What is "keyhole urchin"

Keyhole \Key"hole`\, n.

1. A hole or apertupe in a door or lock, for receiving a key.

2. 1. (Carp.) A hole or excavation in beams intended to be joined together, to receive the key which fastens them.

   2. (Mach.) a mortise for a key or cotter.

      Keyhole limpet (Zo["o]l.), a marine gastropod of the genus Fissurella and allied genera. See Fissurella.

      Keyhole saw, a narrow, slender saw, used in cutting keyholes, etc., as in doors; a kind of compass saw or fret saw.

      Keyhole urchin (Zo["o]l.), any one of numerous clypeastroid sea urchins, of the genera Melitta, Rotula, and Encope; -- so called because they have one or more perforations resembling keyholes.

n. Any of numerous clypeastroid sea urchins of the genus ''Melitta'', ''Rotula'' and ''Encope'', with one or more perforations resembling keyholes.