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, HORNERA LICHENOIDES; 3, DISCOPORA 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 
