Water-vascular system and tube feet, unique to echinoderms, could only exist with the support of the endoskeleton. The ossicles of the endoskeleton form units that end up enlarging in geometrical patterns during growth of the organism. Presumably there was selective pressure for a bottom-dwelling lifestyle, where the radial nature can allow the echinoderm to respond in any direction to food, changing conditions, etc. 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.