CILIOBRACHIATE POLYPES ;-BOWERBANKIA. 531 surrounding it. The food appears to be retained for a considerable time in the stomach, and may frequently be seen to pass back into the gizzard, for the purpose of being again submitted to its operations. After being subjected to the digestive process in the stomach, the particles which remain are transmitted into the intestine; but previously to their entering it, they are seen to rotate for some time about its orifice, the movement being produced by the action of the cilia that fringe it. The granular matter accumulates in little pellets in the intestine; and when these pass out from its extremity, they are immediately whirled off to a distance, by the same action of the cilia upon the arms, as that which produces a current towards the mouth. 1103. Considerable power of selection appears to be enjoyed by these Polypes in regard to their food. The movements of the cilia which fringe the tentacula, seem entirely under the control of the individual; and by their vibrations currents of water are produced, which bring particles of various kinds of living and dead matter to the entrance of the alimentary canal. It has been sometimes noticed that animalcules, which have thus been drawn into the pharynx, escape during the act of swallowing; but they are usually met by one of the tentacula, which bends inwards, and by means of a sharp blow drives them back into the mouth. They do not immediately perish in the stomach, but may sometimes be seen to continue their movements for a considerable period within its cavity. Of the vortex which is drawn by the cilia towards the mouth, a considerable amount passes out between the bases of the arms; and this usually contains the finer particles, which thus escape. Of those which remain within the cone, those of fitting size are received into the pharynx; and, when they have passed downwards to its termination, a more perfect selection appears to take place, by which some are admitted into the stomach, whilst others are rejected. Besides this regular flow of water into the pharynx, the fluid is occasionally introduced in larger quantity, by the sudden dilatation of the tube, which quickly returns to its former diameter, and expels a large part of what it had thus taken in. This movement is not improbably connected with the function of 532 CILIOBRACHIATE POLYPES ;-BOWERBANKIA; FLUSTRA. respiration. That a part of the alimentary canal should be specially modified for this purpose, is by no means surprising; for such a modification, in a somewhat different form, is not at all uncommon in various divisions of the animal kingdom, Thus, in the lowest group of Mollusca, to which these Polypes bear considerable affinity, we have seen that the entrance to the stomach lies at the bottom of a large cavity lined by the respiratory membrane, over the walls of which currents of water are constantly passing, which supply the digestive organs with alimentary materials, besides effecting the aeration of the blood (§ 974). 1104. As in the two preceding Orders, we meet with species among the Ciliobrachiate polypes, which have the power of forming stony cells, by a deposit of lime in their soft tissues. The cells of some of these polypes are covered-in, when the animal is withdrawn, by a sort of lid or flap, which is provided with muscles for drawing it down upon the mouth of the cell. In others of the compound polypidoms, however, the cells overlap one another obliquely; and the orifice is not at their extremity but rather on one side, so that the operculum has only to be slightly upraised, to allow of the protrusion of the animal. This is the case in the common Flustra of our own coasts, which presents a flat expanded surface, so thickly set with these apertures, as to appear like a delicate network. These Sea-mats, as they have been fancifully termed, so much resemble common seaweeds in their general aspect, that they are often mistaken for them by ignorant collectors. They may readily be distinguished, however, by the crispness of their feel, when contrasted with the flabbiness of the Algae, as well as by the polype-cells so beautifully arranged on their surface. The extension of these compound structures takes place by buds which are developed from the outer surface of the cells. From one original cell of the Flustra, five such buds may be sent off, which develope themselves into new cells around it; and these, in their turn, produce buds from their unattached margins, so as rapidly to increase the number of cells to a very large amount. This extension may go on almost without limit; and it often happens CILIOBRACHIATE POLYPES ;-FLUSTRA. 533 that the cells in the centre of a leaf-like expansion of the Flustra are devoid of polypes, and have lost their vitality, whilst the edges are in a state of active growth. Although this mode of multiplication appears to be the one most characteristic of the group, the production of gemmules or ova occasionally takes place, as in other Polypifera. These are developed from the lining membrane of the cell, and gradually increase so as to fill up its cavity, and to cause the death of the contained polype. When mature, they escape from the cell, and swim about for some time, after which they fix themselves, and lay the foundation of new structures. 1105. The polypes forming these compound structures are usually packed closely together; and are of very minute size; FIG. 625.-PLUMATELLA: a, natural size; b, a group enlarged; c, anal orifice. so that a single polypidom of very moderate dimensions must contain an enormous multitude of individuals. Dr. Grant has reckoned that, on an ordinary specimen of Flustra carbasea, there are about ten square inches of surface; in each square inch there may be about 1800 polype-cells, thus making altogether 18,000 within this small space. Each of these polypes has 22 tentacula; so that there will be about 396,000 of these minute arms 534 upon CILIOBRACHIATE POLYPES. -FOSSIL CORALS. this little specimen. If each of these tentacula has only 100 cilia upon its edges (and there are probably many more), the whole polypidom will have 39,600,000 of these minute but important organs. Other species certainly contain more than ten times these numbers. Dr. Grant has computed about 400,000,000 cilia to exist on a single Flustra foliacea. In the Alcyonella, a fresh-water species composed of long membranous cells arranged side by side so as to form a spongy mass, the number of tentacula on a moderate-sized specimen may be computed at nearly five millions and a half; and the cilia are certainly not less than a hundred times that number. The figure in the preceding page represents another very beautiful fresh-water polype of this Order, which is by no means uncommon. It is found upon the surface of leaves, &c., of aquatic plants. 1106. From the very imperfect degree in which the greater part of the polypes of this group have been yet examined, the subdivisions of it are at present necessarily founded only upon the arrangement of the cells in regard to one another. Such an arrangement is liable to this great objection, that the mode in which they are disposed will vary extremely with the surface upon which they grow. Hence it is not advisable for us to enter more particularly into the classification of this group; but the study of it may be especially recommended to those who have opportunities at their command, as a most interesting pursuit, which, with but a moderate amount of previous information, and of skill in observation, is sure to be productive of much that will be alike interesting, novel, and important. 1107. The most extended survey we can take of the operations of the Polypifera upon the surface of the globe at the present time, will give us but a very inadequate idea of the important part which they performed, in the remoter epochs of the history of the earth. Our wonder is excited when we hear of a continuous reef of coral more than a thousand miles in length; yet what is this to the formation of limestone strata, covering superficial areas, not only of thousands, but of tens of thousands, of square miles, to a thickness, in many instances, of METAMORPHOSES OF CORAL. 535 3000 feet? Yet the Geologist of the present day has little hesitation in regarding these formations, as having taken their origin from the labours of these apparently-insignificant and simplyorganised beings. As at the present time, the greater proportion of these structures appears composed of the Lamelliform corals (§ 1069); but the remains of Alcyonian polypes are by no means unfrequent in the limestone rocks, and are especially abundant in particular strata; and some of the harder forms of the Ciliobrachiate group are occasionally found. 1108. There are many instances in which the Coral structures of comparatively recent origin have undergone a metamorphosis, which causes them to lose, in greater or less degree, their original aspect. Large masses, when long exposed to the air, become changed into a solid, often somewhat crystalline, rock; in which the traces of organic structure are very indistinct, and with which the Mountain or Secondary Limestone closely corresponds. This is observed in the Bermudas, a group of islands, which seems to have been for the most part formed by Coral Polypes of the same species with those now existing in the seas around.-Moreover, the Coral Sand, formed by the action of the waves upon the living structure, often becomes consolidated into a hard stone by the filtering of water through it; a small quantity of the carbonate of lime being probably dissolved at the surface (where the carbonic acid of the air increases the solvent power of the water), and set free again below, so as to glue together the separate particles. It is in such a mass, that the human skeleton is imbedded, which was found on the shore at Guadaloupe, and is now placed in the British Museum. This stone, when minutely examined, is found to consist of a number of rounded grains, cemented, as it were, together; and it closely resembles the rock known to the Geologist as Oolite.Further, where shallow water exists around Coral islands, the bottom is found to be covered with a layer of white mud, which is formed by the decay of the animal matter that held together the particles of carbonate of lime in the stony corals; and these, being thus set free in a finely-divided state, fall to the bottom in a form which, if dry, would constitute Chalk. Thus we may |