Arachnomorpha Lameere 1890 is a subdivision or clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 of Arthropoda, comprising the monophyletic group formed by the trilobites, other great appendage arthropod

Megacheira

Megacheira is an extinct class of predatory euarthropods that possessed a pair of short enlarged appendages . They were found in marine environments throughout the world from the lower to middle Cambrian, and were important components of several faunas, including the Burgess and Maotianshan...

s and trilobite-like families (Helmetiidae, Xandarellidae, Naraoiidae

Naraoiidae

The Naraoiidae is a family of soft-shelled trilobite-like arthropods that are known only from the early and middle Cambrian -- primarily from the Burgess Shale of British Columbia and the Maotianshan Shale of China....

, Liwiidae

Liwiidae

Liwiidae is a family of nektaspid in the order Nectaspida. Below are the genera and species of this family:Buenaspis , Liwia , Soomaspis , and Tariccoia ....

, and Tegopeltidae), and a diverse sister clade including the chelicerates

Chelicerata

The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites...

. 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).

The arachnomorph concept has been challenged by suggestions that the trilobites in fact fall in the mandibulata

Mandibulata

The Mandibulata or mandibulates is a clade of arthropods that comprises the extant subphyla Myriapoda , Crustacea and Hexapoda . Mandibulata is currently believed to be the sister group of the clade Arachnomorpha, which comprises the rest of arthropods...

 stem-group.

There is no consensus as to assigning Arachnomorpha a formal Linnean category

Linnaean taxonomy

Linnaean taxonomy can mean either of two related concepts:# the particular form of biological classification set up by Carl Linnaeus, as set forth in his Systema Naturæ and subsequent works...

.

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
  Aglaspida
  The Aglaspida, more correctly termed Aglaspidida, were a group of small, horseshoe crab-like arthropods that were once regarded as basal or ancestral horseshoe crabs. However they cannot be accommodated within the chelicerates. Now, though, they are regarded as a distinct group, possibly being...
  
   Walcott, 1911
• Cheloniellida
  Cheloniellida
  The Cheloniellida Broili, 1932 are an unranked taxon of extinct Paleozoic arthropods, currently thought to be a member of the Arachnomorpha and related to the Chelicerata and the Trilobita, while not included in any of those.There are six included genera, dating from Ordovician, Silurian and...
  
   Broili, 1932
• Chelicerata
  Chelicerata
  The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites...
  
  Heymons 1901 [= Euchelicerata Weygoldt & Paulus, 1979]
  • Class Xiphosura
    Xiphosura
    Xiphosura is an order of marine chelicerates which includes a large number of extinct lineages and only four recent species in the family Limulidae, which include the horseshoe crabs...
    
     Latreille, 1802
  • Class Chasmataspida
    Chasmataspida
    Chasmataspidida are a rare, extinct group of chelicerate arthropods. They are probably related to either horseshoe crabs and/or sea scorpions...
    
     Caster & Brooks, 1956
  • Class Eurypterida Burmeister, 1843
  • Class Arachnida Lamarck, 1801