"The terror continues . . . promise newspaper ads for "Friday the 13th, Part 2." Display posters at the theaters get down to the nitty-gritty: "The body count continues . . ."

A definitive catch line, it sums up both the content of the movie and the curiously raucous, convivial sporting interest the public seems to bring to the spectacle of victims piling up methodically. Would the Paramount copywriter responsible care to step forward and take a bow? incidentally, why not make up a sassier title to accompany the sassy slogan? This lame "Part 2" stuff has to stop. Something like "Friday the 13th Strikes Again" would be far more appropriate.

A low-budget horror thriller about the slaughter of seven unlucky counselors at a summer camp, "Friday the 13th" was the biggest sleeper of 1980, placing 20th on Variety's annual survey with rentals of $16.5 million, probably 50 times the production cost, estimating conservatively. The audience for the sequel, now at area theaters, seems to be packed with a returnees in a holiday mood, explosively vocal when their expectations are teased or fulfilled.

With the exception of the occasional unnerving customer who can be heard demanding sadistic gratification in voices that seem to erupt from primordial depths, the responses suggest a knowing and relatively harmless suspension of disbelief. There are mocking moans and wolf calls whenever the nubile counselors go into their obligatory strips and outbursts of laughter whenever characters place themselves in obviously exposed positions, begging to be assaulted while dallying in the woods or reclining near open or easily breakable windows.

The interesting thing is that being on to such deceits doesn't prevent people from shrieking when the assaults are eventually launched.an effective jolt invariably transcends an awkward setup. The failure to be concerned about separates the short of people inclined to deplore exploitation movies from the public inclined to derive gratuitous enjoyment from them.

Moreover, the visceral nature of the sensation in horror thrillers tends to remind you of susceptibilities that are intellectually embarrassing and leveling. despite all sophisticated defenses, you find yourself prey to primitive fears, superstition and basic technical tricks of the movie medium. Rejecting the premise and laughing at the blunders and cliches don't necesarily provide immunity from the shocks.

If you missed or avoided the original, there's no cause to feel left out, because the sequel begins with a reprise of the concluding stalking and atrocity sequences from "Friday the 13th." A kind of built-in preview trailer, these flashbacks serve to set up the derivative plot mechanism of the sequel, in which copycat murders are committed by a direct, bogeyish descendant of the killer in the original, while also giving the audience a taste of the scarifying, ritualistic sensation to come.

Director Steve Miner and writer Ron Kurz introduce more potential victims than they have time or invention to knock off. Moreover, their hit list leaves plenty of room for argument. Amy Steel and John Furey, the callow Aryan leads reserved for the climactic showdown with the bogeyman, seem like better candidates for early dismissal in my book. One of the nastier brainstorms is to introduce a handicapped character who ends up on the victim list, a choice that the audience acts a little reluctant to accept, although less reluctant than the prospect of a victimized pet dog.

Miner doesn't linger over the multiple throat-slashings and skull-splittings. Comparatively speaking, he seems less bloodthirsty than the directors of "Friday the 13th," "The Exterminator" or "Mother's Day," to name only a few competitors of grosser gruesomeness. His most conspicuous bad habit is the facetious shock cut: from, say, a possibly mangled doggie to hot dogs roasting on a grill or from a freshly sliced head to sexual partners climaxing. That sort of Big Joke.