122 CORALS AND CORAL ISLANDS. 



and many worms, of which the Serpula is the largest and most 

 destructive, inasmuch as it extends constantly through the liv- 

 ing part of the coral stems, especially in the Masandrina. On 

 the loose basis of a Mseandrina, 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 com- 

 pletely rotten through." 



On the other hand Serpulas and certain kinds of barnacles 

 (of the genus Creusia, etc.) penetrate living corals without in- 

 jury to them. They attach themselves when young to the sur- 

 face of the coral, and finally become imbedded by the increase 

 of the zoophyte, without producing any defacement of the sur- 

 face, or affecting its growth. Many of these Serpulas grow with 

 the same rapidity as the zoophyte, and finally produce a long 

 tube, which penetrates deep within the coral mass ; and, when 

 alive, they expand a large and brilliant circle or spiral of deli- 

 cate rays, making a gorgeous display among the coral polyps. 

 Instinct seems to guide these animals in selecting those corals 

 which correspond with themselves in rate of growth ; and 

 there is in general a resemblance between the markings of a 

 Creusia and the character of the radiations of the Astraea it in- 

 habits. 



In recapitulation, the three most influential causes of the 

 exclusion of reef-forming corals from coasts are the following : 



I. The too low temperature of the waters along shores. 



II. The too great depth of the waters. 



III. The proximity of the mouths of rivers, on account of 



