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. CLASS CRINOIDEA.— The crinoids are commonly called _sea-lilies_ because of their flower-like appearance. The _calyx_ is composed of symmetrically arranged calcareous plates, and most crinoids have a long stem. Other crinoids are free-swimming in the adult stage and are attached only during the earlier phases of their development. The crinoid calyx is typically cup-shaped (fig. 23) and five grooves radiate out from its center. These grooves continue outward along the complexly segmented arms and are used as channels to convey food to the mouth. [Illustration: Fig. 23. Typical modern crinoid, or “sea lily,” showing principal parts.] Calyx Arm Plate Stem Columnal Cirri Holdfast (root) The crinoid stem is attached to the base of the calyx and serves for purposes of support and attachment. This stem consists of a relatively long flexible stalk composed of numerous calcareous disk-shaped segments called _columnals_ (fig. 23; Pl. 35), each of which contains a round or star-shaped opening in its center. Many crinoids have very long stalks (some are as much as 50 feet in length), and when the animal dies the columnals become separated and are scattered about on the ocean floor. Many Paleozoic limestones contain such great numbers of crinoid columnals that they are referred to as _crinoidal limestones_ (fig. 8). Crinoidal limestones occur in some of the Mississippian and Pennsylvanian formations of central Texas and in the Pennsylvanian of north-central and Trans-Pecos Texas. The stalk is attached to the sea floor or some other object by means of a root system called the _holdfast_ (fig. 23). This structure commonly branches out into the surrounding sediments, and in this manner the crinoid animal is firmly anchored to the bottom of the sea. Crinoids, like most echinoderms, are gregarious animals—that is, they commonly live together in large numbers, and for this reason great numbers of crinoid remains are commonly found concentrated in relatively small local areas. Most fossil crinoids are found as stem fragments because the more fragile calyx and root system are less likely to be preserved.