Pill bugs are arthropods like insects, centipedes, and spiders, but they aren’t closely related to them. They are actually crustaceans, isopods, more closely related to shrimp and lobsters. And of course, trilobites were arthropods too, and they shared with pill bugs the ability to roll themselves up into a defensive posture.
—Richard I. Gibson
Ordovician trilobite photo by Vassil, under GNU free documentation license. Flexicalymene photo by Steve Henderson, used by permission.