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In the opening sequence of the 100th episode of "The Simpsons," Bart wrote on the chalkboard, "I will not celebrate meaningless milestones."

Well, they're celebrating on Sunday — celebrating a milestone that definitely isn't meaningless.

"The Simpsons" is airing its 600th episode (Sunday, 7 p.m., Fox/Ch. 13), a number only one other scripted prime-time American TV show ("Gunsmoke") has ever achieved.

"We've made sure to really celebrate this one," said executive producer Al Jean. "To get this far is really, I have to say it, we're proud of ourselves."

Coincidentally, it's also the annual Halloween episode. Work had already begun on "Treehouse of Horror XXVII" when the writers and producers were made aware that it would be No. 600. So it's your average "Treehouse of Horror" show — meaning it includes several unrelated stories that are considerably more crazy than your average installment of "The Simpsons" — along with a few surprises.

"There's a special ending, which, actually, we shot at [San Diego] Comic-Con," Jean said. "There's a special beginning. And there's a third thing" — that he "can't say."

It parodies "Planet of the Apes," "Hunger Games," "Fury Road" and "Goldfinger." Drew Carey, Kelsey Grammer and Ken Fagen of the band Steely Dan make cameo appearances, and Sarah Silverman voices Lisa's murderous imaginary friend.

The (rather brief) opening bit includes the return of Frank Grimes, who appeared and was killed in Episode 176 — titled "Homer's Enemy" — way back in May 1997. Grimes (voiced by Hank Azaria) was a new plant employee who deeply resented Homer's lack of professionalism on the job at the nuclear power plant — and who died in a work accident.

Grimes is a ghost in Episode 600, and he can't believe there have been that many episodes.

"In hell, they make you watch them all in a row," he says.

Or … you can tune to FXX beginning on Thanksgiving (Nov. 24) at 10 a.m. as the cable channel kicks off another 300-hour Every Simpsons Episode Ever marathon that will continue through 10 p.m. on Dec. 16 — 12 ½ days later.

It doesn't look as if the end is near, either. "The Simpsons" hasn't been officially renewed beyond the end of the current season, but the voice cast has been signed for an additional two years.

"I think we will be [picked up], but that's just my guess," Jean said. "So, no, can't plan it yet. But hoping — fingers crossed."

"Gunsmoke" produced 635 episodes from 1955-75. You could stretch the point a bit and make "The Simpsons" No. 1 on that list, because there were also four dozen 1-minute shorts featuring Homer and Co. that aired in 48 episodes of "The Tracey Ullmann Show" from 1987-89. And don't forget the 2007 theatrical film.

Assuming it's renewed, "The Simpsons" will pass "Gunsmoke" 18 episodes into Season 29. And that would also be a meaningful milestone.

Jean acknowledges that one of the toughest parts of writing new "Simpsons" episodes is avoiding repeating all the episodes that have gone before, since it premiered Dec. 17, 1989.

The "Simpsons" writers are still "trying to say to people it's still great and we want you to watch it," Jean said, "and even if you've seen 600 of them, the 601st will be worth your while."

On TV

The 600th episode of "The Simpsons" — "Treehouse of Horror XXVII" — airs Sunday, Oct. 16, at 7 p.m. on Fox/Ch. 13.