104 ELEMENTARY ZOOLOGY 



some other members of the class which are of unusual 

 interest. They are the corals, or coral polyps. We 

 know these animals chiefly by their skeletons (fig. 17). 

 The specimens of corals which one sees in collections, or 

 made into ornaments, are the calcareous skeletons of 

 various kinds of the coral polyps. Some of the corals 

 live together in enormous numbers, forming branching 

 colonies fixed as closely together as possible, and secrete 

 while living a stony skeleton of carbonate of lime. These 

 skeletons persist after the death of the animals, and 

 \)f cause of their abundance and close massing form great 

 reefs or banks and islands. These coral reefs and islands 

 occur only in the warmer oceans. In the Atlantic they 

 are found along the coasts of Southern Florida, Brazil 

 and the West Indies ; in the Pacific and Indian Oceans 

 there are great coral reefs on the coast of Australia^ 

 Madagascar and elsewhere, and certain large groups of in- 

 habited islands like the Fiji, Society, and Friendly Islands 

 are exclusively of coral formation. Coral islands have a 

 great variety of form, although the elongated, circular, 

 ring-shaped and crescent forms predominate. How such 

 islands are first formed is described as follows by a well- 

 known student of corals: 



"A growing coral plantation, with its multitudinous 

 life, oftentimes arises from great depths of the ocean, and 

 the sea-bed upon which it rests is probably a submarine 

 bank or mountain, upon which have lodged and slowly 

 aggregated the hard skeletons of pelagic forms of life. 

 W T hen, through various sources of increase, this submarine 

 bank approaches the depth of from one hundred to one 

 hundred and fifty feet from the surface of the water, there 

 begins on its top a most wonderful vital activity. It is 

 then within the bathymetric zone of the reef-building 

 corals. Of the many groups of marine life which tnen 

 'cake possession of the bank, corals are not the only 



