Echinodermata (echinoderms)Collection Highlights | Updated 7 years ago Pentagonaster stibarus Photo by Clay Bryce, image copyright WA Museum The name Echinodermata is derived from the Greek word for "spiny skin", although not all of the approximately 6000 species have skin that is spiny. Echinoderms are simple animals that live in the sea - none are found in freshwater or on land. All (mature) echinoderms have an obvious central mouth, and radial symmetry (the body has a central point with body parts extending outwards from it). Another characteristic shared by all echinoderms - although less obvious - is their water-vascular system used for "walking". Most mature echinoderms live on the bottom of the ocean and have hundreds of tiny tube feet, called podia, filled with seawater. Certain podia can be extended by the echinoderm when it expands and contracts chambers within its water-vascular system. Muscles in the podia are used to retract these tube feet, and the expansion and retraction of the appropriate podia in the correct order allows the echinoderm to walk. Many echinoderms have suckers on the ends of their podia which they use to capture food or to anchor themselves to rocks. There are five different classes of echinoderms: Asteroidea, Ophiuroidea, Crinoidea, Echinoidea and Holothuroidea. Marine Invertebrates Section View the discussion thread.