Waters, Johnny A. Department of Geology, Appalachian State University, Boone, North Carolina.
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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. See also: Pelmatozoa
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