Millipedes, centipedes, scorpions, crabs, lobsters, crayfish are all arthropods. Evolutionary, arthropods are notable because they were the first animals to inhabit land. They were preadapted to land because they had hard shells for support and to slow dehydration, an excretory system that conserved water and a good respiratory system . They hit land at about 400 MYA and exploded from there. Even today, Phylum Arthropoda has more species than any other phylum.
Arthropods are animals, so they go in Kingdom Animalia.
They have organs so they go in Subkingdom Eumetazoa. They have bilateral symmetry, so they go in Branch Bilateria. They are protostomes and have spiral, determinate cleavage so they are placed in Subgrade Schizocoela. These sections are specialized for different things. A Chitinous exoskeleton - a tough shell made of chitin , a tough carbohydrate .