Part 21

P. H. GOSSE, del. LEIGHTON, BROS.

PURPLE-TIPPED URCHIN. ROSY FEATHER-STAR. STARLET. ]

ITS EYES.

It is of a pentagonal figure, with the margin a little receding between the angles, but not so as to cause the latter to form distinct arms. The body is flat below, and plump and cushion-like above, of a yellowish olive hue, with the very edge of a golden orange tint, while a spot of the latter colour, a little out of the centre, marks the situation of a remarkable organ called the madreporiform tubercle, the proper use of which has not as yet been satisfactorily determined. Just at the extremity of each angle, but a little below, is situated a wart of rich crimson hue, which is supposed to be an eye, being seated on a small ganglionic swelling of a nerve that passes along the ray. It is true no crystalline lens has as yet been detected on the pigment dot, either in this species or any other (for the specks are found in the same position in all the proper Star-fishes), but they are manifestly of the same character as similar specks in Rotifera, and other humble animal forms, which in some examples are connected with an indubitable lens. It might seem at first as if the situation of these eyes were not very favourable to vision; but, in truth, they command the ground just before and around the ray-tip, and also the water in a horizontal direction; and as there is one at each of the five angles, the entire circumferent space is viewed at each moment. Add to this, that the animal is in the habit of very frequently turning up the tip of one or other of its rays, when the range of vision would take in the zenith; and we shall perceive that no position in any other part of the body could be so suitable for the location of eyes as these selected. It is not to be supposed that distinct definition of objects is attained by these rudimentary organs; but the animal is probably conscious of the difference between light and darkness, and may also discern the sudden approach of any object, either by its interception of light, or by its colours, though its features and form may be indistinct. Such a degree of visual perception, though very imperfect in our esteem, may be of great use to this sluggish creature, and amply sufficient for its need.

The whole upper surface of this Starlet is covered with six-sided convex plates, each of which is crowned by a group of short blunt spines; the number in each group varying from one to nine; four to six, however, most commonly; arranged in a diverging tuft. Over the madreporiform tubercle, the tufts are stouter, and bend towards each other, as if to protect this delicate organ, which, whatever its function, is grooved with sinuous furrows, visible only with a considerable magnifying power, like the rounded masses of coral from the tropical seas, called brain-stones.

On the under surface similar stout short spines are arranged in transverse bands across each ray, interrupted, however, by the central furrow, which is perforated with two ranges of orifices to give issue to the sucker-feet, which thus form a double row. These organs do not differ importantly either in structure, function, or appearance, from those in the Cross-fishes, in the Sea-cucumbers, or in the Urchins.

In health and activity the whole upper surface is covered with a forest of short pellucid tubes, closed at the tips, which protrude from pores in the plates, and stand erect, moving, however, to and fro at the will of the animal; they are evidently filled with fluid. I cannot find any notice of these organs: they are probably connected with respiration.

SEA-URCHIN.

What a wonderful piece of mechanism is a Sea-Urchin! Accustomed as I am to the multitudinous contrivances and compensations that present themselves at every turn to the philosophic naturalist, often as surprising and unexpected as they are beautifully effective, I am yet struck with admiration at the structure of an Echinus whenever I examine it anew. A globular hollow box has to be made, of some three inches in diameter, the walls of which shall be scarcely thicker than a wafer, formed of unyielding limestone, yet fitted to hold the soft tender parts of an animal, which quite fill the concavity at all ages. But in infancy the animal (and, of course, its box, as this must be full) is not so big as a pea; and it has to grow till it attain its adult dimensions. The box is never to be cast off, and replaced by a new one; the same box must hold the infant and the veteran Urchin. The limestone, not being a living tissue, but an inert earth, can grow only by being deposited. Now the vascular tissues are within, and the particles they deposit must be on the interior walls. This would indeed augment the amount of limestone in the box, but it would be at the expense of the contained space. The thicker the walls, if thickened from within, the less room in the cavity; but what is wanted is more room, ever more, and more. The growing animal feels its tissues swelling day by day, by the assimilation of food, and its cry is, “Give me space! a larger house, or else I die.”

How is this problem solved? Ah! there is no difficulty. The inexhaustible wisdom of Jehovah the Creator has invented a beautiful contrivance for the emergency. The box is not made in one piece, nor in ten, nor in a hundred; six hundred distinct pieces go to make up the hollow case; all so accurately fitted together, that the perfect symmetry of the outline is not broken; and yet, thin as their substance is, they retain their relative position with unchanging exactness, and the slight brittle box possesses all requisite strength and firmness.

Each of these symmetrical pieces of shell is enveloped by a layer of living flesh, a vascular tissue of exceeding thinness, which passes up between the joints where one meets another, on every side, and not only so, but actually spreads itself over the whole exterior surface. So that when you take up an Urchin into your hand, and having rubbed a small space clear of spines, look on it; you have not, as you may suppose you have, exposed the surface of the shelly box, but only the flesh that covers it; yet this is so transparent and colourless, so inconceivably thin, so absolutely adherent at every point, that its presence will not be discernible to feeling or sight, without the aid of high microscopic powers.

This being so, the glands of the investing fleshy tissue secrete lime from the sea-water, which holds it in solution, and constantly deposits it, after a determinate and orderly pattern, on every part of the surface of each shelly piece; the inner face, the outer face, and each of the sides and angles of the polyhedron grow together, and all so evenly, that while the dimensions increase, both of thickness and superficies, the form characteristic of that individual piece is maintained with immutable mathematical precision. Thus the volume and capacity of the box grow with the growth of the individual segments, and it ever keeps the globose shape at first imposed upon it.

ITS MECHANISM.

But this is but a small part of the mechanism of this interesting tribe. If you put into a basin of sea-water one of the pretty kind which we find so abundantly under stones at low water,--whose green spines are tipped with rosy purple, like the tentacles of an Anthea,--you will presently observe it marching majestically along by means of the hundreds of sucker-feet, which it possesses in common with the Star-fish. Now, if you have in your cabinet the empty box of an Urchin of this same kind, and taking it in your hand, hold it up to the light, and look into the cavity from the under or mouth side, you will have a very interesting spectacle. The light streams in through a multitude of minute holes, as smooth and regular as if drilled with a fairy’s wimble; and these holes are arranged in a pattern of elegant symmetry. They run in lines, like meridians, from pole to pole of the flattened globe; but instead of being set at uniform intervals, they constitute five principal sets or bands, with blank intervals between, about twice as wide as the drilled bands. Then each band comprises two series, each of which contains a double row of orifices. These last, again, do not constitute a single unbroken line, but an interrupted or zig-zag line, which is, in fact, made up of a number of short diagonal rows; three holes in each diagonal, set one after another.

Put the living and the dead together. These tiny orifices, as minute as the point of the finest cambric-needle could make in a bit of paper, afford exit to the suckers, which are, of course, equally numerous. Through these pass the slender pellucid tubes, filled with elastic fluid, which carry at their tips a flat ring of calcareous shell, affording to each the form and firmness to make each one an adhesive sucking disk, in the centre of which a tiny vacuum is created at will by muscular retraction.

ITS SPINES.

But this is not all. Again, look at the living Sea-Urchin. It bristles with the rosy-tipped spines, which have a satiny lustre, owing to the reflection of the light from the delicate ridges and furrows with which the whole is fluted, like an Ionic column in miniature. How they are all moving, and swaying to and fro on their bases, quite independently of each other, however, making circles and traverses in the water with their points, as the mast-heads of a ship do among the clouds in a gentle swell, when seen from the deck. Professor Agassiz fell into the egregious blunder of supposing that the spines were the organs of locomotion in the Urchins, denying, with much contempt, the theory which attributed this office to the suckers. One can only wonder whether he ever saw a living Urchin in motion, as one moment’s glance at the phenomenon is sufficient to prove how utterly his theory was false, as Forbes has well shown.

However, let us once more observe the empty denuded box from the cabinet; and now on the exterior. Between the principal bands of pores (called ambulacra, from their fancied resemblance to walks in a garden), as also between the two series which constitute each band, the space is studded with hemispherical warts, of very diverse size, which look as if turned in ivory, unpolished; and each wart is crowned with a smaller wart of like material, but bearing the most perfect polish.

At the bottom of each spine there is a cavity exactly fitting this second wart, and equally highly-polished in its interior. During life the spine was seated on the wart, not united to it, but moving upon it in all directions, with perfect smoothness and freedom; a ball and socket joint, in fact. It was held in its place by an investment of muscle, which completely enclosed both the wart and the base of the spine, having one insertion in the unpolished wart, and the other in a remarkable ring or shoulder of the spine, visible just above the socket.

These are but two or three salient points of interest in the structure of this little unvalued, disregarded creature. I could relate much more; indeed, I think it would not be difficult to write a bulky volume of the history and biography of a single Sea-Urchin, of which every page would display the glory of God. But I have not space for that here.

We sometimes, but very rarely, find on this coast a very lovely form of this class of animals, the Rosy Feather-star. It consists of ten long attenuated arms, radiating from a common centre, composed each of about forty slender joints of stone, and each joint carrying a pair of diverging beards, also many-jointed, all of which together, by their number and arrangement, give to the arm the aspect of a beautiful feather. Around the central point of radiation, a small cup-like body gives origin to the arms, which are double, a pair springing from a single basal joint. Within the cup the soft parts of the animal are chiefly located, the organs of the vital functions; and from the convex surface spring a number of jointed stony threads, like necklaces, much shorter and slenderer than the arms, which serve as claspers, gripping and holding firmly the projections of the rock, by means of strong curved claws with which they are terminated, in shape like those of an eagle.

The whole elegant creature is of a lively rose tint, interrupted by patches of bright yellow, disposed with no regularity or apparent order; the whole, both the yellow and the rosy portions, studded with crimson dots. Edward Forbes, if I rightly understand him, considers these dots to be ovaries, which he estimates at upwards of 57,000 in number.

INFANT FEATHER-STAR.

In infancy the Feather-star is seated at the extremity of a long slender jointed stalk, attached at its lower end, whence it rises erect, like a plant. Indeed the whole animal, in this condition, with its cup-like base and elegantly incurving arms, seated on its tall stem, has so close a resemblance of outline to a flower, that the fossil specimens, which are very numerous, and of large size, are known as Lily-stones, and technically as Encrinites, which word has the same allusion. After a while, the radiating portion, or flower, separates from the stalk, and swims freely, contracting its arms to give the impulse, in the manner of a Medusa.

Who, on looking at these two creatures side by side,--the Sea-Urchin and the Encrinite,--would imagine that they possessed any close natural relationship, or would suspect that they could have been framed on the same model? Yet it is really so; there is a common plan of structure in both; pervading, too, many intermediate forms, which at first sight would seem to manifest as little resemblance to the one as to the other. It would, in fact, be easy to select from any well-furnished museum a continuous chain of specimens, whose links approach each other so closely as to form an unbroken series from the Urchin to the Feather-star.

CONNECTING LINKS.

Among the Urchins proper there are some species, such as the Sphere Sea-egg, and the one known as Fleming’s, which have a figure not far from that of a globe. Others are much more depressed, of which the little Purple-tipped is a notable example. Still the spherical shape is conspicuous. From this rounded form other species, more and more flattened, gradually lead to the Scutella, which takes the form of a thin round plate, quite flat beneath, but slightly convex on the upper surface. The structure is the same as before, but the spines appear to the naked eye only as very minute hairs; but, when magnified, are found to be of the most elaborate workmanship, each having a movable socket-joint. In the genus Clypeaster, the round outline is changed for a five-sided figure, the angles of which in succeeding species project more and more, and the spaces between become more and more indented, till we arrive at the Starlets, and at length to the Cross-fishes (Asteriadæ). The rays gradually becoming longer and more slender, we are brought to those in which they are so lengthened as to resemble the tails of so many serpents, whence they are named Ophiura. In succeeding genera, such as that called Medusa’s head (Gorgonocephalus), the central part is still further diminished, and the rays are divided into branches of great length and number. Each ray, soon after its commencement, separates into two more; these again into two others, and so on to an astonishing extent. Upwards of 2500 ramifications have been counted on a single specimen, presenting a living net, by the contraction of which any small animal once touched would inevitably be detained. The sucker-feet are no longer found, these animals changing their position by dragging themselves along by their flexible arms. Finally, we have the Feather-stars, which, as we have seen, in their infant condition, and the Lily-stars, which throughout life (as the abundant fossil species in our own land, and that noble one which still exists in the West Indian seas), consist of slender-jointed arms, with feather-like filaments, seated at the free extremity of a tall jointed stalk, also furnished with whorls of filaments, which is fixed by its base to the solid rock.

Altogether, the series presents us with one of the most instructive and most marvellous examples of the vast variety of external form and internal structure which may be assumed by almost insensible modifications of one plan of organization; and so of the unfathomable resources of wisdom in the ever-blessed God. For every one of these links,--and there are multitudes which I have not named, found either in remote seas or in a fossil condition, that fill up the gaps with close gradations--displays an essentially common model; and the least fragment of the stony skeleton of any one would be sufficient to enable a competent naturalist to decide authoritatively, the instant he looked at it beneath his microscope, that it belonged to the great class ECHINODERMATA. The calcareous shell of which the framework is composed,--a glass made of lime,--is deposited in a fashion which, while common to all, is found nowhere else throughout the whole animal kingdom.