"I am born without ambition," says the modest Echinoderm. "I ask for none of the brilliant gifts possessed by those gentlemen the molluscs. I would neither make mother-of-pearl nor pearls; I have no wish for brilliant colours, a luxury which would point me out; still less do I desire the grace of your giddy Medusas, the waving charm of whose flaming locks attracts observation and exposes one to shipwreck. Oh mother! I wish for one thing only: to be—to be without these exterior and compromising appendages; to be thick-set, strong, and round, for that is the shape in which I should be the least exposed; in short, to be a centralized being. I have very little instinct for travel. To roll sometimes from the surface to the bottom of the sea is enough of travel for me. Glued firmly to my rock, I could there solve the problem, the solution of which your future favourite, man, seeks for in vain—that of safety. To strictly exclude enemies and admit all friends, especially water, air, and light, would, I know, cost me some labour and constant effort. Covered with movable spines, enemies will avoid me. Now, bristling like a bear, they call me an urchin."

Fig. 112. Echinus mamillatus (Lamarck), natural size.

Let us now look a little more closely at the general structure of the sea-urchins—in zoological language, Echinidæ.

Fig. 113. Echinus mamillatus. Sea Urchin, without spines, natural size.

The body of the sea-urchin is globular in form, slightly egg-shaped, or of a disk slightly swollen. It consists essentially of an exterior shell or solid carapace, clothed in a slight membrane furnished with vibratile cilia. This carapace is formed of an assemblage of contiguous polygonal plates, adhering together by their edges. Their arrangement is such that the test or shell may be divided into vertical zones, each springing from a central point on the summit terminating at a point of the spheroid diametrically opposite—namely, the circumference of the buccal orifice. These vertical zones are of two kinds, some larger and others straighter, each zone consisting of a double row of plates, the first charged with movable spines, the second pierced with holes disposed in regular longitudinal series, from which emerge certain fleshy tentacula, which, as we shall see presently, serve as feet to the animal. When armed with these bristling spines, the sea-urchins resemble the hedgehogs; but when the spines are down, they look very much like a melon or an egg, to which their shape and calcareous nature have sometimes led to their being compared by the vulgar as well as by the learned. We shall give a tolerably exact idea of the two different aspects which the carapace of the urchin presents when the spines are erect and lowered, by reference to Fig. 112 (Echinus mamillatus), which represents the animal bristling with spines, and Fig. 113, in which the same species is represented after death, when deprived of these weapons of defence: and how complicated these defences must be! It has been calculated that more than ten thousand pieces, each admirably arranged and united, enter into the composition of the shell of the sea-urchin, to which no other can be compared. To abbreviate slightly Gosse's description of that wonderful piece of mechanism, the sea-urchin: "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 fills the cavity at all ages. But in infancy the animal is not so big as a pea, and it has to attain its adult dimensions. The box is never to be cast off or renewed; the same box must hold the infant and veteran urchin. The limestone can only increase in size by being deposited. Now the vascular tissues are within, and the particles they deposit must be on the interior walls. To thicken the walls from within leaves 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. Its cry is, 'Give me space! a larger house, or I die!' How is this problem solved? Ah! there is no difficulty. The inexhaustible wisdom of the Creator has a beautiful contrivance for the emergency. The box is not made in one piece, nor in ten, nor a hundred. Six hundred distinct pieces go to make up the hollow case; all accurately fitted together, so that the perfect symmetry of the outline remains unbroken; and yet, thin as their substance is, they retain their relative positions with unchanging exactness, and the slight brittle box retains all requisite strength and firmness, for each of these pieces is enveloped by a layer of living flesh; a vascular tissue passes up between the joints, where one meets another, and spreads itself over the whole exterior surface."

This being so, the glands of the investing tissue secrete lime from the sea water, and deposit it after a determinate and orderly pattern on every part of the surface. Thus the inner face, the outer face, and each side and angle of polyhedron, grow together, and the form characteristic of the individual is maintained with immutable mathematical precision. The dimensions and shape of these prickles are very variable. In certain Echinidæ they are three or four times the diameter of the body. In the urchin, properly so called, they are only three-fourths or four-fifths that diameter. They sometimes resemble short bristles. These defensive weapons have tubercles for supports, which are arranged on the surface of the animal with perfect regularity. At the base they present a small head separated by compression. This head is hollow on its lower face, presenting a cavity adapted to a tubercle of the shell. Each of the prickles, notwithstanding its extreme minuteness, is put in action by a muscular apparatus.

Fig. 114. Echinus esculentus (Lamarck), natural size.