222 CORALS 



forming corals, llie genera Distichopora and Stylaster, 

 for example, are not uncommonly found in quite shallow 

 pools at low tide in the Tropics, but species of Distichopora 

 are found at a depth of 100-260 fathoms in the Indian Ocean 

 and the West Indies, and species of Stylaster are found in 

 the Malay Archipelago in depths of 0-1038 fathoms.^ The 

 other genera of this Order are principally confined to deep 

 water. 



The reason for the limited distribution of the more 

 important reef-forming corals cannot be determined with 

 certainty. It may be that their lateral distribution north 

 and south of the tropical zone is checked by the lower 

 temperature of the water, a minimum temperature of about 

 18° C. being necessary for their continued existence. 



The range in depth may be determined by the power of 

 direct sunlight to penetrate sea-water. There can be no 

 doubt that the coral Algae are entirely dependent upon 

 sunlight for their continued vitality, and if a depth of 40 

 fathoms be taken as the maximum depth at which living 

 coral Algae are found, it will be found to agree with the 

 maximum depth at which effective rays of the sun can 

 penetrate sea-water. The other reef-forming corals are not, 

 perhaps, so entirely dependent on sunlight as the coral Algae 

 are, for they are provided with tentacles and other organs 

 for catching and digesting animal food ; but still, a majority 

 of them are also provided with the chlorophyll-bearing 

 zooxanthellae which require sunlight, and it is highly 

 probable that these corals do not flourish unless their animal 

 food is supplemented by the food supplied by the zooxan- 

 thellae. In support of this conclusion it may be pointed out 

 that the Stylasterina which are not provided with zooxan- 

 thellae are independent of the action of direct sunHght, and 

 extend from shallow water to the great depths of the ocean. 



The rate at which corals grow has also an important 

 bearing on many of the problems connected with coral reefs. 

 On this point a great deal of interesting information has been 

 collected in recent years. By the measurement of corals 

 found on anchors and cables which were sunk at a known 



^ Stylasterina of the Siboga Expedition, livr. xix., 1905. 



