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In Real Life, a turtle's shell is fused to its spine (It is, after all, a modified rib cage). Removing a shell in Real Life, if possible at all, will kill the turtle in seconds. But in fiction, it is often treated as a removable article of clothing. Expect the turtle, when out of the shell, to be wearing little more than Goofy Print Underwear.
Alternately, the shell may be large enough to not only accommodate the resident turtle, but also allow somebody else to hide inside it at the same time. In some cases, the Removable Shell may also be portrayed as having a furnished apartment (and often Bigger on the Inside). Another variant will be for the turtle to withdraw all of its limbs (and its head) to make them pop out so that it does a 180 degree turn without moving from the spot.
A variant can apply to other shelled animals like snails, although snails with removable shells are less common.
Sub-Trope of Art Major Biology, Appropriate Animal Attire, and Funny Animal Anatomy.
Compare Fur Is Clothing (the mammalian equivalent).
Contrast Eggshell Clothing (the shell that is supposed to come off does not).
The obscure Japanese series Oraa Guzura Dado had one turtle character, for whom this trope was a running gag. (The series is animated, but doesn't look like anime at all.)
Dragon Ball's Master Roshi, though not a turtle, is the turtle hermit, and wears a heavy shell on his back for training purposes which is fully detachable.
In Pani Poni Dash!, the cast travels inside Himeko's dreams, wherein the Archangel Michael has a turtle companion who keeps losing her shell.
Back in the Eighties, there was a newspaper cartoon (Some Thoughts by Eric Johnson) in the UGA college newspaper, The Red and Black, that featured a series of strips about a turtle jumping out of his carapace (wearing the requisite wife-beater and boxers), later suggling inside the carapace with a turtle-honey, and finally, about some Georgia Tech students who indignantly wrote in to the cartoonist to complain that they had opened up a real turtle and not found a little animal in boxer shorts.
The Terrific Whatzit, a DC ComicsGolden AgeFunny Animal superhero, is a turtle who possessed a costume similar to that of the Golden Age Flash, and powers similar to those of Golden Age speedster Johnny Quick. When in costume, he removed his shell (his superhero name stemmed from the difficulty of telling what species he was without the shell).
Films — Animation
Verne in Over the Hedge often loses his shell. After this happens several times, R.J. asks Verne, "What is the point of this thing again?"
In Cats Dont Dance, T.W. Turtle never actually gets out of his shell, but he did duck into it on one occasion and re-emerge with a scrub brush and soap, as though he'd been cleaning the inside.
One scene in Robin Hood had Toby Tortoise pull his head into his shell and peek out from one of his armholes.
Clothahump the turtle wizard in Alan Dean Foster's Spellsinger series doesn't have a removable shell, but he has enchanted his plastron to install transdimensional cabinet drawers in his chest.
A board book by Sandra Boynton featured a turtle inside its shell looking at another turtle that's outside its shell.
A picture book by Eric Carle was actually about a turtle who took off his shell.
Mystery Science Theater 3000: During one of the Gamera movies, the bots decided to make a model of Gamera to show what's inside his shell. As they imagined it, he has a fully furnished apartment, complete with a gym, inside there. As well as another door which should never be opened, because that's where Gamera keeps all his internal organs.
Subverted in Red Dwarf: Lister recounted how, when he was a kid, he once opened a turtle's shell with a can opener. Said he, in his defence: "Well, I didn't know!"
Subverted in a Noodle Incident mentioned by Darnell on My Name Is Earl. Randy apparently pulled the original Mr. Turtle out of his shell to see what was inside, then crammed him back into it hoping Darnell wouldn't notice. The original Mr. Turtle didn't survive.
Dungeons & Dragons has the Tojanida; a monster that resembles a cross between a sea turtle and a lobster. It has eight holes in its shell, out of which it can freely stick its flippers, claws, and head in any configuration it desires.
Dub from Die Anstalt: Treating his neuroses required you to literally get him out of his shell, and it's later revealed that his problems stem from having it on inside out.
Well, it's reversible. Having it on inside out isn't so much the source of the problem as a hint to what the root of the problem really is (the racing stripe indicating his need to always be faster).
In American McGee's Alice, Mock Turtle's shell gets stolen by the Duchess, and Alice gets to become a honorary reptile by helping recover it.
Carapacion-type enemies in Final Fantasy XIII might qualify, as when "Broken" their carapace vanishes, which removes their Nigh Invulnerability until they restore the carapace.
It's only really iffy because even when they actually have the carapace it's not a true shell, but more closely resembles a pangolin or man-made scale/plate mail.
The Polish puzzle game Zagadki Lwa Leona features a puzzle where you have to help a turtle whose shell apparently was stolen and inexplicably got stranded on a branch.
Rare gastropod example: According to its Pokedex entry, the Pokémon Shelmet is a snail-like Pokemon whose helmet-like shell is often removed from its body by an beetle-like Pokemon called Karrablast so it can evolve into Escavalier (which resembles an armored insect who has spears for arms and said armor being made from said shell). Fortunately, Shelmet evolves into Accelgor, a Ninja snail.
The Oshawott family (based on sea otters and other aquatic mammals) also have removable shells, called Scalchops (for Oshawott and Dewott) and Seamitars (for Samurott). However, unlike most of the examples listed here, the shells are too small to hide inside and are instead used solely to be removed as weapons.
Mega Man X2 has Crystal Snail. If you hit him with the Magnet Mine, his shell flies off, causing him to lose his only method of defence and causing him to focus on trying to reclaim it over attacking you...which you can prolong indefinitely by knocking the shell around. Justified because he's a robot.
In Aquaria, Naija can remove the shells of small turtles with her Bind song, which would otherwise damage her.
The Escarglow dream eaters in Kingdom Hearts 3Dthrow their shells like boomerangs for a number of their attacks. They have a serious flaw in that their shells will be smacked away if they receive a solid hit, which causes them to prioritize getting them back over anything else and being snails, they do this slowly.
Turtles in Darkwing Duck for NES sneeze their shells at Darkwing, at which point they are vulnerable (but, interestingly, still really tough, requiring 6 pellets to go down). The shell then comes back like a boomerang, so watch your back while attacking them.
Simonla of "Sonichu" assurs the other Rosechus, and the audience, that her shell is removable so nothing will get in the way of her poorly-drawn breasts when she poses nude.
Gary the snail in Sponge Bob Square Pants, depending on the episode. Sometimes the shell lifts up to show something hidden inside; sometimes the shell is Bigger on the Inside and SpongeBob is able to crawl into it; and sometimes cracking the shell exposes Gary's veiny, throbbing innards.
Franklin and all of his family members have shells that are removable. Played with in a fire safety story in which Mr. Turtle found that he could not evacuate his house by the window unless he first removed his shell.
Happens in the very first episode of Dink, the Little Dinosaur. Crusty takes off his shell to take a bath. When the resident badguy, Trannor shows up, one of the characters hides in the shell and he carries both the shell and the character off.
The hermit crab. It's quite an oddball in that it's the only arthropod that uses a calcified shell for protection. Except, it doesn't produce its own, it simply looks for an abandoned gastropod shell. As it grows, it needs to look for bigger and bigger shells to accommodate. Quite hilariously, they also use manmade rubbish - like plastic bottlecaps, for instance.
The Turtle's shell is the last part of the reptile's body to deteriorate.
Sadly, this trope is responsible for an awful lot of badly-injured turtles turning up at vet clinics and wildlife rehab centers, after ignorant people's attempts to "take off" their shells to see what they look like without them. Answer: They're dead without them.