The term ONE-CELLED CREATURE can be summarized by two crossword solutions: AMOEBA (6 letters) and AMEBA (5 letters). An AMOEBA is a single-celled organism that moves by extending its cytoplasmic extensions, known as pseudopodia. Amebas are similar to amoebas, but they are typically larger and have more complex structures. Both amoebas and amebas are classified as protists, which are eukaryotic organisms that lack the characteristics of plants, animals, and fungi.