locomotion of a type found in both terrestrial and aquatic animal groups. Some fossorial animals dig short permanent burrows in which they live; others tunnel extensively and nearly continuously. In relatively soft substrates, such as soil, burrowers tend to be limbless (lizards, snakes) or equipped with powerful forelimbs (moles, badgers, mole crickets). In either group the animal's exterior is usually relatively smooth; burrowing lizards and snakes are especially smooth-scaled, and moles have short, velvety fur. The eyes of burrowing animals tend to be reduced or absent, and the ears often lack external openings.