any marine crustacean of the subclass Cirripedia, usually having a calcareous shell, being either stalked (goose barnacle) and attaching itself to ship bottoms and floating timber, or stalkless (rock barnacle or acorn barnacle) and attaching itself to rocks, esp. in the intertidal zone.

a person or thing that clings tenaciously.

noun

appendage , bloodsucker , crustacean , freeloader , hanger-on , leech , parasite