The white cliffs of Dover are composed of a variety of protist fossil shells, including coccolithophores (a type of algae) and foraminiferans. Their process of formation took millions of years: After these protists died, their shells were deposited on the bottom of the ocean in a fine gray mud; after time, layer upon layer of sediment deposited above compressed the mud. After even more time, the mud hardened, forming a type of limestone we call chalk. Eventually, geologic processes—such as the uplift of the land and erosion by water or ice—exposed the outcrop of white limestone.

Protists move in several different ways. Because they are exclusively found in aquatic or very moist environments, they need certain appendages to remain motile. Two of the most common ways are with flagella and cilia, both used by cells to move through watery environments. Cilia move back and forth, while flagella undulate in a whiplike motion, moving in the same direction as the cell’s axis. (For more about flagella and cilia, see the chapter “Cellular Basics.”) Other protists use pseudopods (or “false feet”) to move, and are large, lobe-shaped extensions of the organism.

But note: not all protists move. Some are sessile—attached using certain structures (usually stalks) that adhere to a substrate. And some protists are both sessile and mobile; for example, many of the brown algae have free-floating sperm, whereas mature algae are attached to rock or other substrate—and are not mobile.