106 



CORALS AND CORAL ISLANDS. 



slender tentacles around the mouth. But, in internal struc- 

 ture, and all of the animal below the head, they are Mollusks. 

 They form delicate corals, membranous or calcareous, made up 

 of minute, cabin-like cells, which are either very thin crusts on 

 sea- weeds, rocks, or other supports, or slender moss-like tufts, 

 or graceful groups of thin, curving plates, or net-like fronds ; 

 and sometimes thread-like lines, or open reticulations. 



Occasionally they make large, massive corals, from the 

 growing of plate over plate. 



The first of the following figures, represents one of the 

 delicately branching species, of natural size ; and the second, 

 a portion of the same, much enlarged. The latter figure 

 shows that the branches are made up of minute cells. From 

 each cell, when alive, the bryozoum extends a circlet of ten- 

 tacles, less than a line in diameter. 



1, 2, nOKNERA LICHENOIDES ; 3, DISCOPOKA SKENEI, SMITT. 



The encrusting kinds are common in all seas. The crust 

 of cells they make is often thinner than paper. A portion of 

 such a crust is represented, enlarged, in figure 3. When ex- 

 panded, the surface is covered over with the delicate flower-like 

 bryozoa. A low magnifying power is necessary to observe them 



