Scorpion - Class Arachnida of Subphylum Chelicerata; Crab - Class Malacostraca of Subphylum Crustacea; Ant - Class Insecta of Subphylum Hexapoda; There are all sorts of differences among these groups - number of legs, exoskeleton material, habitat, method of reproduction, number and placement of antenna, manner of body segmentation, and the list goes on. read more