Thought to be one of the oldest living reptiles, turtles appeared on earth more than 250 million years ago and have changed little. Though their body plan is foreign to us, it has served them well during this time, and they have adapted to live on both land and sea.
The turtle shell is made up of two parts: the carapace (top, rounded shell) and the plastron (the bottom shell). Both shells are made of fused bone. In fact, turtle have vertebrae making up their spinal cord, just as humans do, but their vertebrae are fused to their shell.
Turtle Skeletal Structure
Turtles are vertebrates, so they have similar skeletal structure to humans. However, the pubic bone is inverted, pointed inward rather than outward, and the pelvis is fused to the plastron. They also have an elongated scapula to aid in movement with the shell. Probably the most notable difference is in the skull: Turtles don't have any teeth. They use beaks and strong jaw muscles to crush food.
Turtles have a three-chambered heart that consists of two atria and one ventricle. Oxygenated blood and deoxygenated blood is separated by membranes and blood flow in the ventricle, but there is some mixing. When turtles are diving for food, they can bypass the lungs and send blood directly to the rest of the body. Because turtle ribs are fixed to their shells, they don't move for the lungs to expand. Thus, turtles have special muscles that allow the lungs to inhale and exhale air.
The turtle digestive system is not that different from most other vertebrates. Since they have diverse diets, though, they often consume plant matter, which is difficult to digest. To account for this, they have an elongated small intestine with bacteria in the caecum that help them digest. This slows digestion but allows them to retain more nutrients.