Arachnomorpha ("Spider Form") Lameere 1890 is a subdivision of Arthropoda, containing the monophyletic group formed by the trilobites and other trilobite-like families (Helmetiidae, Xandarellidae, Naraoiidae, Liwiidae, and Tegopeltidae, and a diverse sister clade including the chelicerates . Great debate is held on the position of the Pycnogonida, which are currently thought not to be placed in the immediate vicinity of the Chelicerata

There is no consensus as to assigning Arachnomorpha a formal Linnean category. It could be a Subphylum or a Superclass and then its subgroups Cheliceromorphora and Trilobita would be ranked as various infrataxa and the Chelicerata sensu stricto would be a Class.

A proposal to consider the Olenellinae as sister group to the Chelicerata has been refuted, which contraposed many synapomorphies uniting them to the Trilobita instead.

Synonyms include Arachnata Paulus, 1979 and Palaeopoda Packard 1903.

Classification of the Arachnomorpha

• ???Pycnogonida Latreille, 1806 (incl. Pantopoda Gerstaecker 1863)

Arachnomorpha Lameere 1890 [= Arachnata Paulus, 1979, = Palaeopoda Packard 1903]

• Trilobita Walch 1771
• Strabopida Hou & Bergström, 1997
• Aglaspida Walcott, 1911
• Cheloniellida Broili, 1932
• Chelicerata Heymons 1901 [= Euchelicerata Weygoldt & Paulus, 1979]
• Class Xiphosura Latreille, 1802
• Class Chasmataspida Caster & Brooks, 1956
• Class Eurypterida Burmeister, 1843
• Class Arachnida Lamarck, 1801

References