Metamorphosis has also been extensively studied in amphibians, a class of vertebrates which includes frogs, toads, and salamanders. "Amphibian" means dual (amphi-) life form (-bian) and refers to the typical life history of these animals, in which an aquatic larva metamorphoses into a terrestrial adult. The reptiles, such as turtles, lizards, and snakes, is another class of vertebrates whose species superficially resemble adult amphibians, but do not undergo metamorphosis.
Metamorphosis differs in the many different amphibian species. In frog development, the eggs hatch and give rise to tadpoles, small aquatic larvae that have external gills and are mainly vegetarian. As the tadpole grows, internal gills and limbs form. Several significant changes occur during metamorphosis into the adult, including growth of a large mouth and tongue, loss of gills, formation of lungs, growth of the front legs, and resorption of the tail. Numerous biochemical changes accompany these morphological changes, such as synthesis of a new visual pigment in the eyes and a new oxygen-binding hemoglobin protein in the blood. The adult is mainly insectivorous and partly terrestrial.
Interestingly, the sexually mature adults of some amphibians, such as the axolotl, have a larval morphology. The retention of larval or juvenile characteristics in adulthood is defined as neoteny. Neoteny is apparently caused by a genetic mechanism which uncouples development of body cells and the development of the sexual organs. Although neoteny is most apparent in amphibians, because they are normally metamorphic, changes in developmental timing may underlie the evolution of many species, including humans.