CCELENTERATA : ACTINOZOA. 115 



CHAPTER XVII. 



DISTRIBUTION OF ACTINOZOA. 



1. DISTRIBUTION OF ACTINOZOA IN SPACE. 2. CORAL REEFS. 

 3. DISTRIBUTION OF ACTINOZOA IN TIME. 4. APPENDIX. 



DISTRIBUTION OF ACTINOZOA IN SPACE. The Zoantharia mala- 

 codermata appear to have an almost cosmopolitan range, sea- 

 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 

 Hydrozoa, swimming near the surface far from land. Peuna- 

 tulidte and Gorgonidcs are found in the seas of the temperate 

 zone, but the latter attain their maximum within the tropics. 

 The Red Coral of commerce (Corallium rubrum) is derived 

 from the Mediterranean. 



The so-called 'reef-building' Corals have their distribution 

 conditioned by the mean winter temperature of the sea, a 

 temperature of not less than 66 being necessary for their 

 existence. 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 Actlnozoa 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, 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. The head-quarters of the reef- 

 building corals may be said to be around the islands and con- 

 tinents 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 : 



1. Fringing -reefs (fig. 31, 1). These are reefs, seldom of 

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



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