A cilium which may have a variable arrangement of axonemal microtubules and also contains molecular motors. It may beat with a whip-like pattern that promotes cell motility or transport of fluids and other cells across a cell surface, such as on epithelial cells that line the lumenal ducts of various tissues; or they may display a distinct twirling motion that directs fluid flow asymmetrically across the cellular surface to affect asymmetric body plan organization. Motile cilia can be found in single as well as multiple copies per cell. [ http://www.ncbi.nlm.nih.gov/pubmed/17009929 http://www.ncbi.nlm.nih.gov/pubmed/22118931 http://www.ncbi.nlm.nih.gov/pubmed/20144998 ]