129 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 Meandrina. On 
the loose basis of a Meandrina, 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- , 
ae 
| 
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 zodphyte, without producing any defacement of the sur- 
face, or affecting its growth. Many of these Serpulas grow with 
the same rapidity as the zodphyte, 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 Astrea 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. 
I]. The too great depth of the waters. 
IIL. The proximity of the mouths of rivers, on account of 
