C(ELENTERATA : ACTINOZOA. l8l 



Red Coral of commerce is a Mediterranean species/and occurs 

 principally at depths of from 5 to 6 fathoms, though occurring 

 at 120 fathoms or more. The Organ-pipe Corals (Tubipord] 

 are confined to the warm seas of the " coral region ; " and the 

 genus Heliopora, the only recent representative of the family 

 H(lioporid(K> is confined to the Pacific and Indian Oceans. 

 The only living corals referred to the Rugosa are Guynia, 

 which is found in the Mediterranean, and the Haplophyllia of 

 the Florida seas. The Ctenophora are pelagic, free-swimming 

 forms, and appear to be cosmopolitan in their distribution. 

 Lastly, the Zoantharia sclerodermata are partly inhabitants of 

 deep water, and partly shallow-water forms; and the latter, as 

 commonly forming " coral-reefs," are so important as to de- 

 mand special consideration. 



The so-called " reef-building " corals have their distribution 

 conditioned by the mean winter temperature of the sea, a tem- 

 perature of not less than 66 being necessary for their existence. 

 The seas, therefore, which possess the necessary temperature 

 may be said to be all comprised within a distance of about 

 1800 miles of the equator on each side. Within these limits, 

 however, apparently owing to the influence of arctic currents, 

 no coral-reefs are found on the western coasts of America and 

 Africa. They are found chiefly on the east coast of Africa, 

 the shores of Madagascar, the Red Sea, and Persian Gulf, 

 throughout the Indian Ocean and the whole of Polynesia, and 

 around the West Indian Islands and the coast of Florida. 

 The headquarters of the reef-building corals may be said to 

 be round the islands and continents of the Pacific Ocean. A 

 " coral-reef" is a mass of coral, sometimes many hundred 

 miles in length, and it may be two thousand feet or more 

 in thickness, produced by the combined growth of different 

 species of coralligenous Actinozoa. As before said, a mean 

 winter temperature of not less than 66 is necessary for their 

 existence, and therefore nothing worthy of the name of a 

 " coral-reef" is to be found in seas so far removed from the 

 equator as to possess a lower winter temperature than the above. 



According to Darwin, coral-reefs may be divided into three 

 principal forms viz., Fringing reefs, Barrier-reefs, and Atolls, 

 distinguished by the following characters : 



i. Fringmg-reefs (fig. 91, i). These are reefs, seldom of 

 great size, which may either surround islands, or skirt the 

 shores of continents. These shore-reefs have no channel of 

 any great depth intervening between them and the land, and 

 the soundings on their seaward margin indicate that they re- 

 pose upon a gently sloping surface. 



