The lining of the larynx is stratified ciliated columnar epithelium. Pseudostratified columnar epithelium often has hairlike projections or cilia that extend into the lumen of the tubes in which they are present. These cilia beat back-and-forth and can move substances through the tubes. For instance, the ciliated cells lining the female reproductive tract help create currents in the fluid filling the tract; these currents move an ovum from the ovary through the oviduct and into the uterus. Ciliated cells lining the respiratory passages help to prevent mucus from clogging the tubes and also help to sweep dust particles, microbes, and other potentially harmful substances out of the air as it passes through the tubules.