Spiders (class Arachnida, order Araneae) are air-breathing arthropods. They have eight legs, and mouthparts (chelicerae) with fangs that inject venom. Most make silk. The arachnids are seventh in number of species of all animal orders. About 48,000 spider species, and 120 families have been recorded by taxonomists. Over twenty different classifications have been proposed since 1900. Spiders live on every continent except for Antarctica, and in nearly every habitat with the exceptions of air and sea.