The amphibians include three modern-day groups of animals: the legless, wormlike caecilians found in the tropics, the tail-less frogs and toads (collectively called anurans), and the tailed salamanders. Many of these animals live on dry land as adults, but the typical amphibian life cycle occurs at least partly in the water or in a moist environment. The dual life of these animals, on land and in the water, gives this group its name—Amphibia, meaning "double life."
Frogs, toads, salamanders, and caecilians all belong to the Amphibia. Amphibia translates to "double life"—a reference to the amphibian's life cycle, which occurs partly in water and partly on land. Although most amphibians live this double life, a few complete their cycles entirely on land or entirely in water. Our study of the frog's life cycle begins with sexual reproduction. A male entices an egg-carrying female to mate and stimulates her to release her eggs into the water. The male overlays these eggs with a cloud of sperm. Frogs and most other amphibians reproduce through external fertilization.
The fertilized egg, called the zygote, contains all the fat and proteins required for the early phase of frog development. Whether laid on land or in water, the egg must remain moist during development; the egg readily loses water through its delicate envelope if its surroundings are dry.
The single-celled zygote divides many times to form an embryo with thousands of cells. As it develops, the embryo elongates and forms a tailbud. After embryonic development is complete, the embryo hatches and becomes a larva, also called a tadpole. The tadpole has external gills for respiration. These external gills will disappear as the animal continues to develop.
The tadpole will transform itself in a process called metamorphosis, which is controlled by hormones. During metamorphosis, the external gills are replaced by internal gills and hind legs begin to appear. Virtually every organ in the frog changes and becomes suitable for life on land.
Metamorphosis continues as front legs appear and the internal gills are replaced by lungs. The tail begins to be resorbed. The mouth of the larva widens and develops an insect-catching tongue. The tadpole's gut even shortens, preparing the animal for the transition from an herbivorous diet to a carnivorous diet. After a complete metamorphosis takes place, the adult frog is specialized for life on dry land. When the frog reaches reproductive maturity, it completes the life cycle by producing a new generation of frogs.
Amphibians typically live in water or on land during the different phases of their life cycles. An amphibian's early development must take place in a moist environment, because the jelly envelope surrounding the developing embryo easily dries out. In the case of the frog, the fertilized eggs then develop into water-bound tadpoles. These swimming creatures are well adapted to their watery environment—they initially lack legs, they use gills to respire, and they are herbivores, eating the underwater flora available to them. However, as they transform into adults, their bodies prepare for a totally different lifestyle. Practically every organ of this animal transforms, or metamorphoses, as the tadpole prepares for life as a land-dwelling, carnivorous frog.