Plates in the test are arranged into columns radiating from an aboral apical zone to the mouth. In regular echinoids the test is globular and perfectly pentaradiate, but in irregular echinoids there ...
ambulacra polyporous, uniserial; ambulacral plate compounding phymosomatid type (occluded both on aboral and oral sides); each plate has single, imperforate and crenulated primary tubercle lying near ...