The term SIMPLE ANIMAL can be summarized by the crossword solution AMOEBA, which is a six-letter word. An AMOEBA is a single-celled organism that belongs to the protozoa family. It is considered to be one of the simplest animals on Earth, as it lacks a true cell nucleus and other organelles. Amoebas are found in freshwater, saltwater, and soil environments, and they feed on bacteria and other small organisms.