|-Entirely marine organism with test made up of calcareous plates or spicules.
-Different from other invertabrates in that test is secreted from middle body layer rather than outer layer, thus becoming enveloped by soft tissue.
-Each plate or spicule is a single crystal of calcite.
-Presumed to have evolved from cystoids (primitive echinoidermata, became extinct in Carboniferous).
-Consists of stalk surmounted by a calyx which contains 13 plates arranged in 3 rows.
|-Have unusual vacillitating history with local bursts and extinctions.
-Became extinct in epoch just before P-T boundary.