(A) Cilia and flagella stir fluid by moving back and forth in their own plane (top), or in a rotary fashion (bottom). (B) The flagella of single-celled organisms exhibit diverse beat patterns, sometimes on the same cell. For example, dinoflagellates have a transverse flagellum (red) which loops around the cell’s circumference and propagates helicoidal waves, and a longitudinal flagellum which moves back and forth behind the cell. (C) The alga Euglena gracilis has one short flagellum (inside the cell body), and one long flagellum, which contains a core axoneme (white) and a paraflagellar rod (lattice pattern). When the two incompatible structures ‘glue’ together via bonding links (right side image), this creates a mechanical frustration that forces both structures to bend together. As a result, the flagellum bends out of the plane and generates a lasso-like motion which pulls the alga along a helical path.