Populations of thin, motile processes found covering the surface of ciliates (CILIOPHORA)
or the free surface of the cells making up ciliated EPITHELIUM. Each cilium arises
from a basic granule in the superficial layer of CYTOPLASM. The movement of cilia
propels ciliates through the liquid in which they live. The movement of cilia on a
ciliated epithelium serves to propel a surface layer of mucus or fluid. (King & Stansfield,
A Dictionary of Genetics, 4th ed)