Crinoid (black) attached to a fan coral. A crinoid (also called a featherstar) is a primitive, free-swimming marine invertebrate, a member of the group that includes the starfish. It uses its feathery and fragile arms to catch and convey microscopic plankton and detritus to its mouth at the base of its arms. Fan corals belong to the order Alcyonacea, or soft corals, which do not produce calcium carbonate skeletons. Photographed off the coast of Rajah Ampat, Indonesia.