Aristotle's lantern

An Aristotle's lantern is a chewing device found in some echinoids. Spatangoida species do not have an Aristotle's lantern. It is a jaw (=triangular pyramid that ends in a pointed tooth); a plume (=soft and curved piece located at the base of the jaw); an external opening (=large opening located near the external base of the jaw); epiphyses (two appendices, which may or may not be fused, which more or less close the external opening); a joint (=piece joining the pyramids); a compass (=curved piece located underneath the joint).