An amoeba was a type of unicellular organism.
According to Spock in 2267, the Organians were as far above Humans and Vulcans in evolution as those species were above amoebae. (TOS: "Errand of Mercy")
In 2269, Spock compared the cosmic cloud seeking planets to consume with the instinct of an amoeba seeking food. (TAS: "One of Our Planets Is Missing")