(subphylum Echinozoa, class Echinoidea)
A subclass of sea urchins in which the normally rigid test is composed of ambulacra and interambulacra, made up of two columns of plates. Spheridia are present. The subclass includes both regular and irregular types of echinoid. They first appeared in the Late Triassic.
Subjects: Earth Sciences and Geography — Zoology and Animal Sciences.