The Chelicerata are one of the four major living divisions of arthropods. This clade contains at least three major subgroups, the Arachnida (arachnids, including spiders, mites and scorpions), Limulina (horseshoe crabs) and the extinct Eurypterida (sea scorpions). A fourth group, the Pycnogonida (sea spiders), is morphologically very divergent from other arthropods and debate continues as to whether or not they belong to the chelicerates. Marine chelicerates (excluding pycnogonids) first appeared in the Ordovician (earlier taxa previously assigned to chelicerates such as Sanctacaris are no longer regarded as such) and were prominent during the Palaeozoic (particularly the eurypterids), but non-arachnid chelicerates (excluding the pycnogonids) are now extinct except for three genera of horseshoe crabs. The vast majority of living chelicerates belong to the mostly terrestrial arachnids.
The chelicerate body is divided into two tagmata, the prosoma (or cephalothorax) and opisthosoma (or abdomen). The chelicerates receive their name from the chelicerae, pre-oral appendages used for feeding. Plesiomorphically, the chelicerae are pincer-like appendages as in horseshoe crabs and harvestmen, but in many arachnid groups the chelicerae have become modified, such as into the fangs of spiders. As pre-oral appendages, the chelicerae are not homologous to the mouthparts of other living arthropods but to the antennae. The first pair of post-oral appendages are the pedipalps, plesiomorphically leg-like appendages used for food manipulation. The remaining walking legs are restricted to the prosoma, while the opisthosoma lacks appendages.
Age: Ordovician to present.
Description: Two tagmata, prosoma and opisthosoma (fused in some arachnids; opisthosoma severely reduced in pycnogonids). Prosoma with one pair of pre-oral appendages (the chelicerae) and five pairs of post-oral appendages (the pedipalps and four pairs of walking legs - five or six pairs in some pycnogonids). Opisthosoma ancestrally with 11 or 12 segments, without appendages (though opisthosomal structures such as gills may derive from modified appendages). Respiratory organs (gills, book lungs or tracheal spiracles) located on underside of opisthosoma. Post-anal telson present in many groups.
Comments: Older texts unite the marine non-arachnid chelicerates (except pycnogonids) into the Merostomata, but it is universally agreed that this is a paraphyletic group with respect to the arachnids.
The Horseshoe Crab - a detailed guide to the only living "merostomates".
Introduction and description Christopher 21:53, 2 September 2008 (PDT).