THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 45 



form a firm skeleton of lime (calcium carbonate), and after 

 their death these skeletons persist, and because of their 

 abundance and close massing form great reefs or banks and 

 islands. Coral 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 



FIG. 24. Organ-pipe coral. 



groups of inhabited islands like the Fiji, Society, and 

 Friendly Islands are composed exclusively of coral islands. 

 More than two thousand kinds of living corals are known, 

 and their skeletons offer much variety in structure and 

 appearance. Brain coral, organ-pipe coral (Fig. 24), the 

 well-known red coral from Italy and Sicily, used as jewelry, 

 and the sea pens and sea fans are among the better known 

 and more beautiful kinds of coral skeletons. 



26. Colonial jelly-fishes. While many of the medusae or 

 jelly-fishes are another form of individual of a true fixed 

 polyp, many of the larger and more beautiful jelly-fishes do 

 not exist in any other form. Some of these larger jelly- 

 fishes are several feet in diameter, and when cast up on the 

 beach form a great shapeless mass of soft, jelly-like sub- 



