"SEA URCHINS" "The common sea urchin digs a niche in soft rock... or lives freely on the ocean floor." "Varied in color and almost spherical, they measure from 1,5 to 4 inches in diameter." "The shell bristles with spines an inch long." "At a certain magnification, they resemble columns of a temple." "Placed on its back the sea urchin will turn over to press its ventral side against a hard surface." "At the center of its ventral side is its mouth, five teeth allow it to dig out, crush, loosen and absorb all sorts of debris pulled from the rocks." "The complex framework that maintains and operates the teeth fills almost the entire interior cavity in some sea urchins." "Powerful muscles allow these teeth to snap straight through a spine, for example." "lnjecting this sea urchin with gelatin and removing its shell reveals its internal organs." "At the top are the five reproductive glands." "The coiled digestive tube runs from the mouth located at the base," "to turn over or move the sea urchin does not use its stingers but hydraulic feet- hundreds of hollow elastic stems that end in suckers." "Each stem is linked beneath the shell to a muscular pouch filled with water." "All these pouches adjoin each other like pages in a book." "When a pouch contracts, it sends water out through the stem." "The stem, which passes through a hole in the shell, elongates and moves its suction cup forward." "The suckers have powerful muscles, ensuring a solid grip." "The stem extends to almost 100 times its normal length - longer than any of the spines." "When a sucker has attached itself to something, the water in the stem recedes into the pouch." "The stem then shortens and pulls the sea urchin toward the point where it's attached." "In this way, the sea urchin moves about, crawls, and climbs." "It's clear that the movement of the spines has nothing to do with this locomotion." "Here we get an idea of the adhesive power of the suction cup." "The stem may break, but the suction cup will not budge unless the sea urchin itself detaches it." "Amid the forest of spines stir delicate crystalline forms." "These are microscopic organs formed by the sea urchin itself." "Like humans produce nails, they're called pedicellariae, calcareous stems from 1-9 mm that end in three jaws." "The smallest have jaws in the shape of clover leaves and seem to clean the sea urchin's surface and spines." "The jaws are always moving, independently of each other, constantly stirring up the water." "Let's take a closer look." "Others look like a long beak a top a long neck." "The jaws, connected only at the base, cannot seize or hold on to anything." "Perhaps they serve as sensory organs, informing the sea urchin of what's happening on its surface." "There are many variations in form." "Here's one with very short jaws but a very long stem." "Even more rare is this variety with four jaws." "The protective pedicellariae have rigid stems, but the jaws can all move Ìn different directions." "Each jaw has a venomous gland and sharp injector teeth from 1/300 mm to 1/500 mm in length." "When the jaws close, the teeth hook together in a very effective spherical triangle." "In some pedicellariae, the stems too have venomous glands," "Lit from behind, the jaws call to mind a stained-glass window," "Pedicellariae around the mouth may help carry debris to or from the mouth." "Their stems are flexible along almost their entire length." "Their jaws, sturdy and strong yet craned like the finest lace, look like serpents' heads." "They too have many different forms." "The pedicellariae and spines, and the entire sea urchin are covered with billions of vibrating cilia .001 mm long whose incessant motion stirs up constant eddies, thus ensuring that the water around and inside the sea urchin" "is constantly and vigorously renewed." "A vital requirement for the sea urchin's life."