Among the following organisms, which one does not belong to the class of other three?

The crab belongs to the class **Malacostraca** (subphylum Crustacea), while the mite, scorpion, and spider all belong to the class **Arachnida** (subphylum Chelicerata). Although all four are arthropods, arachnids are characterized by having eight legs and no antennae, whereas crabs typically have ten legs and two pairs of antennae. This question tests the core concept of **biological classification and taxonomic hierarchy** within the Phylum Arthropoda.