Spider Spider Spiders (order Araneae) are air-breathing arthropods that have eight legs and chelicerae with fangs able to inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other orders of organisms. Spiders are found worldwide on every continent except for Antarctica, and have become established in nearly every habitat with the exceptions of air and sea colonization. , at least 45,700 spider species, and 113 families have been recorded by taxonomists. However, there has been dissension within the scientific community as to how all these families should be classified, as evidenced by
Opabinia unusual lateral lobes with gills persuaded him that it may have been a representative of the ancestral stock from which both the annelids and arthropods arose. In 1985 Derek Briggs and Whittington published a description of "Anomalocaris", also from the Burgess Shale. Swedish palaeontologist Jan Bergström suggested that the two animals were related, as they shared lateral flaps with gills, stalked eyes and other features; and he classified them as primitive arthropods, although he considered that arthropods are not a single phylum. In 1996 Graham Budd found what he considered evidence of short, un-jointed legs in "Opabinia". His examination of