A class of extinct Pelmatozoa in the subphylum Blastozoa which arose in the Ordovician and flourished in the Carboniferous and Permian. Blastoids did not survive the Permo-Triassic mass extinction event. Historically, two orders of the Blastoidea have been recognized: the Fissiculata with exposed hydrospire slits and the Spiraculata with concealed hydrospires. In recognition of their polyphyletic origins, the Spiraculata are now subdivided into the Troosticrinida, Nucleocrinida, Granatocrinida, and Pentremitida.