i64 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. 



The feeding of corals is of the greatest importance in 

 the study of coral reefs. Corals feed in the same manner 

 as sea anemones, seizing animal prey by means of tentacles 

 armed with batteries of stinging cells. They pass this 

 into the stomach cavity where it is digested. Corals are 

 specialised carnivores living primarily on the animal 

 plankton ; they will not accept plant material and if this 

 is introduced into the stomach cavity it cannot be digested. 

 In a few corals food is conveyed to the mouth by the 

 hair-like cilia which are usually concerned with cleansing. 

 In some, where the cilia normally beat away from the 

 mouth, these change the beat when animal material is 

 placed on the surface and carry this to the mouth. All 

 true reef-building corals possess within their tissues vast 

 numbers of minute, single-celled plants or " zooxanthellae." 

 This strange association between animals and plants, 

 called symbiosis, is discussed in Chapter IX. The plants 

 are all contained within cells of the animal and are normally 

 so numerous that, in the absence of other colouring matter, 

 they give a brown colour to the coral. This association is 

 so universal that it is difficult not to believe that it is 

 of value to both. It is certainly essential to the plants 

 which never occur free in the sea, being carried from genera- 

 tion to generation by the " planulae." The plants gain 

 protection and are able to tap at the source not only carbonic 

 acid gas, which they need to build up starch, but also 



