In fact, it is their rib cage, and their spine, and their vertebrae, and their sternum.

Basically, a turtle's skeleton is inside out.

And just like you can't take a skeleton out of a person, right, you can't take a turtle out of its shell either.

But if you could, you'd probably be surprised by what you'd discover.

Here's the inside of a turtle.

That's Maria Wojakowski, a biologist who's been studying turtle ecology for more than a decade.

Here's your shoulder girdle.

Here's your hip girdle.

Notice how those hips and shoulders are actually inside the turtle's rib cage?

Turtles are one of the only land animals on the planet with this feature.

They're also some of the only animals that can breathe with their butts.

You see, inside a turtle shell is a very particular respiratory system.

You will see the lungs towards the top here.

Now, most land animals breathe by expanding and contracting their ribs, which creates a natural pump that guides air in and out of their lungs.

But turtles can't do this because their rigid shells don't expand.

So instead they rely on sheets of muscles within their shell to pump in oxygen through their mouths.

That is, most of the time.

Then there are other times when turtles breathe out the other end, more specifically, through what scientists call the cloaca.

It's the same opening that turtles use to urinate, defecate, and lay eggs.

And in some cases, it can double as a set of gills, sucking in water and absorbing the oxygen within.

Scientists think that turtles do this when they're spending long periods of time underwater, like when they're hibernating.

And if you look really closely at the inside of a shell, you'd discover another feature that helps with hibernating underwater: a scaffold-like structure that can store and release chemicals.

That structure actually helps turtles breathe without any oxygen at all.

It works like this:

Many turtles hibernate in frozen ponds that are starved of oxygen, and to survive, their metabolism switches over from aerobic to anaerobic.

That means they stop using oxygen for energy and start using glucose instead, via a process called anaerobic respiration.

And the byproduct of that is lactic acid.

Now, theoretically, this acid could build up in a turtle's body and kill it.

That's where the shell's structure comes in.

It can absorb the lactic acid as well as release a bicarbonate to neutralize that acid.

It's essentially Tums, but for turtles.

So as it turns out, having a shell is pretty handy for certain situations.

In fact, scientists think that turtles originally got their shells for digging, likely more than 200 million years ago.

They dig, like, really, really complex burrowing structures underground.

And of course, shells are incredibly useful for defense against predators, no matter how fierce they may be.

Turtles are amazing.