CORAL REEFS 163 



— probably the most widespread of reef-building corals — 

 appears, for example, as a much branched, arborescent 

 coral in smooth waters (Plate 65), as a single cylinder often 

 of great length in deeper water, and as a solid, irregular 

 mass in the rough water on the outer edge of the reef within 

 the action of the perpetually pounding surf. The reason 

 for these varied forms is to be found in the influence of these 

 different conditions on the growth processes of the coral ; 

 in deep water which is always calm, there is nothing to 

 prevent the original coral from developing on the " ideal " 

 plan and growing straight upwards with a single apical 

 point of growth from which polyps are budded off on all 

 sides, but in the rather rougher conditions nearer the surface 

 even in the most sheltered conditions, injury of the apical 

 polyp leads to the establishment of new points of growth, 

 each of which forms a new branch, while in the most exposed 

 conditions perpetual injury of all projecting growths leads 

 to the formation of a solid, irregular mass. At one time 

 it was thought that these different types were all different 

 kinds of corals, but now we know that there are relatively 

 few kinds but unlimited variations in them all. The most 

 striking example is that of the hydroid coral, Millepora, of 

 which there are all manner of different forms but apparently 

 only the one species in the world. 



The effect of sediment is of the greatest importance. 

 The coral polyps are able to expand and contract, and their 

 exposed surfaces are usually covered with fine, rapidly 

 moving hairs, the action of which helps to cleanse the 

 surface, but these are their only means of protecting them- 

 selves from sediment and, like all attached animals, they 

 are essentially at the mercy of material which drops on them 

 from above or collects around them. A branching coral 

 growth has less to fear from falling sediment than a massive 

 growth which offers a wide surface on which it can collect. 



M 



