Kingdom Animalia Linnaeus, 1758 - animals
   Phylum Arthropoda Siebold, 1848 - arthropods
     Subphylum Chelicerata Heymons, 1901
       1 Class Pycnogonida Latreille, 1810 - pycnogonids
       2 Class †Aglaspidida Walcott, 1911 see Order Aglaspidida Walcott, 1911
       3.1 Class †Xiphosura Latreille, 1802 - horseshoe crabs
       3.2.1 Class †Eurypterida Burmeister, 1843 - sea scorpions
       3.2.2 Class †Chasmataspidida Caster & Brooks, 1956
       3.3 Class Arachnida Lamarck, 1801