Fig. 5. Measurement of flexural stiffness (EI) of a kinocilium compared with that of a flagellum. (A) The schematic illustration of a portion of an axoneme shows major features of the ultrastructure of flagella and kinocilia. (B) Arrows directed at the axonemes above each bar indicate the direction of loading for each measurement of stiffness. Flagellar stiffness was measured in demembranated sperm of the sand dollar (Clypeaster japonicus) activated by ATP (Ishijima and Hiramoto, 1994). (i) The stiffness of a flagellum when loaded along its beating plane is shown in comparison to measurements of stiffness for (ii) a kinocilium from the present study (error flags denote 95% confidence intervals) and the stiffness of (iii) a flagellum when loaded perpendicular to the beating plane.