Spiders are arthropods.
Spiders ( order Araneae ) are air-breathing arthropods that have eight legs and chelicerae with fangs that inject venom . Anatomically , spiders differ from other arthropods in that the usual body segments are fused into two tagmata , the cephalothorax and abdomen , and joined by a small , cylindrical pedicel . In all except the most primitive group , the Mesothelae , spiders have the most centralized nervous systems of all arthropods , as all their ganglia are fused into one mass in the cephalothorax . Unlike most arthropods , spiders have no extensor muscles in their limbs and instead extend them by hydraulic pressure .