A cilium (plural cilia) is an organelle found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body.
There are two types of cilia:
- motile cilia, which beat against fluid outside the cell.
- non-motile, or primary cilia, which typically serve as sensory organelles.
In eukaryotes, cilia and flagella together make up a group of organelles known as undulipodia.Eukaryotic cilia are structurally identical to eukaryotic flagella, although distinctions are sometimes made according to function and/or length.