Echinoderms are characterized by radial symmetry, several arms (5 or more, mostly grouped 2 left – 1 middle – 2 right) radiating from a central body (pentamerous). The body actually consists of five equal segments, each containing a duplicate set of various internal organs. They have no heart, brain, or eyes, but some brittle stars seem to have light sensitive parts on their arms. Their mouth is situated on the underside and their anus on top (except feather stars, sea cucumbers, and some urchins).
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