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 zoophyte 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 



