Describe the structure and function of cilia and flagella.

Cilia and flagella are slender, hair-like projections that extend from the surface of many eukaryotic cells. Both are composed of microtubules arranged in a '9+2' array, covered by the cell membrane. Flagella are typically longer and fewer in number, providing propulsive force for cell movement (e.g., sperm cells). Cilia are shorter and more numerous, often moving fluids or particles across the cell surface (e.g., lining of the trachea) or enabling cell motility (e.g., paramecium).