CORALS— CORAL REEFS. 



709 



Fig. 19.— Sea-Pbn 

 \PeiinaXula), 



living jelly that covers them often glistening with metallic lustre. Some- 

 times the delicate branches unite to form networks, as in the Sea-fans 

 (Gorgonia). In other cases fine branches stand out from a central shaft, and 

 the whole colony resembles a feather with its barbs. 

 Fig. 19 represents one of the Sea-pens belonging to 

 this group. The lower end of the colony does not 

 carry any polyps, but is buried in sand or mud. This 

 is a peculiarly beautiful object when alive, especially 

 in the dark, as waves of phosphorescent light sweep 

 over the surface of the colony and make it glow with 

 indescribable beauty. It is these graceful and vividly- 

 coloured corals which form the chief adornments of 

 the coral reefs, the solid rocks of which are derived 

 from the harder and more massive skeletons produced 

 by the six-rayed corals. 



The marvellous beauty of the banks covered with 

 living corals in the Red Sea and elsewhere, where 

 shallow water enables the traveller to feast his eyes 

 on these exquisite growths, has often roused the 

 enthusiasm of naturalists. They have been compared 

 with fields or gardens of the choicest and most 

 brilliantly-coloured flowers growing in the richest 

 profusion, all the intervals between the larger growths 

 being filled up with bright moss, which is itself com- 

 posed of minute corals. Among the elegant flowering 

 shrubs and bushes of this garden brilliantly-coloured fishes, glittering with 

 metallic lustre, dart to and fro ; Star-fishes, Sea-urchins, and Snails climb 

 about among the branches, while transparent Crustaceans and Jelly-fish 

 swarm in the crystal water around. 



Before quitting the Corals we must briefly recall the important part they 

 have played in Nature in helping to build up the continents which we now 

 inhabit, a process which can still be seen going on in the 

 great coral reefs and islands of tropical seas. With the Coral Eeefs and 

 exception of a few unimportant forms. Corals are now Islands. 



restricted to regions within 30° on either side of the 

 equator, the more important reef-building forms being able to carry on their 

 life activities only in water above a certain temperature. Even within these 

 latitudes corals are not found everywhere. They flourish best in the Indian 

 Ocean, the Pacific Ocean, and the Carribean Sea, where the reef-builders are 

 at work over thousands of square miles. 



A coral reef is a bank of coral rock built upon the shallow sea-bottom 

 immediately surrounding the shores of tropical lands. Where the land is 

 edged by a reef, as in the island of Mauritius, the ordinary 

 beach passes into a flat irregular bank terminating at a The Fringing 

 varying distance from land,- in a ridge over which the Reef, 



sea continually breaks. This bank, which is called a 

 " fringing reef," is almost entirely made up of the skeletons of colonies of 

 corals, with which, however, are mixed the calcareous remains of myriads of 

 molluscs, sea-urchins and crabs, and the tubes of the tube-dwelHng worms 

 which haunt every cranny and crevice of coral growths. At low tide broad 

 expanses of the coral rock are to be seen just above water level, strongly 

 contrasting with the usually steep shore of the land to which the reef forms 



