Near the end of it's first season Star Trek: The next Generation did something different, and something they haven't tried since.
In the episode "Conspiracy" Captain Picard and crew learn that a race of alien parasites has infiltrated the Federation. Picard and crew of course stop this invasion, no surprise there. This could have been standard Star Trek fare, but they managed to make one of the goriest episodes of TV to that point.
The episode is of note for horror fans for several reasons.
First of all, one of the people who tells Picard about the conspiracy is Captain Rixx, played by Michael Berryman. Berryman played a Bolian (named for the shows director Cliff Bole) and this race would be featured a lot in TNG and other Star Trek shows.
This was one of the few Star Trek episodes that featured non-humaniod aliens. The parasites were all puppets and were stop motion animated in some scenes.
We were also treated to this scene where the parasite infected officers eat live grub worms for dinner.
The mother creature of the parasites was a pretty gruesome creature.
What this episode is most remembered for hough is this scene.
Hands down this is one of the best head explosions ever. This is at least as good as the Scanners one.
Reaction to the episode was mixed and the show never did another gory episode like this again. It's a shame, I would have liked to seen more horror in Star Trek.