CAUSES AFFECTING GROWTH OF CORALS. 95 



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 

 consequence of evaporation, — a condition of the lagoon of 

 Knderby'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, bryo- 

 zoans, &c., 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 185 1) : " Innumerable 

 boring 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 pene- 

 trating far into the living portion of these compact communi- 

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

 and they belong to difterent families of the animal kingdom ; 

 among the most active and powerful we would mention the 

 date-fish or Lithodomus, several Saxicavae, Petricolce, Acae, 

 and many w^orms, of which the Serpula is the largest and most 

 destructive, inasmuch as it extends constantly through the 

 living part of the coral stems, especially in the Mseandrina. On 

 the loose basis of a Maeandrina, measuring less than two feet 

 in diameter, we have counted not less than fifty holes of the 

 date fish — some large enough to admit a finger — besides hun- 

 dreds of small ones made by worms. But however efficient 

 these boring animals may be in preparing the coral stems for 

 decay, there is yet another agent, perhaps still more destruc- 

 tive. We allude to the minute boring-sponges, which pene- 

 trate them in all directions, until they appear at last completely 

 rotten through. ' 



On the other hand, Serpulas and certain kinds of barnacles 



