Cystoids (sis'-toids) are related to the crinoids and blastoids but are more primitive than either. The body, or calyx, is not nearly so well developed, and the arms are irregular and rarely preserved. The body plates are quite irregular in arrangement.

The cystoids lived from the Ordovician Period, 490 million years ago, until the latter part of the Devonian Period, about 370 million years ago.

Many cystoids found in Illinois come from quarries in Silurian rocks of the Chicago region and in the Mississippi River bluffs of northwestern Illinois.

images of echinoderm fossils