Turtles, tortoises, and terrapins alike possess a shell. This much you know. But to understand why they never leave the house without it, it's important to understand this organic armor's inner workings.
Turtle shells aren't like the exoskeletons found on insects, or the upcycled outer crusts adopted by hermit crabs. As Business Insider points out, these shells are built right into the structure of the reptiles themselves. They're never shed, they don't molt off, they aren't abandoned to make room for more turtle. Rather, they're a complex and specialized combination of extended rib and pelvic bone intermingled with what's called "dermal bone," a membranous, hardened skin. In short, they're just as much a part of the lizards' bodies as your silently screaming vestigial twin is a part of yours.
Inside the shell, turtle bits are packed in tight, expertly crammed together like a moist game of Tetris in such a way as to allow many species to retract their heads and limbs. This gives the illusion of a hollow interior, but in reality, that iconic turtle crust is housing about as much reptilian intestine as will fit. Basically, to answer the original question, a turtle without its shell would look red, wet, and exceedingly dead.
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