Arachnid Arachnid Arachnids () are a class (Arachnida) of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Almost all adult arachnids have eight legs, although the front pair of legs in some species has converted to a sensory function, while in other species, different appendages can grow large enough to take on the appearance of extra pairs of legs. The term is derived from the Greek word ("aráchnē"), from the myth of the hubristic human weaver Arachne who was turned into a spider. Spiders are the largest order in the class, which also includes scorpions, ticks, mites, harvestmen, and solifuges. Almost
Opabinia family tree is that modern tardigrades may be "Opabinia"s closest living evolutionary relatives. Although Zhang and Briggs (2007) disagreed with Budd's diagnosis that "Opabinia"s "triangles" were legs, the resemblance they saw between "Opabinia"s lobe+gill arrangement and arthropods' biramous limbs led them to conclude that "Opabinia" was very closely related to arthropods. In fact they presented a family tree very similar to Budd's except that theirs did not mention tardigrades. "Opabinia" made it clear how little was known about soft-bodied animals, which do not usually leave fossils. When Whittington described it in the mid-1970s, there was already a vigorous debate about