THE LAGOON 189 



of types and not of species; and exactly the same assertion 

 holds good with regard to the lagoon. The same corals that 

 live on the barrier are also found in the lagoon. At the same 

 time it is quite correct to say that some types are definitely 

 confined to certain definite environments, and this forms one 

 of the most interesting pages in the study of living corals. 

 This subject has already been discussed and so no further 

 details will be added here : but it must be insisted that sand 

 and silt are the most important agents in the causation of 

 death of corals, and that their presence acts in two ways : 

 (1) It may fall on them and choke their zooids from above, 

 or (2) It may overtake them from below. Some idea of the 

 rate of the silting of the lagoon may be gathered by measuring 

 the dead lower portions of the great branching colonies of 

 Madrepora ; for these corals do not normally die in their lower 

 portions, and the dead area represents the amount of silting that 

 has taken place, in the immediate neighbourhood of the colony, 

 since its first establishment. 



Sand then is the great architect of the lagoon, and while it 

 inevitably kills the coral growths, it more than counterbalances 

 for this by its own building efforts. 



The still waters of the lagoon are harmful to the coral 

 colonies in another way, for large areas of the shallow 

 water are covered by great beds of algse. Where algse exist 

 abundantly corals do not flourish. Agar-agar and many 

 other seaweeds cover a considerable portion of the lagoon, 

 making characteristic dark patches on the bottom. One such 

 large bed exists in front of the eastern end of Pulu Tikus, and 

 many are found in the southern portion of the lagoon, and 

 these beds flourish to the absolute exclusion of coral growth. 

 There is in particular one very fine green alga that occurs 

 even in the barrier pools, and is particularly inimical to coral 

 growth. At certain seasons of the year these fine green 

 threads are found, as drift-weed, in every pool ; and as they 

 wash to and fro in the tide they become entangled among the 

 branches of the corals. Wherever the threads obtain a fair 

 lodgment, a green mat-like growth is spread on the surface of 



