(a) A specimen of freely swimming Euglena gracilis, and (b) a sketch of the cross-section of its flagellum, as seen from the distal end. The flagellar inner structure is composed by the paraflagellar rod (PFR, textured), and the axoneme (Ax). The PFR is connected via bonding links to the axonemal doublets 1, 2, and 3. The inner structure of the flagellum is enclosed by the flagellar membrane (dotted contour). By inhibiting MTs’ sliding, the PFR selects the spontaneous bending plane of the Ax (dashed line). As a key geometric feature, the solid line that joins the Ax center and the PFR center crosses at an angle the spontaneous bending plane. Doublets are numbered following the convention adopted in the electron microscopy studies Melkonian et al., 1982 and Bouck et al., 1990, to facilitate comparison. The opposite convention, in which microtubules are numbered in increasing order in the anti-clockwise direction when seen from the distal end of the Ax, is far more common in structural studies of cilia.