Here's why:

* Structure: Cilia and flagella are extensions of the cell's cytoplasm. They are covered by a plasma membrane, just like the rest of the cell.

* Internal Structure: Within the plasma membrane lies a core structure called the axoneme. The axoneme is composed of microtubules, which are themselves enclosed by another membrane, the axonemal membrane.

So, you have the plasma membrane surrounding the entire cilium/flagellum and the axonemal membrane surrounding the internal microtubules.