Sea-Urchins are frequently taken in dredging. Several common species, usually of a small size, are often found among the rocks situated between tide marks.

Into the aquarium no specimens larger than from one to two inches in diameter should be introduced, and even these require to be closely watched, for if afflicted with a fatal illness, I know of no animal whose remains sooner taint the water. Almost before life is extinct, the Urchin throws out a light-coloured nauseous fluid, that speedily poisons the surrounding water, and, of course, causes the destruction of any inhabitants of the tank who may neither have the sense nor opportunity to inhale copious draughts of fresh air. As a rule, if the suckers are motionless, or if on touching the animal it is found not to be adherent to any object, transfer it at once to your 'infirmary' for further observation.

I have always found small specimens to be much more lively and walkative, (if I may be allowed the expression) than their more corpulent brethren.

The inflexible, mail-like crust, or shell, as it is commonly called, of the Echinus is perhaps one of the most marvellous objects on which the eye can rest. Although at first sight it appears to be a solid calcareous box, it is in reality composed of several hundred pentagonal plates,[17] of various sizes, so closely dove-tailed together that their marks of junction are scarcely perceptible. Upon a superficial examination we are apt (most erroneously) to consider this wonderful piece of work to be more elaborate than the wants of the animal demand. The fact of the Lobster or Crab throwing off its entire shell at certain seasons, to admit of the increased growth of the animal is a truly marvellous phenomenon, still, it would more excite our wonder were we to find that, instead of being cast away at all, the hard, inelastic envelope which surrounds the bodies of crustaceans was made to swell or expand proportionately with the soft parts of the animal! Now, the mosaic-like shell of the Sea-Urchin, though built up, as before stated, of several hundred pieces, is by a beautiful process slowly and imperceptibly enlarged correspondingly with the growth of the animal.

The gradual enlargement of the Echinus shell takes place in the following manner:—

Over the entire surface of the globular shell, spines, and joints of the living Urchin, there exists a delicate membrane that insinuates itself between the pentagonal plates above mentioned, and continually deposits around the edges a certain portion of calcareous matter (carbonate of lime). The same process being also carried on by the fleshy covering that surrounds the spines, &c., it must be evident that so long as the vital power of the animal exists, each plate and spine, still keeping to its original form, must be daily and hourly augmented in size until the Sea-Egg has attained its full and mature dimensions.

As to how the spines retain their relative position in each plate, as the latter gradually becomes enlarged, I cannot positively state; but may be permitted to mention, that, judging from carefully prepared sections of the plates when submitted to the microscope, each spine appeared to my eye to be by some singular process urged along in a kind of groove to its proper place.

The hedgehog-like spines that surround the globose body of the Sea-Urchin are all moveable at the will of the animal,—each prickle being connected by a ball-and-socket joint to a pearly tubercle, which acts as the 'socket' on which the 'ball' of the spine revolves. If the spine be removed, a comparatively smooth surface will be left, on which are various sized tubercles systematically arranged. Situated at regular intervals between the tubercles are ten broad bands, disposed in pairs, and containing many hundreds of very minute perforations, or ambulacral orifices, as they are generally termed by naturalists.

Through these apertures issue numerous sucker-like feet, closely resembling those of the Star-fish, but endowed with far greater powers of contraction and extension.

The number of suckers is very great. In an Urchin measuring exactly three inches in diameter, by aid of a hand lens, I counted no less than 3300 pores in the ten avenues. Now, these pores are always situated in pairs, and as each sucker occupies a pair of pores, it will give 1650 as the total amount of suckers.

There is no doubt that it is almost entirely by means of these curious organs that the Sea-Urchin is enabled to move about from place to place, although no less an authority than Professor Agassiz asserts to the contrary. 'How, in fact,' says this author, 'could these small tentacula, situated as they generally are in that part of the body which is never brought into contact with the ground when the animal moves, and overhung by calcareous solid spines—how, I ask, could these flexible tubes be used as organs of motion? It is an undeniable fact, and I have often observed it myself, that it is with their spines the Echini move themselves, seize their prey, and bring it to their mouths by turning the rays of their lower edge in different directions. But the correction of an error respecting the functions of the ambulacral tubes does not solve the problem relating to their nature and use. This problem we are yet unable to solve, as we know nothing more respecting them than that they are connected with the aquiferous system.'

Many other writers, among whom is Professor Forbes (from whose work on Star-fishes I have transferred the foregoing extract), assert, in opposition to the great Swiss naturalist, that the EchinidÆ move by the joint action of their suckers and spines. 'The argument,' says the great British naturalist, 'against the suckers being organs of motion, founded on their position above as well as below, would equally apply to the spines, to which organs Professor Agassiz has attributed all progressive powers in these animals.'

The fact is now so well established, that it is scarcely necessary for the writer to state, that from personal observation he can fully confirm the evidence of Professor Forbes relative to the functions of the suckers of the Sea-Urchins. But although that talented author entertained no doubt as to the organs in question being powerful locomotive agents, he evidently seems to have felt himself unable to suggest any purpose they could possibly serve when situated on the back or upper part of the animal.

My own experience incontestibly proves that the suckers in question are used for precisely the same purpose as those situated in any other part of the body. I am enabled to state, from having repeatedly witnessed the phenomenon, that the Echinus can walk about with equal facility while lying on its back as in its more natural position. The advantage of this power to the animal under certain circumstances will be apparent upon a little reflection.

With regard to the spines, I fancy their purpose is almost solely to assist the Urchin to burrow in the sand, and to protect it from the attacks of its enemies. It may be, however, that at particular times they serve as aids to locomotion, but that their assistance can be, and is, often dispensed with entirely by the animal, I can most positively assert.

My experiments were always conducted in glass vases, up the smooth, polished sides of which my specimens frequently advanced. Upon reaching the surface of the water, I have seen an Urchin roll completely round and move along on its back, then after a time change its position, and travel round the circumference of the vessel while attached by its side, the body of the animal being sometimes inverted.

At such times as these it must be quite evident that the spines would be totally useless, and that by the suckers alone did the animal perform its interesting movements.

According to a certain writer, there are some foreign species of the Echini remarkable for possessing spines, which act both as offensive and defensive weapons. 'On one occasion' (this writer says) 'when searching for a fish in the crevice of a coral rock, I felt a severe pain in my hand, and upon withdrawing it, found my fingers covered with slender spines, evidently those of the Echinus, of a grey colour, elegantly banded with black.

'They projected from my fingers like well-planted arrows from a target, and their points being barbed could not be removed, but remained for some weeks imbedded as black specks in the skin. Its concealed situation did not permit me to examine this particular Echinus. In some experiments I approached the spines with so much caution, that had they been the most finely pointed needles in a fixed state no injury could have been received from them, yet their points were always stuck into my hand rapidly and severely.'

In addition to those above described, the Sea-Urchin is provided with other organs, in shape somewhat resembling minute pincers, supported on fleshy stems, which always keep up an incessant motion when the animal is in a healthy condition. They are scattered in great numbers over the surface of the body, among the spines, and around the mouth of the Urchin.

The use of these singular objects—by naturalists termed PedicellariÆ—is totally unknown. Some writers think they are an integral part of the Echinus, others describe them as distinct and parasitic animals. There is good reason to believe that the former will eventually be proved to be the correct explanation of the matter.

Its masticatory apparatus is not the least wonderful portion of the Sea-Urchin. The teeth, five in number, which may frequently be seen protruding from the mouth, are of extreme hardness, and of seemingly disproportionate length. They are not fixed in sockets as ours are, or they would be speedily worn away by their action on the shelled mollusca upon which the animal feeds, but fresh substance is added to each tooth as fast as it is worn away by use, as in the case of many gnawing animals. 'In order to allow of such an arrangement, as well as to provide for the movements of the teeth, jaws are provided, which are situated in the interior of the shell, and these jaws, from their great complexity and unique structure, form perhaps the most admirable masticating instrument met with in the animal kingdom. The entire apparatus removed from the shell consists of the following parts. There are five long teeth, each of which is enclosed in a triangular bony piece, that for the sake of brevity we will call jaws. The five jaws are united together by various muscles, so as to form a pentagonal pyramid, having its apex in contact with the oval orifice of the shell, while its base is connected with several bony levers by means of numerous muscles provided for the movements of the whole. When the five jaws are fixed together in their natural position, they form a five-sided conical mass, aptly enough compared by Aristotle to a lantern, and not unfrequently described by modern writers under the name of "the lantern of Aristotle." The whole of this complicated machinery is suspended by muscles from a frame-work fixed in the interior of the shell, and may often be picked up upon the beach, or still better exposed in situ in a dead Echinus, by those who would examine closely this wonderful piece of mechanism.'[18]

I have made two careful drawings of the jaws and teeth of the Echinus. No. 1 represents, as it were, the 'elevation' of the pentagonal pyramid above described, while No. 2 constitutes the 'plan' of the same object.

The Echinus sphÆra, or common Egg-Urchin, may often be seen forming a curious ornament in the drawing-rooms of the "West End," and also in the dwellings of the poorer classes, who, according to some authors, boil it like eggs, and so eat it. Hence its popular title. Among the ancients the EchinidÆ were accounted a favourite dish. 'They were dressed with vinegar, honied wine or mead, parsley and mint. They were the first dish in the famous supper of Lentulus when he was made Flamen Martialis. By some of the concomitant dishes they seemed designed as a whet for the second course to the holy personages, priests and vestals, invited on the occasion.'

The illustration on Plate 10 was drawn from a living specimen, and gives a somewhat unusual representation of a Sea-Urchin. In general the spines alone are shown, but I have endeavoured to give the uninitiated reader some faint notion of the appearance which the suckers present when extended from the surface of the shell.

The young Urchin sat very quietly while I was engaged in taking his portrait, but continually extended crowds of his slender tubular legs in all directions, as above indicated, much to my gratification and apparently to his own.

In preparing a Sea-Urchin for a chimney ornament, the most important point is to remove the spines so as to let the tubercles remain entire. In performing this operation some little experience is necessary. Several times I attempted the process by aid of a pen-knife and a pair of pliers, but not with a satisfactory result. Having mentioned my difficulty to a friend, he laughingly asked me if I had ever heard of a certain pilgrim who, for some peccadillo he had committed, was doomed to perform penance by walking to Loretto's shrine with peas in his shoes? Of course I was acquainted with the story, but could not see what it had to do with Sea-Urchins, and told my brother naturalist so. Still smiling, he said, 'Do you remember the relief that was said to be afforded to the humorous rascal, both mentally and bodily, by boiling his peas?' Yes. 'Well, then,' was the reply, 'do you boil your Sea-Eggs, and you will find your troubles speedily cease.' I did as I was directed, and found the advice of great service; for, after being an hour or two in the 'pot,' the spines of the Urchin may be totally rubbed off by aid of a nail brush, or some such instrument. Moreover, the colour of the shell is improved, and the dental apparatus may be drawn out entire, with the greatest ease.

I may here take opportunity to mention, that the student who may think proper to act upon the hint above given, should not boil the Urchin too long, or the fleshy parts will become dissolved, and the entire shell fall into a multitude of fragments.

This unfortunate result actually happened on one occasion to a genial, clever friend of mine, much to his chagrin and my malicious delight.

There are several other species of Sea-Urchins whose forms are tolerably well distinguished by their popular appellations. Thus we have the 'Silky Spined Urchin;' the 'Green Pea-Urchin,'—the latter is the commonest and prettiest of all its kindred, its back being covered with a kind of powdery green, as is seen on the elytra of many beetles; the 'Cake-Urchin,' which from its flattened form may be regarded as a link between the Sea-Urchins and the true Star-fishes; the 'Purple Heart-Urchin,' and the pretty 'Rosy Heart-Urchin,' appropriately named from the brilliant crimson hue that its body presents during life.