Cilia and flagella are related microtubule-based and membrane-enveloped extensions of the plasma membrane that project from many protozoan and most animal cells. Abundant motile cilia are found on the surfaces of specific epithelia, such as those that line the trachea (see Figure 4-35), where they beat in an orchestrated wavelike fashion to move fluids. Animal-cell flagella, which are longer than cilia but have a very similar structure, can propel a cell, such as a sperm, through liquid. Cilia and flagella contain many different microtubule-based motors: axonemal dyneins are responsible for the beating of flagella and cilia, whereas kinesin-2 and cytoplasmic dynein are responsible for flagellum and cilium assembly and turnover.