Introductionreptile, name for the dry-skinned, usually scaly, cold-blooded vertebrates (see Chordata) of the order Reptilia. Reptiles are found in a variety of habitats throughout the warm and temperate regions (except on some islands), with the greatest variety in the tropics. Reptiles differ from other terrestrial vertebrates (birds and mammals) in that they are cold-blooded, that is, they lack an effective system for regulating their body temperature, which tends to approach that of the environment. For this reason reptiles are not found in the coldest regions of the world, and they hibernate in cool winter areas.
They range in size from 2-in.-long (5-cm) lizards to 30-ft-long (9-m) snakes. They typically have low-slung bodies with long tails, supported by four short legs that project outward from the sides of the body; however, all snakes are limbless. Although reptiles are fundamentally a terrestrial group, some are adapted to living in water. All breathe air by means of lungs and have thick, waterproof skins designed for retaining body moisture. Unlike amphibians, they do not possess gills or breathe water at any stage of their development, and nearly all lay their eggs or bear their young on land.
The reptilian egg has a porous shell and a system of membranes designed to protect the embryo from desiccation. It also has a large quantity of yolk for nourishment. This type of egg is typical of terrestrial vertebrates, and is very different from the simple, unprotected eggs of fishes and amphibians, which are laid in the water. Fertilization is internal in reptiles, and males have copulatory organs. Females of most species lay eggs, but in some the egg is incubated and hatched internally. In a very few there is true live birth, with the young nourished by a primitive placenta instead of an egg yolk.
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