Echinoderms are one of the most distinctive and common groups of invertebrates in all marine environments from the intertidal zones to deep-sea trenches. The name "echinoderm" means "spiny skin" and comes from their appearance of having spines or tubercles covering their body. Most echinoderms are bottom feeders. This phylum contains over 6000 species and most can be recognized by their pentamerous radial symmetry meaning that they can be divided into five equal parts around their central axis. Echinoderms start life as bilaterally symmetric larvae, but metamorphose into their pentamerous body plan as they mature.
All echinoderms have an internal skeleton composed of calcareous ossicles. These ossicles can be hinged (in sea stars and brittle stars), fused together to form a hard shell (in urchins), or almost undetectable (in sea cucumbers). All echinoderms have a water-vascular system composed of fluid-filled canals and surface appendages. Ancient echinoderms used this system to collect and transport food, but present echinoderms use it to power their tube feet which are used for locomotion, gas exchange, feeding, attachment and sensory reception. Echinoderms have no specific excretory organs. Most are dioecious, with fertilization taking place in the sea water. Larvae may be free-living or may have a yolk sac to provide nourishment throughout their larval period. The larvae settle to the bottom to metamorphose into adults.