CORAL REEFS 165 



mination. The corals found in deep water, significantly 

 enough, never have these associated plants. It appears that 

 in many cases the corals are not absolutely dependent on the 

 plants, for if there is animal food to be obtained — and this 

 will usually consist of members of the minute drifting life 

 of the sea — this will be seized and swallowed and the plants 

 left unmolested in their sheltered home where they grow 

 and increase by dividing. But should there be any shortage 

 of animal food, the corals fall back on the plants, which are 

 brought into the stomach cavity and there digested. In the 

 cases where the stomach is cut off entirely from the outside, 

 the coral must depend absolutely on the plants. 



It is difficult to estimate the growth of corals, for this 

 depends so much on conditions, especially temperature and 

 the supply of food, but it has been found that the branches of 

 Madrepora may increase by one or two inches in length 

 per annum, while a mass of Porites increased its diameter 

 by thirty inches in twenty-three years. Other estimations 

 have shown that a shallow-water reef might, under favour- 

 able conditions, grow upwards at a rate of about one foot 

 in eleven and a half years, or fourteen and a half fathoms 

 in one thousand years. In Samoa, Dr. Mayer came to the 

 conclusion that the corals added some 840,000 pounds of 

 limestone to the reef annually, although about four times 

 this amount is removed in the same period by currents, 

 sea cucumbers and other coral-eating animals. This is not 

 the only evidence we possess which indicates that coral 

 reefs, in some areas at any rate, may, at the present day, 

 be decreasing rather than increasing. 



The Nullipores or calcareous seaweeds grow as sheets 

 (Plate 61), usually comparatively thin, though masses two 

 feet thick have been reported from the Indian Ocean. They 

 vary in abundance from place to place, being clearly of first- 

 rate importance in some areas, such as the Maldive and 



