laterally, the appearance is such that we cannot but firmly believe, that it is grooved throughout with straight and deep longitudinal furrows. But if we break off the same spine transversely, and so exhibit it that the broken end shall be presented to the eye, we perceive that there are no grooves; but that the points in the circumference, which seemed to be the summits of the ridges, which are very narrow, are really lower than the intermediate spaces, which we supposed to be the grooves, and that the surface of these spaces is really convex in a slight degree.

The explanation of these contradictory appearances is easily given. Meanwhile, however, they read an important lesson to the inexperienced microscopist, not to decide too hastily on the character of a surface or a structure, from one aspect merely. So many are the chances of illusion, that the student should always seek to view his subject in different aspects, and under varying conditions of light, position, &c.

It is by making a thin transverse section of a spine,— cutting off a slice of it, to speak in homely phrase,—-that we shall demonstrate the structure, which is very beautiful. This is an operation requiring much delicacy and practice, and implements for the special purpose ; and hence it is best performed by professional persons, who prepare microscopic objects for sale. You may see such a section, however, on this slide ; but I do not know whether the spine belongs to the species we are examining.

The whole central portion is formed of the sponge-like calcareous matter, which, from the variously reflected and refracted rays of light, appears nearly opaque, and of a bluish colour by transmitted light. This structure sends forth radiating points (making longitudinal ridges, of course, in the perfect spine); and it is the opacity of these points (or ridges) which reach the circumference, that gives to the spine the appearance of being fluted. Indeed it would be fluted if this were the entire structure; but the open space left between these projecting radii is filled with the solid glassy matter, having, as we see, a convex surface. This, however, from its perfect transparency, is not seen when we look at the side of the spine, the eye going down through the transparent matter to the bottom of the space. The spine is, in fact, a fluted column of spongy glass, with the grooves filled with solid glass.

We have not yet seen, however, the beautiful mechanism appropriated to the movement of these spines. You can hardly see this to advantage in the living animal, but here is the entire shelly box of a dead Echinus, on which, while for the most part the surface is denuded of spines, a few dozen remain sufficiently attached to show what I wish to demonstrate, viz. , the mode of articulation. You observe that th« whole globose shell is covered with tiny knobs, differing in size, and not set in very regular, or at least not very obbut a SPINE OF ECHINUS. vious, order, showing Segment of Section. tendency to run in lines from ;pole to pole of the globe. Giving attention to one of tlie larger of these knobs, under a lens it is seen to be a hemispherical eminence on the shell, the very summit of which is crowned by a tiny nipple of polished whiteness, resembling ivory. Xow if we carefully lift one of the still remaining spines from its attachment, which in its present •dried state is so fragile that the slightest touch is sufficient for the purpose, we shall note that its base rests on this tiny nipple ; and, on turning it up, and bringing the magnifying power to bear upon its base, we see that this is •excavated with a hollow, whose dimensions exactly corre-

spond with those of the nipple. It is indeed a true "balland-socket" joint, like that of the human hip or shoulder, and is surrounded by a capsular ligament to keep it in place ; the muscles which sway the spine from side to side and cause it to rotate being inserted outside the capsule or case. Professor Edward Forbes calculates that upon a large Echinus, such as this dried specimen of E. sphcera, there are more than four thousand spines, every one of which has the structure, the mechanism, and the movements that we have been examining. Well may he say, that " truly the skill of the Great Architect of Nature is not less displayed in the construction of a Sea-urchin than in the building up of a world ! "

To return now to our little E. miliaris, which has been all this time coursing round and round his saucer, wondering, perchance, at the narrowness and shallowness of the White Sea in which he finds himself. Again we peer, lens to eye, over the bristling surface, and discern, shooting up amidst the spines, and almost as thickly crowded as they, multitudes of the tiny organs which have caused so much doubt and discussion among naturalists. Midler, the great marine zoologist of Denmark, who first discovered them, thought them parasitic animals, living piratically upon the unwilling Urchin, and accordingly gave them generic and specific names. The term ])edicellaria, which he assigned to his supposed genus, is that by which modern naturalists have agreed to call them still, though the word is not now used in a generic sense, since it is indubitably established that they are not independent animals, but essential parts of the Urchin itself. Miiller described three distinct sorts, and I have added a fourth to the number ; they are named P. triphylla, tridens, globifera, :— and stereopliylla. They all agree in these particulars That each has a long, slender, cylindrical, fieshy stem, through the centre of which runs an axis or rod of calcareous substance ; that the base of the stem rests on the skin of the Urchin ; that on the summit is placed a head consisting of three pieces, which are capable of being widely opened and of being closed together, at least at their tips; that the edges of these pieces, which come into mutual contact, are furnished with teeth, which lock into each other; that the head-pieces (like the stem) consist of calcareous centres, clothed with flesh; that, besides the opening and shutting of the head, the stem can be swayed from side to side ; and that all these movements are spontaneous, and apparently voluntary. It appears that the head-pieces close on any object presented to them, such as the point of a needle, and hold with considerable force and tenacity, so that the pedicellaria may be drawn out of the w Tater without relaxing its grasp.

Looking at one of the first-named kind, the pedicellaria triphylla, of this Echinus miliaria, we see that it consists of three broad and thick somewhat triangular pieces, jointed into a head, set on a thickish stem of transparent gelatinous fibrous substance, through which a slender core of calcareous matter runs that looks fibrous and blue. The three movable pieces or blades are convex externally, concave internally ; thin in substance, furnished along their opposite or concave sides with two longitudinal ridges or keels, each of which is cut into the most beautifully fine teeth, so that the edge of each ridge looks like a shark's tooth ; the edges of the pieces are also similarly toothed : these shut precisely into each other.

In the larger E. sphcei'a, the head-blades of this kind have one stout central ridge, which is rounded and not toothed. It forms the front of a great interior cavity which opens by two orifices on each side of the column.

The movable pieces inclose a skeleton of calcareous substance, glassy, colourless, and brittle, in which, according to the plan I have already described, are excavated a multitude of oval cavities which form irregular rows ; a central line runs down each piece, that is solid and free from cavities. This calcareous skeleton is -encased in a gelatinous flesh, similar to and continuous with that of the stalk.

This is the smallest kind, the head being about -=-Vth of an inch in height.

Considerable modifications are found to exist in the details of each form, in the relative proportions which the parts bear to each other, and so forth ; so that two forms, which in their extreme conditions widely differ, mutually •approach, and appear to run into each other. This is the case with the present, and with the form which I -will now show you.

P. tridens is much larger than any of the other forms, the movable head being about -^V^h of an inch in length, and the wdiole organ about -^th of an inch. This may be considered as essentially P. trip/iylla, modified by the blades being greatly drawn out in length, and at the same time rendered quite slender, so that they may be called pins; they meet only at the points, where they often cross ; the spaces between the basal parts being open. The inner edges of these are notched with teeth as in P. triphyUat of which those near the tips are larger, and cut into subordinate teeth of exquisite minuteness. We have here an opportunity of seeing that the oval or square

HEAD OF PEDICELXARIA markings, which are thickly placed TBIDENS. throughout the calcareous substance of the blades, are certainly cavities in it; for in dhose examples in which the pins, which are very brittle,

are broken, the edge of the fracture is not ev ren, but jagged, with holes exactly corresponding with the marks in question; so that the structure is the same as that of the spines and of all the other solid parts of the Urchin.

We will now examine some specimens of P. tridens, treated with potash, which will enable us to see the calcareous support better. The head-blades expand at the base into three-sided figures, each of the two interior sides of which is indented with a large cavity, leaving a projecting, dividing rid^e, armed with teeth somewhat remote from each other. The one exterior angle is toothed in a corresponding manner, but the opposite angle appears plain. The angle of one blade-base fits into the cavity of itsneighbour ; and, so far as I have observed, when the two edges thus overlap it is the toothed one that is on the outside. Looking from the circumference towards the centre of the head, it is the left angle that is toothed and external, the right being plain and sheathed. This observation,, however, applies only to E. miliaria j for, in the corresponding organs of E. sphrera, both sides of the threecornered base appear untoothecl, except close to thebottom, where a deep notch indents each margin.

Viewed from beneath, the head assumes an outline which is rondo-triangnlar; but yet such that each side of the triangle has a very obtuse projecting angle in themiddle, where the blade-bases meet each other. They fit accurately, and each has a deep oblong cavity in its bottom, which does not, as I conceive, communicate with the interior.

By selecting one of these heads, which has been divested of its fleshy parts by immersion in caustic potash, and then well cleansed by soaking in clean water, and placing it under a low power of the microscope— 100 diameters, for example—with a dark ground, and the light of the lamp cast strongly upon it by means of the Lieberkiihn, or the side condenser, we shall have an object of most exquisite beauty. The material has all the transparency and sparkling brilliance of flint-glass, while the elegantly-shaped pins, the perfect symmetry of the bases, the arch which is lightly thrown across their cavity, the minute teeth of the tips locking accurately into each other, and the oval cavities in the whole structure set in regular rows, and reflecting the light from thousands of points, constitute a spectacle which cannot fail to elicit your admiration.

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