Fig. 126. Red Coral Branch.
The red coral generally grows on the under-side of ledges or rocks, in a pendent position, and at considerable depths. It is not found at 15 or 20 fathoms; they only begin to fish for it at from 30 to 60 fathoms; and it is brought up from even 100 or 120, while the strong reef-building corals cannot exist below 25, or at most 30 fathoms; being immensely superior in vigour, these require a greater supply of air, light, and heat. The red coral is generally fished for along the coasts of Algiers and Tunis; it is also found in the seas round Sicily and Sardinia, and in the Grecian Archipelago. The red coral is always irregularly branched. The branches are sometimes white, supposed to be from disease; the white coral of commerce is a species of Caryophyllia, an Actinian, and not an Alcyon, zoophyte.
Fig. 127. Red Coral (greatly magnified), from ‘Histoire Naturelle du Corail,’ par M. Lacaze Duthiers.
The Corallium Johnstoni, a native of the Atlantic, has a white axis, with branches spreading flatly and horizontally like a fan from the rock to which it is attached; it is entirely covered with a yellowish flesh, but the polypes only inhabit the upper surface, as if they could not live in shade. The Corallium secundum, a similar zoophyte, was discovered by Professor Dana near the Sandwich Islands, with a white or rose-coloured fan-shaped stem and branches, covered by a scarlet coat, having the polypes also only on the upper surface.
The Pennatulidæ, or sea-pens, which are the third family of the Alcyon zoophytes, bear a great resemblance to a goose’s feather. The genus Pennatula has a flatly-feathered, upright, calcareous axis, the bare part of which is analogous to the quill; but, instead of being fixed like the stem of a Gorgon, it is merely stuck into sand or mud at the bottom of the seas, while the upper feathered part, containing the polypes, remains in the water. The axis decreases in thickness upwards, and the pinnules, which diverge from it transversely like wings, are angular, thin, membranaceous, and strengthened by spicules. The whole animal is covered with a soft fleshy tissue; the polypes, which have eight pinnated tentacles, are arranged in a single row along the edges of the pinnules, with their visceral extremities prolonged into the soft tissue, so as to give it a tubular structure, through which the nourishing juice prepared by the polypes is carried for the maintenance of the general envelope, the refuse being thrown out at their mouths. When the sea-pens leave the mud or sand, they do not swim actively with their pinnules, but move languidly at the bottom. The Pennatulæ are phosphorescent; they are of a dull reddish brown during the day, but at night they shine with the most brilliant iridescence. In the tropical seas they occasionally exceed a foot in length; in the cool latitudes they are not more than five or six inches. The Pennatula phosphorea, found on the British coasts, has a hollow axis, occupied by a well-developed stylet; long pinnulæ symmetrically disposed on each side of the middle and upper part of the axis; the polypes, which are very contractile, are arranged transversely on their upper and anterior edges; the pinnæ of the wings are scythe-shaped, and furnished with a vast number of sharp spicules, and these combine in bundles at the base of the cells, in which the polypes live. The back of the pen, lying between the feathery wings, is sometimes smooth, sometimes crowded with scales, arising from the development of the spicules with which it is filled. The eggs of this animal are yellow, and have the size and form of poppy seeds. They are developed into ciliated larvæ within the polypes, which come out at their mouths, and swim away; but their activity is much diminished when they have acquired their mature form. These Pennatulæ increase also by a kind of budding. There are species of phosphorescent sea-pens in all the European seas and Indian Ocean.
The Virgulariæ are sea-pens which have long slender stems, with short transverse pinnules, on both sides of their extremity: they have no spicules, and are remarkable for the contractile power both of their axis and polypes. Mr. Darwin mentions a species he met with during his voyage in the Southern Ocean, which seems to be akin to the Virgularia juncea common in the Indian Seas. They were long and slender, projecting in vast numbers like stubble above the surface of muddy sand. When touched or pulled, they suddenly shrunk down with such force as to disappear partly or altogether. Sensitive as these animals are, they have no nerves; hence their motions must be owing to the irritable nature of muscular fibre. The eggs of the Virgularia mirabilis, native on the Scotch and Norwegian coasts, are formed in the fleshy coat at the base of each polype. As soon as they acquire their yellow colour and ciliated surface they enter into its body, and revolve in it for a little time before they come out at its mouth.
The family of the Tubipora, inhabitants of warm seas, are the most beautiful of the Alcyons. They consist of rounded masses of considerable size, formed of fragile, hollow, and nearly parallel calcareous tubes. The tubes do not touch one another, but they are united at intervals by horizontal plates, formed of an extension of their bases, dividing their mass into stages. In the Tubipora musica, a native of the Indian Ocean, there are several superincumbent series of equal and parallel tubes, exactly like the pipes of an organ. The whole compound fragile mass is of the richest crimson, and the polypes spread their white tentacles like stars over the mouths of the uppermost pipes, or retreat into them. Buds spring from the upper part of the tubes, and the result is the death of the parents, which are succeeded by a young living race a stage above them. The Tubipora purpurea lives in the Mediterranean and Red Seas. The polypes of a species found by Professor Dana, at the Feejee Islands, have their centre and mouth of a brownish red, and their tentacles yellow, edged by a double fringe of violet-coloured pinnules.
Fig. 128. Tubipora musica.