At the top: front part of the flagellate Chlamydomonas. At the left: usual movement of the two flagella, the stroke causes a forward movement. At the right: backward movement by wavelike strokes of the flagella (W. HAUPT, 1977) At the bottom: different stages of the stroke of the flagellate Monas. At the left: active power stroke (energy-consuming), at the right: inactive recovery stroke (B. M. KRIJKSMANN, 1925)