A shell that is part of the animal
Can a Tortoise Come Out of Its Shell?
No. The shell is not a house it climbs into — it is the tortoise's own ribs and backbone, on the outside.
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Step 1 of 5
A snail's shell and a tortoise's shell are different things.
There are about fifty bones in a shell, and you can trace the seams between them. A snail builds its shell out of hard stuff it makes and adds to the rim. A tortoise's shell grew as part of its skeleton.
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Step 2 of 5
The ribs moved outwards and flattened.
In most animals the ribs curve round inside the body. In a tortoise they spread out and broadened into plates, and the backbone is fused to them along the top.
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Step 3 of 5
So there is no gap to climb out of.
Pulling a tortoise out of its shell would mean pulling it away from its own back. The shell is as much part of it as your ribs are part of you.
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Step 4 of 5
The shoulders ended up in a very odd place.
The embryo shows it happening: the ribs grow out past the shoulder and swallow it Because the ribs went outwards, the shoulder blades finished up inside the ribcage. That is true of no other animal with a backbone, and has been the arrangement for some 220 million years.
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Step 5 of 5
The cost is that breathing had to be reinvented.
You breathe by moving your ribs. A tortoise cannot — its ribs are a solid box — so it pumps air with muscles that squeeze its insides instead.
The shell grows with the animal, in rings at the edge of each plate, so a tortoise never has to swap it for a bigger one.
The short version
A tortoise can never leave its shell, because the shell is its own ribs and backbone spread out and fused — not a house it carries.
Try it yourself
Feel your ribs curving round your chest, then breathe in and feel them lift. A tortoise's ribs are flat plates that cannot do either.