The blastoids are a widespread and long ranged class of echinoderms. Their taxonomic diversity is low at their first appearance (M. Ordovician?; Silurian) and geographically restricted but they had become more diverse and cosmopolitan by the Devonian. They were widespread until the end of the Paleozoic. Approximately eighty genera and several hundred species are known. Most blastoids were attached to the substrate but several experimented with an eleutherozoic life style. Blastoids were passive filter feeders, usually living in shallow water (5–50m) communitites. The preservation of parts of most of the principal organ systems in the theca allows detailed studies of the evolution and functional morphology of blastoids.