Nested Mouths

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 Wingwong). Most of the time, it will just screech at the terrified protagonists and do nothing effective.

Note that some fishes actually have something like this, most notably the moray eel. It's called a pharyngeal jaw.

This is one way of having Too Many Mouths. See also Multipurpose Tongue.

Examples of Nested Mouths include:

Anime and Manga

All hollows in Bleach, below Menos-class, have a smaller mouth (that of the original human soul) within their much more visible, larger one. 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.
In Cells at Work!, on at least one occasion, AE3803 (Red Blood Cell) does a scream take that gives her tongue or uvula a mouth of its own (also screaming, naturally).

Comic Books

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).

Film

The most iconic example is from the Alien series.
In a possible parody of the above and/or Tremors, 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).
In the restaurant scene in Monsters, Inc., one of the monsters has a second monster for a tongue, which eats the food.
The Nightmare Before Christmas: Although Oogie Boogie is actually made up of insects, he 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.
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.

Live-Action TV

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.
In an episode of X Files features a demon with a snake for a tongue.

Newspaper Comics

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.

Tabletop Games

The Yu-Gi-Oh! trading card game has "Saint Dragon - The God of Osiris" (known to English-language readers as Slifer the Sky Dragon), who has a pair of stacked jaws. "Uria, Lord of Searing Flames", its Sacred Beasts counterpart, has a pair of nested jaws.

Video Games

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 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

Web Original

In Dragonball Z Abridged, Frieza's third form whips out a mouthed tongue for a half-second, poking fun at its Alien-like appearance.

Western Animation

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, with the last one 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 Flowers and 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.

Real Life

The above-mentioned moray eel, and a number of other species of aquatic creatures.
- Most sea urchins have "Aristotle's lantern" — mouth sac with a pyramid of five teeth (and tongue). It may look at rest like a hybrid of sphincter and Dilating Door, but it got 60 muscles to extrude, retract, scrape or grind as needed… and can regrow teeth.
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.