Amoeba

Webster's Dictionary

(n.) A rhizopod. common in fresh water, capable of undergoing many changes of form at will. See Rhizopoda.

The Nuttall Encyclopedia

A minute animalcule of the simplest structure, being a mere mass of protoplasm; absorbs its food at every point all over its body by means of processes protruded therefrom at will, with the effect that it is constantly changing its shape.