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 
Fie. 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 meduse 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- 
