The star-fish's mouth is on its lower side, and almost directly abuts on its stomach. It is a voracious feeder, and will even attack molluscs. Formerly it was believed that the animal would open an oyster with one of its rays, or legs, but this was unlikely, as the oyster might be likely enough to have the best of it in such a case by shutting his shells on the intruder. It is now pretty well understood that it injects an acrid poison into the oyster's shell, which obliges it to open.
The "Urchins" seem to owe their name to Aristotle, and their spiny covering and armature have in all ages attracted the attention of naturalists. Some of them have 3,000 or 4,000 prickles, and their organisation is really wonderful. They are enclosed in a globular hollow box, which grows with their growth. Gosse explains how. The box can never be cast off, and it is obvious that the deposits made from inside would only narrow the space, which really requires to be enlarged. "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 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." Their spines are instruments of defence and of locomotion; each has several muscles to work it.
The poet-scientist, Michelet, has beautifully painted the animal's nature, and makes it describe itself as follows. "I am born," says the unobtrusive Echinoderm, "without ambition; 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 thickset, strong, and round, for that is the shape in which I should be the least exposed; in short, to be a centralised 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 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."(34)