But, lowly organized and apparently insignificant as these animals are, the part they play in the operations of Nature is in the highest degree imposing. Not only do they produce these exquisite arborescent forms, but they build gigantic structures with caverns, grottoes, and mighty arch-work, and raise rocky walls, which rival in extent and massive grandeur the noblest mountain scenery upon the land. Though these constructions proceed but slowly, yet in numbers that are inconceivable, and through ages that are incalculable these tiny beings have been engaged in the work of rock-manufacture, until they now rank with earthquakes, the rising and sinking of continents, and other stupendous agencies by which the crust of the globe has been shaped. Multitudes of islands, hundreds and thousands of feet above the surface of the sea, and multitudes of others sunk thousands of feet below it; stony reefs, along which the navigator sails hundreds of miles; the sheet of rock through which Niagara is slowly cutting its way, and extensive beds of limestone scattered over the continents—all have a common origin—all have been extracted from sea-water and secreted by animals low in structure, and chiefly by these jelly-form polypes, many kinds of which are so minute as to be hardly visible to the naked eye. Living, working, multiplying, and dying like ourselves, building blindly but grandly in the final result, perhaps here also not unlike 
Fig. 1. Fresh-water hydra. ourselves, these humble creatures illustrate the method of Nature, and their works and ways are of inexhaustible interest. Their instructive story has just been told by Mr. Dana, with the fascination of romance and the fidelity of science, in his charming book on "Corals and Coral Islands." In the present brief presentation of some of the facts of the subject we shall chiefly follow Prof. Dana, and we are indebted to the courtesy of his publishers for the accompanying illustrations from his work.
The animal kingdom is divided into several sub-kingdoms, one of which comprises numerous species of animals termed Radiates, because their parts are arranged radially round a centre. One division of the radiates is known as polypes, and they have the faculty of secreting a stony frame or skeleton, which is termed coral. The polypes are the most important coral-making animals, but this substance is produced also by other radiates, by some of the lowest tribes of mollusks, and a kind of coral is made by lime-secreting sea-weeds.
There is a group of radiates termed Hydroids. One of these, the fresh-water hydra, is represented in Fig. 1, as it is often seen attached to the under surface of a floating leaf. Prof. Dana says: "It is
