Echinoidea (sea urchins and sand dollars) lack arms entirely. They have a globular or flattened body enclosed in a rigid test (shell) with spines. In contrast, Asteroidea (sea stars) have 5 arms radiating from a central disc; Ophiuroidea (brittle stars) have 5 long, flexible arms; Crinoidea (feather stars, sea lilies) have multiple feathery arms used for filter feeding.