Major Subgroups of Chelicerata

Xiphosura:

Horseshoe crabs, and extinct relatives

Xiphosurans are aquatic chelicerates retaining many 'primitive' features, e.g. book gills, and leg bases modified for grinding food (=gnathobases). They have a prominent prosomal carapace (='shell'). They are relatively large, reaching 60 cm long.

Arachnida:

Spiders, scorpions, mites, ticks and relatives

Arachnids are a very diverse group with thousands of living species. They are terrestrial and show adaptations for living on land, such as book lungs and other mechanisms for breathing air.

Eurypterida:

Sea "scorpions"

Extinct aquatic to amphibious chelicerates - see Fossil Forms for more information.

Live pycnogonid in marine habitat (courtesy of AMNH)

Pycnogonida:

Sea "spiders"

Sea "spiders" are a strange group of marine arthropods with unknown affinities. They have often been placed in or near to the chelicerates, and although were not included in Dunlop and Selden's (1997) analysis, are probably related (Dunlop, 2002). They have some chelicerate features, such as clawed head appendages equivalent to chelicerae. However, they also have many unique derived characteristics (autapomorphies - such as a feeding proboscis) which make them difficult to compare with other groups. Until more work is done, they shall remain enigmatic, and recorded as Arthropoda incertae sedis.