|Turtles, a generic name for the group of reptiles which includes
tortoises and terrapins, are reptiles most of whose body is shielded by
a special bony shell developed from their ribs. All extant, or living, turtles
are members of the order Testudines, which includes both living and extinct
varieties of turtle.
There are two major groups of turtles: sea turtles, which grow to large sizes and live in the oceans in the temperate and tropical regions of the earth, and fresh-water turtles.
Fresh-water turtles which spend the majority of their time on the land are generally called tortoises. In the United Kingdom aquatic fresh-water turtles are known as terrapins. Fresh-water turtles are generally much smaller, ranging in size from a few centimeters to a few feet long. All turtles have a protective shell around their bodies. The top part of their case is called the carapace, the underside is the plastron, and the two are connected by the bridge.
The size of turtles can vary from a few centimetres to up to two meters. Turtles generally live a long time; some individuals are known to have lived longer than 150 years.
The first turtles already existed in the era of the dinosaurs, some 200 million years ago. Turtles are the only surviving branch of the even more ancient clade Anapsida, which includes groups such as the procolophonoids, millerettids and pareiasaurs. All anapsid skulls lack a temporal opening. All other extant amniotes have temporal openings (although in mammals the hole is obscured). Most of the anapsids became extinct in the late Permian period, with the exception of the procolophonoids and the precursors of the testudines (turtles).
Even though they spend large amounts of their lives underwater, turtles are air-breathing reptiles, and must surface at regular intervals to refill their lungs with fresh air. They also spend part of their lives on dry land. Sea turtles lay their eggs on dry sandy beaches, and are highly endangered largely as a result of beach development and over hunting.
Aquatic respiration in Australian freshwater turtles is currently being studied. Some species have large cloacal cavities that are lined with many finger-like projections. These projections, called "papillae", have a rich blood supply, and serve to increase the surface area of the cloaca. The turtles can take up dissolved oxygen from the water using these papillae, in much the same way that fish use gills to respire.