DESTRUCTION OF CORAL REEFS ' 415 



of coral occur formed from the heads of large mseandrinas and 

 astrseans torn by the waves from their anchorage and rolled about, 

 grinding down the rock beneath them and also being worn them- 

 selves. 



Perhaps by far the largest contribution of coral sand and mud 

 is made by the activities of organisms. Darwin found that two 

 species of the Parrot fish (Scarus), "one inhabiting the surf outside 

 the reef and the other the lagoon, subsist entirely ... by browsing 

 on the living polypifers." (21.) On opening several of these fish, 

 he found their intestines distended by small pieces of coral and 

 finely ground calcareous matter, which must pass daily from them 

 as the finest sediment. On Funafuti, on the other hand, no evi- 

 dence of such destruction of coral rock by fish was found, though 

 they apparently fed to some extent on the Lithothamnion. In like 

 manner the reported destruction of coral masses by holothurians 

 found no confirmation on Funafuti, where these echinoderms live 

 wholly on the organic matter of the sand and mud. Worms, mol- 

 luscs and crustaceans, however, appear to be active destroyers of 

 coral rock, though at Funafuti the molluscs were found to live 

 among the coral masses without interfering with them. On these 

 reefs, however, the Lithothamnion rock was found riddled by 

 Gephyrean worms and an Aspidosiphon "from two to three inches 

 in length, but very thin, and possessing a long retractile proboscis, 

 by which it is seen at low tide to feed ofif the Lithothamnion-cov- 

 ered rock immediately surrounding its abode. ... A small piece 

 of reef rock, a few inches in diameter, when broken off, will show 

 as many as 10 or 15 of these animals." Echinoderms also dwell 

 in cavities which they have hollowed out of the reef rock, and 

 which, except for the recuperative growth of the Lithothamnion, 

 they would rapidly destroy. Among the most efficient coral de- 

 stroyers are probably the Crustacea, and they are generally credited 

 with the production of much of the coral sand. Not only the corals 

 themselves, but the shells of molluscs and the tests of echinoderms 

 are ground up by these voracious feeders, the resultant being a 

 mixture of lime sand and flour, which by the sorting action of the 

 waves is distributed in beds around the reef. 



Much coral sand will become lodged in the interstices of the 

 reefs between the growing coral masses, thus binding the whole 

 into a solid mound. One of the marked features of this mound 

 of coral rock is the absence of stratification, and when solidified the 

 mound will be an irregular, structureless, often lens-shaped mass 

 of lime rock embedded in stratified calcarenytes and calcilutytes. 

 Along the margin of the reef the clastic lime rocks will be deposited, 



