132 
NATURAL HISTORY. 
[N. 200L. GAL, 
axis, and which has sometimes, though erroneously, (from its being 
commonly seen in collections without the remains of the investing 
animal,) been considered the entire coral. This axis is thickened by 
depositions of fresh layers of horny matter on its surface as the mass 
increases in size and requires more support, the increase of the thickness 
and length of the axis being always simultaneous with the growth of the 
mass. In some kinds, the axis is only formed of an immense number 
of spiculae, like those in the substance of the skin, being crowded 
together in the centre, as in the genus Briareum (Case 28). 
In general it is formed of a quantity of horny matter, which is de¬ 
posited in successive layers, as in Gorgonia (Cases 24 to 28). The 
axes of some of these kinds have been called, from their colour, Black 
Coral , and were formerly much esteemed for their supposed magical 
and medicinal qualities. They are now chiefly used to make riding whips 
and whisks. 
In some genera a large quantity of calcareous matter is deposited 
along with the horny matter, forming a stone-like axis, as in the Red 
Coral, ( Corallium , Case 21); and in other kinds the calcareous matter 
is only deposited in certain parts of the axis, leaving the rest simply 
formed of the homy animal matter, as in the genus Isis (Case 21). 
In this kind the axis has been considered as jointed, because the stony 
and the horny parts easily separate from each other when the mass of the 
animal has been removed and the axis is dried ; but a larger and larger 
quantity of stony matter is gradually deposited as the mass increases in 
size, and in the large masses, the axis of the lower part is almost entirely 
stony, like the axis of Corallium. Specimens shewing all the changes 
are in the collection (Case 21). Lamarck, not being aware of this 
change, considered the axis of the old specimens as a different genus. 
The Antipathes (Case 27) have the same kind of animal and axis as 
the Gorgonia , but the skin of the animal is thin, cellular, and easily 
leaves the axis bare, when the mass is taken out of the water. 
In the genus Hyalonema, (Case 28,) the animal mass, instead of 
forming a single axis for its support, secretes in the middle of its sub¬ 
stance a bundle of thin transparent fibres looking exactly like a rope of 
spun glass; each of these fibres is formed of numerous concentric coats, 
like the axis of Gorgonia , and they are each of them enlarged in size 
by a new layer of mineral matter, secreted by the thin membranes which 
surround each of them, as the mass requires more support. Unlike the 
axis of Gorgonia , the fibres are formed of nearly pure flint, and are 
hard enough to scratch glass, and the mass, instead of forming for itself 
an expanded base, lives with the end of the axis sunk in a species of 
sponge. This genus is found in the Japanese seas. It is the only one, 
whose animal nature is well determined, that secretes pure silex, for the 
Tethya probably belongs to the vegetable kingdom. 
The Sea Pens ( Pennatulidce , Case 28) have many characters in 
common with the Gorgonice. They have only a single axis, which 
pervades their central stem, but they live floating free in the sea or with 
the naked part of the base of their stem sunk into the sand and mud on the 
coast, as the Hyalonema is into a sponge, and they are easily known 
from both by their symmetrical form, which, in the true Pennatula, re¬ 
sembles that of a pen with the animals coming out from the upper part 
of the side branches. Virgularia differs from the former in the stem 
