We had to put our cat to sleep last week. Tallulah was nearly 20 years old, deaf, toothless, incontinent and riddled with some kind of nasty tumors. It was, as they say, time. In retrospect it was perhaps well past time.

Neither Tallulah nor her demise have anything to do with television, but I couldn’t help thinking of her long, sad decline as I watched the eighth season premiere of “That 70s Show” (7 p.m., Fox). After all, this comedy has been kept alive by artificial means for some years now.

Despite its often crude dialogue, consisting largely of one-liners about sex and stoners, one couldn’t help liking this show. Its cast was arguably the best in network comedy. And now at least two members of this strong ensemble have gone on to better things. Topher Grace (“Traffic”) and his character Erik have already departed. Ashton Kutcher’s (“Punk’d”) dim Kelso is being phased out.

The most grievous symptom of this show’s creative rigor mortis is the onset of terminal guest-star-itis. “Will & Grace” has been wracked with this syndrome for some years now. But the upcoming guests on “That ’70s Show” also exhibit a secondary symptom: a festering case of insiderism. Bruce Willis will grace the show sometime this month. As any tabloid reader knows, he is the former husband of Demi Moore, now romantically linked to Kutcher. Speaking of Moores, Mary Tyler Moore will also appear in an episode sometime in 2006. She may not be connected to cast gossip, but “’70s” is shot on the same soundstage as the old “Mary Tyler Moore Show.” Isn’t that special?

Eight seasons is an eternity on network television. There are plenty of classic comedies that have never reached that milestone. The folks at Fox should do the decent thing and put this sad, old cat to sleep.

Tonight’s other highlights

¢ Angie can’t talk about her troubles on “E-Ring” (7 p.m., NBC,).

¢ A brazen shooter kills by day on “Criminal Minds” (8 p.m., CBS).

¢ The teams tangle to sell a new stain removal product on “The Apprentice: Martha Stewart” (8 p.m., NBC). Try to contain your excitement.

¢ The same, old thing on a brand new season of “Trading Spouses: Meet Your New Mommy” (8 p.m., Fox).

¢ Secrets of the hatch emerge on “Lost” (8 p.m., ABC).