. These Zoaiitharia sclerobasiea require no notice, 

 except simply to remai'k tliat they are distinguished from other 

 sclerobasic Corals (such as the Oorgomda^ by the fact that each 

 polype possesses tentacles which are a multiple of sLv in number. 



Order II. Alcyoxaria. 



The second great order of living Aiiinozoa is distinguished by the 

 fact that the polypes are furnished ^\\i\ifrin<jed tentacles, and that 

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

 eiyJit in number. AVith few excejitions, the Alcf/onan'a are com- 

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

 body-cavities of the ]i(dyjies are connected with a system of com- 

 miuiicatinjLj cannls which are exca.vatetl in the C(enosarc, so that a 

 free circulatioJi of nutrient fluids is tiius kept up. The structure of 



