a class of extinct marine animals of the phylum Echinodermata. They lived from the Ordovician (possibly from the Cambrian) to the Devonian. They either attached themselves to the sea floor or simply lay on the bottom. The body consisted of a calyx and a stem. The calyx was spherical or ovate and consisted of numerous polygonal ossicles with complexly structured pores. On the calyx were a few appendages, or brachioles. There were two orders, comprising approximately 40 genera). The Cystoidea are important in explaining the phylogeny of Echinodermata and the stratigraphy of the Ordovician.