258 



THE POPULAR SCIENCE MONTHLY. 



Fig. 1. 



But, lowly organized and apparently insignificant as these animals 

 are, the part they play in the operations of Nature is in the highest de- 

 gree 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 incon- 

 ceivable, 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 stu- 

 pendous agencies by which the crust of the globe has been shaped. 

 Multitudes of islands, hundreds and thousands of feet above the sur- 

 face of the sea, and multitudes of others sunk thousands of feet below 

 it ; stony reefs, along which the navigator sails hundreds of miles ; the 

 6heet 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 



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 accom- 

 panying illustrations from his work. 



The animal kingdom is divided into 

 several sub-kingdoms, one of which com- 

 prises 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 se- 

 creting a stony frame or skeleton, which is termed coral. The polypes 

 are the most important coral-making animals, but this substance is pro- 

 duced 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 at- 

 tached to the under surface of a floating leaf. Prof. Dana says : " It is 



Fresh-water Ilydra. 



