Stubborn Crustacean, It Clings Onto Rocks
Puzzle 16 of Groups of Three Easy Pack
Question is: Stubborn Crustacean, It Clings Onto Rocks
and the answer is: BARNACLE
Word Meaning:
BARNACLE — A marine crustacean of the subclass Cirripedia that attaches itself to submerged surfaces such as tidal rocks or the bottoms of ships.
Definitions from Wiktionary.