"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." [Illustration: 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. [Illustration: 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