Most Ophiuroidea (brittle stars and basket stars) move by making rowing motions along a surface with their long, flexible arms. Typically, 2 arms row, while the other 3 arms stabilize, but arms may switch roles from stroke to stroke. A system of muscles and nerves controls arm movement. Support is provided to each arm by an internal skeleton of tiny calcium carbonate plates (vertebral ossicles) linked together by ball and socket joints. Four outer chains of calcium carbonate plates run down the top, bottom, and each side of the arm. The side plates are often spiny, giving the animal traction as it rows. Despite the layers of support, Ophiuroidea arms are skinny and frequently break (thus, brittle star). Indeed, when attacked by a predator a brittle star will lose (autotomize) part or all of an arm as a defense tactic, and later regrow it.