Extinct group of echinoderms, ranking in some classification schemes as a class or superclass, and ranging from the Lower Ordovician to the Upper Devonian. The body is spherical or ovoid, covered with a plated theca, and was usually attached to the substratum either directly or by a short stalk. The thecal plates are calcitic and perforated by characteristic pore structures that functioned in respiration. The plates are arranged in circlets but vary in number from around 13 to more than 100, giving an irregular appearance. The mouth, on the upper surface, is surrounded by ambulacral food grooves that in some cystoids were equipped with food gathering appendages called brachioles. These are rarely preserved. In many cases radial symmetry is poorly developed.