Centriole-based, motile cell extensions. These organelles are usually indistinguishable in fine structure as seen with the electron microscope, but quantitatively there are many (several hundred) cilia and few or fewer (usually one or two) flagella, on one cell. Bacterial or prokaryotic flagella are entirely different organelles that are not considered in this article, which concerns only eukaryotic (higher-cell) flagella.