94 THE SHORE 



soft tissues. For this purpose fixed colonies should be 

 selected, such as by growing together might form a solid 

 platform. Massive forms with large anemones will be 

 found most useful for the purpose, being easier to 

 observe, and giving the roughest surfaces for the 

 lodgment of sediment. At the same time observations 

 on the expansion and contraction of the tentacles of 

 the coral anemones, and so of the whole animals, might 

 be added, especially in connection with the ebb and 

 flow of the tide, with the amount of sediment in 

 suspension in the water, and with darkness and light. 



Whatever be the cause, it seems certain that the 

 ordinary reef corals do not grow in any luxuriance at 

 a greater depth than 30 fathoms, though isolated 

 colonies have been dredged as deep as 50 fathoms. 

 Down to 30 fathoms the larvae of coral anemones 

 affix themselves in vast numbers on any tropical 

 coast or suitable bank, if there exist rocks or stones for 

 their attachment. Each rapidly becomes a full-grown 

 anemone, and buds off fresh anemones, quickly forming 

 a colony supported on a massive skeleton of carbonate 

 of lime. As some of the anemones die or are killed, 

 fresh surfaces of rock are left for the fixation of further 

 coral larvae, and finally a rough platform is built up 

 to the surface of the sea, beyond which growth is, of 

 course, impossible. In this way a fringing reef may be 

 formed round any island with a depth of 30 to 40 

 fathoms off its outer face, or a reef may be built up on 

 any shallow bank to the surface. Corals are the most 

 important organisms in its structure, but the remains 

 of molluscs, starfishes, soft corals (Alcyonidse), Polyzoa, 

 and a host of other animals assist as well. Algae also 

 are important, both calcareous leaved forms, such as 

 Halimeda (Figs. 50, 51) and the more massive skeletons 

 of Lithothamnia (Fig. 75) as firm and strong as those 



