'Keyhole limpet' definitions:

Definition of 'keyhole limpet'

noun

Marine limpet having a conical shell with an opening at the apex [syn: keyhole limpet, Fissurella apertura, Diodora apertura]

From: GCIDE

Keyhole \Key"hole`\, n.
1. A hole or apertupe in a door or lock, for receiving a key. [1913 Webster]
2. (a) (Carp.) A hole or excavation in beams intended to be joined together, to receive the key which fastens them. (b) (Mach.) a mortise for a key or cotter. [1913 Webster]
Keyhole limpet (Zool.), 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 (Zool.), 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. [1913 Webster]

From: GCIDE

Fissurella \Fis`su*rel"la\, n. [NL., dim. of L. fissura a fissure.] (Zool.) A genus of marine gastropod mollusks, having a conical or limpetlike shell, with an opening at the apex; -- called also keyhole limpet. [1913 Webster]