Sand dollars and their relatives (clypeasteroids, cassiduloids) have a bilaterally symmetrical test composed of TEN double columns of plates, five interambulacral columns (labelled 1-5) and five ...
Regular sea urchins have a pentaradially symmetrical test composed of ten double columns of plates, five interambulacral zones and five ambulacral zones, which together make up the corona. Ambulacral ...