Īnatomically, spiders (as with all arachnids) differ from other arthropods in that the usual body segments are fused into two tagmata, the prosoma, or cephalothorax, and opisthosoma, or abdomen, and joined by a small, cylindrical pedicel (however, as there is currently neither paleontological nor embryological evidence that spiders ever had a separate thorax-like division, there exists an argument against the validity of the term cephalothorax, which means fused cephalon (head) and the thorax. However, there has been dissension within the scientific community as to how all these families should be classified, as evidenced by the over 20 different classifications that have been proposed since 1900. As of August 2021, 49,623 spider species in 129 families have been recorded by taxonomists. 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. They are the largest order of arachnids and rank seventh in total species diversity among all orders of organisms. Spiders ( order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk.
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