CORAL

the hard, chiefly limy skeletons of certain Coelentera, notably the millepores, the madrepore or reef corals, the horny Anti path aria, and the Alcyonaria, which include the deadmen’s-fingers (Alcyonium), the organtula), the red c. (Corallium) and blue c. (Heliopora). The most important c’s are the Mad rep oraria, which form c. rocks, reefs, and islands in tropical seas ordinarily between 30° N. and S. of the | equator. Reefs may either be fringing | an island or be separated from the lana Con-|by a shallow lagoon channel (barrier reef), or from a ring-shaped atoll with a work|central lagoon. As c’s are killed by there-|fresh water and deposits of mud and sand, thev find more food and grow | fastest on the seaward side of the reef, of] and a channel is left opposite to the of| mouth of a creek or river. Since c. of| cannot live below a certain depth, c. | islands must have been formed on submarine peaks rising to about 40 fathoms below sea-level, or raised to that height | by the constantly falling débris of marine | organisms or by volcanic action. Another | explanation (Darwin’s) holding good for | a few atolls is that islands have gradually it} subsided, the fringing reef c. growing | accordingly, and thus forming an atoll. | It is certain, however, that the rate of | growth of c. is considerable, under favor- | able conditions up to 9 in. per annum.