Sudden Anatomy

In a lot of comics and animation, the anatomy of living creatures tends to be simplified in a variety of ways. Some of these are common, such as people not having any discernible lips. Others are more unique to their artists, like how the children in South Park don't seem to have legs, just feet connected directly to their torso.

However sometimes the piece of anatomy that's usually missing becomes important to the story, and so a character will suddenly gain that piece of anatomy for exactly as long as they need it. For example, if characters in a series don't usually have lips, they'll suddenly sprout a pair if they need to kiss someone. Or, if they're normally drawn without eyebrows, but they're doing a Spock imitation, they'll suddenly grow some just for the Fascinating Eyebrow moment.

Depending on how entrenched the art style is in the audience's mind, this can seem rather jarring. And sometimes that's actually the desired effect.

Anime and Manga

 * In the Dragon Ball manga, Demon King Piccolo (who has Four-Fingered Hands) temporarily grows a fifth when he tells Goku "You shall die in five seconds". This is actually Lampshaded by the author. (In the anime, Piccolo has five fingers, so the trope is averted.)

Comic Books

 * In the Fritz the Cat comics and both movies, Fritz (who wears no pants) is only... er, anatomically correct during sex scenes. The female characters, however always are.
 * One comic issue featuring Donald Duck shows the characters experiencing a sound tremor that causes everything to vibrate, with one of the duck triplets making a comment going "If I had teeth, they would rattle". The interesting thing is that in that very panel, he does. And this is not uncommon - the Disney duck characters are drawn with teeth at many other occasions in the comics, usually gritting them or brushing them.

Film

 * Verne the turtle in the movie Over the Hedge mentions his tail a lot, and it's prefectly visible whenever he turns his back. Unless he loses his shell.
 * None of the characters from Sleeping Beauty have visible irises, but when Princess Aurora finally awakens from her deep sleep at the end of the film, her eyes actually turn dark blue but turn black again during the epilogue.
 * B.O.B. from Monsters vs. Aliens is able to grow arms when needed. Justified in that he's a Blob Monster.

Video Games

 * Monkey Island: The only time Guybrush Threepwood has appeared with five fingers was during the banjo duel with Van Helgen in the third game.
 * Most Sonic the Hedgehog characters seem to only have mouths when they're talking.

Web Animation

 * Averted in Homestar Runner, where many characters have no visible arms. Rather than sprouting arms when necessary or being hindered by their character design, these characters Take a Third Option and seem to manipulate objects through telekinesis (though it's sometimes implied they simply have invisible arms).
 * Madness Combat gave an unfortunate Mook an ear just so the protagonist could rip it off.
 * The klaymen of Klay World, who often grow fingers just to point at or stroke each other, and don't talk with a mouth but grow one whenever they feel like chewing on something.

Web Comics

 * In Sluggy Freelance Bun-Bun is never drawn with a mouth. It's quite clear he has a mouth, since he chews on stuff a lot and (ahem) mouths off any time he feels like it, but as of 2010, we have seen it only three times. In 13 years.
 * MS Paint Adventures: The characters tend not to have visible arms unless they're using them. This birthed the running gag of "quickly retrieve arms", generally producing a response to the tune of "You've already got arms, numbnuts!"
 * In City of Reality, Mayor Rabbit's arms seem to just mysteriously disappear whenever he's not using them.
 * In Scandinavia and The World, the characters are drawn without arms, except when they need to make gestures or hold things.
 * Characters in UG Madness are usually drawn without fingers (there are two guest comics that poke fun at this), but will suddenly have them if they need to point at something.
 * Order of the Stick has the hands of its stick-figure characters go from three lines for fingers to recognizable fists when someone punches another person.
 * In Rescuing Dara, Dara only has arms when she's using them, which seems to be a trait of her species, as seen in this sprite animation from the Pokemon games.

Web Original
"Joel: I wish I had arms for this scene.
 * The Ask Jappleack tumblr makes a point of how Jappleack, possessing hooves, grabs things...HilarityEnsues.
 * In its parent series Pony Dot Mov, Fluttershy was briefly seen with fingers so that she could snap them in SHED.MOV, and in MAGIC.MOV Twilight is seen actually sprouting a finger out of her hoof with which to press a button.
 * Characters in Bonus Stage will often be without arms in inaction scenes. Lampshaded at one point:

His arms appear from the top of the screen.

Joel: Oh, there they are! They descend and attach to his body"

Western Animation

 * One episode of The Simpsons had a B-plot where Homer couldn't remember what color Marge's eyes were, and she refused to take off a pair of sunglasses until he could remember. However, since their eyes are always drawn as simply a black dot in the middle of a white orb, the audience had no clue what color Marge's eyes were either. At the very end of the episode, though, the camera zooms in on Marge's eyes to reveal a thin (and I mean very thin) hazel-colored iris. The idea here seems to have been, yes, Simpsons characters do have irises, it's just that they're so small we can't see them outside of an extreme closeup. In another episode, a boy hits on Lisa by calling her "Blue-Eyes".
 * The characters are not usually drawn with nails, but in "Trilogy Of Error", Homer's severed thumb has a fingernail on it. Plus, Homer's toenails are visible during an extreme closeup of his weight scale in "King Size Homer".
 * In The Grim Adventures of Billy and Mandy, Mandy is always drawn without a nose. Her lack of nose is even an important plot point in one episode. However, when Billy briefly takes over Mandy's body, he decides to pick her nose, and when he does nostrils suddenly appear on her face in order to allow it. Only the nostrils, though.
 * This is also the case when Mandy sniffs Billy's stink bomb in "The Really Odd Couple".
 * Looney Tunes
 * In "Rabbit of Seville", Bugs Bunny is drumming on Elmer's head as if playing a piano. A close-up shows that Bugs' normally Four-Fingered Hands now have five fingers, because the piano piece couldn't be shown accurately otherwise.
 * Gossamer's arms appear from and disappear into his body as needed.
 * In "High Diving Hare" after Yosemite Sam pushes Bugs off the diving board Bugs grows a pair of toe claws to catch onto the board, this is the first and only time we ever see them.
 * In the Dilbert TV series, characters that normally have No Mouth suddenly develop one whenever they need to say something, and it then disappears as soon as they stop. In the comic, Dilbert's mouth is only visible when he's gritting his teeth or shouting.
 * The Veggie Tales characters. It started out avoiding obvious manipulation of objects, but as time went on and available plots where this was still an option got used up, the animators started fudging. Especially amusing is the fact that in the opening, Larry still points out that he can't play the guitar because he doesn't have hands... and in the "Silly Songs With Larry" segment of Lord of the Beans, guess what we see him do.
 * They recently gave the opening sequence a major overhaul (new animation, new recording of the song) that finally takes out that joke.
 * See also the Flash-animated spinoff Larry-Boy series, where the animators came up with every excuse they could to let the characters physically manipulate things: Larry-Boy's utility belt has robot arms for every gadget, Professor Bok Choy wears a robe with empty sleeves to hold his pointer, various villains either have natural or technological arms, and a scientist was shown wearing a levitating "empty" glove.
 * South Park
 * Most of the kids in only have eyebrows when making certain expressions such as anger or sadness. Most of the adults, however, always have visible eyebrows, as well as some additional facial features.
 * The kids have no visible fingers just thumbs usually, however whenever they need to emphasize something or hold on to something they have them. Most of them don't have ears or noses either but sometimes they appear suddenly when they use them only to disappear seconds later. For example, hey suddenly gain noses when they are shown pressing their faces up against the glass.
 * In "A Very Crappy Christmas", when Stan does an impression of Cartman, he suddenly has a double-chin just like Cartman has.
 * Stan was also noted to have blue eyes once.
 * Canadians have a whole different set of Missing/Sudden Anatomy.
 * The Powerpuff Girls:
 * In "Criss Cross Crisis", Buttercup, realizing she's in Professor Utonium's body, flexes the fingers on the Professor's hands (Buttercup's usual body has no fingers). In the accompanying closeup, Four-Fingered Hands is averted (like in the Bugs Bunny example above).
 * "Mr. Mojo's Rising" features a dramatic close up of the Professor's eyes as he's remembering when (Mojo) Jojo was his lab assistant and he suddenly has both irises and eyelashes.
 * In a Garfield TV special when the announcer says that a dog can play five instruments at once he grows an extra finger to emphasize this, the fifth finger disappears seconds later.
 * In SpongeBob SquarePants it is often pointed out that Patrick doesn't have a nose. But, in "Wet Painters", after seeing Spongebob make a paintbrush with some nose hairs he suddenly gains a nostril and produces a giant mess of nose hairs. And, in "Battle of Bikini Bottom" he makes himself grow one in order to fling boogers at Spongebob. There's also Squidward, Patrick and Spongebob growing hair on more then one occasion. And of course, the infamous toenail removal scene in the episode "House Fancy". In the episode "Sailor Mouth," Spongebob counts the swear words on his fingers during Mr. Krabs' cluster F bomb, announcing that Mr. Krabs' said all thirteen bad words, holding up his hands with a combined total of thirteen fingers. They go away after the gag is over. Patrick has also been capable of growing hair by sheer will.
 * In an episode of Camp Lazlo involving serial pantsing, the Long Pants the characters usually wore became well-defined shorts just seconds before they were pulled down.
 * Phineas and Ferb's Perry the Platypus only has teeth when he wants to bare them.
 * One episode of Cat Dog played this for laughs by having Cat hold up seven fingers.
 * On one episode of The Amazing World of Gumball, Richard is able to grow a moustache to make himself look tough.
 * When Gumball is listening to something across a room in "The Robot" he suddenly grows a large human-like ear on the side of his head and the cat ears he normally has on the back/top of his head disappear.
 * While not a human, Bender of Futurama tends to suddenly have plugs, switches, buttons and other devices on his body whenever a joke calls for it.
 * My Little Pony Friendship Is Magic: Unlike the usual case, this is done constantly and subtly with eyebrows in the show, which for most characters only appear when an expression requires them.
 * Frosty the Snowman has Four-Fingered Hands, but sprouts a fifth finger when he tries to count.
 * Done often in Adventure Time, usually with eyebrows in a wide range of styles or chin hair on Finn for humor/emotion. Marceline also picks her nose, once by growing one and another time with her apparently invisible one.