The word "shells" has a few synonyms that can be used interchangeably depending on the context. One possible synonym is "seashells," which specifically refers to shells that are found on beaches and shorelines. Meanwhile, "exoskeletons" can also refer to shells, specifically the hard, protective outer covering of certain animals like crabs and insects. "Hulls" are basically the shells of seeds like nuts, beans, and grains. "Carapaces" has a similar meaning as exoskeletons, pertaining to the hard-cased outer layer of certain animals like turtles. Lastly, "cocoons" could also be considered a synonym for shells, as it denotes the protective outer covering of certain insects during their pupal stage.