• 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 zones are made of plates that are pierced by single or double pores for the tube-feet.

• These pore-pairs are situated along the outer (adambulacral) margins of the ambulacral zone.

• The central suture line between the two columns of plates in each ambulacral zone is the perradial suture line.