It may be interesting to compare with this the structure of the more solid shells of bivalves, which have been so elaborately studied by Dr. Carpenter. In general, these consist of two very distinct layers, well seen in the valve of the Pearl Oyster and its allies. The Pinna, or Wingshell, the largest of our native bivalves, affords us a good example, especially of the external layer, since here this layer projects beyond the inner one, in thin transparent edges, which gives us an opportunity of examining their structure without any artificial preparation. This fragment, taken from the edge of one of those leafy expansions, we will examine with a low magnifying power.

Each oHts surfaces has a sort of facetted, or honeycombed appearance ; and the broken edges, which even to the naked eye appear fibrous, are seen to resemble a number of basaltic columns. " The shell is thus seen to be composed of a vast number of prisms, having a tolerably uniform size, and usually presenting an approach to the hexagonal shape. These are arranged perpendicularly, or nearly so, to the surface of the lamina of the shell ; so that its thickness is formed by their length, and its two .surfaces by their extremities."*

SECTIOX OF SACKE FROM 1'EAKL OYSTEK.

The inner layer of such shells is remarkable for possessing in different degrees the property of reflecting rainbow-like colours, often with great delicacy and splendour ; and this is termed nacre, or familiarly " mother-of-pearl." This iridescent lustre depends, as Sir David Brewster has shown,f upon a multitude of grooves or fine lines, which run in a very waved pattern, but nearly parallel to each other, across the surface of the nacre. " As these lines are not obliterated by any amount of polishing, it is

Carpenter, " The Micro cope," 590. + " Phil. Trans.," 1814.

obvious that their presence depends upon something peculiar in the texture of this substance, and not upon any mere superficial arrangement. When a piece of nacre is carefully examined, it becomes evident that the lines are produced by the cropping out of lamina? of shell, situated more or less obliquely to the plane of the surface. The greater the dip of these lamina?, the closer will their edges be ; whilst the less the angle which they make with the surface, the wider will be the interval between the lines. When the section passes for any distance in the plane of a lamina, no lines will present themselves on that space. And thus the appearance of a section of nacre is such, as to have been aptly compared by Sir J. Herschel to the surface of a smoothed deal board, in which the woody layers are cut perpendicularly to their surface in one part, and nearly in their plane in another." *

Those beautiful objects,—so much prized for personal adornment,—pearls, are concretions accidentally formed within the shells of such mollusks, and are wholly composed of the inner layer. Drs. Kelaart and Mobius have recently published some highly interesting observations on the causes both of the iridescence and of the pearly lustre ; and these I will cite from the abstract translation of them made by Mr. Dallas.

" The surface of pearls is not perfectly smooth, but covered with very fine microscopic elevations and depressions. These are more or less irregular in their altitude, but approach most nearly to equality in pearls of the finest water. In pearls which exhibit a certain iridescence, and which, when turned in different directions towards the eye, present even very faint bluish, greenish, and reddish tints, the surface is found to present delicate irregular curved furrows, which either run tolerably parallel to each other, or form small irregular closed curves. This is due to the mode of growth of the pearl, in which thin layers * Carpenter, "The Microscope," 5<H.

of nacre, of small dimensions, have been laid over each other. There is no continuous layer over the pearl, but a number of small portions which sometimes overlie the margins of the subjacent layers, and sometimes leave them uncovered. This structure is seen most distinctly in the pearl shell, where the conditions are rendered more simple by the layers being deposited on a flat, or but slightly curved surface. The distance of the furrows from each other is not always the same ; sometimes they may be recognised with the simple lens, while on other parts they approach within —J^th of an inch of each other. That the iridescence of nacre, or the nacreous colour, as distinguished from pearly lustre, is caused by the interference of the light reflected from these furrows and the intervening edges of the strata, is proved by the circumstance, ascertained by Brewster, that impressions of mother-of-pearl taken in red or black sealing-wax exhibit the same phenomena of colour distinctly. In pearls, in consequence of their spherical form, the different masses of coloured light are so diffused that they unite to form white light; and this takes place with the greater perfection in proportion as the furrows are lost, and become converted into a surface of fine elevations and depressions.

" For their lustre, pearls are indebted to their being composed of fine layers, which allow light to pass through them ; whilst the numerous layers, lying one under the other, disperse and reflect the light in such a manner that it returns and mixes with that which is directly thrown back from the outer surface. It is the co-operation of light reflected from the surface, with light dispersed and reflected in the interior, that gives rise to lustre ; for this reason the knots of window-glass exhibit pearly lustre, and the membranes of pearls deprived of their line are almost as lustrous as solid pearls, except that their whiteness is destroyed. ' The two masses of light entering the eye, act upon it from different distances. Now, as it adapts itself to the body seen through the transparent layer, it cannot distinctly see the light reflected from the surface, and the consciousness of this infinitely perceptible reflection produces the phenomena of lustre.' * The thinner and the more transparent the layers of which the pearl consists, the more beautiful is its lustre ; and in this respect the sea-pearls excel those of our river-mollusks." f

We will pass now, by an easy transition, from the shells of the Mollusca to their tongues. Who that looks at the weather-worn cone of the Limpet, as he adheres sluggishly to the rock between' tide-levels, would suspect that he carries coiled up in his throat a tongue twice as longas his shell 1 And that this tongue is armed with thousands of crystal teeth, all arranged with the most consummate art in a pattern of perfect regularity 1 It sounds almost like a fable to be told that the great Spotted Slug, which we sometimes find crawling in damp cellars, carries a tongue armed with 2G,800 teeth ! Yet there is no doubt of the fact.

You see on this slip of glass a very slender band about two inches in length. This is the tongue of the common Periwinkle. While it was in the living animal, its forepart occupied the floor of the mouth, whence it passed down below the throat, and, turning towards the right side, formed a close spire of many whorls, exactly like a coil of rope, which rested on the gullet. Here we have it extracted, uncoiled, cleansed, and affixed to a slip of glass for microscopical examination.

Only a small portion of the band is visible at a time with such a power as is necessary to display the structure; but by means of the stage-movement we can bring the whole in succession under the eye, and discover that, with some modifications of form, the same essential plan of * Dove, " Farbenlehre," 117. t "Ann. & Mag. N.H." ; Feb. 1S58.

structure, and even the same elements, exist throughout. Concentrating our attention on a single transverse series of the numerous curved lines that at first sight bewilder the mind, we perceive, by delicate focussing, that the object before us consists of a number of hooks projecting from the surface of the translucent band, and arching downward. In this case a single row consists of seven such hooked plates or teeth ; one in the centre and three on each side. Each hooked plate has its arching tip cut into five toothlets, of which the central one is the largest ; and its base is united with the cartilaginous or gristly substance of the band. Only the middle plate is symmetrical ; the lateral ones bend inwards towards the central one, and are symmetrical only when considered in pairs, each associated with its opposite. The plates are perfectly transparent, but of a yellow horny colour; they are very hard; and, as they are not dissolved by acids, it has been supposed that their substance is siliceous (having the nature of flint) ; but they are more probably chitinous, or formed of the substance of which the hard parts of insects are composed. The tongue before us has 600 rows such as these, each, as we see, closely following, and indeed overlapping, its predecessor ; so that we can never look at a single row without at the same time seeing others which it overlaps, or by which it is overlapped.

The specimen which I will now show you is broader, but shorter. It is the tongue of Trochus ziziphinus, a large and handsome shell of regularly conical form, not uncommon on our rocky shores. It is perhaps a more interesting study than that of the Periwinkle. There are here, you observe, three constituent elements in the pattern. First, a delicate glassy central tooth, tapering to a fine point, and cut into minute saw-teeth along each edge. Then a series, of five on each side, of similar glassy pointed leaves, bending inward ; and outside these, on either hand, are a great number of stout dark-coloured hooks, arching forward and inward, each notched with saw-teeth, and diminishing in thickness as they recede from the centre.

The manner of using this elaborate organ is no less curious than is its structure. During life it is only the front portion—not more than one-third— of the band that is in use ; this is spread out on the floor of the mouth, with the teeth projecting and hooking backwards. The remainder has its edges rolled over towards each other, forming a tube closed at its extremity, which, as I have

TOXGUE OF TROCHUS.

already observed, is coiled away (in the long-tongued kinds) among the viscera.

The mode in which the tongue is used may be readily seen by watching the actions of a Periwinkle in a marine, or a Pond-snail in a fresh-water aquarium. When the conferva has begun to form a thin green growth on the glass sides of the tank, the Mollusca are incessantly engaged in feeding on it, and rasping it away with this toothed band. "The upper lip with its mandible is raised ; the

ower lip expands; the tongue is protruded, and applied to

the surface for an instant, and then withdrawn ; its teeth glitter like glass-paper, and in the Pond-snail it is so flexible that frequently it will catch against projecting points, and be drawn out of shape slightly as it vibrates over the surface."*

Perhaps every variety is accompanied by some variation in food or manner of feeding. With the Trochus, the proboscis, a tube with thick fleshy walls, is rapidly turned inside out to a certain extent, until a surface is brought into contact with the glass, having a silky lustre: this is the tongue; it is moved with a short sweep, and then the tubular proboscis infolds its walls again ; the tongue disappearing, and every filament of conferva being carried up into the interior from the little area which had been swept. The next instant, the foot meanwhile having made a small advance, the proboscis unfolds again, the tongue makes another sweep, and again the whole is withdrawn ; and this proceeds with great regularity. I can compare the action to nothing so well as to the manner in which the tongue of an ox licks up the grass of the field, or to the action of a mower cutting down swathe after swathe as he marches along. The latter comparison is more striking, on account of the marks of progress which each operator leaves behind him. Though the confervoid plants are swept off by the tongue of the Mollusk, it is not done so cleanly but that a mark is left where they grew ; and the peculiar form and structure of the tongue, which I have above noticed, leave a series of successive curves all along the course which the Mollusk has followed, very closely like those which mark the individual swathes cut by the mower in his course through the field.

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