Turtles are reptiles of the order Chelonia, most of whose body is shielded by a special bony or cartilagenous shell developed from their ribs. The term turtle is usually used for the aquatic species, though aquatic fresh water turtles are also called terrapins. more...
The term is sometimes used (esp. in North America) to refer to all members of the order, including tortoises, which are predominantly land-based. The order of Testudines includes both extant (living) and extinct species. About 300 species are alive today. Some species of turtles are highly endangered.
All turtles have a protective shell around their bodies. The top part of the shell is called the carapace, the bottom is called the plastron, and the two are connected by a bridge.
Sea turtles grow to large sizes and live in the oceans in the temperate and tropical regions of Earth. Pond turtles (terrapins) are usually much smaller, while some land terrapins (tortoises) are as large as sea turtles. The sizes of turtles vary from a few centimetres (forest and jungle species) to two meters (the leatherback turtle and the Galapagos tortoise).
Turtles generally live a long time; some individuals are known to have lived longer than 150 years. The oldest tortoise on record is Tui Malila, known to have lived at least 188 years.
Sea turtles lay their eggs on dry sandy beaches. The eggs of the largest species are spherical, while the eggs of the rest are elongated. Their albumen is white and contains a different protein than do bird eggs, such that it will not coagulate when cooked. Turtle eggs prepared to eat consist mainly of yolk. In some species, temperature of the egg during development determines whether an egg develops into a male or a female: a higher temperature causes a female, a lower temperature causes a male.
Although they spend large proportions of their lives underwater, turtles are air-breathing reptiles, and must surface at regular intervals to refill their lungs with fresh air. However, aquatic respiration in Australian freshwater turtles is currently being studied. Some species have large cloacal cavities lined with many finger-like projections. These projections, called "papillae", have a rich blood supply, and increase the surface area of the cloaca. The turtles can take up dissolved oxygen from the water through these papillae, in much the same way that fish use gills.
Turtles have a gelatinous substance in their upper and lower shell, called calipash and calipee respectively, the calipash being of a dull greenish and the calipee of a light yellow color.
The first turtles are believed to have existed in the era of the dinosaurs, 200 million years ago. Their exact ancestry is disputed. It was believed that they are the only surviving branch of the ancient clade Anapsida, which includes groups such as procolophonoids, millerettids, protorothyrids and pareiasaurs. All Anapsid skulls lack a temporal opening, while all other extant amniotes have temporal openings (although in mammals the hole has become the zygoid arch). Most anapsids became extinct in the late Permian period, except procolophonoids and possibly the precursors of the testudines (turtles).
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