Cystoidea

Class of extinct stemless echinoderm, with a globular exoskeleton made up of numerous asymmetrically arranged calcareous plates. The plates were perforated by tiny pores, which were probably used for gas exchange. Cystoids lived from the Ordovician to the Permian periods, and were probably suspension feeders, trapping particles with short, slender arms (brachioles) and then passing them along food grooves to the mouth.