What class are spiders, insects, crabs?

There are hundreds of thousands of species of insects. And this is a separate class of arthropods. This class includes a grasshopper or an ant. But spiders and crabs belong to other classes. It can be assumed that spiders are insects, but in fact they are classified as arachnids. Although at first they were not isolated from other insects. Crabs are crustaceans. That is, when many species of animals, birds, insects, etc. have common features, they are combined into separate classes to simplify classification.