Five arms blend into central disc. Tough but flexible skins since ossicles se parately embedded. Ossicles form a continuous fused armor in the form of plates. Long protruding spines are protective in sea urchins and short spines used for burrowing in sand dollars. Mobile flexible arms clearly demarcated from central disc.
Dermal ossicles form a continuously articulated armor, especially along the arms.
Tend to be secretive small particle feeders, in contrast to sea stars. Central body is elongated. Stalked, mouth directed upwards, food collected by tube feet in the branched arms. Ossicles almost continuous but articulated. Reduced central disc.