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Welcome to Brain Stuff, a production of IHeart Radio.

Hey, Brainstuff, Lauren Vogelbaum here.

Turtle is the common American English word for the 350-some species of reptiles known for their characteristic shells.

Sometimes you might see, especially in British English, distinctions between land-based tortoises,

freshwater terrapins, and marine turtles. But today I'm talking about the whole mess of them

using the word turtle, okay?

Okay.

They live in oceans, lakes, streams, forests, and deserts all over the world, except Antarctica.

They might be in a yard or pond near you.

And those shells are amazing adaptations.

The shape of a turtle shell depends on the species and the

turtle's habitat. Most land-dwelling turtles have a high-domed shell, which helps protect them from

the jaws of predators, while most aquatic turtles have a more streamlined, flattened shell

that lets them glide through the water. Turtletles shells are made up of two parts.

The carapace, which is the upper half on the turtle's back, and the plastrin, which is the lower

half under the turtle's belly. Structures called bridges fasten the two together at the turtle's

sides. They're generally bony, but in soft-shell turtles,

they're flexible. For the many species of turtles able to retract into their shells,

a hinge joins the two halves together that allows the carapace and plasterin to close tightly

when the turtle draws itself in. Both the carapace and plastrin are made of bone, including about 50 to 60 rib and backbones in the upper shell, and a fusion of clavicle and rib bones in the lower shell.

On the outside of the bone, each shell half is covered in scutes, sometimes called shields, which are overlapping pieces of

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