Coral reefs are masses of limestone originally secreted, in the form of
coral, by minute polyps which live in countless numbers in the tropical
seas. The coral produced by a single community of polyps grows chiefly
upward; but multitudes of distinct communities often live so near
together that the small lateral growth of each brings them into contact.
Their separate, fragile structures, gradually broken up and compacted by
various means, are in time transformed into a solid mass, forming walls
of coral rock frequently of enormous extent. The great barrier reef near
the northeastern shores of Australia, the longest known, is not less
than one thousand two hundred and fifty miles in length.
[Illustration: =A LIVING SINGLE CORAL FROM THE PACIFIC OCEAN=
The coral polyp is one of the master-builders of the world. It may be
likened to a sea-anemone, but is inferior in muscular organism, and
immensely superior in defensive organization.]
Reef-building polyps do not live below the depth of one hundred or one
hundred and twenty feet, and hence require a foundation near the
surface. This is supplied by submarine mountains and plateaus, or the
slopes of those volcanic cones which form the high islands.
Growing vertically, the reefs repeat at the surface the outlines of
their bases, which fact gives rise to the circular figure both of atolls
and reefs in mid-ocean, and to the elongated, wall-like form of reefs
adjacent to the continents, like those of Florida and of Australia.