As the name indicates (Greek amphi: dual, bios: life), amphibians can live in aquatic as well as terrestrial habitats. Most of them have two pairs of limbs. The body is divisible into head and trunk, and a tail may be present in some. The amphibian skin is moist and without scales. The eyes have eyelids. A tympanum represents the ear. The alimentary canal, urinary and reproductive tracts open into a common chamber called the cloaca, which opens to the exterior. Respiration is by gills, lungs and through the skin. The heart is three-chambered, with two auricles and one ventricle. These are cold-blooded animals. Sexes are separate. Fertilisation is external. They are oviparous and development is indirect. Examples include Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra (salamander) and Ichthyophis (limbless amphibia). Amphibians belong to super class Tetrapoda because they bear limbs. The points most often tested are the moist scaleless skin, the common cloaca, the three-chambered heart with two auricles and one ventricle, external fertilisation and indirect development with a larval stage, and the existence of limbless forms such as Ichthyophis, which shows that not all members have two pairs of limbs.