They are only very simple structures, but without them we could not survive: Countless tiny hairs (cilia) are found on the outer wall of some cells, for example in our lungs or in our brain. When these micrometer-sized hairs coordinate their movement and produce wave-like movements together, they can cause currents on a microscale and thus pump fluid from one place to another. Paramecia—unicellular organisms with numerous cilia—also use such effects to move around.
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Source: Phys.org