202 FOUNDERS OF OCEANOGRAPHY 



form dry land and may be habitable. The majority of 

 coral islands are on atolls. 



Before passing to the theories which have been put 

 forward to account for these forms of coral structures, let 

 us consider what the reefs are made of. They are wholly 

 produced by living organisms, animals and plants, and 

 especially by the coral animals or polypes. 



Huge coral structures of carbonate of lime are built up 

 by innumerable minute polypes, each of which is like a 

 small sea-anemone, and has a mouth surrounded by tentacles. 

 There are some solitary corals, formed of single polypes, 

 comparable with sea-anemones which have deposited lime 

 skeletons in and around their bodies ; but the majority of 

 corals are colonies formed of an immense number of polypes 

 produced by continuous budding. 



There are certain deep-sea corals which do not form reefs, 

 but may be of importance in helping to build up platforms 

 upon which reefs can grow. 



The true reef-forming corals live only in shallow water, 

 as a rule not deeper than 30 fathoms, and in water which 

 is never colder than about 68° F. They are, therefore, 

 tropical animals, limited by the isotherms of 68° North and 

 South of the Equator, a zone lying for the most part between 

 30° N. latitude and 30° S. latitude. 



It must not be supposed, however, that coral reefs are 

 wholly, or even chiefly, formed of coral skeletons produced 

 by coral polypes. There are in addition many other cal- 

 careous organisms present, including Foraminifera, Molluscs, 

 Polyzoa, and even NuUipores and other calcareous sea-weeds 

 (such as Halimeda), and in some cases these form the greater 

 part of the reef. 



Once the facts of distribution are ascertained, there is 

 no mystery in regard to the formation of the fringing reef. 

 It merely grows and spreads under suitable conditions 

 wherever it can in shallow water. It hugs the coast-Hne 

 because the living organisms which are forming it cannot 



