It’s a little confusing. Just look at ’em. You would think, superficially, that a barnacle is a kind of clam, oyster, or mussel. They cling to rocks for their entire lives, filter feeding from the high tides. But as it turns out, all those other things are molluscs, related to slugs, snails, octopi and squid to name a few. But a barnacle is actually a crustacean, like crabs, lobsters, and shrimp. Barnacles actually have legs inside of their shells, and show the same segmentation as all crustaceans. There’s a lot of other things that one can confuse, such as roly polies can be a variety of insect or crustacean species. These animals all show analogous external structures because for the lifestyle and environment in which they live, that body plan is perfectly adapted.