sea lily, stalked echinoderm of the class Crinoidea. Sea lilies are ancient, having reached their peak in the Middle Mississippian period; about 5,000 fossil species are known. About 80 modern species remain. Marine animals, like all echinoderms, they are largely restricted to fairly deep water, from about 35 to 1,000 fathoms (60–300 m). Small organisms and particles are trapped in mucus in the water-vascular (ambulacral) grooves located on the feathery, branching arms and are conveyed to the mouth. Unlike the free-swimming feather stars to which they are related, sea lilies remain permanently attached to the ocean bottom. Two genera, Rhizocrinus and Cenocrinus, occur at moderate depths in the Atlantic and Caribbean. Sea lilies are classified in the phylum Echinodermata, class Crinoidea.