The most primitive living class of echinoderms, whose members are either stalked (sea lilies) or unstalked (feather stars). The body is contained within a cup-like calyx, composed of regularly arranged plates, consisting of a lower dorsal cup which is covered by a dome (the tegmen). There are usually five plated and branching arms (brachial processes, or brachia) that articulate freely with the calyx. The upper surface contains the mouth and anus. There are tube feet along each arm with a median food groove between them leading to the mouth. The stem, when present, consists of a column of calcite discs (ossicles or columnals) each with a central hole (lumen) for extensions of the soft parts. All Palaeozoic forms were stemmed (sometimes of considerable length), but most modern forms are free swimming. They first arose in the Lower Ordovician, and fossil crinoids are an important constituent of Palaeozoic limestones.

Crinoidea