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. 22b). 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.
[Illustration: Fig. 22. Two extinct attached echinoderms. (a)
_Pentremites_ (Mississippian). (b) _Caryocrinites_ (Silurian).]