The blastoids are extinct short-stemmed echinoderms with a small,
symmetrical, bud-like calyx. The blastoid calyx is composed of 13
calcareous plates arranged in a typical five-sided pattern (fig. 22a).
The _mouth_ is located in the center of the calyx and is surrounded by
five openings called _spiracles_. Five distinct _ambulacral_ or _food
grooves_ radiate outward from the mouth.
Blastoids range from Ordovician to Permian in age and were especially
abundant during the Mississippian period. No blastoids have been
reported from any of the rocks of Texas.