TEXTURE AND COMPOSITION OF CORALS. 89 
an anus, as well as a mouth; the intestine curves around and termi- 
nates in the disk. They are widely removed from true Zoophytes, both 
by this character, and also by having the tentacles furnished with 
vibratile cilia—that is, minute appendages resembling short hairs, 
which are kept in nearly constant vibration. 
Some species of Bryozoa form thin crusts over rocks or sea-weeds, 
consisting of united cells, scarcely distinguishable unless magnified. 
The coralla of other species are branching or thin foliaceous; and 
these also consist of series of minute cells. 
2. TEXTURE AND COMPOSITION OF CORALS. 
The texture of calcareous corals is in general quite porous or cellu- 
lar. Small stars or rounded depressions are scattered over the surface, 
and sometimes these stars form the centres of small prominences, 
called calicles (little cups). Besides these polyp-cells, which mark the 
position, each of a separate polyp, there are pores or cellules penetrating 
the texture of the coral mass ; yet in some zoophytes, the coral secretions 
continue increasing in the animal till the pores are almost or quite 
obliterated, and the texture is nearly compact, the polyp-cells alone 
remaining. In many species, wherever there are concavities of much 
depth in the surface of a zoophyte, the coral of these concavities is 
looser or more spongy than elsewhere, for the reason, apparently, that 
the polyps in such parts have a poorer chance for securing food and 
fresh portions of water. 
In the Gorgonie, and other species forming a distinct axis to the 
branches, this axis is solid, without a trace of a cell, and usually with 
faint evidences of a concentric structure. It is thus that the red coral 
of commerce, used in jewellery, differs from the Madrepore or common 
white coral: it is the azs of a species of Corallium; and the polyps 
constituted a layer about it, in the same manner as the polyps of a 
Gorgonia cover the horny axis of these species. 
In hardness, the common calcareous corals are a little above ordinary 
limestone or marble, the degree being represented in the mineralogical 
scale of Mohs by 3-5 to 4, while, in limestone, it is about 3. The 
ringing sound given when coral is struck with a hammer, indicates 
this superior hardness. It is a common error of old date to suppose 
that coral when first removed from the water is soft, and afterwards 
hardens on exposure. In fact, there is scarcely an appreciable dif- 
ference; the live coral has a slimy feel in the fingers; but if washed 
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