ioo THE SHORE 



loose masses are not found on the lagoon flat,* it being 

 generally a bare or slightly hollowed-out area, which is 

 alternately covered and uncovered by the tides. The 

 latter must be the ultimate agents which effect the 

 removal of the boulders, and this they are supposed 

 to do by removing the coral, which is composed of 

 almost pure carbonate of lime, either in solution or as 

 fine mud in suspension in the water, or by both means. 

 The solvent action of sea water on coralline material is 

 well determined, and organisms are likewise known to 

 break up coral rock into fragments — gravel, fine sand, 

 and, lastly, mud. The removal of material in suspension 

 is evidently of as great importance as solution, or even 

 greater, hundreds of thousands of square miles of ocean- 

 floor near coral reefs being charted as covered with 

 coral mud. How the latter is produced requires further 

 investigation. Probably there is some animal matter 

 left in the coral limestone. In search for this the 

 masses of rock are riddled in the first instance with 

 the fine borings of algse (Achyla) and sponges, their 

 borings invisible to the naked eye. These are followed 

 by worms, Sipunculids, and hosts of other organisms, 

 which cause the rotting to pieces of large masses. 

 They are further broken down by the waves into gravel 

 and sand, as such being ingested by other worms, sea- 

 urchins, Holothurians, Sipunculids, and Balanoglossids, 

 which in their intestines further grind them up into 

 the finest mud ; the latter readily passes into suspension 



* Boulders formed of the skeletons of the blue coral, 

 Heliopora, often persist for long periods of time, resisting most 

 of the ordinary agencies which in such positions cause decay. 

 The same is also true to some degree of Millepora, the stinging 

 "coral," which is a form of peculiar anatomical interest, as 

 having medusae bearing the generative products (Figs. 27 

 and 28). 



