thick skin, as in the Star-fish,-or is only to be discovered in the rudimentary condition of scattered patches of calcareous deposit, as in the Holothuria and its allies.

The elementary structure of the skeleton in this class may be described as a network, composed of calcareous and animal matter intimately united; the former, however, being greatly predominant. In this network, the interspaces or openings, and the solid structure which surrounds them, may bear an extremely variable proportion to one another. The predominance of the latter gives great solidity and strength to those parts of the skeleton where strength is principally required; whilst the texture is very loose and porous, with but a small amount of solid materials, in those parts where the object is rather to fill up space. In either case, however, the strength of the whole is much greater than would be inferred from the apparent slightness of its intimate structure, as seen in a thin section. For the network is not on a single plane, but extends in all directions, every part supporting every other part, and being supported by it in turn.

But this structure is seen to most advantage in the spines or prickles, with which the shell of the Echinus is beset. The strength of these organs is so increased by being connected with solid ribs or pillars, as to constitute a regular and distinct pattern, which appears to differ in every separate species. The spines are, for the most part, cylindrical or conical in their form; their transverse sections, there fore, are usually of a circular shape. Reduced to a sufficient thinness, and examined with the microscope, they exhibit a regular concentric arrangement, resembling that of an Exogenous stem, and proceeding from the same cause -the addition of successive layers, each on the outside of the preceding one. Each layer of the calcareous network is surrounded by a row of solid pillars or ribs, which may be seen running from end to end on the external surface of the spine; and as these are composed of a very transparent substance, their cross-sections might be mistaken at first sight for open spaces, in those species at least in which the substance is colourless. In most cases, however, it is tinged with some decided hue. The brilliancy of these hues, when shown in a strong light, and the marvellous regularity and variety of the pattern, render these sections,

unattractive and even uncouth in external appearance as are the bodies from which they are derived, perhaps the most beautiful objects which the microscope has revealed As much of beauty is found in the infinity of shapes and hues, as of exquisite fitness and harmony in the forms and structures of the new creation thus brought within our survey.

to us.

We have already alluded to the importance of the microscope to the geologist; how it has verified the true nature of fragmentary remains of the animals and plants of former epochs, and how it has shown us vast stratified deposits almost entirely made up of the accumulated remains of animals and plants which, even in their living state, were of microscopic minuteness. We still look forward to further discoveries in other cases, from a microscopic examination of rocks, at present believed to be inorganic. At the request of Mr. C. Darwin, Dr. Carpenter minutely examined the physical composition of the extensive calcareous deposit which covers the surface of the Pampas, and in which the remains of the great Megatheroid quadrupeds are imbedded; and compared it with the composition of the calcareous tufa now in progress of formation along the coast of Chili. The latter was found to be obviously composed in great part of fragments of shells, distinguishable by the naked eye: and the dense matrix in which they are imbedded is chiefly made up of minuter fragments, only distinguishable as such by the microscope; while the amorphous deposit of calcareous particles that cements the whole together has every appearance of having been the result of the attrition or decomposition of the same organic structures. On examining the Pampas deposit, the principal part of it was found to be composed of amorphous particles, so similar in aspect to those of the Chilian rock that their identity could scarcely be doubted; whilst, scattered at wide intervals, were discovered particles of shell, distinctly recognisable by the microscope, though invisible to the naked eye. From these indications it seems a probable inference that the sources of both deposits were the same; but that the materials of the Pampas deposit had undergone a much greater degree of comminution than those of the Chilian rock. This inference agrees with Mr. Darwin's hypothesis, "that the Pampean formation was slowly ac

cumulated at the mouth of the former estuary of the Plata, and in the sea adjoining it;" and appears to render it much more probable than M. D'Orbigny's theory of a great debacle, or Sir Woodbine Parish's idea of a fluviatile deposit.*

We observe also in Mr. Williamson's microscopic analysis of the Levant mud, already alluded to, that he has been able to recognise minute fragments of the shells of Mollusca and Echinodermata, mingled with the Infusoria and Foraminifera, of which that deposit is chiefly composed; and it further appears, that fragments of this description form no inconsiderable proportion of some varieties of chalk. Dr. Carpenter's attention has recently been given to the minute examination of those vast deposits of Nummulitic Limestone in different parts of the world, which have been represented by many geologists as equivalent to the higher beds of chalk of Northern Europe; but which are now generally regarded as belonging to the earliest ages of the tertiary period. In a paper on this subject recently communicated to the Geological Society, Dr. Carpenter has not only described an entirely new type of structure, presented by bodies which have been hitherto, for want of microscopic analysis, classed among the Nummulites; but he has also proved that the calcareous matrix in which the Nummulites and their allies are imbedded, and which forms continuous strata of hundreds of miles in length and breadth, and sometimes of three thousand feet in thickness, is itself composed of the comminuted remains of similar bodies, mingled with the shells of minuter Foraminifera.

Similar illustrations of the importance of microscopic inquiry in adding to our knowledge of details, in every department of the science of organization, might be multiplied indefinitely. But the microscope has in fact wrought a complete revolution in the general aspect of the science; and we are desirous of laying before our readers a sketch of what may be termed the Philosophy of Physiology, as this fine instrument has now enabled us to understand it.

* Darwin's Geological Observations on South America, p. 77.

We have already pointed out how microscopic investigation has demonstrated, that the simplest form of a living independent structure is a solitary cell; and that this cell lives for and by itself-drawing its nourishment from the elements around, converting them into the materials of its own growth, multiplying itself by self-division, and originating an entirely new generation by the union of its contents with those of another cell of a similar kind. Now if we analyse the structure of the most complex and perfect plant, we shall find that it may be reduced to an assemblage of the same elements; among which the whole series of vital actions is distributed, in such a manner that each cell, whilst going through its own succession of changes, at the same time performs some office which is essential to the existence of its fellows, and which is thus subservient to the common life of the whole fabric. A great part of that fabric, in the long-lived forest tree, consists of a structure whose offices are rather mechanical than vital;namely, the woody fibre, which constitutes the chief portion of the solid axis of the stem, roots, and branches; and which also enters the leaves and fruit, forming a skeleton for the support and protection of the softer parts. Yet this woody fibre is itself made up of lengthened cells, whose cavity is filled with a hard deposit, just in the same manner as the cells of shell are consolidated by carbonate of lime. We find, too, that the higher plants are provided with a set of vessels, for the rapid and uninterrupted conveyance of fluid from one part to another; while these vessels are themselves formed by the coalescence of cells arranged in a linear series, the partitions which originally separated them having disappeared.

Thus even in these two portions of the fabric, which seem to depart most widely from the simple type of vegetable life presented by the Cryptogamia, there is no real infraction of the general principle that the entire organism is composed of an assemblage of cells; and the modification has reference solely to the mechanical purposes to which these tissues are to be subservient. In those parts of it, however, in which active changes of a strictly vital nature are taking place, we uniformly find these operations effected by the instrumentality of cells, which retain their original simplicity and independence of character, and CHRISTIAN TEACHER.-No. 47.

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which go through their term of life with comparative rapidity. Thus it is well known that the chief agent concerned in the first absorption of fluid through the roots, is neither the woody fibre which forms the principal mass of their fabric, nor the vessels they contain; but the succulent cellular tissue at the growing extremities of their ultimate ramifications. This fluid undergoes no essential change during its ascent to the leaves. But in these organs it is completely transformed,-a large part of its superfluous water being set free by exhalation, and its solid matter receiving great addition in the carbon fixed from the atmosphere. It is through this process of elaboration, effected by the cells of the leaves, that the crude watery fluid which is brought up to the leaves, is converted into the viscid descending sap, which now contains all the organic substances necessary for the extension of the fabric, as well as those numerous materials, known as vegetable secretions, so essential to animal life and human welfare. We usually find these products separated from the general current of the circulation, and stored up in some particular part of the fabric. Thus, the nutritious properties of the seeds of the grasses and leguminous plants, are derived from the combination of azotised compounds and starch: in other cases, the seeds are valuable on account of the quantity of fixed oil contained in them; essential oils, again, are usually kept apart in special receptacles, either in the seed-coats, or in the envelope of the fruit, or in the leaves; colouring matters, though sometimes obtained from the general mass of the soft tissue, are very commonly preserved in the wood, as also resinous compounds; while the medicinal properties of plants in their vast variety are discovered hidden, some in one part of the structure, some in another, but nearly all in particular situations appropriated to them. Now the cavities of cells are the storehouses of these several products; from every class of which a certain group of cells procures the peculiar nourishment required for its development.

In the higher plants the enlargement of their fabric, by the growth of new parts, is always effected by the production of new cells, in continuity with those previously existing, and in fact generated from them by the same process of multiplication as in the lowest forms of vegetation ;