Apical System

Apical System, the system of plates found in the Echinozoa, regarded as homologous with the basals and radials of the Crinoidea. They are usually ten in number, and arranged in a double circle, in the centre of which the anus opens (as in the common Sea Urchin). The plates of the inner circle are known as the costals or basals, and the outer the radials.