ORDER ACTINOIDEA. 65 
interrupting ridges are prominent and large, to others, where the 
surface is smooth. Some traces of them are seen in the recent Muss 
and Euphylle.* 
The transverse dissepiments secreted across the cells of the Pocil- 
lopore, Favosites, and many Cyathophyllide () 46), appear to be 
connected, as suggested by Ehrenberg, with this process of dying or 
removal below. ‘The base of the polyp, or, at least, the central part 
of it, is withdrawn at intervals, and after each withdrawal, a new plate 
is secreted by the base of the animal. 
It is obvious from the preceding, that the polyp, which is the germ 
of a compound zoophyte, loses its identity, and cannot be said, in any 
proper sense, to have the long life which is attributed to the full- 
grown zoophyte itself; or else, we might have, among the huge 
Astreas of the Red Sea, polyps that were cotemporaries with the 
builders of the pyramids. 
C. CoaLESCENCE oF BRANCHES. 
63. The forms of zoophytes are farther modified by the frequent 
coalescence or growing together of branches. A clump is sometimes 
so united in this way, that only the branchlets at the extremities 
are entirely free; and occasionally a branching corallum finally be- 
comes nearly solid, a few holes intersecting or riddling the mass, 
being the only indications within that it was a ramose species. 
When the species ramifies in a plane, the coalescing branchlets some- 
times produce a complete network, as in the sea-fan (Gorgonia 
flabellum) of the West Indies. The vase Madrepores are other 
examples of the same. ‘This coalescence is so complete in some of 
the horizontally growing Madrepores (M. palmata and flabellum), 
that they form broad solid plates or folia, with perhaps an inch or so 
of the coalesced branches, free at the margin of the plate. 
In foliaceous zoophytes, the same coalescence may take place. In 
certain species, the folia curve around until the edges meet and 
grow together, and produce a chimney or tubular form, as in the 
Echinopora reflexa. Again, a plate folds upon itself, and the parts 
unite, back to back, so that a species, which usually has polyps only 
on one surface,—unifacial,—may change its character and resemble 
hifacial species, in which polyps open on both sides. 
A broken piece of live coral, placed against another of the same 
species, will soon grow to it and continue its existence as if unin- 
* See plate 6, figure 30, 
17 
