THE BENTHAL 209 



and the west coast of South America, limit the occurrence of coral. 

 The cold southward current on the east coast of Florida prevents their 

 development, but they flourish on the west coast. On the other hand 

 many corals are little sensitive to increase of temperature above their 

 optimum. Coral polyps, withdrawn into their cups, may withstand a 

 temperature of 56° in the direct sun, when they are exposed at low tide. 



Within this tropical belt, further subdivisions (with reference to 

 the coral) may be made. 48 A narrower equatorial belt, with water 

 above 23.4°, harbors all the large branched forms, while the two 

 bordering areas, with temperatures between 23.4° and 20.5°, have 

 principally the encrusting forms, which are more resistant. The variety 

 of species is also greater in warmer water. At Port Galero (Mindoro) 

 Griffin listed 180 species of Madreporaria ; 49 Klunzinger 50 described 

 128 species from the very warm Red Sea; and Stuhlmann 51 collected 

 51 species at Zanzibar. On the Tortugas, at 25°N. latitude, the number 

 of species is scarcely 30, and in the Bermuda reefs (latitude 32°N.) 

 only 10 species of madrepores and 2 of millepores have been found. 52 



Stony corals exist, it is true, in higher latitudes. The solitary species 

 Flabellum and Caryophyllia and the colonial Astroides (p. 199) occur 

 in European waters, and Astrangia extends along the North American 

 coast from Florida to Cape Cod. Corals also appear in deep water as 

 banks of considerable extent, though without forming reefs, as for 

 example the "Coral Patch" in the Atlantic at a depth of 800 m., which 

 drops off to 1000-1500 m. at its edges, covered with Lophohelia 

 prolifera, and other forms. 53 The same Lophohelia and Amphelia ramea 

 appear on the Scandinavian coast north to the Lofoten Islands and 

 are especially well developed in the Trondjem fiord, where they form 

 banks at 200 m. depth, differing from reefs in that they do not rise 

 to the surface. 54 Reef-building corals extend to a depth of 40-60 m., 

 rarely down to 74 m. 55 



The growth of reef corals in waters close to the surface is intimately 

 correlated with the penetration of light. Their polyps are filled with 

 symbiotic algae, Zooxanthellae, either embedded in the body wall or 

 free in the stomach. Gardiner 56 has shown that in several genera a 

 considerable amount of oxygen is given off, indicating active assimila- 

 tion. No animal matter which could be regarded as food was to be 

 found in the entera of many and in Prionastraea the digestive cavity 

 has disappeared, so that the food of the polyps seems to be wholly 

 supplied by the symbiotic algae. Thus the basic food supply, which 

 must always be derived from plants by animals, is here cultivated by 

 the animals themselves. It becomes evident that they require light in 

 order to flourish, and extend in depth as far as the seaweed or 



