Turtles are adjusted to living in all types of environments. Freshwater turtles can be found in ponds, lakes and rivers, marine turtles in the oceans and seas, and terrestrial turtles on land. There are also turtles that cross the borders between these environments, making them semi-terrestrial.
Turtles come in a great variety of shapes and sizes. There are species only 6 inches in diameter (for example the North American box turtle), and the giant leatherback sea turtle, which may attain lengths of up to 8 feet.
The upper shell of the turtle is called the carapace, and the lower is called the plastron; their structures varying among different turtles. Some of the aquatic turtle species, the soft-shelled turtles, have their carapace covered with leathery skin.
Anatomically, turtles and tortoises are divided into two basic groups:
- side-necked turtles - these are freshwater species that hide their heads by bending their necks sideways
- arch-necked turtles - these pull their heads into their shell by retracting their necks in an s-shaped fashion