CORAL-MAKING ACTINOID POLYPS. 55 
regularity that these hemispheres are perfectly symmetrical, 
even when enlarged to a diameter of ten or fifteen feet. A 
portion of the surface of one of these massive species, called 
Orbicella cavernosa, from the West Indies, is represented in 
the annexed figure. In the growth of these hemispheres, the 
enlargement takes place in the spaces between the polyps; and 

ORBICELLA CAVERNOSA. 
whenever these spaces begin to exceed the width usual to the 
species, a new mouth opens, commencing a new polyp; and 
thus the growth of the mass involves multiplication by buds. 
The small calicle near the centre of the figure is from one of 
the new interstitial buds. 
Species of Porites also grow into hemispheres and rude hil- 
lock-like forms, through the same method of budding, and 
some of the masses in the tropical Pacific have a diameter of 
even twenty feet. Myriads of living polyps are combined in a 
single such mass, for each is but a fifteenth or a twentieth of an 
inch in diameter. 
Often there is a lateral growth of the polyp and thereby of 
the zodphyte without much upward growth; and spreading 
leaves are thus made, and bowl-like shapes. Where there is 
lateral budding, the leaves have generally an edge of young 
polyps from the new buds that are there opening, as in the 
