POLYP SKELETONS AND MATCBLE. 483 



are found hundreds of miles from any ocean. Coral 

 reefs and coral islands have essentially the same composi- 

 tion as limestone or marble, being composed mainly of 

 carbonate of lime. 



From these facts, and many others, geologists believe 

 that a large part of the limestones of the globe are made 

 out of the coral reefs that were formed in the old oceans, 

 which long ages ago covered the countries where limestones 

 are now found. 



If this be true, many of the rocks which underlie vast 

 countries, the marble temples and palaces of the East, the 

 marble monuments and public buildings of our own coun- 

 try, the mortar upon the walls and ceilings of our houses, 

 and the marble tables and mantels so highly prized 

 all have come from the skeletons of these and other 

 low animals of the sea. 



And perhaps it is not too much to say that their skele- 

 tons have furnished even the blocks of marble which the 

 sculptor chisels, and that thus the polyps are connected 

 with the highest department of culture and of art in 

 which the mind and hand of man can engage. 



PRINCIPAL TOPICS CONSIDERED IN CHAPTER v. 

 SECTION I. 



The Radiates considered as a Branch. The arrangement of their parts, etc. 



Classification. 



SECTION II. 



SUB-SECTION I. 



The Echinoderms considered as a Class. Meaning of the name. Classifica- 

 tion. 



SUB-SECTION II. 

 The Holothurioidea as an Order. Plates in the skin. Size. 



