​CYSTOIDEA, an order of extinct echinoderms. They are spherodial animals. They have a mouth above; the arms are rudimentary. Von Buch first elucidated their structure and affinities at Berlin in 1845, and gave them the name of cystidese in place of sphseronites; their original appellation. Now cystidese has become cystoidea. They range from Upper Cambrian to the Silurian, being especially prominent in the Bala Limestone.