Zoologists of the last century confounded all the species of this genera inhabiting the tropical seas, making only one species, to which they gave the name of Tubipora musica. But it is now known that there are many species of Tubiporæ, readily distinguishable in a fresh condition by a difference in the colour of the polyps. The tissue of these singular beings is of an intensely red colour. The disposition of their tubes in the style of organ pipes has always attracted the attention of the curious inquirer into the secrets of Nature.
II. GORGONIADÆ.
Milne Edwards divides this order into three natural groups:—I. The Gorgoniadæ. II. The Isidians. III. The Corallines.
Fig. 42. The Sea Fan, Gorgonia flabellum (Linn.).
The Gorgonians are composed of two substances: the one external, sometimes gelatinous and fugitive; sometimes, on the contrary, cretaceous, fleshy, and more or less tenacious. Animated with life, this membrane is irritable and encloses the polyp; it becomes friable or arenaceous in drying. The second substance, internal and central, sustains the first, and is called the axis. This axis presents a horny appearance, and was formerly believed to possess chemical characters analogous to the horns and hoofs of some of the vertebrated animals. It has recently been asserted that the tissues of these corals consist essentially of a particular substance which resembles horn, but which is called Corneine. A little carbonate of lime is sometimes found united with this substance, but never in a sufficient quantity to give it a stony consistence. This outer covering developes itself in concentric beds, between the portion of the axis previously formed and the internal surface of the sclerotic covering.
Fig. 43. Fan Gorgon, magnified.
The mode of growth in this axis presents great variations. Sometimes it remains simple and rises like a slender rod, sometimes it has numerous branches. It is arborescent when the branches and their accompaniments take different directions so as to constitute tufts. It is panicled when they arrange themselves on both sides of the stem or principal branches, after the manner of the barbs of a feather. It is flabelliform when the branches rise irregularly under the same plane; reticulated, when branches are so disposed as to be attached to each other by network in place of remaining free.
The Gorgoniadæ are found in every sea, and always at considerable depths. They are larger and more numerous between the Tropics than in cold or even temperate climates. Some of these corals scarcely attain the twelfth of an inch in height, while others rise to the height of several feet.