This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(143). This singular mode of reproduction, M. Milne-Edwards observes, seems at first sight to be very different from that observed in the Alcyonidium; but on reflection, a considerable analogy may be traced between them. In Alcyonidium the internal tunic of the abdominal cavity fulfils the functions of an ovary, and produces at determinate points both gemmae and ova; in Alcyon, on the contrary, while the internal membranous layer gives birth to ova, the gemmae are developed elsewhere, from the canals which permoate the common mass. But the membrane which forms these canals, and which is the seat of this kind of vegetative reproduction, is merely a continuation of the internal tunic; and hence it is easy to understand how it may fulfil analogous functions.
"Were we to imagine one of the Alcyonidae capable of secreting not merely the calcareous spicula that are mixed up with the softer portions of its body, but abundant quantities of carbonate of lime, which, being stored up in the centre of its substance, should form a dense calcareous axis incrusted with the uncalcified part of the living animal, and perforated at its surface so as to form innumerable cells or lodges adapted to contain the polyps that provide nourishment for the common mass, we should have a good general idea of the structure of the tribe of polyps that next comes beneath our notice.
(145). The shallower parts of the tropical seas contain countless forms of madrepores, known to us, unfortunately but too often, only by detached fragments of the earthy skeletons which the beauty of their appearance induces the mariner to bring to our shores. These calcareous masses generally assume more or less an arborescent appearance, spreading to a considerable extent, so as to cover the bottom of large tracts of the ocean, and not unfrequently they play an important part in producing geological changes, which are continually witnessed in the regions where they are abundant.
(146). In the hot climates in which the saxigenous corals abound, they are found to frequent shallow bays and sheltered spots, where they can enjoy the full influences of light and air, unexposed to the agitation of the ocean, which, were it to beat continually upon them, would infallibly destroy their delicate substance: in such situations, the submarine rocks become gradually incrusted with the calcareous skeletons which they produce; and if undisturbed, in the lapse of years successive generations will of course deposit such large quantities of calcareous matter as to form beds of considerable thickness. That there are at the bottom of the ocean bold and precipitous cliffs, rising from a depth of 1000 or 1200 feet, their broad tops approximating the surface of the ocean, every one will admit, without having recourse to the labours of madrepores to account for their formation, although the sheltered portions of the summits of such mountain-ridges afford an eligible position for their increase. In such situations, therefore, they accumulate, and slowly deposit continually increasing masses of earth upon the brow of these submarine mountains, until at last the pile approaches the surface of the sea, and even, at low water, remains uncovered by the waves.
The further elevation of the rock, as far as the polyps are concerned in its construction, here ceases; but a variety of causes tend gradually to heap materials upon the newly appearing island: storms, which tear up the bottom of the sea, perpetually throw to the surface sand and mud, which becoming entangled among the madrepores, and matted together with sea-weed, forms a solid bed over which the waves have no longer any power. The circumference of the islet is perpetually augmented by the same agency: sea-weeds and vegetable substances cast upon it, by their decay cover its top with vegetable mould; and if its proximity to other land permit the united action of winds and currents to bring the germs of vegetation from neighbouring coasts, they take root in the fresh soil, and soon clothe with verdure a domain thus rescued from the ocean.
The Corallidae are compound polyps of apparently more perfect organization than those forming the last family: The polypary or central axis, which supports the external or living crust, is solid, without cells, and variously branched, - the larger species resembling shrubs of great beauty, frequently coloured with lovely hues, and studded over their whole surface with living flowers; for such the polyps which nourish them were long considered even by scientific observers. The central stem of these zoophytes differs much in its composition in different families, sometimes being of stony hardness; in other cases it is soft and flexible, resembling horn; and not unfre-quently it is formed of both kinds of material: it is, however, always produced by the living cortex, which secretes it in concentric layers, the external being the last deposited.
(148). The example which we shall select for special description is the coral of commerce, Corallium rubrum (flg. 29), from which we derive the material so much prized in the manufacture of ornaments.
(149). The red coral is principally obtained in the Mediterranean. When growing at the bottom of the sea, it consists of small branched stems, incrusted with a soft living investment, by which the central axis is secreted, and studded at intervals with polyps possessing eight fringed arms, and capable of being contracted into cells contained in the fleshy covering, but not penetrating the stem itself. The skeleton or polypary of the coral is of extreme hardness, and susceptible of a high polish - a circumstance to which the estimation in which it is held is principally owing. But in other genera of this family, the central axis, instead of being constructed of calcareous matter, is formed of concrete albumen, and resembles horn both in appearance and flexibility; such are the Gorgoniae of the Indian Ocean. In the Isis hip-puris (fig. 30, b) the central axis is alternately composed of both these substances, exhibiting calcareous masses united at intervals by a flexible material, allowing the stem to bend freely in every direction. The object of such diversity in the texture of the polypary of the Corallidae will be at once apparent when we consider the habits of the different species. The short and stunted trunks of Corallium, composed of hard and brittle substance, are strong enough to resist injuries to which they are exposed; but in the tall and slender stems of Gorgonia and Isis, such brittleness would render them quite inadequate to occupy the situations in which they are found, and the weight of the waves falling upon their branches would continually break in pieces and destroy them; this simple modification, therefore, of the nature of the secretions with which they build up the skeleton which supports them allows them to bend under the passing waves, and secures them from otherwise inevitable destruction.