Limpet

In a general sense, any hat-shaped, or conical, gastropod shell.
Any one of many species of marine shellfish of the order Docoglossa, mostly found adhering to rocks, between tides.
1. Hence: Something or someone that clings tenaciously to another object or person; specifically a military explosive device having magnets allowing it to cling to a metallic target object, such as the hull of a ship.