Protozoa
From Memory Alpha, the free Star Trek reference
Protozoa are simple microorganisms comprised of a single cell. There are many variations of protozoa – for example, some move through the act of flagellation, and some contain no nuclei. (VOY: "Message in a Bottle")
The space-dwelling lifeforms encountered by the USS Voyager in 2371 reminded Captain Kathryn Janeway of protozoa due to the way they propelled themselves through space. (VOY: "Elogium")
