You're likely thinking of cilia or flagella. These are short, hair-like structures that protrude from the surface of some cells.

Here's a breakdown:

* Cilia are short, hair-like projections that beat rhythmically in a coordinated manner. They are found in many organisms, including humans, and play a role in movement, sensing, and filtering. For example, cilia in your respiratory system help sweep mucus and debris out of your airways.

* Flagella are longer, whip-like projections that help cells move. They are typically found in single cells, such as bacteria and sperm cells. They move in a wave-like motion.

Both cilia and flagella are composed of microtubules, which are protein filaments that provide structural support and facilitate movement.

Let me know if you'd like to know more about cilia or flagella!