ALIEN - Uptown.

If you think you would be thrilled and horrified and excited by seeing a particularly vicious order of seafood, you could go to the space'n'horror film "Alien." But if you're all that squeamish, you could probably get the same effect at the wharf, and put garlic butter on it after the thrill wears off.

The creature from outer space that terrorizes the crew of a commercial space ship by hiding in the boiler room and popping out and yelling "Boo!" (or whatever) is slimy, crusty, wriggly and graspy. Unlike the Friday Blue Plate Special, however, it changes shapes, beginning as a crab and working its way through the bouillabaisse until it ends up as a kind of frogman with a tail.

No attempt is made to establish a cultural exchange with it. The crew members stalk the passages of their craft hoping to get it before it gets them and most of the film is taken up by individuals walking through darkened passages, their eyes searching from left to right, thus clueing in the audience that it's about to drop from the ceiling.

Nevertheless, this is considered a classy production of its kind. Of both of its kinds - horror and space. What this means, besides the fact that the voyage is accompanied by snatches of Mozart, is that many of the obvious tricks of the genres have been avoided, and there is a certain amount of restraint. It is not restraint that you would notice if you had not seen other films of these types.

There is much less blood than in many horror films: one big spurt and it's pretty much over. The concentration is on apprehension, rather than gore.

A great deal of trouble has been taken with the alien object, so that it's not obviously plastic, and the space ship has a more practical look to it than the usual space-film vehicle.

And there's more of a concern to make the characters seem realistic rather than comicstrip heroes and villains. The crews, played by Tom Skerritt, Sigourney Weaver, Yaphet Kotto, Veronica Cartwright, Harry Dean Stanton, John Hurt and Ian Hol, is a tired, businesslike group given to complaining about the shortcomings of the company they work for. They're all equally good at depicting the various degrees of competence and boredom of people doing a routine but demanding job, and some of them can also scream.

It might be noted that there are two women and one black among these six crew members, and they all behave as sensible colleagues, without friction, romance or giggles. This is a cinematic breakthrough.

In fact, the only character who turns out to be reprehensible is the science officer on the ship. There is some feeling among the others in favor of doing the job as cursorily as possible so they can all get home, and it's the scientist, with his constant probing, who is making life difficult.

It's a concept of "alien" that would have made a scary picture. CAPTION: TOM SKERRITT FINDS THE SOURCE OF DISTRESS CALLS IN "ALIEN".