Chelicerata Heymons, 1901
   01 Pycnogonida Latreille, 1810 - pycnogonids
   02.1 Genus †Sanctacaris Briggs & Collins, 1988
   02.2 Euchelicerata Weygoldt & Paulus, 1979
   03 Xiphosura s. lat. Latreille, 1802 - horseshoe crabs see Class Xiphosura Latreille, 1802
   04 †Chasmataspidida Caster & Brooks, 1956
   05 †Eurypterida Burmeister, 1843 - eurypterids
   06 Scorpiones Koch, 1837 - scorpions
   07 Opiliones Sundevall, 1833 - harvestmen
   08 †Phalangiotarbida Haase, 1890
   09 Pseudoscorpiones De Geer, 1778
   10 Solifugae Sundevall, 1833
   11 Palpigradi Thorell, 1888 - palpigrades
   12 Acari Leach, 1817 - mites and ticks
   13 Ricinulei Thorell, 1876 [monotypic] - ricinuleids
   14 Arachnida and/or Pantetrapulmonata Shultz, 2007
   15 †Trigonotarbida Petrunkevitch, 1949
   16 †Uraraneida Selden & Shear, 2008
   17 Araneae Clerck, 1757 - spiders
   18 †Haptopoda Pocock, 1911 [monotypic]
   19 Amblypygi Thorell, 1883 - whip spiders
   20 Uropygi Thorell, 1882 [monotypic] - whip scorpions see [unranked] Uropygi Thorell, 1882
   21 Schizomida Petrunkevitch, 1945 - schizomids