Arthropods are bilateral; their appendages come in pairs. Legs, antennae, pedipalps, and mouth-parts. Each segment has two and only two. We don’t count gills or wings. Gills are organs, that may stick out at the base of the legs; wings are outgrowths of the exoskeleton. Much may be written about the legs. After all, “Arthropoda” means “jointed legs.”