Sea Urchins

A close-up look at sand urchins and rock urchins. At the seashore, a man digs up a sand urchin. We look closely. He sets it back in the sand, and it burrows out of sight. Its intestines take nutrients out of sand. Using magnification 200,000 times normal size, we see a rock urchin’s spines with suckers on the end; a drawing illustrates how they work. A sea urchin walks toward a rock. We see three-fingered jaws—pedicellaria at the end of flexible stems—take in algae and other bits. We also see cilia less than 0.001 ml in length; their motion constant, creating whirlpools. On the shore again, we watch the setting sun. Occasional titles in French tell us what to watch for.