RADIATES. 179 



enormous quantities of fossil coral that are met with in 

 calcareous rocks have led to the belief that the pro- 

 ductions of these zoophytes form a large portion of the 

 mass of our continents, and that these small beings may 

 modify the outlines of the earth's crust in a rapid and 

 remarkable manner. If we may judge by the results of 

 investigations made during less than a century just past, 

 we are justified in the conclusion that, in a time rela- 

 tively short for geological change, the greater number 

 of the islands of Oceanica will be united together. 



Eed coral is the earthy axis of a polyp that grows in 

 abundance in the Persian Gulf, the Red Sea, and various 

 parts of the Mediterranean, such as the Straits of Mes- 

 sina, off the coasts of Sardinia, Tunis, and Algeria. The 

 stem or axis adheres to submarine rocks, generally 

 has a beautiful red color, and the form of a little tree 

 without leaves or small branches, and a height vary- 

 ing between six and twenty-five inches. A single polyp 

 like that shown in Fig. 106 is the starting-point of the 

 coral stem represented alongside of it. On the smaller 

 branches of the stem are seen young individuals, whose 

 development will produce new branchings. This method 

 of development is quite analogous to that of vegetables. 



The class of porifera includes the sponges, beings 

 formed of a gelatinous mass, supported by a solid frame- 

 work of variable composition. Sometimes this frame- 

 work is composed of long silicious needles interlaced and 

 resembling spun glass ; sometimes it consists of calca- 

 reous spiculae or of horny filaments. The common sponge 

 is of the latter variety, and it is the horny skeleton that 

 is used. The holes that run through it in all directions 

 are the canals through which water circulates during the 

 life of the creature. 



