These are primitive attached echinoderms which were relatively common in early [Paleozoic] time. The typical [cystoid] has a somewhat globular or sac-like [calyx] (the main body skeleton) composed of numerous, irregularly arranged, [calcareous] plates ([fig. 22]b). The plates composing the calyx are usually perforated by pores or slits which were probably used in excretion or [respiration]. The calyx was attached to the sea bottom by a short stem.
Cystoids range from [Cambrian] to [Devonian] in age and were especially abundant during [Ordovician] and [Silurian] time. Their remains are rare or absent in the rocks of Texas.
Fig. 22. Two extinct attached echinoderms. (a) Pentremites ([Mississippian]). (b) Caryocrinites ([Silurian]).
CLASS BLASTOIDEA.—
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. 22]a). 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.