8 J. D. Dana on Zoophytes. 
The secretions producing it are of two kinds, either basal and 
epidermic or internal. ‘The former are either calcareous or horny, 
rarely siliceous ; and with the exception of the Antipathi, they 
are confined to the Alcyonaria. 'The latter are calcareous. — 
37. Internal secretions.—These take place through the inner 
tissues of the polyps; and as the internal cavity of these animals 
is radiated with compartments, it is natural that the cells of 
their coralla should be radiated likewise. Each compartment in 
fact corresponds to a calcareous lamella, these lamelle being form- 
d between two fleshy lamelle. The existence of cells on the 
surface of a corallum is owing simply to the fact that the upper 
and central portions of the polyp do not secrete lime, while the 
sides and parts of the fleshy lamelle do; the consequence is that 
there is a surface concavity in the corallum, into which the disk 
of the polyp falls on contraction. There can be no such thing 
as a disappearance of the polyp in the cell, for the coral as we 
have said is wholly within the exterior. skin of the polyp; yet 
the tentacles and disk may disappear; and this they do also in 
the fleshy Actinia. Conceive of a fleshy Actinia secreting lime 
throughout the tissues of its sides, (excepting the exterior skin, ) 
and between its fleshy lamelle, and the reader will correctly com- 
prehend the relation of coral to the animal. careous se- 
cretions may thus form a solid structure penetrated by the animal 
tissues ; and when separated from the animal there will be cel- 
ules where the animal tissues remained ; and under a microscope 
a thin polished plate would show other animal fibres wholly en- 
closed in the coral. The animals represented in figures 12 and 13, 
differ in this single particular,—that in the latter this very process 
has taken place as described, and in the former ithas not, There 
are many corals without surface-depressions or cells, and in these 
there is no semblance even of a retreat of the polyp. 
_ The Madrepores, Astraas, Caryophyllias, and Cyathophylla are 
instances of this mode of coral secretion. 
38. In the cells of most Caryophyllia, Dendrophyllie, é&e., 
three smaller calcareous lamella (the middle one broadest ) alter- 
nate with one larger lamella, This results from the arrange- 
Fig. 97, ‘ment of the fleshy lamella of the polyp, as shown in 
Hay 
ne eae 
| 
Will 
figure 19, and more correctly in the annexed figure, 
narrower intervals, thus corresponding exactly with the calca- 
exhibiting the position of these lamelle as seen 
broader in This arrangement of the fleshy lamellae may 
be seen in the external markings of many Actinie. This ex- 
