Cystoids
As a water dwelling creature, Cystoids ( sis'-toids ) are related to the crinoids and blastoids, but are more primitive than either.
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This ancient group is widely distributed in Early and Middle Paleozoic rocks. The cystoid theca (see above photo) is usually globular or saclike, with radial or, rarely, bilateral symmetry.
It is made of numerous plates, regularly or irregularly arranged, that are pierced by a characteristic pore system. The stem, or column, is made of numerous circular or elliptical plates, known as columnals.
Not all cystoids had a column, and many apparently did not use it for attachment.
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The feeding system of a cystoid consisted of a set of brachioles, which were free appendages on the dorsal surface used for gathering suspended material.
Long, slender arms give many a flower-like appearance, while those without arms often resemble a flower bud.
The body, or calyx, is not nearly so well developed, and the arms are irregular and rarely preserved. However, the body plates are quite irregular in arrangement. (See the plates below.)
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The cystoids lived from the Ordovician Period – 490 million years ago – until the latter part of the Devonian Period, about 370 million years ago.
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