A cilium is a short, hair-like cellular appendage (up to 10 microns long) specialized for the purposes of locomotion or feeding. Cilia are present, usually in large groups, on the surface of many cells, notably certain protozoans, called ciliates, and some types of vertebrate epithelium such as those along the respiratory tract. They beat to produce cell movement or create a current in fluid surrounding a cell. Cilia and eukaryotic flagella have the same structure and are collectively called undulipodia.