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Turtles have an unmistakable appearance, with a head, tail, and four legs projecting from a broad bony shell. The domed upper carapace and flattened lower plastron serve to protect the torso and its organs. In most land-dwelling and amphibious turtles the head, limbs, and tail can be withdrawn inside this shell if danger threatens. A horny beak (like that of a bird) covers the jaws. The head, legs, and tail are covered by horny scales, and the feet have horny nails. The bony shell is covered with an epidermal layer of scutes in most turtles, occurring in a regular pattern that may be diagnostic of the species. Only the leatherback sea turtle and the softshell freshwater turtles lack these horny plates.

The internal anatomy of turtles is rather typical of vertebrate animals, with two lungs, a reptilian three-chambered heart and associated circulatory system, and an unremarkable digestive system with an esophagus, stomach, small and large intestines, and an associated liver. As with other reptiles, the digestive, urinary, and reproductive systems vent to the outside through a chamber known as the cloaca, which also encloses the penis of male animals. The nervous system comprises a well-developed brain, a spinal cord enclosed in a vertebral column, and peripheral nerves. The senses of vision (including color vision) and hearing are acute.

Species of turtles have an extreme size range. Adult American mud turtles (Kinosternon subrubum) are less than 5.0 in (12.5 cm) long, while the gigantic leatherback sea turtle (Dermochelys coriacea) can attain a length of more than 6 ft (183 cm) and weight up to 1,500 lb (680 kg).

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Science EncyclopediaScience & Philosophy: Toxicology - Toxicology In Practice to TwinsTurtles - History And Fossil Record, Morphology, Ecology, Behavior And Life History, Side-neck Turtles - Classification, Turtles and humans