DEATH-PROCESSES OF CORAL COLONY 125 



boring alg£e completed the ruin. If any colony of branching 

 coral be removed entire from the lagoon, it will be found that 

 tliG lower portion is invariably dead ; but this death is in most 

 cases not a natural one resulting from the senility of the 

 zooids, but merely an index of the amount of silting up of 

 the lagoon which has taken place since the establishing of the 

 colony. Sand is ever being washed into the lagoon through 

 the numerous gaps in the island ring, and most decidedly the 

 tendency all over the lagoon is a gradual filling up, by the 

 deposition of finely triturated fragments. The floor of the 

 lagoon is fairly steadily rising, and those colonies of corals 

 growing in its bed are for ever being encroached upon by the 

 gradual rising of the sand level. The deposited sand most 

 certainly kills the zooids with which it comes in contact, and 

 the result is that the lower portion of every lagoon colony is 

 killed. Silt then, in this atoll, is the most potent factor in 

 causing coral death, and next in importance to the silt conies 

 the seaweed. 



There is a green alga which, at some seasons of the year 

 more than at others, comes to the barrier pools in great 

 quantities : it is a growth of fine green threads, and its effect 

 on coral growth is really wonderful. A pool in which nume- 

 rous flourishing colonies live, quite on a sudden may show 

 the advent of this alga, and every portion of every colony, 

 which may receive a chance injury, at once becomes the site 

 of the growth of these fine threads. I believe that it is 

 always at the site of injury that the attack of the alga starts, 

 but its growth soon invades, and invariably kills, the living 

 healthy portions of the colony. A colony once fairly invaded 

 by this parasite rapidly dies, and yet it never succeeds in 

 obliterating coral growth, for as suddenly as it came to the 

 barrier pools it goes. Spring tides and hot weather seem to 

 promote its growth, or perhaps lower the resistance of the 

 coral colonies, for when the rock-pools are left long to swelter 

 in the sun, with but little depth of water in them, then the 

 alga seems to be most active. It is a great factor in causing 

 the death of the atoll corals, and ranks in Cocos far in advance 



