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.
![Arthropod tree](https://evolution.berkeley.edu/wp-content/uploads/2021/05/25-Arthropods-e1620421247469.png)