Cup composed of radial and basal ossicles without anal plates in the adult. Infrabasals present in many fossil taxa; reduced or absent in extant taxa. Mouth exposed on the surface. Axial nerves enclosed in a canal penetrating the basals, radials, and brachials. Brachials generally joined by muscular articulation, but non-muscular (ligamentary) articulations are present in almost all species. Arms bearing pinnules in all extant taxa.