with lighthouses and light-ships, and
is protected from the huge rolling billows of the ocean by the reef
itself. There are several breaks in the reef through which vessels can
pass out into the open ocean.

This mighty barrier, the work of coral polyps, is of special interest
not only on account of the curious shapes and varied kinds of sea life
it presents, but because of the commercial value of its products. The
beche-de-mer, pearl, oyster, and sponge fisheries yield an annual
revenue of upward of half a million dollars, and when all of the
resources of the reef are properly exploited the returns will be more
than doubled.

The habitat of the reef-building coral is in clear tropical waters. The
polyps thrive best near the surface; they cannot live at a depth
exceeding one hundred and twenty-five feet. The reef-building coral must
not be confounded with the precious, or red, coral, which flourishes in
a muddy sea-bottom and is found chiefly in the Mediterranean Sea.

When alive and in the water, coral polyps present a variety of beautiful
forms and colors. Living polyps are composed of limestone skeletons
covering and permeating a soft gelatinous substance which corresponds to
the flesh of animals. When the polyps are removed from the water this
soon decomposes and disappears; in certain species a part of it flows
off as a thick liquid.

Fish fantastically striped and of brilliantly variegated colors are seen
swimming among the coral. In tropical waters many of them have
fascinating colors and patterns. By simulating the colors of the coral
polyps they escape the species that prey upon them.

The different kinds of coral are generally designated by common names
according to the different objects which they resemble. Thus, by
similarity of form we have _brain_ coral, _organ-pipe_ coral, _mushroom_
coral, _staghorn_ coral, etc.

Some of the islands and reefs are the homes of sea fowl and at the
nesting season are literally covered with their eggs. These fishers of
the sea have marvellously well-developed faculties for location, since
each bird goes directly to her nest when returning to the islands. As
night approaches, when all the birds seek the land, their wild cries are
deafening.

Some of the islands are turned to profitable account by the export of
guano. On Raine Island, so extensive are the deposits of guano that a
railroad has been built to facilitate handling the product.

Beche-de-mer, or trepang, i