[Free Registration/Login may be required to access all resource tools.] Arachnids are a class of joint-legged invertebrates in the subphylum Chelicerata. They live mainly on land but are also found in freshwater and in all marine environments, except for the open ocean. There are over 100,000 named species, and there may be up to 600,000 species in total, including the unknown ones. Learn more about arachnids in this learning module produced by CK-12.