Most often a form of Bizarre Alien Biology, and usually played for horror or at least a thrill: A creature has inside its mouth, rather than a tongue, another mouth, face or even a whole tiny head. Expect it to be nestled snugly among More Teeth than the Osmond Family, and it may have a few choice fangs of its own.
The interior mouth may be a parasite, an offspring, or an actual part of the larger organism, such as a reproductive organ (the better to give you a Face Full of Alien Wing-Wong). Most of the time, it will just screech at the terrified protagonists and do nothing effective.
Note that some fish actually have something like this, called a pharyngeal jaw. While most of these are simply immobile bony blocks in their throats to "chew" food, in the case of the moray eel, it has the unique ability to actually slide its forceps-like pharyngeal jaw up its throat and out of its mouth to grab food and pull it down its esophagus. (Amusingly, this ability was only discovered in 2007, three decades after Giger designed the xenomorph.)
- All hollows in Bleach, below Menos-class, have a smaller mouth (that of the original human soul) within their much more visible, larger one (that of the mask). With some, especially those with beaks or abnormal mouths, this is more noticeable.
- In a special manga Crossover pairing the characters from One Piece and Toriko, one of the creatures encountered on Gourmet Island was a giant tiger with a vicious-looking fox jutting out of its mouth, which in turn had an angry scarred pig jutting out of its mouth. The creature is most notable for having an unmeasurable capture level.
- In Heroic Age, Cervius's frenzied form has this.
- Zeruel from Evangelion: 2.0 has a rather horrific case of this. His outer 'skull' splits apart to reveal an extendable mouth with a tripartite jaw, out of which comes a purple, vaguely tongue-looking protrusion, out of which comes an enormously long 'inner mouth' with gigantic cutting teeth. All of this lovely display is accompanied by loud and unpleasant vomiting/retching sounds.
- Uria, Lord of Searing Flames from Yu-Gi-Oh! GX has a pair of nested jaws; its counterpart among the original God Cards, Saint Dragon - The God of Osiris (Slifer the Sky Dragon), has a pair of stacked jaws.
- Parallax from the Green Lantern comics has nested jaws.
- Bliss the Morlock, from X-Men has an extra mouth on her tongue.
- In the graphic novel Black Hole (about teens who get an STD which mutates them in various ways) one of the major characters has another mouth in his throat. It has a tongue and can talk a little.
- One issue in The Simpsons comic series had an Alien Spoof with an Alien Mr. Burns who sported a tongue that looked like Mr. Smithers, who himself sported a smaller tongue looking like the chipper Ned Flanders (which he promptly bites off in annoyance).
- The hideous-looking but ultimately heroic Wrarth in the Fourth Doctor's comic-strip adventures have something like a cross between this and a prehensile tongue. Amusingly, one of them eats a slice of cake with it.
- In the restaurant scene in Monsters, Inc., one of the monsters has a second monster for a tongue, which eats the food.
- In The Nightmare Before Christmas, Oogie Boogie is actually made up of insects, but for some reason has a snake for a tongue.
- Planet 51 had a 1950's era alien society, where one female citizen kept a four legged alien dog based off the Xenomorph from Alien, complete with acid pee and second tongue.
- This is one of the features of the monsters in the Alien series.
- There's actually a reason for this beyond Rule of Scary. Originally the "tongue" was meant to be the head of the Chestburster, with the "adult" Alien being a biomechanical shell it built around itself. However, this was dropped during the design phase when the Chestburster was redesigned to look like a fetal version of the adult because Geiger's earlier design looked like an undead roast chicken.
- In a possible parody, the sandworms in Beetlejuice have these, which look like a miniaturised version of the same creature with a different colour pattern.
- In Evolution, when a party of women encounter what most resembles a green pig, they are unnerved but also endeared- until it opens its mouth and something like a tiny vulture's head reaches out to bite one of their fingers.
- Played for laughs in Kung Pow! Enter the Fist, where one of the signs of the Chosen One is that he has a face on his tongue (which speaks, though not coherently).
- The Graboids from the Tremors films and series have multiple extensible mini-mouths on their tongues. In the first film, a single severed tongue is initially mistaken for an entire creature.
- Pacific Rim: Raiju, one of the Kaiju, has what seems to be a crocodile-like head. However, that isn't his real head: it's just a three-part outer shield that splits open to reveal his small, fleshy true head inside.
- King Kong (2005): The Carnictis worms from the Insect Pit have extensible, toothy maws on fleshy tubes that protrude from their ends.
- The Shapeshifter Default Form of the Polymorph in Red Dwarf is very similar to the creature in Alien. Its inner mouth is used to suck out people's emotions.
- An episode of The X-Files features a demon with a snake for a tongue.
- In a parody of Alien at the end of the Mystery Science Theater 3000 episode Fire Maidens from Outer Space, Timmy (Crow's evil doppelganger) is revealed to have one within his beak. It is very disturbing and very funny.
- In Calvin and Hobbes, Calvin once built a giant "snow snake" out of snow and pretended he was being swallowed by it, claiming that "horrible inner jaws" were dragging him into its gullet, since its visible mouth clearly wasn't moving. Susie wasn't fooled.
- Guerrero Maya Jr. of CMLL (and others) has a toothed mask to give himself this appearance when he speaks.
- Some Piranha Plants in Super Mario Bros. games have a small inner mouth that shoots fireballs.
- In American McGee's Alice the Queen of Hearts monster has the Mad Hatter's face inside her mouth, with Alice's face inside his. See this at 4:11 in this video.
- Kog'Maw, the baby Eldritch Abomination from League of Legends, has a strange proboscis-like mouth inside of his already gaping maw. He uses it to turn his mouth into a biochemical artillery cannon.
- The materialized Dragon Stream in Devil Survivor 2 is a literal chain of heads linked by mouth after mouth.
- The Leviathans in Gears of War have an inner circular mouth. This is seen in the second game when the player must jump inside of the monster's first mouth and chuck grenades into the inner mouth.
- Lionotus from Vindictus.
- Barnacles get this in Half-Life 2.
- In Demon's Souls the Dragon God boss has a second row of teeth on a set of jaws separate from its normal jaws.
- In Doom 3 and its expansion packs, the Maledict has the head of Dr. Betruger on its tongue. And Dr. Betruger appears to be the one in control.
- Venom, main antagonist of Spider-Man: Web of Shadows, opens his mouth at one point early on to reveal the mouth of his still-conscious human host, who gives a quick speech before retreating back into Venom's body.
- The animatronics in Five Nights at Freddy's; a good example would be Chica.◊
- Abyss, the corrupted form of Zasalamel from the Soul Series has two rows of teeth, one inside the other.
- In Dragon Ball Z Abridged, Freeza's third form whips out a mouthed tongue for a half-second, poking fun at its Alien-like appearance.
Freeza: So, Namekian, what do you think of my third form?Piccolo: I think Ridley Scott's gonna sue somebody.Freeza: What are you talking about? (mouth pops out, then retracts) Let me touch your skin!
- SCP-1132 is a Mexican woman who has a venomous snake for a tongue.
- In The Tick cartoon, the alien Thrakkorzog has a little face on his tongue that talks about eating brains.
- Chip 'n Dale Rescue Rangers has a scene from an obvious Alien parody which takes nested mouths Up to Three. The alien in question has three mouths nested in one another, the last one is just big enough to pinch the Ripley look-alike's nose.
- The Critic also had an Alien spoof, where the second mouth kissed Ripley.
- Similar to Oogie Boogie, the evil tree in Flowersand Trees has a snake for a tongue.
- In Animaniacs, Dot counters a Xenomorph with her boxed pet, which appears to be a giant Jerry Lewis caricature with a smaller head inside.
- Bounty Hamster. Another Alien spoof, where a baby alien resembling a small xenomorph shoots out a dozen nested mouths in an attempt to bite Cassie. However in "Save the Whale", Marion is surrounded by a whole bunch of baby xenomorphs who all do this trope... The next time we see Marion, he's giggling over being licked by multiple tongues.
- Transformers Prime, this happens to CYLAS / Breakdown, when Knock Out infuses him with Synthetic Energon mixed with Dark Energon, and his mouth turned into a vampire xenomorph, and is out to suck out energon.
- Hunson Aberdeer (Marceline's dad) from Adventure Time has a whole second head nested inside of his neck. It opens sideways and when he's using his full power his "regular" head splits open to leave the inner one on full display. Disturbing phallic (and related) imagery very much present in this case.
- The Boogey Man from The Grim Adventures of Billy & Mandy displays this when angry.
- The above-mentioned moray eel, and a number of other species of aquatic creatures.
If its mouth opens wide, and there's one more inside, that's a moray...
- The tongue-eating louse is a parasite that infests fish. It consumes the tongue and replaces it, grabbing a bite of whatever the fish eats. For better or worse (?), though, the fish can use the louse like a regular tongue.
- Dragonfly nymphs have an extensible second pair of mandibles used to grab prey, otherwise kept tucked under their actual feeding mandibles.
- Bloodworms have retractable biting mouths with four venomous teeth, normally kept drawn up inside their toothless outer mouth. This lets them shoot out their predatory biting mouths at prey, yet keep their teeth safely withdrawn as they're burrowing mouth-first in seabed mud.
- Human children are born with both their child and adult teeth already developing inside the jawbones, so x-rays taken before they lose their first set show a mouth surrounding another mouth.