72 CORAL AND ATOLLS 



five different types of vegetative growth. It may grow as 

 (I) a spherical mass, (2) an encrusting layer, (3) a free plate, 

 (4) a branching tree-like growth, or (5) a mere amorphous 

 lump ; and though a definite inherent growth tendency is 

 strongly implanted in the embryo, still the demands of the 

 environment may call forth any type of vegetative growth. 



The growth-forms are purely the results of repeated 

 divisions of the zooids, and so it will be seen that the relative 

 value, from a reproductive point of view, of the zooids in a 

 colony is of the greatest importance. This consideration 

 brings in its train the division of all the colonial corals into two 

 groups of normal growth -forms ; for all the zooids may take 

 an equal share in the asexual reproduction, or, again, some 

 may be of greater importance than others, and the asexual 

 reproductive functions may be lodged in a very few individuals 

 only. These two great divisions must be considered separately, 

 for the rules that may be applied to their respective methods 

 of growth are widely different. 



Taking first the class in which every unit is of equal value, 

 and going back to the earliest origin of the colony, it is easily seen 

 that a zooid " A " settled on a nucleus will divide into zooids 

 " B " and " C," and " B " will further divide into " D " and " E," 

 and " C " into " F " and " G," and so on ; each newly divided 

 individual taking its equal share in future divisions. The 

 natural outcome of this state of thins^s is that, if the site of 

 election of grov\^th be a prominence, or, as is not uncommon, a 

 small isolated fragment, then the equal divisions will tend to 

 form a spherical mass. The rapidly growing colony will tend 

 to surround the nucleus on all sides, and in this manner are 

 formed those rounded masses of Poritcs and Adrceopora com- 

 monly found lying free in sandy pools, which, when broken 

 across, are seen to be formed around a central nucleus, which 

 generally consists of a fragment of dead and altered coral. 



It is of course but natural that the true spherical form 

 cannot long survive in very large colonies, for the zooids 

 growing below are of necessity killed by pressure. The mass 

 will therefore become a hemisphere, and continue its growth 



