SPONGES AND CORALS 



tudes of tiny sea animals are still building the Reef up- 

 wards, generation after generation leaving their limy 

 skeletons behind as they die, to add their extremely thin 

 layers to the deposits of countless ages. These are the 

 years (a microscopic period of time in the eons during 

 which generations of these amazing organisms have lived 

 and died to produce the Reef) which mark the emergence 

 of the vast structure from the sea. In other parts of the 

 world many other coral reefs have already emerged or are 

 slowly coming to the surface. 



Coral reefs can be built only when the animals are 

 washed by waters warm enough to assist the secretion of 

 the calcareous substance which forms their skeletons. The 

 living reef-structures, with all their associated life forms, 

 must therefore be confined to waters with temperatures 

 which do not fall below seventy degrees for more than 

 very brief periods. They therefore occur only in the area 

 bounded by the Tropics of Cancer and Capricorn, and 

 mainly on the eastern shores of continents, bathed by 

 currents of warm water carried towards the poles by the 

 world's wind and tide patterns, under the influence of the 

 earth's rotation. 



Taking North America as an instance of this principle : 

 the Pacific coast lacks corals, because they are aflfected 

 by uprisings of cold water, while the north-eastern shores 

 of Australia, washed by warmer currents, are the site of 

 the Great Barrier Reef The only coral coast within the 

 United States is the two hundred-mile group of islands 

 (actually two groups) known as the Florida Keys — 

 second only to the Australian Reef itself for strange and 

 beautiful corals in vast profusion. 



Although coral reefs^ consisting of the more massive 

 kinds of coral, are confined to the sea's warmer waters, 

 the more primitive kinds, already mentioned, are very 

 widely distributed and are found at all depths, both in 

 warm and cold waters. Certain varieties form dense beds 

 oflf the coasts of Scotland, Norway and Portugal — but it 



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