124 CORAL AND ATOLLS 



sheltered sandy pool of the barrier flats. In the same pool, 

 which is almost completely cut off from the sea at low tides, 

 and then contains about 2-3 feet of water, and which is about 

 100 yards long by 20 wide, numerous corals live and flourish; 

 calm-water forms of Madrepora and Pocillopora, capable of resist- 

 ing silt, being the most abundant. The conditions of life in these 

 barrier pools are peculiar. The pools are filled with sand, for 

 the fragments which are triturated by their journey to and fro 

 over the barrier are deposited in them, they contain the 

 minute green filaments of the boring algse, and at midday low 

 tides they become heated by the sun to 93° Fahr. and more. 

 Their coral fauna is practically always the same ; Madrepora 

 flourishes in its most highly branching forms, Pocillopora always 

 has a good foothold, and the other species are in plenty, but 

 hardly in a state of luxuriance. Beche-de-mer in hundreds 

 live in these pools, and Crustacea, polychsetes, mollusca, eels, 

 and the myriad brilliant fish make up the conspicuous fauna. 

 Of the many rough-water colonies which were transplanted 

 experimentally into this environment, not one remained alive 

 at the end of 50 days, and most were dead within a month. 

 The first sign by which a colony shows that its environment 

 is not suitable is by becoming highly pigmented. Both rough- 

 water forms are, when flourishing, very pale corals, being 

 usually of a light buff colour ; but within a fortnight the 

 transplanted colonies had become of a dark yellow-brown, and 

 in Madrepora there was a more than usual tendency to lateral 

 branch formation. In 20 days most colonies had some portion 

 dead, and the dead parts became rapidly the site of growing 

 algse. In 30 days nearly all the numerous transplanted 

 colonies were dead or dying, and by 50 days no portion of 

 any colony remained alive. It was silt that had determined 

 the death of them all ; the stunted, flattened types of 

 Madrcpora and Pocillopora are both corals of a rough-water 

 habitat, they are used to clear water in which there is little or 

 no suspended matter, and not a single colony was able to with- 

 stand the slow but certain sedimentation of the barrier pool. 

 When the silt had once fairly determined their death, the fine 



