96 CORALS AND CORAL LSLANDS. 



(of the genus Creusia, &c.) penetrate living corals without 

 injury to them. They attach themselves when young to the 

 surface of the coral, and finally become imbedded by the in- 

 crease of the zoophyte, without producing any defacement of 

 the surface, or affecting its growth. Many of these Serpulas 

 grow with the same rapidity as the zoophyte, and finally pro- 

 <luce a long tube, which penetrates deep within the coral mass ; 

 and, when alive, they expand a large and brilliant circle or 

 spiral of delicate rays, making a gorgeous display among the 

 coral polyps. Instinct seems to guide these animals in select- 

 ing 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 Astrsea it inhabits. 



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. J 



II. The too great depth of the waters. 



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

 which sediment is distributed along the coast adjoining and 

 over the sea bottom. 



IV. Rate of Growiii of Corals. 



The rate of growth of coral is a subject but Httle under- 

 stood. We do not refer here to the progress of a reef in 

 formation, which is another question complicated by many 

 co-operating causes ; but simply to the rapidity with which 

 particular living species increase in size. There is no doubt 

 that the rate is different for different species. It is moreover 

 probable that it corresponds with the rate of growth of other 

 alHed polyps that do not secrete lime. The rate of growth 

 of Actiniae might give us an approximation to the rate of 

 growth in coral animals of like size and general character ; 

 for the additional function of secreting lime would not neces- 

 sarily retard the maturing of the polyp ; and from the rate of 

 growth of the same animals in the young state, we might 



