The structure of the shells of the testa ceous Annelida, and of the peduneulate Cirrho poda, does not essentially differ from that of Mollusca; but in most of the sessile Cir rhopods, such as the common Balanus, we find a cancellated structure or diploe in tervening between the inner and outer plates of the shell (vol. i., p. 685). A less regular diploe has been described by Mr. J. E. Gray,* as existing betWeen the laminw of Ostrea purpurea ; but in no other shells of existing Mollusca has any approach to it been yet discovered. A very regular cancellated structure, however, is exhibited in the singular extinct group of Rudistes, where it makes up nearly the entire thickness of the shell (fig. 421.). The cancelli are usually short hexa gonal prisms, terminated at each end by' a flat partition ; consequently, a section taken in one direction (fig. 421.) will exhibit the walls of the chambers disposed in a hexagonal net work ; whilst a section that passes at right angles to this will bring into view the trans verse partitions ( fig. 422.). The cancelli are frequently occupied by calcareous infiltra tion ; which might lead to the belief that, like the cells of the Pinna, they were so consolidated in the living state. But they are also to be met with entirely empty, or with their walls merely lined by calcareous crystals ; so that there can be no doubt that they were originally hollow. The presence of this structure assists in determining the zoological position of the curious group in question, which many considerations would lead us to regard as having been interme diate between the Bivalve Mollusca and the sessile Cirrhopoda. And it may be added that, by the same evidence, the place of the curious Pleurorhyncus hibernicu.f, a fossil which has been assigned to a different tribe by almost every naturalist who has examined it, would unhesitatingly be determined as amongst the Rudistes.
Echinoderwata.— The structure of the skeleton in this class is entirely, different from that which we have found to be cha racteristic of the Mollusca ; whilst, in its essential features, it presents a remarkable uniformity throughout the various members of the group. The general arrangement of its components is the same, for example, in the firm plates which make up the testa of the Echinida, in the joints of the stems and branches of the Crinoidea, and in the scattered calcareous deposits which are met with in the integuments and in the tentacula of the Holy thltrida.
The elementary structure of the skeleton of the Echinodermata may be described as a net-work, composed of calcareous and animal matter intimately united ; the former, however, being greatly predominant. In this net-work, the interspaces or areolw, and the solid structure which surrounds them, may bear an extremely variable proportion to one another ; so that, in two masses of equal size, the one or the other may greatly pre dominate, and the texture may have either a remarkable lightness and porosity, or a con siderable degree of compactness and brittle ness. We may take the plates making up the shell of the Echinus as presenting a t3pical form of this structure ; from which the transition is easy towards either the more solid or the more open character which it elsewhere presents. When we obtain a very thin slice of one of these plates, taken parallel to the surface of the shell, we find that it is composed of a lamina, apparently in itself destitute of structure, perforated with con siderable regularity by apertures of a circular or oval form. The diameter of these aper tures (fig. 423.) varies to a certain extent in different parts of the same shell, the reticula tion being much coarser in the inner than in the outer layers : from numerous measure ments, the extremes may be stated at about 1-450th and I-2500th of an inch. The en tire thickness of the shell is made up of an immense number of such plates, which lie parallel to each other, but not in contact ; for they are separated from each other by little pillars, which rise up vertically from each plate to support the next, and which thus connect the different plates whilst hold ing them apart. The broken bases or ends of these minute pillars are commonly to be seen upon the surfaces of the perforated plates, at the spots intermediate between three or four of the apertures (fig. 423. b, b). The suc cessive plates are always so disposed, that the centres of the perforations of one shall corre spond with the intermediate solid structure of the next (fig. 423. a, a); and their trans parency is such, that, when we have reduced a section to such a degree of thinness as to contain a small number of the reticulated layers, it is easy, by a proper adjustment of the focus of the microscope, to bring either one of them into distinct view. In whatever direction we slice 'the shell of the Echinus, we alwa3s meet with a sort of reticulated structure ; for if our section be parallel to the surface of the plates, it brings into view one or more of the perforated laminw just described; whilst, if it be per pendicular to the surface, it passes vertically through a series of these laminw; and in the direction of the pillars that connect them, which thus constitute an areolar structure of a tolerably regular form. The testa is thus
of an extremely porous character, the areolw having the freest communication with each other. Even in the living state, however, the areolm appear to be empty, the ingress of the fluid with which the surface of the shell is in contact being prevented by the delicate me:nbrane that covers it. At the same time, it possesses a remarkable degree of strength, in proportion to the aniount of solid matter employed in its construction ; for every part at the same time supports, and is supported, by the surrounding fabric.
The skeleton of the Echinodermata con tains very little organic ;natter. When it is submitted to the action of dilute acid, so that the calcareous matter is removed, the re siduum is very small in amount ; indeed, unless the acid be so weak as only just to dissolve the carbonate of lime, the organic matter also will be dissolved, and no animal basis will be apparent. When, however, it is obtained in a state fit for examination, it is found to possess the reticular structure of the calcareous shell ; the meshes or areolm being bounded by a substance in which a fibrous appearance, intermingled with granules, may be discerned under a sufficiently high magnifying power, as was first pointed out by Professor Valentin. This tissue bears a close resemblance to the areolar tissue of higher animals ; and the shell may probably be con sidered as formed, not by the consolidation of the cells of the epidermis, as in the Mol luscs, but by the calcification of the fibro areolar tissue of the true skin. This calcifi cation of areolar or simply fibrous tissue, by the deposit of mineral substance, not in the meshes of areol, but in intimate union with the organic basis, is a condition of much interest to the physiologist ; for it presents us with an example, even in this low grade of the animal kingdom, of a process which seems to have an important share in the formation and growth of bone, viz. the progressive calcification of the fibrous tissue of the periosteum.# Not only the entire shell, but the frame work by which the teeth of the Echinus are enclosed and supported, is composed of a calcareous reticulation similar to that now described ; nor is it confined to these solid structures. It has been pointed out by Pro fessor Valentin, that the buccal membrane contains isolated patches of extreme de licacy ; and the same eminent observer has detected a most beautiffil example of this structure in the calcareous rosette, vvith which, as long since observed by Monro, the sucker at the extremity of each am bulacral tube is furnished. But it is in the spines with which the shell is beset, that the most remarkable displays of it are to be met with ; for it is there disposed in connection with s.olid ribs or pillars, which increase the strength of these organs, in such a manner as to constitute a most regular and elaborate pattern, which appears to differ in every distinct species. When we make a thin transverse section of almost any spine be longing to the genus Echinus, we are at once made aware of the existence of a number of concentric layers, arranged in a manner that strongly reminds us of the layers of wood in the stem of an exogenous tree. The number of these layers is extremely variable ; depending, not merely upon the age of the spine, but upon the part of its length from which the section is taken. The centre of the spine (fig. 424. a.) is filled up with the same kind of calcareous net-work as that of which the shell is composed ; and this is sometimes so delicate, as to appear as if made up by the interlacement of mere threads. This medullary centre is bounded by a row, more or less circular according to the form of the spine (which is sometimes angular), of open spots (b, b, b), in which it is deficient : these, on a cursory inspection, might be supposed,-from their transparency, to be void spaces ; but a closer inspection makes it evident that they are the sections of a circular row of solid ribs or pillars, which form the exterior of every layer. Their solidity becomes very obvious when we either examine a section of a spine whose substance is pervaded (as frequently happens) with a deep colour, or when we look at a thin section of any spine by polarised light. Around the first circle of these solid pillars, we find another layer of the fibro-calcareous net-work, which again is bounded by ano ther circle of solid pillars, whose transverse sections are seen at c, c, c. The same ar rangement may be repeated many times, (dd, e e). On looking at the outer border of the section, we observe that the rounded sides of these pillars (f, f) form a series of pro jections with hollows between them ; and these exactly correspond vvith the projecting ribs and furrows which we may notice run ning along the natural surface of the spine when we examine this with a magnifying glass, or even (in some instances) with the naked eye.