FORMATION OF CORAL REEFS. 165 



especially adapted to give a strong, solid base to 

 the whole structure, are known in our classifica- 

 tions as the Astraeans, so named on account of 

 the star-shaped form of the little pits crowded 

 upon their surface, each one of which marks the 

 place of a single more or less isolated individual 

 in such a community. 



Thus firmly and strongly is the foundation of 

 the reef laid by the Astraeans ; but we have seen 

 that for their prosperous growth they require a 

 certain depth and pressure of water, and when 

 they have brought the wall so high that they have 

 not more than six fathoms of water above them, 

 this kind of Coral ceases to grow. They have, 

 however, prepared a fitting surface for different 

 kinds of Corals that could not live in the depths 

 from which the Astraeans have come, but find 

 their genial home nearer the surface ; such a 

 home being made ready for them by their prede- 

 cessors, they now establish themselves on the top 

 of the Coral wall and continue its growth for a 

 certain time. These are the Meandriuas, or the 

 so-called Brain- Corals, and the Porites. The Me- 

 andriuas differ from the Astraeans by their less 

 compact and definite pits. In the Astraeans the 

 place occupied by the animal in the community 

 is marked by a little star-shaped spot, in the cen- 

 tre of which all the partition-walls meet. But in 

 the Meandriuas, although all the partitions con- 



