94 INVERTEBRATE ANIMALS. 
The second group of the Zoantharia is termed that of the 
Zoantharia sclerodermata, from the nature of the skeleton or 
coral. In this group are all the so-called ‘“‘reef-building” corals, 
which are the makers of the well-known “coral-reefs.” The 
members of this group all possess the power of secreting car- 
bonate of lime within their tissues, so as to form a more or less 
continuous skeleton or corallum. From the fact that this 
corallum is secreted by the inner layer of the polypes, and is 
therefore truly zwzthzz the body, it is said to be ‘‘ sclerodermic,’ 
in opposition to the kind of coral produced by other forms (such 
as the red coral), in which the coral forms an internal axis, 
over which the ccenosarc is spread, much as the bark encloses" 
the wood of a tree. In this latter case the coral is said to be 
‘“‘sclerobasic.” (For illustrations of these different kinds of 
corals, see figs. 34 and 39.) In the typical form of sclerodermic 
coral, the skeleton is in the form of a conical cup (fig. 34), the 
upper part of which is hollow, and is called the “ calice.” The 
lower part is divided into a series of com- 
partments by vertical plates, which are 
called the “septa,” and which correspond 
to the mesenteries of the living animal. 
Sometimes the space contained within the 
walls of the cup or *corallite” is broken 
up by horizontal plates called “tabule ;” 
but when these are present, there are gene- 
rally no septa. In the coral just described 
we have a single corallite, produced by one 
Fig. 34.—Cyathaxonia polype, and this simple condition may be 
Dalmani, a simple maintained throughout life. In the great 
sclerodermic coral, ese 
consisting ofa single Majority of cases, however, the polypes bud, 
corallite. A portion s0 as to form a colony all bound together 
eee eee by a common flesh or ccenosarc. When 
in order to show the such a colony, therefore, produces a sclero- 
interior of the calice, dermic coral, in place of a single corallite, 
with the septa. : 
we have a composite skeleton composed of a 
number of little cups or corallites, each of which was produced 
by one polype, and all of which are united by means of a com- 
mon calcareous basis, secreted by the ccenosarc (fig. 39, a). 
As before said, the ‘“‘septa” are vertical partitions, which 
divide the space contained within the walls of the coral (the so- 
called “ theca”) into a series of compartments. The septa 
