Purple Sea Urchin

Description:

"A purple sea urchin's pin cushion appearance comes from its round inner shell, called a "test." The test is covered with pincers (pedicellariae), tube feet and purple spines that move on ball-and-socket joints. Young urchins sport green spines. The spines spear food and protect an urchin from predators. Tiny hairs (cilia) covering the spines create a water current that carries food to the urchin and washes away wastes. An urchin uses its many tube feet to move along rocks, sand or other surfaces. And if food lands on an urchin's back, all those tube feet pass the food down to the urchin's mouth like a bucket brigade. Surprisingly, an urchin also "breathes" through its tube feet—that's where gases are exchanged, instead of in gills or lungs. Five toothlike plates, called "Aristotle's lantern," surround an urchin's mouth on the bottom of its shell. An urchin uses its teeth and spines to dig holes in stones, which become the sea urchin's hideaway. Sometimes a sea urchin grows larger than its dugout and is "in for life"—then it must depend on food drifting to it. An urchin's teeth and spines can even drill through steel pilings by flaking away the rust that coats them."