150 THE ANATOMY OF INVERTEBRATED ANIMALS. 



miles. It is by no means certain, however, that any one 

 species of West India reef-coral is identical with any East 

 Indian species, and the corals of the central Pacific differ very 

 considerably from those of the Indian Ocean. 



Different species of corals exhibit great differences as to 

 the rapidit}' of their growth, and the depth at which they 

 nourish best ; and no one must be taken as evidence for anoth- 

 er in these respects. Certain species of Perforata (Madre- 

 poridoe and Poritidce) appear to be at once the fastest grow- 

 ers, and those which delight in the shallowest waters. The 

 Astrceidce among the Aporosa, and Seriatopora among the 

 Tabulata, live at greater depths, and are probably slower of 

 increase. 



Under the peculiar conditions of existence which have 

 just been described, it would seem easy enough to compre- 

 hend, a priori, the necessary arrangement of coral-reefs. As 

 the reef-building Actinozoa cannot live at greater depths than 

 twenty fathoms, or thereabouts, it is clear that no reef can 

 be originally formed at a greater depth below the surface, and 

 such a depth usually implies no very great distance from land. 

 Furthermore, we should expect that the growth of the coral 

 would fill up all the space between the shore and this farthest 

 limit of its growth ; so that the shores of coral seas would 

 be fringed by a sort of flat terrace of coral, covered, at most, 

 by a very few feet of water ; that this terrace would extend 

 out until the shelving land upon which it had grown descended 

 to a depth of some twenty fathoms ; and that then it would 

 suddenly end in a steep wall, the summit and upper parts of 

 which would be crowned with overhanging ledges of living 

 coral, while its base would be hidden by a talus of dead 

 fragments, torn off and accumulated by the waves. Such a 

 "fringing reef" as this, in fact, surrounds the island of 

 Mauritius. The beach here does not gradually shelve down 

 into the depths of the sea, but passes into a flat, irregular 

 bank, covered by a few feet of water, and terminating at a 

 greater or less distance from the shore in a ridge, over which 

 the sea constantly breaks, and the seaward face of which 

 slopes at once sheer down into fifteen or twenty fathoms of 

 water. 



The structure of a fringing reef varies at different dis- 

 tances from the land, and at different depths in its seaward 

 face. The edge beaten by the surf is composed of living 

 masses of Porites, and of the coral-like plant, the Nullipore ; 

 deeper than this is a zone of Aporosa (Astrceidce), and of 



