CCELENTERATA : ACTINOZOA. I2Q 



CHAPTER XVII. 



DISTRIBUTION OF ACTINOZOA. 



i. 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 Hy- 

 drozoa, swimming near the surface far from land. Pennatulidce 

 and Gorgonida 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 tem- 

 perature 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 Actinozoa 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 re- 

 moved from the equator as to possess a lower winter tempera- 

 ture 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. 36, 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 



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