145 
N. ZOOL. GAL.] NATURAL HISTORY. 
different genus. The Antipatkes (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 substance 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 Gor¬ 
gonia , 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 symmetri¬ 
cal form, which, in the true Pennatula, resembles 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 being longer, more slender, and in having the side 
branches smaller. The Renillce are fleshy, with a kidney¬ 
shaped expanded head, and the animals coming out on 
the broad flat sides ; and the Veretillurn differs from the 
rest in being club-shaped, with the animals emitted on all 
the sides of the club. 
The Polypi aria, (Cases 29 and 30 ,) or third order of 
Zoophytes, like the Actiniae , have numerous simple ten¬ 
tacles round their mouth, but the stomach is long, simple, 
and not, or only very slightly, longitudinally folded. 
The fresh water Polype, (Hydra,) which belongs to 
this group, is peculiar for being free and having a soft 
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