The class Amphibia has >7500 described species, including 3 orders: Anura (Salientia), Caudata (Urodela), and Gymnophiona (Apoda). Anura refers to tailless amphibians and is the largest order, comprising >6000 species, including frogs and toads ( see table Similarities and Differences Between Frogs and Toads . Caudata refers to amphibians with tails (salamanders, newts, and sirens) and comprises >700 species ( see table Some Common Salamanders and Newts ). The order Gymnophiona is made up of >100 species of caecilians. Caecilians are wormlike amphibians that inhabit both aquatic and terrestrial environments. They are rarely kept in the care of humans. Amphibian populations are declining due to a complex mix of factors including habitat loss, climate change, infectious disease outbreaks, toxin exposure, and pollution.