The Echinoid Directory

The perignathic girdle is found on the inner side of the test at the peristome edge. It is composed of calcitic projections that act as platforms for the attachment of lantern retractor and protractor muscles. In cidaroids (above right) these projections are made entirely from interambulacral plates, and these are termed apophyses (ap). Protractor muscles attach near the base centrally while retractor muscles attach to their inner edge. In non-cidaroid regular echinoids (above left) there are generally both interambulacral elements and ambulacral elements to the perignathic girdle. The ambulacral elements are tall, finger-like structures termed auricles (au) and it is here that the retractor muscles attach. Protractor muscles attach to the low interambulacral wall in between.