To take the inhabitant of the madrepore as an exam ple of the animal itself, it may be considered as formed of three parts, the shell, the head, a centre, and the feet, or hands. The latter are very numerous, and are divided, or split at the extremities, while they surround the body of the animal in the form of a circle. Each of these feet, or hands embraces a lamella of the star of the madrepore, so that they serve both for the construction of the cell, and for fixing the animal in it. The pedicle, or single part of the hand, appears to be a muscular body, and is fixed in a cylindrical tube, which is properly the body of the animal. Within this is a stellated body, which is supposed to be the head, quick in its motions; while the rays seem to be the tentacula by which it feeds itself.
We remarked just now, that all the different species of corals engaged in the formation of the coral banks were not known, but some of the genera, at least, and a few of the species, have been ascertained. The chief of these are Madreporx of different kinds, Milleporx, among which the cxrulea has been observed, the Tubipora mu sica, a Caryophyllia, a Distichopora, and a Corallina. Astreie, Echini, and other shells, living and dying on the banks, add to the heap of calcareous matter, without be ing properly concerned in the erection of the structures. Frequently, also, Holothurix, and other soft worms, are found in the reefs, and have by careless observers been mistaken for the coral insects.
Nearly all the islands that lie on the south of the equa tor, between New Holland and the western coast of America, derive either the whole, or a great part of their structure, from these animals. The whole of that sea, and, indeed, of some others, abounds in coral rocks and reefs, which are in a state of daily and rapid increase, and which are probably destined, at some future day, to ele vate themselves to the level of the water; to become the seats of vegetation, in process of time the habitations of man ; and ultimately, perhaps, to produce scarcely less than a continent in this extensive ocean.
Among other places, these reefs abound particularly between New Holland, New Caledonia, and New Gui nea; and they are well known to exist in great abundance in the seas of the Indian Archipelago, as at Chagos, Juan de Nova, Cosmoledo, Assumption, Cocos, Amirante, and the Laccadive and Maldive islands. They are also nu merous in the east side of the Gulf of Florida, and it is well known that they form a daily increasing impediment to the navigation of the Red Sea.
The extent of these reefs and islands is an object of great curiosity and stn prise, when we consider the appa rent feebleness of the means by which they are produced, and the minuteness of the agents. An instance or two of
this will suffice for our present purpose. One of the Tonga islands, the Tongataboo of Cook, is an irregular oval of twenty leagues in circumference, while its eleva tion above the level of the water reaches to ten feet. The soundings from which the thickness of this bed of rock might be estimated have not been given, but they are known to be deep in all this sea, and may safely he taken at not less than a hundred fathoms, so that the whole forms what may be considered an enormous stratum of organic limestone. But the largest which appears to have been ascertained, is the great reef on the east coast of New Holland, which extends unbroken for a length of 350 miles ; forming, together with others that are more or less separated from it and from each other, a nearly continu ous line of 1000 miles or more in length, with a breadth varying from twenty to fifty miles. Before such a moun tain of limestone as this, even the Appenine shrinks in comparison ; and that such a mass should have been pro duced by such insignificant means, is a just subject of ad miration to philosophical minds, and of wonder to those which have not considered the indefinite powers of units in endless addition.
Although the greatest depths of these submarine moun tains have not been ascertained, they have been sounded to 200 fathoms and more. It is not uncommon for navi gators to say, that they lie in depths that are out of sound ing: a vague term among mariners, as it is now known that the lead can be sent down without difficulty to a thou sand fathoms. The reefs or the islands which they after wards form, are sometimes disposed in rows, or in lines more or less straight ; at others they are accumulated in groups; and, not unfrequently, they are disposed in a cir cular or oval form : the latter disposition, whether on the small or great scale, having a material influence on the form and nature of the future island.
It is imagined that their production is very rapid ; but n this part of the subject there is some uncertainty, and there is also reason to think that it has been somewhat ex aggerated. These seas cannot, from their extent, be in timately known; nor is it possible that the infinite num bers of the reefs that exist in them should have been noted down. Even if they had, it is always a good excuse for an incorrect chart or a bad reckoning, to assert that a new rock was found where the old one had been overlooked.