P. globifera is formed on the same model as P. tripliylla, but is more globose, and each piece appears to have a deep cleft at the point, which does not extend to the interior side, where a thick ridge runs down from the point to the base. At the summit of this ridge, in each of the three divisions, there is set a strong acute spine, directed horizontally inwards, so that the three cross each other when the blades close, which they do energetically,—a formidable apparatus of prehension ! The stem is much more slender than in P. triphylla, and the height of the head of one of average size is only ^rd of an inch. It is peculiar also in being slender throughout, and in having the knobbed calcareous stalk extending up to the head, which appears to work on it. In each of the other sorts the stalk extends only through a part of the distance, above which the investing fleshy neck becomes wider and empty.

But the internal structure is not quite the same as in the others. The main portion of the head is composed of gelatinous flesh ; the calcareous support being reduced to that ridge which runs up the interior side of the blade. This is somewhat bottle-shaped, with a bulbous base, and a long slender neck, with two edges on the inner face, which are armed with horizontal hooked spines, some of which are double ; and the whole terminates in a sort of ring, formed by the last pair of spines, which unite into the acute horizontal point that I have already mentioned. The skeleton is filled with oval cavities, like that of the others.

The fourth kind of Pedicellaria, which I call P. stereojJii/lla, is quite distinct from any of the others. It is very minute, the head being only v^roth °f an inch in height. The head is a kind of lengthened sphere, cut into three segments, exactly as if an orange were divided by three perpendicular incisions, meeting at the centre. Thus the blades meet accurately in every part when closed, but expand to a horizontal condition. These are almost entirely calcareous, being invested but thinly with the gelatinous flesh. They are filled with the usual oval cavities, set in arched rows.

The head is set on a hollow gelatinous neck nearly as Avide as itself, andthrown into numerous annular wrinkles ; its walls are comparatively thin, disclosing a wide cavity, apparently quite empty, as the blue calcareous stem extends only half-way from the base to the head. At this point the neck contracts rather abruptly, and continues to the base, but just wide enough to invest the stem.

This sort is confined, so far as I have seen, to the ovarian plates and their vicinity, where they are numerous.

Thus these tiny organs, so totally unlike anything with which we may parallel them in other classes of animals, do not merely afford us amusement, and delight us by their elegance of shape and sparkling beauty, the variety and singularity of their forms, the elaborateness of their structure, and the perfection of their mechanism, but excite our marvel as to what can be the object which they subserve in the economy of the creature,—what purpose can be fulfilled by so many hundreds of organs, so singular and scattered over the whole surface of the shelly body.

It is very difficult to answer this question. The only organs with which they can be compared are the singular " birds' heads " in so many of the Polyzoa, which we looked at some time ago. But, unfortunately, a like mystery enshrouds the use of those processes, and the only light that we have as yet upon either form is that of dim conjecture. It has been supposed that, in both cases, the function of the prehensile forceps is to seize minute animalcules or floating atoms of food, and pass them to the mouth : but the supposition is involved in great difficulties ; as the organs, however fitted for prehension, seem peculiarly un suited for transmitting objects ; besides that the great majority of them are placed very remote from the mouth. I can only repeat the conjecture which I hazarded in the case of the Polyzoan "bird's heads," viz., that the pedicellarice are intended to seize minute animals, and to hold them till they die and decompose, as baits to attract clouds of Infusoria, which, multiplying in the vicinity of the Urchin, may afford it an abundant supply of food.

There is yet another series of organs which stretch out from every part of the surface of this living box; scarcely less numerous than either the spines or pedicellarice, but very different from both. They are what I alluded to just now as the feet. Let us pay a moment's attention to their appearance and action, before we examine their structure.

We see, then, extending from various points of the shelly case of the Urchin, and reaching to twice or thrice the length of the longest spines, slender pellucid tubes, slightly tapering towards their free extremity, which then abruptly dilates into a hemispherical knob, with a flat end. These very delicate organs are extended or contracted at the will of the animal, and turned in every direction, waved hither and thither, and evidently have the faculty of adhering very firmly by their dilated tips to any object to which they are applied.

So much we can discern as we watch the creature disporting in this vessel of water; but we will now endeavour to learn a little more about its structure and economy. Selecting for this purpose a sucker which is extended to great length, I snip it across with a pair of sharp scissors, as near the base as I can. Mark the result. The terminal knob which was attached to the bottom of the saucer maintains its hold, but the tube has suddenly shrunk up to a sixth part of its former length, exchanging at the same time its smooth slen. derness and translucency for a wrinkled semi-opacity. I push the knob aside with a needle's point and thus destroy its adhesion ; which done, I take up the severed and SUCKER OF TJRCHIIT. shrunken sucker, and lay it in a little sea-water in the live-box.

Under a power of 180 diameters we see that the tube is composed of two series of muscular fibres, the one set running lengthwise, the other transversely or in rings ; the former by their contraction diminishing the length of the tube, the latter diminishing its calibre. The muscular Avails are covered with a transparent skin, studded with round orange-coloured spots, perhaps glandular, exactly similar to those we saw on the exterior of the spines and pedicellarice.

Now, to illustrate the action of these tubular feet, I must a^ain have recourse to the denuded shell of a preserved Echinus. Taking this globose empty box in your hand, hold it up against the light, looking in at the large orifice, which was once occupied by the mouth :—you see that the whole shell is pierced with PORES OF URCHINS u

rminute holes or pores, which are arranged in ten longitudinal lines, associated so as to make five pairs of lines. Now with a lens examine more minutely a portion of any one of these lines, and you discern that it is composed of a multitude of pores, which have a peculiar order of arrangement among themselves; that is to say, they form minor rows which cross, obliquely or diagonally, the course of the longitudinal line. These rows are themselves double, the pores running in pairs, not however with mathematical symmetry. In this species, there are three pairs of pores in each row, and so there are in the one which I have here alive, but in other of our native species the rows consist of five pairs.

These pores are intimately connected with the tubular feet, each of which springs from a portion of the shell that is perforated with a pair of pores; so that the cavity of every tube communicates with th3 interior of the shelly box by two orifices.

Now, on the interior side of these two pores,—that is, within the cavity of the shell,—there is placed a little membranous, or rather muscular, bladder, filled with a fluid which is not materially different from sea-water. There is a free communication between the bladder within ^ind the tube without the shell, by means of the pair of pores, through which the fluid passes. By means of the muscular fibres, which are under the control of the Urchin's will, any portion of this double vessel can be contracted to a certain extent. Suppose it is the interior bladder ; the effect of the contraction of its walls is to diminish its capacity, and the contained fluid is forced through the pores into the tube without. The longitudinal fibres of this part being at the same moment relaxed, the tube is lengthened, because of the injected water. Suppose, now, in turn, the fibres of the tube contract, while those of the bladder relax ; the fluid is driven back, the bladder dilates, and the tube shortens, until, if the animal so please, its swollen tip is brought close up to the pores. By mechanism so beautiful and simple is the prolongation or abbreviation of these organs effected.

"We noticed, however, that the extremities of the tubes had an adhesive power, which faculty it is that constitutes them feet. They are prehensile ; and thus they afford, as we observed in the living Urchin, the means by which it takes hold of even a smooth and vertical surface, as the side of a glass tank, and drags up its body thereby.

Putting, now, the extremity of this cut-off tube under graduated pressure, having first applied to it a drop of caustic potash, we see that it carries a beautiful glassy plate of extreme thinness, which lies free in the swollen cavity of the termination of the tube. This plate is circular in form, apparently notched at the margin, and cut with four or five (for the number varies) incisions, which reach almost to the centre. The substance is formed of the common, clear, brittle, calcareous matter of the skeleton, hollowed into numberless cavities, according to the general plan. The centre is perforated with a larger round orifice. The appearance of marginal notching is deceptive; and indicates a structure analogous to what we see in the spine. The notched line indicates the extent -of the spongy structure ; but beyond this the plate extends into a perfectly circular smooth edge, but is constituted of a layer of calcareous substance so thin that there is no room for the ordinary cavities within it.

The round aperture in the centre plays an important part in the function of the organ. The foot adheres on the same principle as that by which children take up large flat stones with a piece of wetted leather, to the middle of which a string is attached. The boy drops his sucker on the stone, and treads firmly on it, to bring it into close contact with the surface; then he pulls at the string perpendicularly, by which the central part of the leather is lifted a little way from the stone, leaving a vacuum there ; u 2 since the contact of the edges with the stone is so perfect that no air can find entrance between them. Now the pressure of the atmosphere upon the leather is so great that a considerable weight, perhaps half-adozen pounds, may be lifted by the string before the union yields* Well, the very counterpart of this amusing operation is repeated by the clever "Urchin" whose

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