1 64 THE SEAS 



The effect of this is finally to kill, by a kind of suffocation, 

 the polyps in the centre ; growth consequently ceases in this 

 region but is continued round the edges where the polyps 

 are uninjured, the sediment having slipped off. and so the 

 coral changes from a globular mass into a great, shallow 

 cup. Another way in which sediment may affect corals is 

 by accumulating around their bases and killing the lower- 

 most polyps. 



Of the greatest significance in the study of coral reefs 

 are the feeding habits of corals. We should naturally 

 expect that, being so like anemones, they would feed in the 

 same manner, seizing living or dead animals by means of 

 their tentacles armed with batteries of stinging cells, and 

 later digesting them in the stomach cavity. We do find 

 this type of feeding exclusively in some corals and to some 

 extent in practically all, but they have added to this another 

 mode of feeding in which they rely on the assistance of 

 plants. This strange association between animals and 

 plants is called symbiosis, and is explained in more detail in 

 Chapter IX. In the majority of corals, of all types, the 

 stomach cavity and also the tissues, especially around the 

 mouth and tentacles, contain great numbers of minute, 

 single-celled plants or algae. So numerous are they that in 

 some kinds of corals they practically obliterate the stomach 

 cavity and in one most striking instance this cavity is 

 completely shut off and left without an opening to the 

 exterior, so that food cannot be swallowed in the normal 

 manner. Often they are so numerous as to give a green 

 colour to the coral. Like all plants they feed by taking 

 carbonic acid gas and various salts from solution in the 

 sea, for which purpose they must have light, a fact of the 

 greatest importance in the distribution of corals, for all 

 those dependent on the plants must live in water shallow 

 enough for the latter to obtain the necessary degree of illu- 



