Sea urchins use the motion of the ocean to disperse eggs and sperm into the current.
A 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.
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