One-celled protozoans, also known as protists, are single-celled organisms that can be found in a variety of habitats. One type of protist is the ameba, which is a 6-letter word that can be used to solve the crossword puzzle. Amebae are unicellular organisms that move by extending and retracting their pseudopodia, or false feet. They feed on bacteria and other small organisms, and reproduce by splitting into two parts.