Paramecium is a unicellular organism that swims in fresh water by beating thousands of
cilia. When it is stimulated (mechanically, chemically, optically, thermally…), it often swims
backward then turns and swims forward again. This “avoiding reaction” is triggered by a
calcium-based action potential. For this reason, some authors have called Paramecium a
“swimming neuron.” This review summarizes current knowledge about the physiological
basis of behavior of Paramecium.