CAUSES AFFECTING THE GROWTH OF CORALS. 121 



ment of sediment, The same takes place with the hemispheres 

 of Astrsea ; and it is not uncommon that in this way large 

 areas at top are deprived of life. The other portions still live 

 unaffected by the injury thus sustained. Even the Fungia), 

 which are broad simple species, are occasionally destroyed over 

 a part of the disk through the same cause, and yet the rest re- 

 mains alive. It is natural, therefore, that wherever streams or 

 currents are moving or transporting sediment, there no corals 

 grow ; and for the same reason we find few living zoophytes 

 upon sandy or muddy shores. 



The small lagoons, when shut out from the influx of the 

 sea, are often rendered too salt for growing zoophytes, in con- 

 sequence of evaporation, — a condition of the lagoon of Ender- 

 by's Island. 



They also are liable to become highly heated by the sun, 

 which likewise would lead to their depopulation. 



Coral zoophytes sometimes suffer injury from being near 

 large fleshy Alcyonia, whose crowded drooping branches lying 

 over against them, destroy the polyps and mar the growing 

 mass. Again, the dead parts of a zoophyte, though in very many 

 cases protected by incrusting nullipores, shells, bryozoans, etc., 

 as already explained, in others is weakened by boring shells 

 and sponges. Agassiz states, in his paper on the Florida 

 Reefs (Coast Survey Report for 1851) : " Innumerable bor- 

 ing animals establish themselves in the lifeless stem, piercing 

 holes in all directions into its interior, like so many augurs, 

 dissolving its solid connection with the ground, and even pen- 

 etrating far into the living portion of these compact communi- 

 ties. The number of these boring animals is quite incredible, 

 and they belong to different families of the animal kingdom ; 

 among the most active and powerful we would mention the 

 date-fish or Lithodomus, several Saxicava?, Petricolas, A cae, 



