and tortoises are basically divided into two groups: The
first comprising species that pull their heads into the
shell by retracting their necks into an s-shaped curve
and the second comprising freshwater species that hide
their heads by bending the neck sideways. Turtles adjusted
to living in all types of environment, and can be found
in a freshwater or terrestrial habitat, and two families
in the sea.
The turtles vary widely in size, from species of only
6 inches (15 cm) (like for example North American box
turtle), to the giant sea turtle leatherback, which may
attain a length of about 8 ft (2.4 m). The upper shell
of the turtle is called the carapace, and the lower is
called the plastron, and due to adaptive changes and mode
of life, their structure vary among different turtle species.