Echinoderms are marine invertebrates with spiny skin and five-part radial symmetry. They include sea stars, brittle stars, sea urchins, sea cucumbers, and sea lilies. Sea stars have tube feet and a water vascular system to move. They are predators that eat mollusks and crustaceans. Sea urchins are herbivores that graze on algae and plants using their tube feet. Sea cucumbers extract nutrients from sand and release toxic organs as a defense.