Cilia are microscopic, hairlike structures that project from the edges of certain types of cells (the building blocks of all living things) and allow them to move themselves or things that are close by. Not all cells have cilia, and those that do are usually animal cells rather than plant cells. In higher animals, such as humans, cilia also refer to the hairlike lining of the nose, ear, and trachea (the air passage to the lungs) that keep those passages clean from dust, pollen, bacteria, and mucus.
Animal cells must often move about, and cilia are the primary means by which they achieve movement. Cilia are composed of microtubules or extremely tiny tubes whose action or movement can be controlled by the cell. By coordinating the wavelike action of its cilia, the cell can either send itself through its environment or help to move the environment past itself. If one-celled organisms like the protozoan Paramecium are observed under a microscope, the rhythmic, wavelike motion of its cilia can be easily seen beating against its liquid environment as if it were rowing in a coordinated way. Certain one-celled organisms also use their cilia to capture food and move it into their gullet.
Certain cells, like gametes or sex cells, only have a single projection that they use to move about. When a cell has this sort of singular, long, hairlike projection that resembles a tail, it is called a flagellum instead of a cilium. Sperm cells are a good example of cells that have flagella.
Besides the epithelial (skin) tissues in most higher animals contain a carpet of cilia whose purpose is to move tiny particles across and away from sensitive surfaces. The wavy motion of the cilia in the uterine tubes in women help move and guide the fertilized ovum (human egg) down to the uterus where it can attach itself and grow into a fetus. Even a clam uses its cilia to fan water containing oxygen into its gills. Cilia are so tiny that just the trachea may contain as many as 1,000,000,000 cilia per square centimeter.
So ciliary XVII.