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(subphylum Blastozoa)

Extinct class of cystoid-like echinoderms, with radial symmetry, which range in age from Lower Cambrian to Middle Silurian (Lower Palaeozoic). Their globular or flattened thecae are composed of numerous, irregularly arranged plates that lack the thecal pores typical of cystoids, and their sutural pores (i.e. those at plate margins) are unlike those of cystoids. Food grooves are extended on an exothecal skeleton of erect, unbranched, biserial, food-gathering appendages (called brachioles), which are not homologous with the arms of crinoids. The eocrinoids are the earliest blastozoans and may have included ancestors to other cystoid groups, possibly to crinoids also, but the question is still unresolved.

Subjects: Earth Sciences and Geography.

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