Response System

Echinoderms have both sensory and motor neurons with varying degrees of complexity in different species. In general, a nerve ring surrounds the mouth with branching nerve cords connecting to other body areas.

Sensory neurons respond to touch, chemicals dissolved in the water currents, and light. At the tips of the arms of sea stars are eyespots, clusters of light-sensitive cells. Many echinoderms also sense the direction of gravity. For example, a sea star will return to an upright position after being overturned by a wave or current.