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, is a name applied to the flesh of certain sea slugs or sea worms found in the Indian seas. Of this substance great quantities are gathered annually. In the water the animals resemble huge cucumbers, and they are therefore sometimes called "sea-cucumbers." They are found clinging to the rocks below low-water mark, and are from one to four feet in length. Their food consists of microscopic shell-fish which live upon the coral rocks. [Illustration: The Great Barrier Reef of Australia, the most remarkable animal structure in the world]