You remember the old nursery rhyme, the worms crawl in, the worms crawl out, the worms play pinochle on your snout? Writer-director James Gunn, who gleefully resurrected George Romero’s zombie franchise with Dawn of the Dead, doesn’t have worms playing pinochle on snouts in Slither, but he does stack the cards against the characters.

Not that many of the characters in Slither are playing with a full deck, which makes them perfect fodder for Gunn’s deliciously decadent salute to the sort of films he used to write for Troma Films, unapologetic exercises in black humor and entrails. Gunn loves horror movies, and it shows in every frame of Slither. He’s making a film for people who love horror movies and, like Willy Wonka guiding the children through his chocolate factory, does so with a wicked wink in his eye.

We’re not supposed to take Slither seriously — goodness knows most of the characters don’t — which makes Gunn’s use of over-the-top blood and guts more gross than gruesome. Gunn really pushes the envelope, exposing us to one wicked tongue-out-cheek shock after another, leaving us squirming in our seats, much like the little critters who steal the show.

Like the salesman at the beginning of The Music Man, Gunn knows the territory and sells the hell out of his product. He doesn’t waste any time setting up the obvious, throwing us right into the action. Once Gunn sets the story into motion, we have no choice but to tag along, even if it requires us to peek through the fingers covering our eyes. There’s something innately icky about slugs, so imagine extra-terrestrial slugs who infest your body and turn you into a zombie.

That’s the dilemma facing the folks of the small town of Wheelsy, who undergo mysterious transformations when a meteorite lands nearby. I guess the people of Wheelsy never saw The Blob, because they would know never to piss of the contents of a meteorite. That doesn’t stop Grant (Michael Rooker), jealous husband of former beauty pageant-winner turned high school science teacher Starla (Elizabeth Banks), who pokes a slug creature with a stick and lives (dies) to regret it.

Gunn wisely turns Wheelsy into a mini-Peyton Place, where personal conflicts frequently sidetrack more pending matters. This tiny hamlet is the perfect place to stage an alien invasion. Everyone knows everyone else, a tight-knit community that takes a while to wake up to the fact that all is not right. So when Grant becomes infected during a hasty make-out session with another woman, we say good riddance. Unfortunately, Grant doesn’t stay down very long, returning to Starla with a fabulous cover story and the intent of infecting her.

Nathan Fillion, of Serenity, plays Starla’s knight in shining armor, the local sheriff who harbors a secret crush. Sheriff Bill (Fillion) and his trusty deputy, Wally (Don Thompson), usually pass the night away measuring the speed of passing birds, so they are oblivious when a meteor crash-lands behind them. Once the contents of the meteor are exposed and put into action, Slither becomes a nonstop barrage of icky special effects and gallows humor. Gunn recently confessed that he enjoys the freedom of working in horror, and he takes full advantage of that freedom here.

Presented as horror-comedy, the splashy moments aren’t intended to make you evacuate your Junior Mints and popcorn. Some are so extreme you can’t help but let loose a nervous laugh or two. Gunn’s characters have been fabricated for maximum effect, personality shorthand so he doesn’t have to waste time introducing each and every player. Almost from the moment they enter the scene, we know and understand who each of these potential victims and heroes are: the earnest sheriff, the damsel in distress, the boorish mayor (Greg Henry), the feisty heroine (Tania Saulnier), the jealous husband.

After a parade of psychological horror thrillers, it’s refreshing to find a monster movie that doesn’t pretend to be anything but. Gunn isn’t making a suspense thriller; he’s turning people into pus, and pours it on. It’s not food to jar your brain out of its pan; it’s a film about people who become infested by alien slugs and turn into creepy crawlers.

Gunn also understands the necessity of moving from point A to point B with as much ferocity as possible. He gets in, makes his statement and gets out. Lingering only allows the audience to second-guess the characters. We know some of these people are going to make bad, even horrible decisions before the film is over. The fun comes in seeing just how badly, how horribly those decisions will play out.

Like Brenda (Brenda James), the woman Grant picks up in the bar for a quickie. When we meet Brenda, she’s complaining about her fat sister, a reference which will come back to haunt her after she becomes infected. One of Slither’s calling cards is Brenda’s comeuppance, a repulsive display of prosthetic rubber and digital magic which transforms Brenda into her worst nightmare.

There have been numerous films of this nature before (David Cronenberg’s They Came From Within instantly comes to mind, with Gunn even aping that film’s infamous bathtub sequence), but none as delightfully adroit as Slither. You can almost picture the filmmaker standing behind the camera, a Cheshire Cat grin stretching from ear to ear. Those with a sense of humor and a strong stomach will do likewise.