Turtles are reptiles whose protection comes from a shell. The shell is composed of hard, bone plates covered by scutes. The scutes are made of keratin, the primary substance in hair, nails and hooves of other animals. The pigment melanin, present in the scutes, may form intricate designs and brightly colored patterns in some species.
The scutes of most turtles are arranged somewhat like the examples below.
Although the scutes form the familiar outer layer of the shell, it is the
bony layer underneath which actually provides the shape, support and protective
qualities of the turtle shell. The inner layer of bone is fused with the
usual bony structures associated with all vertebrates, vthe
vertebral column and ribs. The vertebrae are particularly interesting for
the modifications that have occurred. The vertebrae of the neck and tail
are small, allowing for a high degree of flexiblility, while the vertebrae
of the central portion of the vertebral column are enormously elongated
and inflexible, fused with the bony layer of the shell, acting as a support
for the carapace.
There are many health implications associated with shell anatomy. For instance, if the outer keratin is breached by infection or injury, the turtle can lose its protection and infection can proceed into the bony layer and the body cavity, threatening the turtle's life. Another example is that the rigid shell prevents motion of the ribs. Because of this, turtles lack the diaphragm that allows other animals to cough. If fluid enters the lungs (which are located just under the carapace) pneumonia presents deadly dangers since the turtle will not be able to easily rid itself of the fluid, and infection is likely.
Although the shell provides excellent protection from many predators, it can also make the turtle vulnerable to health problems if it is not given good care in captivity.
For more on turtle anatomy: Euro Turtle's anatomy page
