Let's find out :)The turtle shell is broken up into three parts. We have:
The shell bones of the shell (the osteoderms) can be broken down into even further parts.
The top of the carapace in the middle is made up of many small pieces of bones called neurals. These end in the front at a large piece of bone known as the proneural and at the back in two pieces called the suprapygals.The plastron also has a different array of bony plates.
The midsections of bones on the carapace are called pleurals and on the front and rear of the pleurals are two pieces of bones. In the front there is the axillary buttress, while in the back we have the inguinal buttress. Both pieces are used to strengthen the shell more.
Finally we come to the end of the shell. All around the side are many little pieces of bones called peripherals. In the front the peripherals meet up with the proneural and in the back the peripherals meet with the pygal.
At the very front of the plastron there is two pieces of bone known as the epiplastron. These two pieces then meet up with a third piece near the bottom of them known as the entoplastron.The osteoderms of the shell are then covered by the horny keratinous scutes. These scutes can also be broken into different parts.
After the entoplastron there are two large plates of bone known as the hyoplastron.
Behind the hyoplastron is two more large plates called the hypoplastron.
Finally at the end of the hypoplastron are the last two pieces of bones. They are called the xiphiplastron.
In the middle on the sides is the bridge which composes two pieces only with a notch on both the front and hind ends called the axillary notch.
The piece of scute directly behind the head is known as the nuchal. Then all the scutes directly behind the nuchal are known as vertebrals.
Radiating along the sides of the shell are scutes called marginals. Named of course because they are at the margins or fringes of the shell.
Finally all the scutes inbetween the vertebrals and the marginals are called costals, thus making up the outer surface of the carapace.
The first pair of scutes directly behind the head are known as the gulars.Together these pieces make up the shell, one of the most amazing pieces of defensive machinery ever evolved.
These are then followed by the humerals, then the pectorals, followed by the abdominals and femorals. Note how these scutes are basically named after the part of the body they are located at or near.
The final and most rearward scutes would be, of course, the anals.
On the front of the bridge there is the axillary and on the back of the bridge there is the inguinal.
For instance in some types of chelonians the shell has evolved a cartilaginous hinge inbetween certain bones. This hinge allows the chelonian to actually close up that part of the shell making it that much safer from predators.
There are also species of chelonian who have decided to mess with their protective covering and make it more maneuvarable and less protective. In the tortoise Malacochersus tornieri (Pancake tortoise) the bony casing has been reduced so as to allow the animal to move more freely. The shell has also been widened and flattened so as to accomodate it's new lifestyle. These tortoises live in rocky areas where when danger threatens, they run to the nearest rocky crevice and cram themselves in. The new shape of the shell allows the tortoise to both move quickly and fit in very narrow spaces.Then there are the Trionychids. They are more commonly refered to as the soft shelled turtles. There shells have undergone a major tranformation. Not only has the bony layer been radically reduced, but the horny keratinous covering of scutes has been dumped in favor of a tough leathery skin. The plastron has been modified into a set of strut like supports instead of a solid casing. This relieves a lot of weight of the turtle's backs making them very fast and maneuverable. This change in shell is not doubt due to the turtle's need for speed when catching their prey and avoiding predators.
Another species that has undergone some radical change in shell structure is that of Dermochelys coriacea (The Leatherback sea turtle). It is the only species of turtle to have basically dumped it shell completely. Instead of bony plates there are many small interwoven bones that provide it's support. It's entire outer covering is cartilaginous making it very streamlined in the water but very susceptible to cuts and bruises when hauling itself out on land to lay eggs.Among other changes in chelonian shell structure there is plastron reductions. These reductions can be nearly to the point of nonexistence and are probably made to allow the turtle to move with greater ease as it's chases down frogs and insects.
So in the beginning there was the shell and it was good. But then some chelonians found that they could do better with a softer, lighter shell and one species even decided to basically remove the shell entirely. As I said before, a remarkable evolutionary achievement.
Turtles and tortoises are mostly digitigrade animals. That is they walk on their toes. All chelonians (except fully aquatic of course) are digitigrade in their forelimbs, while most tortoises are digitgrade in the hind limbs also. For the most part their forelimb terminates in five digits while the hind limb terminates in four. There are exceptions with some species have only four digits on the front leg or only three in the hind.Aquatic chelonians have either fully webbed feet or as is the case with marine turtles, flippers.
The beak is a horny piece of keratin that is self sharpening and continually grows. In some species this beak is covered over by a layer of skin which, combined with their strange noses, makes them look like their lips are in a perpetual pucker.
When this was first being considered, researchers thought that, like amphibians, chelonians achieved respiration by gular pumping. That is they thought that the constant throat pumping movements seen in these animals was used to force air into the lungs. This has turned out to be false and gular pumping in chelonians is now known to be of olfactory (smelling) signifigance.So how do they breathe?
They breathe by a process known as negative pressure differential. This is the same process that we humans use, but since they lack diaphragms chelonians had to find a different way to accomplish this.This negative pressure differential (NPD) is partly achieved via the shell. In tortoises breathing is accomplished by the use of the rigid shell and the toroise's musculature. The muscles used for breathing expand into the limb pockets at the borders of the shell and serve to modify the internal pressure within a chelonian's body. So when the tortoise moves it is expelling air in one movement and taking in air with another. This would also explain why resting turtle's and tortoise's forelimbs move in and out.
Now in species like the Chelydra serpentina (Common Snapper) the plastron has been severely reduced, which actually makes breathing on land easy since the weight of the animal's internal organs force air into the lungs in a kinda cantelevering process, which makes the expulsion of air the only muscular activity (imagine that, having to work to keep air out of your system.)
When in the water though, things change and inspiration is the newly required muscular activity, while expiration happens virtually spontaneously due to the water pressure. Of course marine species which dive to great depths have a better handle on this.So in order to deal with a life enclosed within a shell, chelonians had to not only find a way to make such a thing, but also had to modify the ways in which they moved and breathed.