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."
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.
Textbook Reference: Concept 23.6 Life on Land Contributed to Vertebrate Diversification