Arachnomorpha Trilobites Arachnids And Relatives

The subphylum Arachnomorpha is a diverse lineage consisting of those arthropods with the anus in a ventral position in the penultimate somite of the trunk, development of a styliform terminal projection, presence of trunk gut divertic-ulae, a marginal rim on the cephalic shield, the fusion of four (or more) appendages into the head, and the presence of six podites in the inner rami of the appendages (Wills et al., 1998), although each of these traits is secondarily modified or lost in various lineages of the arachnomorphs. The extensive extinction of basal Arachnomorpha has perhaps been one of the greatest obstacles to studies attempting to study arthropod phylogeny based solely on the modern fauna. Two principal lineages comprise this subphylum: the trilobites (Trilobita) and the cheliceriformes (chelicerates and their extinct relatives). The trilobites are most closely related to the Cheliceriformes, a large assemblage containing, among other groups, the familiar arachnids (spiders, mites, scorpions et al.) and horseshoe crabs.

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