92 CORAL AND ATOLLS 



Corals grow luxuriantly in most of them, and they afford the 

 best field for experimental work. Since their conditions 

 change with every cycle of the tides, they form the meeting- 

 ground of nearly all the commoner species, for they afford in 

 turn most phases of natural environment. Here are found 

 the highly branched Madre'jjorm and Pocillopor<B, and many 

 Astrcem. The colonies are distinguished as a rule by having 

 their corallites raised, for in most pools sand is being deposited. 

 They are also highly branched and of a bushy form, for they 

 are exposed to injury by moving fragments, and so branch 

 formation is stimulated, whilst the great development of indi- 

 vidual branches is limited. 



These rock-pool forms show naturally the graduating 

 series of types which connects the rough-water forms and the 

 smooth-water forms ; and they show also the intermediate 

 stages of the development of coral structures which are 

 intended for the resistance of the action of sediment. 



In the smooth-water forms the predominant feature of the 

 colony is the fragile nature of the growth. Contrasted with the 

 rough-water forms, these colonies are extremely lightly calcified, 

 and their branching systems are distinguished by their deli- 

 cacy. Their branches are long and slender, their structure is 

 far more porous, and their whole appearance is quite different 

 from that of the colonies of the same species that chance to 

 reside in wave-beaten areas. 



The smooth-water forms lead the way to those growths of 

 corals which inhabit the deeper pools of the lagoon. The deep- 

 water forms are the most fragile of all : their growths are 

 more attenuated, and their branches are given off at far less 

 frequent intervals. There is practically no damage inflicted 

 on the growing points — whether they be growing clusters, or 

 " dominant apical zooids " — and so lateral branch formation 

 is never stimulated. Several of the forms of Montipora and 

 Madrepora which occur in from 8 to 12 fathoms in the lagoon 

 are mere cylindrical stems, of great length, and with practically 

 no lateral branches whatever. 



Besides the attenuated form and the absence of lateral 



