Nine microtubuli doublets are regularly arranged at the periphery. Each consists of one tubule A and one tubule B. In the centre are two singlet microtubules (type A). the dynein (outer arm: blue, inner arm: green) is always attached at the A tubule. Its tips make contact with the B tubule of the neighbouring microtubuli doublet (N + 1). The structural elements pointing towards the centre of the flagellum are spokes and their heads (violet). The tubules are connected by nexin bridges. The central singlet tubules are enclosed by the inner sheath (grey). The flagellum/ cilium is surrounded by a membrane (grey, too). (H. MOHRI, 1976)