RADIATES. 351 



of the Caryophyllia. Here are 

 two stony columns formed by two 

 Polypes. The animals are ever 

 at the summits, with only a small 

 portion of the columns in their 

 bodies and living. The rest is 

 like dead bone. It differs from 

 the bones of common animals in 

 its composition. Their bones are 

 made of phosphate of lime, while 

 the Polype's skeleton is made of 



Fig. 272._Caryophyllia. ^ car ^ onate o f lime, Or chalk, 



like the shells of the Mollusks. All of this* stony sub- 

 stance forming these columns is supplied from the blood 

 of the Polype. It gets into the blood from the water, 

 and from the food which the Polype eats. The immense 

 masses of coral seen in some localities are formed there 

 in the same way essentially with the bones of the Verte- 

 brates and the shells of the Mollusks and Crustacea. 

 You observe in the figure that on the summit of one of 

 the columns there are two Polypes, one being larger than 

 the other. Here is the beginning of a branching process 

 which is very common. A second Polype has started 

 out of the side of the original one ; and, as the growth 

 and death go on, now there will be two columns instead 

 of one from that point. And as these grow upward, 

 there may be still other divisions in the same manner. 



613. Some species of coral-forming Polypes, instead of 

 being on branches, are distributed over a continuous sur- 

 face of a stony or calcareous mass. This arrangement is 

 represented in the Astrea Viridis, Fig. 273 (page 352). 

 Here is a rounded mass of limestone, made up of the 

 united skeletons of Polypes. Over its upper portion is a 

 fleshy covering connecting the Polypes together, making 

 what is called a polypidom, or household of Polypes. 

 At a a are the Polypes, out of their cells and fully ex- 

 panded. At b b the animals are within the cells. At c is 



