"Cross-section of the axoneme." This picture, produced by Alan H. Barr at RPI's Center for Interactive Computer Graphics, shows the 9 + 2 structure of the axoneme-the motive structure of a cilium or flagellum, which allows a eucaryotic cell to generate the fluid flow used for feeding and locomotion. The ratchetlike activity of the white dynein arms, or cross-bridges, is transformed into a sliding motion between the microtubular doublets (shown in yellow and purple). The arms are attached to the tubulin-A (purple) part of the doublet, and they walk along the tubulin-B (yellow) part of the adjacent doublet. This sliding motion is converted into the bending motion used for swimming and feeding via the geometric linkage of cross-bridges.