Aristotles Lantern

Aristotle's Lantern, the jaw apparatus of Sea Urchins, as in the common English species (Echinus esculentus); it is of five sectors, each of which consists of four pieces, a triangular pyramid or alveolus, perforated by a long keeled tooth. Above is a curved piece, the compass or radius, and along the upper junction of two pyramids is the rotula or brace.