Scorpions are the most ancient form of arachnids.  In fact the scorpion, or rather and ancient relative, was probably the first creature to walk out of prehistoric ocean onto land.  Scorpions have all the same basic parts as other arachnids.  Their pedipalps however, our developed into specialized pinchers used for grabbing prey and defense.  Another adaptation that the scorpion is famous for is their stinger.