Each of these colonies was formed of a cluster of hexagonal, or rather polygonal pillars, fitting closely into each other, like the basaltic columns of Fingal's Cave, and springing from a common base at the sea bottom.[30] Each pillar constituted the abode of a single animal, and resembled generally the stalk of the cyathopsis. It had the same minute diverging partitions running from the outer walls towards the centre, and the same thin diaphragms, which, stretching horizontally across the interior of the column at short intervals, marked the successive stages of the animal's growth. Within these partitions, which vary from forty to eighty in number, there runs an inner circular tube with thin lamellæ and diaphragms. The exterior of the columns is ribbed longitudinally by a set of long fine striæ, which give somewhat the appearance of the fluting on a Corinthian pillar. The columns, moreover, are not straight, but have an irregular, wrinkled outline, so that, by a slant light, they look like some old pillar formed of many layers of stone, the joints of which have wasted away, producing an undulating profile in place of the original even one. But in these ancient coral columns there is no blunted outline, no worn hollow; the sculpturing stands out as sharp and fresh, and the wavy curves as clearly defined, as though the creature had died but yesterday. They resemble no order of human architecture, save faintly, perhaps, some of the wavy outlines of the Arabesque.
[30] Sir Roderick Murchison figures in his Siluria, p. 282, a gigantic specimen, which measured two feet four inches in width.
Despite all the improvements and inventions of modern times, classic architecture has made no progress since the days of Pericles. All that we do now is but to reproduce what the Greeks created 2000 years ago, and he is reckoned the best architect who furnishes the best imitation. Our architects might find some useful hints, however, by studying the lowlier orders of nature. They would see there patterns of beauty far more delicate than the Grecian capital, and more light and airy than the Gothic shaft. And whether or not they could found a new order of architecture, they could not fail to discover many modifications and improvements upon some of the old. They could not readily light upon a more graceful form than that of the lithostrotion, would they but picture it as it grew at the bottom of the old carboniferous sea. A group of hexagonal pillars, firmly compacted together like those of the Giant's Causeway, or Fingal's Cave, rose from a white calcareous pediment, as columns from the marble steps of an Athenian temple. Each side of the pillar had a wavy undulating surface, delicately fluted by long slender striæ, the whole being so arranged that the convexities of one surface fitted into the sinuosities of the adhering one. Each pillar was crowned above by a capital, consisting of the soft vibratile tentacles of the animal, that hung over like so many acanthus leaves. Of the form of these tentacles, their design and grouping, we know nothing save what may be gathered from the analogy of living corals. There can be little doubt, however, that, like the flower-shaped buds of the existing reef-building polyps, they must have been eminently beautiful, and in strict keeping with the graceful column which they crowned.
Another kindred form was that known as the lithodendron. It, too, grew in colonies, and seems to have closely resembled the last, save that the pillars, in place of being six-sided, were round. I have seen a bed of these corals several yards in extent, and seven or eight inches deep, where the individuals were closely crowded together, so as to resemble a series of tobacco-pipe stems, or slim pencils set on end. The tubes, however, were not all quite straight; many being more or less curved, and sometimes crossing their neighbours obliquely. The internal arrangement was on the same plan as in the two previous corals. The same numerous partitions ran from the exterior wall towards the central tube, the same thick-set diaphragms crossed the entire breadth of the column, imparting the same minute honey-combed appearance to a cross section. The exterior of the column (in L. fasciculatum) was likewise traversed by the same longitudinal striæ.
Both these corals seem to have been fissiparous, that is to say, they propagated by splitting into two parts, each of which formed the base of a new column with a new animal. The evidence for this statement rests on the fact, that many of the tubes are seen to bifurcate in their course, so that two new tubes are produced equal in size and completeness to the old one from which they proceed. Another mode of generation which, in at least its earlier stages, would produce a somewhat similar appearance is called gemmation, and consists in the protrusion of a bud or gemmule from the side of the animal, which shortly develops into a new and perfect individual. It is probable, however, that the ordinary mode of propagation among these old corals was the usual one by impregnated ova. These ova, like those of our sea-anemones, were probably generated within the partitions, between the central stomach and the outer wall, whence they passed down into the stomach, and were ejected by the mouth of the parent as little gemmules, furnished with the power of locomotion by means of vibratile cilia. Some of the Medusa family possess this three-fold mode of propagation; but, in all, the last-mentioned is the most usual.
Has the reader ever stretched himself along the shore, while, perhaps, a July sun blazed overhead, and a fitful breeze came over the sea, just strong enough to chase ashore an endless series of rippling wavelets, and breathe over his temples a delicious and refreshing coolness? Thus placed, and gazing dreamily now, perchance, at the distant sails like white specks along the boundary line of sea and sky; now at the gulls wheeling in broad circles through the air, and shooting swift as arrows down into the blue water, he must often have turned to look for a little at the sand which, heaped up in little mounds around him, formed a couch well-nigh as soft as the finest down. Many a varied fragment entered into the composition of that sand. Mingled among the minuter quartzy particles lay scores of shells, some with the colour not yet faded, and the valves still together—the delicate tellina, with its polished surface, and its flush of pink; the cardium with its strong white plaited sides, and the turritella with its circling spire; some were worn down and sorely effaced, others broken into fragments by the ceaseless grinding of the waves. It was pleasant labour in such a sultry noon to pick out the shells of one species in all stages of decay. The Trochus lineatus, or Silver Willie, as young ramblers by the sea-shore love to call it, showed well the process of destruction. The perfect shell, cast ashore, perhaps, by the last storm, and still uninjured by the tides, displayed its russet epidermis, or outer skin, covered with fine brown zig-zag lines, running across the whorls from the creature's wide pearl-lined mouth to the apex. A second shell exhibited a surface that had begun to suffer; the point had been divested of its thin outer skin, and laid bare the silvery coating of pearl below. A third had undergone a still longer period of abrasion, for the whole of the epidermis was gone, and the surface gleamed with a pearly iridescence. In yet a fourth, this bright exterior had been in large measure worn away, and the blunted, rounded shell displayed the dull white calcareous substance of which it was mainly built up. But there were other objects of interest in the sand: bits of tangle, crusted over with a fine net-work of gauze, and fragments of thin leaf-like membrane, consisting of a similar slender network known popularly as the sea-mat, occasionally turned up among the pebbles and shells. No one who met with these organisms for the first time could fail to be struck with the extreme delicacy of finish, if one may so speak, that characterizes them. And yet he might be puzzled to know what to make of them. The leaf-like membrane, at a first glance, looks not unlike some of the flat-leaved algæ, and such the observer might readily take them to be. Such, too, they were long regarded by naturalists; but a more careful examination of them showed that the so-called plants really belonged to the animal kingdom, and that the supposed leaves were, in truth, the organic dwelling-places of minute zoophytes, of which many hundreds lay grouped together on every square inch. For many years these little creatures were called "celliferous corallines," and classed among the polypi, that great tribe which has its representatives in every ocean, from the coral reefs of the Pacific to the little bell-shaped hydra amid the tangle of our own seas. But the microscope—that lamp which lights us into the inner recesses of nature—revealed at last their true character. Fixed to one spot, living in communities, and exceedingly minute, in short, with many of the outward features of the true corallines, they were yet found to possess a structure so complex and highly organized, as to entitle them to rank among the higher tribes of the invertebrate animals, and they are now accordingly pretty generally subjoined to the mollusca, under the name of Bryozoa.
Each bryozoon consists externally of a single horny or calcareous cell, sometimes furnished with a valve-like lid that folds down when the animal withdraws itself. When danger is past, and the creature begins again to emerge, the upper parts, which were drawn in like the inverted finger of a glove, are pushed out until a series of tentacles, covered with minute hair-like bodies, called cilia, are expanded. The vibratile motion of these cilia causes a constant current in the direction of the mouth, which lies in the centre of the hollow whence the tentacles spring; animalcules are in this way brought in rapid succession within reach of the mouth, and form a never-failing source of nourishment. The interior is greatly more complex than that of the polypi. The stomach is connected above with a cavity like the gizzard of a bird, furnished with pointed sides, which serve to triturate the food before it passes into the stomach. There is also a distinct intestine. The muscular action for the expansion and retraction of the animal is highly developed, and the generative system is a greatly more complex one than that of the polyps already referred to. In short, however closely they might be thought to resemble the corals in outward form, their internal structure undoubtedly links them with a much higher type of organization, and justifies the naturalist in subjoining them as a sub-order to the mollusca.
The cells are grouped at short intervals along a horny or calcareous substance, that sometimes encrusts sea-weed, or spreads out as a flat leaf-like membrane, or rises into cup-shaped or dendritic forms. A series of cells constituting a separate and independent colony, is termed a polypidom. The cells are further connected together by an external jelly-like integument, in which they are sunk, and which serves to secrete the calcareous particles from the sea.
It is interesting to know that creatures so minute and yet so complexly organized, existed abundantly in the seas of the Carboniferous period. No less than fifty-four species are enumerated as having been obtained from the carboniferous strata of the British Islands, and scarcely a year passes without one or two new species being added to the list. The most frequent belong to the genus Fenestella, or little window, a name indicative of the reticulated grouping of the branches like the wooden framework of a window. Each of these branches, or interstices, as they are called, was more or less straight, being connected with that on either side by a row of transverse bars, just as the central mullion of an abbey window is connected with the flanking ones by means of cross-bars of stone. Not unfrequently some of the branches subdivide into two, as we saw to be the case among the cup-corals.
