A subphylum of the phylum Arthropoda. The Chelicerata can be defined as those arthropods with the anteriormost appendages as a pair of small pincers (chelicerae) followed usually by pedipalps and four pairs of walking legs, and with the body divided into two parts: the prosoma (corresponding approximately to the cephalothorax of many crustaceans) and the opisthosoma (or abdomen). There are never antennae or mandibles (lateral jaws). The Chelicerata comprise three classes: the enormous group Arachnida (spiders, ticks, mites, scorpions, and related forms); the Pycnogonida (sea spiders or nobody-crabs); and the Merostomata (including the extant Xiphosurida or horseshoe crabs).