What is a reptile?
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The reptiles , which evolved from amphibians during the Carboniferous period, were the first vertebrate group to become entirely independent of water. This was made possible by the development of a scaly, water-resistant skin and of the terrestrial, or amniote, type of egg found in all higher land vertebrates. The amniote egg has an elaborate series of internal membranes (one of which is called the amnion) surrounding a pool of liquid in which the embryo develops; the membranes prevent desiccation and allow inward diffusion of oxygen. Reptilian eggs have porous shells and large amounts of yolk. Fertilization is internal. In most cases the eggs are laid unhatched; in a few species they are retained and hatched in the body. Reptiles, including such forms as turtles and sea snakes that have returned to an aquatic life, are air-breathing at all stages, and nearly all lay their eggs on land. Gill passages appear, as in birds and mammals, only in the embryo.
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