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.

Echinoderms stand out as the only organisms on Earth with five arms or other elements spaced evenly around a central point. This pentaradial symmetry (penta = “five;” radial = “around a central point") is obvious in some echinoderms, such as sea stars or brittle stars that have five arms. It’s more subtle on others, such as the five rows of tube feet on a sea cucumber or five grooves on a sand dollar. This unusual symmetry of adult echinoderms is not found in juveniles. In fact, echinoderm larvae have two-sided (bilateral) symmetry like humans, and must undergo a metamorphosis to become pentaradial adults. Body plans of animals during their development often say something about their ancestry. The bilateral bodies of developing echinoderms are a reminder that, aside from other back-boned animals (vertebrates), echinoderms may be our closest relatives. Unlike most living echinoderms, many fossil echinoderms did not have pentaradial symmetry.