THE CORALS. 



297 



maintained below twenty fathoms, and thus there is a downward depth limit to the reef-building 

 Corals. They fringe certain islands within the West Indian, Atlantic, Indian, and Pacific Oceans, the 

 Corals clinging on in great masses to the shore, some being uncovered a little at very low tide, and some 

 extending to the depth of twenty fathoms. The growth is upwards, and outwards, or seawards, and 

 sometimes to a considerable extent. Such a mass of Coral is alive on the top and where in contact 

 with the sea ; but all the supporting mass which once was alive is dead, and consists of a mass of hard 

 Coral, united by exothecal structures, and much altered by the percolating water. 



In some parts of the Pacific Ocean, an island is seen surrounded by a calm lagoon of sea water of 

 no great depth, and in the offing is a more or less circular reef, with openings in it. This is a Coral 

 structure which is growing, but does not extend deeper than twenty fathoms in the living state. But 

 the foundation of the distant reef extends from it to the island, and underlies the living and dead 

 mass of Coral. These are barrier reefs. In. other parts of the great ocean simple rings of reef, 

 called atolls, are seen. There is no mountain in the midst, but a lagoon. The fringing and 

 barrier reefs are phases in the development of the atoll. The land was once surrounded by a 

 fringing reef, and subsidence commenced. The Coral, ever growing, increased in bulk upwards, 

 growing as the land sank, and this process gradually necessitated a shallow sea between the mountain 

 tops and the reef. This barrier reef, still subsiding with the mountain, on which it hung amidst 

 the waves, yet ever growing upwards, at last witnessed the total submergence of the land. An 

 atoll thus formed is a vast mass of Coral covering a sunken island, the living Coral forming a ring 

 around a lagoon, with openings seawards. This is the theory of Charles Darwin. 



The most rapidly growing Corals live in the surf and most heated water on the outside of the reef, 

 and the more solid reef-builders remain in quieter water, in the lagoon. Many simple Corals are found 

 amongst the reef-builders, and live in company with a vast assemblage of Tubicolar Worms, Echinoderms, 

 Anemones, and Crustacea ; but the beautiful aspect of the reef, with its gorgeous colours of green, 

 yellow, violet, and gold, is produced by the soft discs and stems of the Madreporaria and Sea Anemones. 



The Madreporaria are very numerous in genera and species, and they may be divided into those 

 which have the hard parts dense, and into those with a light skeleton, very porous in its nature, and 

 reticulate in its construction. These divisions are those of the Aporosa and Perforata. A great 

 group, now almost extinct, but which preceded those just mentioned in time, is that of the Rugosa. 



THE GROUP MADREPORARIA APOROSA. 

 FAMILY TURBINOLID^. 



These Corals are usually simple and solitary, but some have offshoots in the form of buds, which 

 resemble the parent. They are 

 not united by exotheca, and 

 there are no internal dissepi- 

 ments in the interseptal spaces, 

 exceptions) to this statement 

 being excessively rare. 



The common so-called 

 Madrepore of the Devonshire 

 coast,"" and those which are 

 dredged up out of moderately 

 deep water in the North Atlan- 

 tic, are common examples of the 

 genus Caryophyllia, which do 

 not usually increase by budding, 

 but by the development of ova. 

 Those species, which are fixed on 

 to substances on the floor of the 

 sea, often have a delicate outer 

 layer of hard tissue, called an epitheca, and nearly all are very beautifully ornamented, and 



* Caryophyllia smithii (Stokes). 

 276 



CARYOPHYLLIA CYATHUS. 



some 



