Inside the shell compartment are the turtle’s internal organs, including the lungs. Because the turtle’s shell is hard, its chest cavity can’t expand and contract as the turtle breathes. It has to push air in and out of its lungs another way—by using strong trunk muscles that expand and contract the lungs with their movement. If you’ve ever picked up a turtle, you may have heard a hissing noise. That’s the sound of air being quickly pushed out of the lungs to make room for the head under the shell.
Some species, such as box turtles and Blanding’s turtles, have hinges of flexible tissue between the bony plates of the plastron. This allows them to close up the shell like a box, providing added protection.