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structure we shall presently notice, would supply a vast variety of patterns of wonderful delicacy and beauty, both of form and colour.

We will instance only another application of the microscope to this branch of inquiry, in a recent example of great interest to economy as well as science. A continuous rock-formation extends over many parts of Russia, bearing a strong resemblance in its mineral characters to members of the Permian and Triassic systems,-the magnesian limestone and new red-sandstone of our own country. But so great was the difficulty of obtaining from stratification any evidence of its proper place in the series, on account of the prevalent horizontality of the strata, that the only hope of settling the question rested upon the organic remains which might be discovered in the formation. Unfortunately these were few and fragmentary,-consisting chiefly of teeth which are seldom perfectly preserved. From the gigantic size of these teeth, together with their form, it was at first inferred that they belonged to Saurians. But the microscopic investigations of Professors Asmus (of Dorpat), Agassiz, and Owen, have proved them to be the teeth of gigantic fishes, and to belong to a genus (Dendrodus) whose presence is restricted to the Paleozoic formations. The deposit in question is therefore a representative of the old, not of the new, red-sandstone,-a determination of the greatest practical moment; for, on the former supposition, that the teeth were the remains of Saurians, coal-measures would have been expected to lie below; and extensive borings might have been carried on, involving great expenses. The discovery that the remains belonged to fishes of the Devonian epoch put an end to all such expectations. The line of inquiry which was opened by these and similar discoveries has been successfully followed up by Dr. Carpenter. He has demonstrated the organic structure of the shells and other external skeletons of invertebrated animals; and has shown that the diversities are, in many instances, sufficiently definite in their characters to determine the family, sometimes the genus, and occasionally even the species-from the inspection of a minute fragment of the shell, fossil as well as recent. The most recent conchological writers had been of opinion, on the contrary, that shell was not merely destitute of blood-ves

sels, but was completely inorganic,-composed, in fact, of an exudation of calcareous particles, cemented together by animal glue. Even the celebrated geologist and palæontologist Von Buch, when first informed of the result of these observations, exclaimed that it was 'impossible' that shells should possess any organic structure. An a priori probability, however, in favour of their possessing a certain degree of organization followed the discovery of the organic structure of the epidermis or cuticle of higher animals; and the progressive elevation of physiology as a science, which now enables it to predict phenomena, as well as to record and generalise them, is well illustrated by the fact that Dr. Carpenter had published in 1841, some time before the commencement of his direct investigations, the following anticipation of their results: "From the analogy which the shells of Mollusca and Crustacea bear to the epidermic appendages of higher animals, there would seem reason to believe that the former, like the latter, have their origin in cells, and that these are afterwards hardened by the deposition of earthy matter in their interior."*

Indeed, the shells of Mollusca may be regarded as strictly epidermic in their character; since they are formed upon the surface of the mantle, which answers to the true skin of other animals. They appear to be always composed, in the first instance, of epidermic cells, consolidated by the deposit of carbonate of lime in their interior; and in many cases such is the structure, distinctly exhibited in the fully formed shell, and visible in the membranous residuum, after the removal of the calcareous matter by dilute acids. It often happens, however, that the original cellular organization is obscured through subsequent changes, by which new forms of tissue are produced; and we sometimes lose all traces of it. Certain shells have a very elaborate tubular structure; the tubes being disposed in an irregular network near the internal surface, with which they appear to communicate, and sending off long straight branches at intervals which pass towards the exterior. These tubes are usually much larger than those of teeth and bone-their diameter being frequently as much as 1.4000th of an inch. Still it does not appear probable that they receive blood;

* Principles of General and Comparative Physiology, 2nd edit. p. 33.

though they may be well supposed to imbibe nutritious fluid from the surface of the mantle for the reparation of the exterior of the shell; more especially as most of the species in which they abound are remarkable for their foliated or sculptured surface. The peculiar group of bivalves, known under the designation of Brachiopoda, is of the greatest interest to the paleontologist. Their proportion to the ordinary bivalves is at present quite insignificant; but it includes nearly all the bivalve shells which were most abundant in the earlier part of the Paleozoic series. A most unexpected structure has lately been discovered in them. Not only is the texture of the shell itself so peculiar, that almost any species may be recognised as a Brachiopod, from the microscopic examination of even a minute fragment; but in a considerable section of the group, the soft organs of the animal within are seen to be actually prolonged into the substance of the shell, passing though it to its external surface.

The real structure of nacre, or mother-of-pearl, to which its iridescence is owing, has also been determined by a similar method of inquiry. The peculiar lustre of this substance was long since discovered by Sir D. Brewster to be due to the striation of its surface by a series of very shallow grooved lines, which usually ran parallel to each other; and it was found that impressions taken from it on sealingwax, gum, or any other similar material, would exhibit the iridescent colours. Many of our readers are doubtless familiar with the fact, that it has been found possible to communicate similar prismatic hues to metallic surfaces, by ruling them with lines by means of a diamond point, at the distance of 1·10,000th of an inch from one another. Indeed, if such lines be ruled upon a die of hardened steel, the iridescence is developed in every metallic surface which receives its impress. As the lines upon nacre are not obliterated by any amount of polishing, it is evident that their presence depends upon something peculiar in the intimate texture of this substance, and not upon any merely superficial arrangement. Sir D. Brewster supposed this peculiarity to consist in an alternation of layers of mineral membrane and calcareous matter,-the grooves being produced by the wearing away of the former. Now, supposing every line upon the nacreous surface to indicate a distinct

layer of shell substance, a very thin section of mother-ofpearl ought to contain many thousand laminæ, in accordance with the number of lines upon its surface. But what is the fact? When the calcareous portion of the nacre is dissolved with dilute acid, no such repetition of membranous layers is to be found. On the contrary, if the piece of nacre be the product of one act of shell-formation, it furnishes but a single layer of membrane. In the investigation of this curious problem, Dr. Carpenter availed himself of the peculiar facilities afforded by the well-known Haliotis splendens, the remarkable variety of hues in whose shell is produced by the alternation of layers of nacre with laminæ of a horny substance like tortoise-shell. And he was thus led to the discovery that the iridescence may continue in the membrane alone, after all the calcareous matter has been removed by acid,-provided that this membrane retain the folds or plaits in which it was originally disposed, and to which the lineation of the nacre is evidently due. On the other hand, if the membrane be extended, so as to obliterate the folds, its prismatic lustre entirely disappears. The conclusion is, a complete confirmation of Sir D. Brewster's optical discovery of the dependence of the iridescence upon the lineation of the surface, and at the same time a correction of his explanation of the cause of it.

As in the case of bones and teeth, the peculiarities of intimate structure displayed by many shells have enabled the microscopist to identify even minute fragments, which had nothing characteristic in their external forms. For instance, there are certain tribes, both recent and fossil, whose other characters leave a doubt respecting their true place in the series-such as the curious extinct group of Rudistes-where the results of a similar examination appear to furnish the decisive evidence required.

The shells of Crustacea are shown by the microscope to be even more highly organised than those of Mollusca. A section taken across the black end of the claw of the common crab exhibits a structure exactly resembling that of dentine; the dense homogeneous substance being traversed by a multitude of tubuli radiating from the central cavity towards the circumference. The same structure is found in other parts of the shell, but the texture is less dense;

the extremity of the claw obviously requiring, for its prehensile purposes, a special degree of firmness. The surface of the shell is covered by a layer of flattened cells, fitted to each other by angular borders, so as to form a continuous layer. These cells have the power of secreting colouring matter; and it is from the secretions which they elaborate, that the hue of the surface, with all its variations, is derived.

The minute structure of the skeleton in the class of Echinodermata presents features of peculiar interest. This class includes, not merely the Echinus (sea-urchin) and Star-fish,-its best known forms,-but also the Holothuria (sea-cucumbers), and the vast group of Crinoidea, which are scarcely at all represented at the present epoch. The forms and conditions of these animals have apparently nothing in them common to all. What is the similarity, for example, between the zoophytic Encrinite,—an animal somewhat resembling a star-fish, but attached by a stony stalk to one fixed point during its whole existence,-and the free-moving, soft-bodied Holothuriæ, which, as their vernacular name implies, are sometimes so elongated as to present an entirely worm-like aspect, and in which we frequently lose all trace of the radial symmetry by which the group, as a whole, is characterised? Notwithstanding this apparent absence of a community of nature between the extremes, the groups in question are united by such a continuous series of intermediate forms, that no naturalist has felt any hesitation in placing them together in the same class. To this class, however, it has been hitherto impossible to assign any precise general character. The character conveyed by the designation Echinodermata (prickleskinned) is of very limited application. For the prickles or spines which cover the surface of the Echini and the like, are scarcely developed in the star-fish, and are entirely wanting in others. By some naturalists the possession of cirrhi or tendril-like tubes proceeding from the skin has been thought to approach most nearly to a criterion of the class. But Dr. Carpenter has pointed out a more extensive agreement in the minute structure of the skeleton,— whether it consists of polygonal plates united at their edges so as to form a complete envelope to the body, as in the Echinus, or of a solid stem and branching arms, as in the Crinoidea,- -or of a jointed flexible framework within a

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