Figure 322 from Chapter 13 (Cilia and Flagella) of 'The Cell' by Don W. Fawcett M.D. Cilia and flagella usually occur on motile cells or on the free surfaces of epithelia. However, a solitary cilium is a normal occurrence on the cells lining the intra- hepatic bile ducts, the intercalated ducts of the pancreas, the rete testis, and certain segments of the nephron. The functional significance of these processes is not evident, since it seems unlikely that their motility could do more than create local turbulence in the fluid contents of these excretory ducts. Such abortive cilia usually lack the central pair of microtubules and are proba- bly immotile. Because modified cilia are found in photoreceptors and in olfactory epithelia, it has been suggested that these solitary cilia might have a sensory function but there is no evidence to support this speculation. It seems probable that they are merely anomalous, rudimentary structures with no function. A copy of the chapter is available on the ASCB's BioEDUCATE website.