Echinoidea

-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.

-Have characteristic spinose, spherical and radially symmetrical test.

-Typically this test is composed of twenty vertical rows of plates arranged in pairs, five rows are perforate alternating with five imperforate ones.
-Emerging from pores on the perforate rows are tube feet, which are connected to an internal water vascular system responsible for locomotion and respiration.

-Extinction is not of special significance in the evolution and development, but did dramatically reduce numbers.

-Few fossils found pre and post extinction.

-Similar morphology and diversity each side of the boundary.

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