128 ZOOLOGY. 



COBHOSarc. "They constitute a true republic, in which 

 each citizen, whether consciously or not, is compelled to 

 support the general state, by which he himself exists." 

 (Haugkton). The ccenosarc secretes a common calcareous 

 structure, which unites all the cups into one strong mass. 

 Coral, formed thus ivithin the tissues of the animal, is 

 called sclerodermic (Gr. skleros, hard; derma, skin). 

 The septa of modern corals are always multiples of Jive 

 or six; while, in extinct corals (Rugosa), they are 

 uniformly multiples of four. 



182. Coral Beefs. The coral polypes, small as they 

 are, have been important rock-forming agents in all 

 periods of the world's history. The stony masses formed 

 by them are termed coral reefs. The coral zone extends 

 about 30 on each side of the equator. The reefs are 

 principally found in the Indian Ocean, on the east coast 

 of Africa, and near the shores of Madagascar; in the 

 Persian Gulf, the Red Sea, and the Bay of Bengal ; in 

 the Atlantic Ocean, round the "West India Islands, and 

 the coast of Florida ; they are especially abundant round 

 the islands and continents washed by the Pacific Ocean. 

 The coral animals require a mean annual temperature of 

 not less than 68. On this account they are rare, and of 

 small size, in our seas at present. No coral reefs are 

 found either along the west coast of Africa, or the 

 corresponding coast of South America, on account of the 

 cold currents which flow in these places. These reefs 

 assume three principal forms. 



183. (1.) Fringing Reefs, which skirt the shores of 

 continents and islands. These reefs have been constructed 

 in water comparatively shallow, and are generally not far 

 from land. 



184. (2.) Barrier Beefs are distinguished from fringing 

 reefs, by occurring at a considerable distance from the 

 nearest land. They are also surrounded by deep water. 

 The great barrier reef on the north-east coast of 

 Australia, is a notable example of this class. This reef 

 is about 1,100 miles long, and 30 broad on an average, 



