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
Arthropod gills that were very different from those of arthropods. This reasoning implies that "Parapeytoia", which has legs and a backward-pointing mouth like that of the earliest arthropods, is a more credible closest relative of arthropods than is "Anomalocaris". In 2006, they suggested that arthropods were more closely related to lobopods and tardigrades than to anomalocarids. In 2014, research indicated that tardigrades were more closely related to arthropods than velvet worms. Higher up the "family tree", the Annelida have traditionally been considered the closest relatives of the Panarthropoda, since both groups have segmented bodies, and the combination of these groups was