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