These dull brown, red or black echinoderms, sometimes mistaken for sea slugs, are actually related to seastars and sea urchins. They possess tube feet and their skeleton is reduced to ossicles (small calcerous plates) imbedded in their skin. Their highly contractile, cylindrical body has a large number of wart-like projections that become more prominent when the body is contracted. They feed on small pieces of organic matter obtained by sweeping the substrate with their tentacles.