A marine invertebrate animal belonging to a class (Crinoidea; about 700 species) of echinoderms, including sea lilies and feather stars. They have a small cup-shaped body covered with hard plates and five radiating pairs of feathery flexible arms surrounding the mouth at the top. Sea lilies, most of which are extinct, are fixed to the sea bottom or some other surface such as a reef by a stalk. Feather stars are free-swimming and are usually found on rocky bottoms. Crinoids occur mainly in deep waters and feed on microscopic plankton and detritus caught by the arms and conveyed to the mouth. The larvae are sedentary. They arose in the Lower Ordovician (between 500 and 460 million years ago), and fossil crinoids are an important constituent of Palaeozoic limestones.