RADIATES. SS Tae | 
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 
the carbonate of 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 0 6 the animals are within the cells. At cis 
——— 
Fig. 272.—Uaryophyllia. 
