CCE LENTEUATA : ACTINOZOA. I35 



anemones being found on almost every coast ; some of the 

 tropical forms attaining a very large size. The Ctenophora, 

 too, have an almost world-wide distribution, occurring in all 

 seas from the equator to within the arctic circle. In habit all 

 the Ctenophora are pelagic, being found, like the oceanic Hy- 

 drozoa, swimming near the surface far from land. Pennatulid(z 

 and Gorgonidm are found in the seas of the temperate zone, 

 but the latter attain their maximum within the tropics. The 

 Red Coral of commerce {Corallium rubruni) is derived from 

 the Mediterranean. 



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. 



All known Adinozoa are marine, no member of the class 

 having hitherto been found in fresh water. 



Coral-reefs. — 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 Adinozoa. 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. The headquarters of the reef-building Corals 

 may be said to be around the islands and continents of the 

 Pacific Ocean. 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. Fringing-reefs (fig. 38, 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 repose 

 upon a gently-sloping surface. 



3. Barrier-reefs (fig. 38, 2). — These, like the preceding, may 

 either encircle islands, or may skirt continents. They are dis- 



