Turtles – or more specifically turtles, tortoises, and terrapins – have arguably some of the most unusual body plans in the animal kingdom. The exterior is simple enough to understand, with features visible to the human eye including four limbs, a head, and some kind of butt emerging from a shell (or hardened skin slab, in the case of softshell turtles), but have you ever wondered what’s inside a turtle’s shell? We decided to find out.

What is a turtle's shell made of

Turtles hatch with a shell, but according to ZSL London Zoo keeper Daniel Kane, it can be a far throw from the animal's final attire. "Small hatching turtles often have fairly soft shells but these tend to harden in the first few days or weeks of life," wrote Kane, in an email to IFLScience. "When spiny hill turtles hatched at ZSL London Zoo a few years ago, for example, the egg was the size and shape of a kiwi, but after a few days the animal itself was as flat as a pancake and spiky."

The shells themselves have several layers which can include bone, live tissue, and cartilage. A unifying feature for most species (excluding the soft shelly bois) is an outer layer of scutes (Latin for "shield") which are made of keratin – a strong, fibrous structural protein that makes everything from horns to even human nails and hair. Strong though it may be, the shell can still be sensitive to touch which may explain why some tortoises tend to dance when under falling water.

Can turtles survive without a shell, or if it gets injured?

Despite what cartoons may have led you to believe, turtles can’t be removed from their shells. The shell isn’t a fashionable bit of armor popped on by turtles so they can sunbathe on the backs of toothy predators, it’s actually a part of their skeleton. They can heal moderate injuries, but if the shell is severely damaged it's unlikely they'll survive, as their internal organs will be exposed. The dome on top, known as the carapace, has a bony inner lining that fuses to the animal’s backbone. The plastron, which is the flat bit on the turtle’s “belly”, is also bony and fuses with the animal’s rib bones and sternum.

Not all of the turtle’s skeleton is on the outside, though, as its shoulder and hip girdles actually sit inside the shell. This makes land-dwelling turtles such as the tortoises unique, as they’re the only terrestrial vertebrates strutting around with their hips and shoulders basically tucked inside their ribs. Carrying around a big, bony dome might sound like hard work, but the shell isn't as heavy as it looks. "The bone in the shell of many, if not all, species is actually really lightweight as it has a sort of honeycomb structure," explained Kane. "Most of the weight of even a really large tortoise such as, for example, our Galapagos tortoises Dolly, Polly and Priscilla, seems not to be from the shell but more from what’s inside it."

How do turtles breathe

It only gets weirder as we start to look at how these animals breathe. Their lungs are positioned towards the top and front of the carapace, a very handy adaptation that allows all turtles, even the chunky land tortoises, to float. According to Kane, the talent is thought to have been pivotal in these shelly reptiles colonizing oceanic islands such as the Seychelles, Mauritius, and Galapagos.

The only downside of these shell-encased lungs is that, since the turtle’s ribs have become fused and keratinized, it can’t rely on negative pressure in the chest cavity to pull air into its lungs. The turtle has two alternatives here; it can use a muscle sling that’s attached to its shell as means of drawing air in and out (a feature which first emerged in Eunotosaurus africanus, an ancient reptile found in South Africa during the Middle Permian, roughly 260 million years ago) – or it can use its butt.

turtles use their butts to breathe?!

In case you need further clarification on “butt breathing,” the physiological wonder is performed via one of nature’s most versatile structures: the cloaca. This all peeing, all pooping, and all egg-laying super-orifice helps turtles (as well as reptiles and birds) excrete all manner of things, but for turtles, it can also be used to take in oxygen and dump carbon dioxide. This happens as aquatic turtles take water in via the cloaca, where gas exchange occurs, and the water is released again. This is the respiration process that allows some turtles to hibernate underwater, but they do have another trick up their shells.

Inside a turtle’s shell is a unique structure that comes into play when some turtles, like the painted turtle, switch from aerobic to anaerobic (with or without air, respectively) respiration while hibernating underwater. In this state, the turtles use glucose (instead of airborne oxygen) and release lactic acid (the biological byproduct that makes us “feel the burn” during exercise). Too much acid is a problem for the snoozy turtle, so its shell absorbs the acid and releases bicarbonate to neutralize it in the same way that taking an antacid can neutralize acid reflux in humans.

Where are the rest of its organs

Things return to a more conventional body plan when you look at the internal organs of turtles, which sit between its bony lower plastron and lungs (you can see a dissection here). Here you’ll find the digestive and reproductive systems as well as the heart, liver, and kidneys.

[H/T: Business Insider]