? Squamata, with about 3000 species of lizards, 2500 of snakes, and 140 of worm
                        lizards (amphisbaenians); 

                        ? Chelonia, or Testudines, comprised of about 250 species of turtles and tortoises; 

                        ? Crocodylia, including 21 species of crocodiles, alligators, gavials, and caimans; 

                        ? Rhynchocephalia, which flourished during the Jurassic period but today is
                        represented only by the tuatara, which lives on several islands off the coast of New
                        Zealand. Like most classes of vertebrates (animals that have a backbone) reptiles
                        vary in size. The increasingly rare Indian gavial may reach a length of 9 m (30 ft) and
                        weigh 540 kg (1200 lb). The gecko lizard, by contrast, is typically 3 cm (1.2 in) long.
                        Reptiles also include a fascinating range in behavior and appearance; yet, from the
                        timid box turtle to the savage Komodo dragon, all share certain traits that set them
                        apart as reptiles. 

                        Physical Characteristics

                        All reptiles have scaly, thick, waterproof skin that is dry to the touch. Turtles and
                        tortoises have wide, flat ribs over which is laid the "shell" of horny or leathery scales
                        that serves as protective armor. Snakes and many other species molt: As they
                        grow, they periodically shed the outer skin, revealing a new layer that has grown
                        under the old. Turtles and tortoises gain new shells in much the same way. 

                        Internal Structure

                        All reptiles, even aquatic species, breathe through lungs. The serpentlike eel may
                        look like a reptile, but it is classified as a fish because it breathes through gills
                        instead of lungs. In most snakes and some lizards only one lung is functional; in
                        crocodiles and other reptiles both lungs are equally developed. The thorax and
                        abdomen in reptiles are not separated by a diaphragm, and breathing is
                        accomplished by muscles of the body wall. The reptilian heart generally consists of
                        three chambers, two auricles and one ventricle, in contrast to the mammalian heart,
                        which has two auricles and two ventricles. In the crocodilian heart, however, the
                        ventricle is almost completely divided into two chambers by a septum, or partition.
                        Blood is pumped from the heart to the rest of the reptile?s body through arteries.
                        Venous blood returns to the heart from the tail and hind limbs through the kidneys
                        by way of a renal portal (vein-capillary); blood from the abdominal region returns by a
                        portal system passing through the liver. Carnivorous reptiles frequently have fairly
                        simple digestive systems; those that are herbivorous have a more complicated
                        arrangement. All have an intestine for excretion; the urinary bladder is present only
                        in turtles and lizards. 

                        Body Temperature

                        Reptiles are commonly described as cold-blooded, although the term can be
                        misleading. When active, some reptiles maintain their bodies at a higher
                        temperature than do most mammals. The important difference in temperature
                        physiology is that reptiles rely on external sources of heat, whereas birds (which
                        may be descended from ancient reptiles) and mammals generate internal heat.
                        Reptiles regulate their body temperatures by taking advantage of sources of outside
                        warmth, such as direct sunlight, warm surfaces, and the earth itself. Often the body
                        temperature of a reptile is well above that of the surrounding air. Only when the
                        animal is inactive or dormant is body temperature approximately the same as that of
                        the environment. 

                        Eating Habits

                        Crocodiles, snakes, and lizards are meat-eaters, as is the tuatara. Turtles and
                        tortoises can be categorized as omnivorous; those that live on land generally eat
                        vegetable matter, but aquatic species are largely carnivorous. 

                        Reproduction

                        Most reptiles reproduce sexually, and most mate in the spring. Female turtles,
                        crocodilians, and the tuatara are primitively oviparous?that is, they lay eggs. Most
                        snakes and lizards lay eggs as well. Other reptiles, such as the anaconda, are
                        viviparous?that is, they give birth to living young; and some, such as vipers, are
                        ovoviparous?that is, they give birth to living young from eggs hatched inside the
                        mother?s body. Some large sea turtles may lay as many as 200 eggs in a clutch;
                        the tiny geckos generally lay two eggs at a time. Reptile eggs have shells and
                        contain two embryonic membranes: a protective amnion, or egg sac, which prevents
                        the egg from drying out, and an allantois, or vascular fetal membrane that serves as
                        a bladder and a kind of lung. In many species the eggs are left to develop by
                        themselves in a sheltered area, where they may draw heat from the sun or the
                        overall environment and be protected from predators. Many, however, are lost to
                        birds, mammals, and other reptiles. 

                        Habitat

                        Reptiles inhabit most parts of the world, but because of their need to obtain warmth
                        from the environment, they do not live or develop in colder regions. Most are found
                        between 40°N and 40°S lat. In temperate regions they may hibernate during the
                        winter, and in hot climates they frequently estivate?lie motionless, in a state of
                        torpor, during periods of intense heat. Thus, reptiles in the Tropics generally are
                        more active at night than during the day, when they tend to lie in the shade.
                        Water-dwelling turtles and tortoises are adapted to the ambient temperature of the
                        environment where they live, but none are found in the Arctic or Antarctic regions. 

                        Evolution

                        The earliest reptiles developed from amphibians during the Upper Carboniferous
                        period of the Paleozoic era about 300 million years ago. Many forms evolved and
                        flourished in the Mesozoic era, the Age of Reptiles. The best-known orders of
                        extinct reptiles are as follows: 

                        Ichthyosauria

                        These large marine reptiles had bodies that resembled those of present-day fish.
                        Some species were more than 4 m (13 ft) long. 

                        Sauropterygia

                        This order included long-necked aquatic forms that had large bodies and limbs
                        adapted for moving through the water. 

                        Saurischia and Ornithischia

                        These orders included the reptiles commonly known as dinosaurs. First found in
                        Triassic strata, they became extinct at the close of the Mesozoic era. 

                        Pterosauria

                        This large, diverse group of flying reptiles is related to the dinosaurs. Their wings
                        were thin membranes attached to the body by the very long fourth finger of each
                        arm. 

                        Therapsida

                        Commonly called "mammallike reptiles," these were not reptiles at all, but
                        relatives of mammals that split from Reptilia by the end of the Carboniferous
                        period. 

                        Recent evidence indicates that at least some extinct reptiles were warm-blooded.
                        In particular, pterosaurs (commonly called pterodactyls), the flying reptiles, had a
                        thick fur coat, indicating the need to insulate body warmth. It is also likely that at
                        least some dinosaurs, with their rapid growth rate and erect posture, were
                        warm-blooded. Because of this some zoologists have proposed reclassifying
                        these fossil animals out of the class Reptilia. 

                        Of the almost 6000 species of reptiles still in existence, about 400 are
                        endangered, most because of human activity. Turtles have been killed for their
                        meat, alligators for their hides, and many people regard snakes, in particular, as
                        dangerous serpents to be eradicated. Probably the most significant problem,
                        however, is the encroachment by humans upon traditional reptile habitats.