Paramecium is a genus of unicellular ciliated protozoa that are found in freshwater, brackish, and marine environments.
It only appeared in the episode "Valentine's Day," when Patrick was guessing what SpongeBob got for him for Valentine's Day. The first guess was a tent. The second guess was a random guy in the carnival. The third guess was a bunch of paramecia under a microscope, with Patrick asking, "This paramecium?" The paramecium that Patrick was looking at was most likely the genus Paramecium Aurelia.