Modern arthropods include insects, spiders, centipedes, shrimp, and crayfish. All arthropods are the descendants of a single common ancestor. Just as you and your biological cousins can trace your ancestry back to a common set of grandparents, all arthropods can trace their ancestry back to a common arthropod ancestor. Any species descended from that ancestor is an arthropod. And any species not descended from that ancestor is not an arthropod.
→ → → The five branches of the arthropod tree