44 CORALS AND CORAL ISLANDS. 
from specimens collected by him at a depth of 100 to 200 
fathoms off the Florida reef. The actual size was one-third 
that of the figure. The second figure represents a living 
species. 
The bottom of the calicle, or polyp-cell, in the corallum is 
sometimes made simply by the meeting of the radiating sep- 
ta; occasionally by the same, with the addition of a point 
or columella at the centre; often by a twisting together of 
this part of the radiated septa. Very often, also, it is a mere 
porous mass. Sometimes there is a circle of prominent points 
about the centre, as seen in the figure of a Caryophyllia on 
page 42, which are the extremities of narrow vertical strips 
(called palz) lying in the planes of the septa. Similar points 
exist in the Thecocyathus on the preceding page, though not 
in sight in the figure. 
in many cases the bottom is quite solid; and this may be 
so either (1), because the coral secretions fill up all the pores 
as the polyp increases in age, and thus make the inte- 
rior of the corallum solid or nearly so; or (2), because there are 
formed periodically, as the polyp grows upward, solid horizon- 
tal plates across the bottom, so that beneath, in the interior of 
the corallum, there is a series of plates or tables with spaces 
between. The Pocilloporze, among recent corals (p. 70), and the 
Favosites among ancient, are examples. Increasing solidity 
with the increasing age of the polyps is also produced at 
times by additions to the exterior ef a corallum. In many 
species, the skin, over part or all of the exterior, gradually 
disappears or dies away and leaves the corallum bare, while 
all is living within; and, in such cases, the skin, before disap 
pearing, often adds a layer of stony material to the exterior, 
viving greater firmness to the whole. An example is shown 
in the figure on p. 42. In such a case, there is no skin or 
