They are mobile structures, found externally in cells of various living beings.
Eyelashes are short and may be related to locomotion and the removal of impurities. In cells lining the human trachea, for example, the ciliary beats push impurities from the inspired air, work facilitated by mixing with mucus that, produced by the trachea cells, lubricates and protects the trachea. In some protozoa, for example the paramecium, the cilia are used for locomotion.
The flagella are long and also relate to locomotion of certain cells, such as some protozoa (eg, trypanosomes that cause Chagas disease) and sperm.
In some multicellular organisms, for example in sponges, flagellar beating creates streams of water that run through channels and internal cavities, for example, bringing food particles.
Structurally, eyelashes and flagella are identical. Both are cylindrical, outside the cells and covered by a plasma membrane. Internally, each cilium or flagella is made up of a set of nine pairs of peripheral tubulin microtubules, surrounding a pair of central microtubules. It is called the 9 + 2 structure.
Both the eyelashes and flagella originate from an organizing region within the cell known as basal corpuscle. In each basal corpuscle there is a set of nine triplets of microtubules (rather than doubles, as in the cilia and flagella), arranged in a circle. In this sense, the structure of the basal corpuscle is similar to that of a centriole.