Phylum Arthropoda Siebold, 1848 - arthropods
   Subphylum Chelicerata Heymons, 1901
     Class Arachnida Lamarck, 1801
     Class †Arthropleurida Waterlot, 1934 see Subclass Arthropleuridea Waterlot, 1934
     Class Merostomata Dana, 1852 - horseshoe crabs see Class Xiphosura Latreille, 1802
     Class Pycnogonida Latreille, 1810 - pycnogonids
     Class Tardigrada Doyère, 1840 see Phylum Tardigrada Doyère, 1840