What distinguishes arthropod subphylums from each other?

Under arthropoda are subphyla Chelicerata, Myriapoda, Crustacea, and Hexapoda. (There is also an extinct class Marrellomorpha and an the extinct Trilobite subphylum). The chelicerata, like Spiders, scorpions, mites, etc., get their name from having appendages appear before the mouth; myriapods like centipedes and millipedes characterized by a high count of body segments and legs; crustaceans like crabs, shrimp, woodlice characterized by their biramous (two-part) limbs and the nauplius larval form; hexapoda named for their consolidated thorax with only three pairs of legs.