The term ONE-CELLED ANIMAL can be summarized by two crossword solutions: AMOEBA (6 letters) and AMEBA (5 letters). An AMOEBA is a single-celled organism that belongs to the kingdom Protista. It is a type of eukaryote, meaning it has a nucleus and other organelles enclosed within membranes. Amoebas are found in freshwater, saltwater, and soil environments. Amebas are similar to amoebas, but they are prokaryotes, meaning they lack a nucleus and other organelles. They are found in soil and water environments, and are known to cause disease in humans.