Arachnomorpha is a subdivision or
clade of
Arthropoda, comprising the monophyletic group formed by the
trilobites, other
great appendage arthropods and 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. Arachnomorpha are considered the sister group to the
crustaceans, which are increasingly being accepted as members of the
mandibulate clade (including insects and myriapods).