The visitor to the sea-side must frequently in his rambles along the beach have picked up specimens of the curious animals which are popularly known as 'Sea-eggs' and 'Sea-urchins.' The former name is applied to these creatures when they are found cast upon the shore and present the appearance of rounded or ball-shaped objects, each inclosed within a hard but brittle limy shell. Whilst the term 'urchin' is given to the same objects when they are seen in their more natural and perfect state, and when the outside of the shell literally bristles with spines. The name 'urchin,' in fact originally applied to the hedgehog, has been extended to denominate the sea-eggs, from their presenting the spiny appearance so familiarly seen in the common tenant of our woods and hedgerows. Thus the sea-egg is the sea-urchin with its spines detached and rubbed off by the unkindly force of the waves; and the animal thus popularly designated is the Echinus of the zoologist, and belongs to the large class of animals of which the Star-fishes are well-known representatives.

The entire history of the sea-egg is of so curious a nature that the most casual reader may well feel interested in the account of the animal's present and past life; whilst the feeling of mere curiosity to know something of one of the most 'common objects of the shore,' should prompt every sea-side visitor to make the closer acquaintance of the Echinus.

Suppose that we begin our examination by looking at the hard case or 'shell' in which the soft parts of the animal are inclosed. We find on referring to the development of the animal, that this 'shell' actually represents the hardened skin of the animal, and that viewed in this light, it closely corresponds to the shell of the lobster or crab. The shell is flattened at each pole, and we can readily perceive that it is composed of rows of little limy plates, which are disposed in a regular manner from pole to pole, or after the fashion of the meridian lines on a globe. Counting the series of plates, we find the shell to be composed of twenty rows; but we may also perceive a difference between certain of the plates of which the rows are composed. Thus we find two adjoining rows of plates, which are perforated with holes. The next two rows are not so perforated; whilst the third two rows possess holes like the first rows. We may, in fact, proceed round the shell, and come back to the point at which our examination began, with the result of finding that we may group the whole of the twenty rows of plates of this curious limy box into two sets—those with holes and those without; and we may further discover that there are five double rows of perforated plates, and that these alternate with other five double rows which do not possess holes.

Each little plate of the sea-egg's shell may be most accurately described as being hexagonal or six-sided in form; but this shape may be more or less modified in certain regions of the shell. The five double rows of the shell which are perforated with holes, it may be remarked, are those through the apertures in which the small 'tube-feet' of the animal are protruded. And it may also be noted that in some of the sea-eggs these perforated rows do not extend from pole to pole of the shell, as in the common species, but are limited so as to form a rosette-like figure, on the upper surface or at the upper pole of the shell. This modification is well seen in a group of sea-eggs, not uncommon round our coasts, and which are popularly named 'Heart-urchins' from their peculiar shape.

The outside of the shell presents us with some curious features; the zoologist's study leading him thus to carefully note points which an ordinary observer would hardly deem worthy his attention. When we examine the outer surface of the shell, we find it to be thickly studded over with little rounded knobs or 'tubercles,' which are, if anything, most numerous on those parts or rows of plates which are not perforated. And if we carefully study one of the spines we shall find that it is hollowed out or is concave at its base. Clearly then, the spines are meant to articulate by means of these hollowed or cup-shaped bases with the rounded knobs on the outside of the shell, and in each case a true ball-and-socket joint is thus formed. The spines are thus intended to be moved, and they are not only firmly attached by a ligament or band of fibres to the surfaces of their tubercles, but appear to be moved by special muscles, which form a thin investing layer on the outer surface of the shell. The spines undoubtedly serve as organs of defence, but in some species they are employed as boring-organs to scoop out holes in the sand or shallow beds in rocks, in which their possessors lie snugly ensconced.

The outer surface of the shell also bears certain very peculiar appendages, known as 'PedicellariÆ.' These little organisms also occur on the outer surface of Star-fishes and other members of the sea-egg's class; but regarding their exact nature and functions, zoologists are still in doubt. The form of one of these pedicellarians may be best imagined by figuring to one's self a small or minute stalk attached to the shell, and bearing at its free extremity two or three little jaws, which move actively upon one another, with a quick snapping motion. These little jaws can be seen to seize particles of food, and there is no doubt whatever that they possess a life and vitality independently of the sea-egg or other organism upon which they reside; since their movements are seen to continue after the death of the animal which affords them lodgment. Some naturalists have regarded them as 'peculiarly modified spines;' but the reasons or grounds for this belief are anything but clear, since it is difficult to imagine any reasonable explanation of the means whereby a spine could acquire an active living and independent nature. By good authorities, who have not ventured to theorise so boldly, the pedicellariÆ have been regarded as parasites of some kind or other; and they may also possibly represent stages in the as yet unknown development of some organisms. Whilst, assuming them to be fully-grown beings, their function, as they exist on the shell of our sea-egg, has been supposed to be that of seizing particles of food, and of removing waste or effete matters.

The internal structure of the sea-egg shews its near relationship with the Star-fishes and Sea-cucumbers. The mouth is the large orifice opening at the lower pole of the shell; so that as our sea-egg crawls slowly and mouth downwards over the bed of the sea, or over the floor of its native pools, it can procure food without any very great trouble as regards its conveyance to the mouth. The internal furnishings of the body include a stomach and complete digestive system, along with a very peculiar set of jaws or teeth, lying just within the mouth, the points or tips of the jaws being usually protruded from the mouth-opening. This arrangement of teeth is named the 'Lantern of Aristotle,' and comprises five conical pieces, so arranged together and so provided with muscles, as to be perfectly adapted for bruising the sea-weeds and other forms of nutriment on which the sea-eggs subsist. Their near neighbours the Star-fishes do not possess any teeth; although curiously enough, the unarmed sea-stars prefer a richer dietary than that which contents their sea-egg neighbours, since they devour large quantities of oysters and other molluscs. Our sea-egg possesses a heart for circulating its blood, in the form of a simple tube; and although no distinct breathing-organs are developed, naturalists believe that the blood may be purified by being circulated through a delicate membrane which is named the 'mesentery,' and which serves to suspend and support the digestive organs to the wall of the shell. The fact that this membrane is richly provided with the delicate vibratile filaments known as 'cilia,' and that it is bathed in the sea-water—necessarily containing oxygen—and which is admitted within the shell, would seem to favour the idea that it constitutes the breathing-organ of these animals.

The sea-egg is not destitute of means for obtaining some degree of knowledge regarding its surroundings; and it obtains its quantum of information through the same channel by which man is brought into relation with the world in which he lives—namely the nervous system. The sea-urchin possesses no structure corresponding to a brain—indeed in all animals of its nature, the nervous system exists in a comparatively low and unspecialised condition. We do not find, in other words, that development and concentration of the parts of the nervous system seen in the highest groups of animals, and which enables these latter to form definite ideas regarding their surroundings and respecting the world at large. A cord of nervous matter surrounds the gullet of the sea-egg, and from this central portion five great nerves are given off; one nerve-trunk passing along the inner surface of each of the perforated double rows of plates of the shell, to terminate at the upper pole of the body. The only organs of sense developed in the sea-eggs appear to consist of five little 'eyes' of rudimentary nature, each consisting of a little spot of colouring matter and a lens. These eyes are situated on five special plates of the shell, developed at the upper pole or extremity of that structure. We thus remark that the parts of the nervous system, along with other portions of the sea-egg's structure, are developed in a kind of five-membered symmetry—if we may so express it. And it is a singular fact that not only throughout the sea-egg's class do we find the number five to represent the typical arrangement of parts and organs—as is well exemplified in the five rays of the common star-fish—but we also discover that this number is one exceedingly common in the symmetry of flowers. This fact apparently struck an old writer—Sir Thomas Browne—as being a curious and noteworthy feature of the Star-fishes and their allies, since we find him inquiring 'Why, among Sea-stars, Nature chiefly delighteth in five points?'—although to this suggestive query, the learned and eccentric author of the Religio Medici gives no exact or satisfactory reply.

The movements of our sea-egg are effected by means of an apparatus, which forms one of the most noteworthy parts of its structure. If a star-fish be dropped into a rock-pool, it may be seen to glide slowly but easily over the bottom of the miniature sea in which we have placed it. When we examine the lower surface of this animal's body, we at once perceive the means whereby its movements are performed; for existing in hundreds, in the deep groove which runs along the under surface of each ray, we see the little tube-feet or ambulacra, each consisting of a little muscular tube, terminated in a sucker-like tip. By means of an apparatus of essentially similar kind, the sea-egg is enabled to crawl slowly over the floor of the sea. The tube-feet existing to the number of many hundreds in the sea-egg, are protruded, as has already been remarked, through the holes existing in each of the five double rows of perforated plates of the shell. The mechanism of their protrusion depends on the presence of a special system of vessels, known as the 'ambulacral' vessels, which carry water to the little feet, for the purpose of their inflation and distension.

Thus on the upper surface of the shell we find a single large plate perforated with holes like the lid of a pepper-box. This plate opens into a long tube called the 'sand-canal'—a name which is decidedly a misnomer, since the function of the plate resembling the pepper-box lid is to allow water to enter this tube, but at the same time to exclude particles of sand and like matters. The sand-canal terminates in a circular vessel, which, like the nerve-cord, surrounds the gullet; and from this central ring a great vessel, like a main water-pipe, runs up each of the five rows of perforated plates in company with the nerve-cord. At the base of each little tube-foot is a little muscular sac or bag, and into these sacs the water admitted by the sand-canal ultimately passes. When therefore the sea-egg wishes to distend its feet for the purpose of protruding them through the shell-pores, and of thus walking by applying their sucker-like tips to fixed objects, the water in the little sacs is forced into the feet, which are thus distended. Whilst conversely, when the feet are to be withdrawn, the water is forced back, by the contraction of the feet into the sacs, or may be allowed to escape from the perforated tips of the feet, so as to admit of a fresh supply being brought in from the interior.

The development of the sea-egg may be briefly glanced at by way of conclusion, along with a few points in its economic history. The animal, solid as it appears in its adult state, is developed from a small egg, which gives origin to a little body, usually named the 'larva,' but which, from its resemblance in form to a painter's easel, has received the name of Pluteus. This little body does not in the least resemble the sea-egg; possesses a mouth and digestive system of its own, and swims freely through the sea. Sooner or later, however, a second body begins to be formed within and at the expense of this Pluteus-larva; whilst as development proceeds and ends, the sea-egg appears as the result of this secondary development, and the now useless remainder of the first-formed being is cast off and simply perishes. Thus the development of the sea-egg is by no means the least curious part of the animal's history, and presents a singular resemblance to the production of the Star-fishes and their neighbours.

The mere mention of the economic or rather gastronomic relations of the sea-eggs may appropriately form a concluding remark to our gossiping remarks concerning these animals. With our British prejudices in favour of eating only what our forefathers were accustomed to consider wholesome, it is not likely that the sea-eggs will appeal with success to be included as culinary dainties. Yet on the continent these animals are much esteemed as articles of dietary and even of luxury. The Corsicans and Algerians eat one species, whilst the Neapolitans relish another kind; and in classic times, when variety rather than quantity or quality was the chief feature of high-class entertainments, the Echini were esteemed morsels at the tables of the Greeks and Romans. Here then is an opportunity for another Soyer to tempt the modern cultivated appetite with a new and wholesome dish. Considering that crabs and lobsters are so highly esteemed, the sea-eggs but wait a suitable introduction to become, it may be, the favourite tit-bits of future generations.

A wise philosopher—the great Newton himself—remarked concerning the limitation of our knowledge, that we were but as children, picking up at most a few stray grains of sand on the sea-shore, whilst around us lies the great region of the unknown. Our present study may not inaptly be related to Newton's comparison, since it serves to shew that even the brief and imperfect history of a stray shell picked up on the sea-beach may teem with features so curious and with problems so deep, that the furthest science may be unequal to the explanation of the one or the elucidation of the other. Whilst the subject no less powerfully pleads for the wider extension of the knowledge of this world and its living tenants—knowledge which in every aspect reveals things which are not only wondrously grand, but also 'fair to see.'