stony fastness : let us now look at the apparatus which effects this movement.

If you look again at this Serpula recently extracted, you will find, with a lens, a pale yellow line running along the upper surface of each foot, transversely to the length of the body. This is the border of an exceedingly delicate membrane; and on placing it under a higher power (say 600 diameters) you will be astonished at the elaborate provision here made for prehension. This yellow line, which cannot be perceived by the unassisted eye, is a muscular ribbon, over which stand up edgewise a multitude of what I will call combs, or rather sub-triangular plates. These have a wide base ; and the apex of the triangle is curved over into an abrupt hook, and then this is cut into a number (from four to six) of sharp and long teeth. The plates stand side by side, parallel to each other, along the whole length of the ribbon, and there are muscular fibres seen affixed to the basal side of each plate, which doubtless give it independent motion. I have counted 136 plates on one ribbon ; there are two ribbons on each thoracic segment, and there are seven such segments ;—hence we may compute the total

HOOKS OF SEBPULA.

number of prehensile comb-like plates on this portion of the body to be about one thousand nine hundred, each

worms. of which is wielded by muscles at the will of the animal ; while, as each plate carries on an average five teeth, there are nearly ten thousand teeth hooked into the lining membrane of the cell, when the animal chooses to descend. Even this, however, is very far short of the total number, because long ribbons of hooks of a similar structure, but of smaller dimensions, run across the abdominal segments, which are much more numerous than the thoracic. No wonder, with so many muscles wielding so many grappling hooks, that the retreat is so rapidly effected !

t 2

Chapter XVI. Sea-urchins and Sea-cucumbers

Peering about among the rocks to-day at low-tide, Z found, on turning over a large stone, an object which; though familiar enough to those who are conversant with the sea and its treasures, would surprise a curious observer fresh from the fields of Warwickshire. It is a ball, perfectly circular, and nearly globular,— only that its under part is a little flattened,— hard and shelly in its exterior, which is, however, densely clothed with a forest of shelly spines, each one of which has a limited amount of mobility on its own base. On attempting to remove it, I find that it adheres to the stone with some firmness ; and that, on the exercise of sufficient force, it comes away with a feeling as if something were torn, and I find that a multitude of little fleshy points are left on the stone. Having dropped my prize into a glass collecting-jar of sea-water, I presently see that it is slowly marching up the side, sprawling out on every side a multitude of transparent hands, with which it seems to feel its way, and which are evidently feet also, for on these it crawls along at its own tortoise-pace. And I now see that it is the knobbed ends of some of these feet which were torn away by my forcible act of ejectment, and left clinging to the stone.

It was not the first time that I had seen the Sea-urchin (Echinus miliaris) ; and I might have passed it by with a feeling of satiated curiosity, had I not recollected our evening's amusement. Oh, ho ! said I, what a fund of microscopic entertainment is inclosed in this stone box !

&So I brought it home, and now produce it as the text of our conversazione.

Every part is a wonder; but we must examine each in •order. Take the spines first.

As, using this triple lens, we examine these organs • on the animal crawling at ease over the bottom of a saucer of sea-water, we see that each is a taper pillar, rounded at the summit, and swelling at the base, where -it seems to be inserted into a fleshy pedestal, on which it freely moves, bending downward in all directions, and describing a circle with its point, of which the base is the centre. Each spine is for the greater portion of its length of a delicate pea-green hue, but the terminal part is of a fine lilac or pale purple. The whole surface appears to be fluted, like an Ionic column, but this is an illusion, as you will see presently.

I now detach one of the spines, cutting it off with finepointed scissors as near the base as I can reach. I put it with as little delay as possible into the live-box, and examine it with a high power, say 600 diameters. Look -at it. You see the ciliary currents very distinctly; and t if you move the stage so as to bring the basal portion .into view, you may discern even the cilia themselves, very numerous and short, quivering with a rapid movement. The currents are not longitudinal, but transverse, and somewhat peculiar. The floating atoms which come within their vortex are drawn in at right angles to the axis of the spine, and are presently hurled away in the same plane; forming a circle, whose plane is perpendicular to the direction of the spine. The surface upon which these cilia are set is a transparent gelatinous skin, of extreme tenuity, stretched tightly over the solid portion, which it completely covers, and studded with minute oval orange-coloured grains.

The substance of which the spines are composed is best seen by crushing a few of these organs into fragments.

We now see a texture beautifully delicate; they are formed of calcareous substance as transparent as glass, and reflecting the light like that material ; hard but very brittle; clear and solid, with a fibrous appearance in some parts, but in others excavated into innumerable smooth rounded cavities which join each other in all possible ways. It is to this structure that the spine owes its strength, its lightness, and its brittleness.

This arrangement of the calcareous deposit in a sort of glass full of minute inter-communicating hollows is very peculiar, but it is invariably found in the solid parts of this class of animals ; so that the experienced naturalist, on being presented with the minutest fragment of solid substance, would, by testing it with his microscope, be able at once to affirm with certainty, whether it had belonged to an Echinoderm * or not. And this uniformity obtains in all the diverse forms which the animals assume, and in all the various organs which are strengthened by calcareous deposits—Crinoid, Brittle-star, Five-finger, Urchin, Sea-gherkin, or Synapta; ray, plate, spine, sucker-disk, lantern, pedicellaria, dumb-bell, wheel, or skin-anchor,—whenever we find calcareous matter, we invariably find it honey-combed, and eroded, as it were, in this remarkable fashion.

Dr. Carpenter has described this texture so well, that I shall not apologise for quoting his words to you, especially as you will have an opportunity here of testing their correctness, by personal observation. " It is," he remarks, "in the structure of that calcareous skeleton, which probably exists, under some form or other, in every member of this class, that the microscopist finds most to interest him. This attains its highest development in the Echinida y in which it forms a box-like shell, or ' test,' composed of

From the Greek Ixlvog (echinos) a hedgehog, and dspfia (derma) sMn. A name given to these animals from their bodies being generally armed with spines.

numerous polygonal plates joined to each other with great exactness, and beset on its external surface with ' spines/ which may have the form of prickles of no great length, or may be stout, club-shaped bodies, or, again, may be very long and slender rods. The intimate structure of the shell is everywhere the same; for it is composed of a network, which consists of carbonate of lime, with a very small quantity of animal matter as a basis, and which extends in every direction (i.e., in thickness, as well as in length and breadth), its areolae or interspaces freely communicating with each other. These 'areola?,' and the solid structure which surrounds them, may bear an extremely variable proportion one to the other ; so that, in two masses of equal size, the one or the other may greatly predominate; and the texture may have either a remarkable lightness and porosity, if the network be a very open one, or may possess a considerable degree of compactness, if the solid portion be strengthened. Generally speaking, the different layers of this network, which are connected together by pillars that pass from one to the other in a direction perpendicular to their plane, are so arranged that the perforations in one shall correspond to the intermediate solid structure in the next; and their transparency is such, that when we are examining a section thin enough to contain two or three such layers, it is easy, by properly ' focussing ' the microscope, to bring any one of them into distinct view. From this very simple but very beautiful arrangement, it comes to pass that the plates of which the entire ' test ' is made up, possess a very considerable degree of strength ; notwithstanding that their porousness is such, that if a portion of a fractured edge, or any other part from which the investing membrane has been removed, be laid upon fluid of almost any description, this will be rapidly sucked up into its substance." *

To return, however, to our spine. When we look at it * ''The Microscope," 553.

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.

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