At first, perhaps, you are at a loss to know what purpose this shoulder can serve ; but, by turning to the shell, and carefully observing the spines in their natural connexion with it, you will observe that the obliquity of its position accurately corresponds with the angle which the individual spines form with the surface of the shell from which they spring; and that the shoulder has its plane exactly parallel with the latter, but raised a little way above it. Now the entire shell, during life, was clothed with a living flesh, having a thickness exactly corresponding to the distance of the shoulder from the shell. This shoulder, then, was an attachment for the muscular bands, whose office it was to move the spine to and fro ; the projection affording the muscles a much better purchase, or power, than they could have had if they had been inserted into the slender stem itself.

The tubercles on the shell show a structure which corresponds with this. They are very minute; but each of them is regularly formed, and is crowned with its little polished nipple, on which, as I have said, the spine works, as by a ball-and-socket joint. These are arranged with perfect regularity in what is called quincunx* fashion; and by close examination, you will see that each is inclosed in a little area formed by a very low and narrow ridge of the shell, which makes a network. On the lateral portions of the under surface the meshes of this net are particularly conspicuous ; and we see that they constitute shallow hexagonal cells, in the midst of which are seated the tubercles; yet not in the exact centre either, but nearer the front than the back of the area inclosed.

Now this elevated ridge affords, doubtless, the insertion of the other end of the muscles that move the spine ; the ridge giving a better purchase than a flat surface, as the keel on the breastbone of birds is deep in proportion to the vigour of the muscles used for flight. And, surely, the apparently trivial fact, that the space behind the tubercle is greater than that in front, is not without significance, since it implies a thicker muscle at that part, which accords with the circumstance that such would be the insertion of the muscle-band whose contraction produces the outward stroke by which the sand is forced away from the bed.

But what is the need of so much care being bestowed upon the separate motion of these thousands of hair-like spines, that each individual one should have a special structure with special muscles, for its individual movement 3 The hairs of our head we cannot move individually : why should the Heart-Urchin move his 1 Truly, these hairs are the feet with which he moves. The animal inhabits the sand at the bottom of the sea in our shallow bays, and burrows in it. By going carefully, with the lens * See page 60 for an explanation of this term.

at your eye, over the shell, you perceive that the spines, though all formed on a common model, differ considerably in the detail of their form. I have shown you what may be considered the average shape; but in some, especially the finer ones that clothe the sides, the club is slender and pointed ; in others, as in those behind the mouth, which are the largest and coarsest of all, the club is dilated into a long flat spoon ; while in the long, much-bowed spines which densely crowd upon the back, the form is almost uniformly taper throughout, and pointed.

The animal sinks into the sand mouth-downwarcls. The broad spoons behind the mouth come first into requisition, and scoop away the sand, each acting individually, and throwing it outwards. Observe how beautifully they are arranged for this purpose ; diverging from the central line, with the curve backwards and outwards. Similar is the arrangement of the slender side-spines ; their curve is still more backwards, the tips arching uniformly outwards. They take, indeed, exactly the curve which the fore-paws of a mole possess (only in a retrograde direction, since the Urchin sinks backwards), which has been shown to be so effective for the excavating of the soil, and the throwing of it outwards. Finally, the long spines on the back are suited to reach the sand on each side, when the creature has descended to its depth, and by their motions work it inward again, covering and concealing the industrious and effective miner.

Thus we have another instance added to the ten thousand times ten thousand, of the wondrous wisdom of God displayed in the least and most obscure things. "All Thy works shall praise Thee, Lord ! " (Ps. cxlv. 10).

There is an order of animals which naturalists put in the same category as the Sea-Urchins, but which an unscientific observer would regard as possessing little or no affinity with them. Some are like long, soft, and fleshy worms, and others, which come the nearest to the creatures we have been looking at, have still the lengthened form, which, however, so closely resembles that of a warty angled cucumber that the animals I allude to are familiarly called Sea-cucumbers (Holothuriadce). The marine zoologist frequently finds them beneath stones at extreme low water ; and larger forms, as big in every direction as a marketable cucumber, are occasionally scraped from the bottom of the deep sea by means of that useful instrument, the dredge. If you drop one of them into sea-water, you will presently see from one extremity an exquisite array unfold, like a beautifully cut flower of many petals, or, rather, a star of ramifying plumes. Soon it begins to climb the walls of your aquarium, and then you catch the first glimpse of its affinity to the Urchins ; for the short warts which run in longitudinal lines down the body corresponding to the angles, gradually lengthen themselves, and are soon perceived to be sucking-feet, analogous in structure and in function to those with which the Star-fish and the Sea-Urchin creep along.

But the relationship becomes more apparent still when we find that the Cucumber has a skeleton of calcareous substance deposited on exactly the same plan as in the Urchin, viz., around insulated rounded cavities. It is true you may cut open the animal and find nothing at all more solid than the somewhat tough and leathery skin ; but a calcareous skeleton is there notwithstanding, though in truth only a rudimentary one. If we were to cut off a considerable fragment of the skin, and spread it out to dry upon a plate of glass, and then cover it with Canada balsam, we should find (assisted by the translucency which is communicated to the tissues by the balsam) that the skin is filled with scattered atoms of the calcareous structure, perfectly agreeing with that with which the solid framework of the Urchin is built up ; but minute and isolated in the flesh, instead of being united into one or more masses of definite organic form.

But the atoms I speak of are still more perfectly seen by dissolving the piece of skin in boiling potash, and -washing the sediment twice or thrice in pure water ; this may then be spread upon a glass slide, and covered with a plate of thin glass, when it forms an interesting and permanent object for study. I have here a slide which is the result of such treatment; to the naked eye it appears sprinkled with the finest dust, but under magnifying power it is seen to consist of numberless calcareous bodies, of great beauty, and very free from extraneous matter.

The elegance of the forms is remarkable, and also their uniformity ; for though there do occur here and there among them plates of no regular shape, perforated with large or small roundish orifices, yet the overwhelming majority are of one form, subject to slight modifications in shape and size.

Neglecting, then, the irregular pieces, we perceive that the normal form is an oval of open work, built up by the repetition of a single element. That element is a piece of clear glassy material, highly refractive, of the shape of a dumb-bell : two globes united by a thick, short column. The oval is constructed thus :—suppose two dumb-bells to be placed in contact, side by side, and soldered together, there would be of course an oval aperture between their -columns. Then two other dumb-bells are united to these in a similar manner, but one on each side, so that the globes of each shall rest in the valley between the former globes now united. These then are soldered fast in like manner ; and the result is that there are three oval apertures. The next step is that on the top of the four united globes two other dumb-bells stand erect, and lean over towards each other till their upper globes come into contact, their lower ones remaining remote; these are soldered to the mass and to each other at the points of contact, .leaving a fourth aperture. The same is repeated at the opposite end by two other dumb-bells ; and the structure is complete as you see it. In almost all cases the two united globes of these terminal elements are fused into one globe, and in not a few instances the appearance is as if these two dumb-bells were but one, bent over in a semicircular form ; but still a

DUMB-BELLS IX HOLOTEVHIA. good many specimens occur in which the two dumb-bells can be quite distinguished from each other. The calcareous matter that solders the elements together seems abundant, and has the appearance that would be presented if they had been made of solid glass, and united by glass in a state of fusion ; the latter having apparently run together, so as to smooth and round angles and fill up chinks, even where, as is often the case, the globes themselves have only mutually approximated, and not come into actual contact.

The average dimensions of these oval aggregations may be .004 inch in length, and a little more than .002 in width ; but some specimens occur which are a little larger, and others a little smaller than this ; while the irregular plates are sometimes three times the length. Some of the more worm-like members of this class have, however, a skeleton composed of pieces imbedded in their skin, of even more remarkable shapes than these. One of these is the CJdrodota violacea ; a native of the southern coasts of Europe. We have indeed a British

WlIKhL UN IH1KODOTA. species of the same genus, a

specimen of which is in my possession, but I have vainly examined the skin for any structure analogous to this.* In the Mediterranean species the skin, especially of the belly-side, is described as filled with plates exactly resembling broad and thin wheels of glass, supported by four, five, or six radiating spokes, and having the inner edge of the hoop cut into teeth of exceeding delicacy.

Another animal remarkable for the furniture of its skin is the genus Synapta, which is very similar in form, and closely allied, to the Chirodota. It is very common in the Adriatic and Mediterranean seas, but has not yet been taken on the British coasts. I would counsel you, however, to have your eyes open if you have the opportunity of searching our coasts ; for, as M tiller found one species, the Synapta inhcerens, on the shores of Denmark, it is not at all unlikely that we may possess either it or some other. Should it ever come into your hands, slit open the skin of the belly, where you will find, imbedded in little papillaor warts, some highly curious spicula or calcareous forms. Each consists of an oblong plate, perforated with large holes in a regular manner, and having a projection on its surface near one extremity, to which is jointed a

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