CCELENTERATA : ACTINOZOA. 147 



CHAPTER XVII. 



DISTRIBUTION OF ACTINOZOA. 



T. 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. Pennatulidcz 

 and GorgonidcR 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, most abundantly in depths of from 25 to 50 

 feet, but extending to a depth of 1000 feet. 



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 Acfinozoa 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 Acfinozoa. 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 



