Protozoa move by cell extension, flagella and cilia; the method of movement is determined by the type of organism and the surrounding environment. Protozoa are classified into three groups: ciliates, amoebae and flagellates. Ciliates move using tiny cilia, flagellates move through water using flagella as oars and paddles and amoeba crawl along surfaces by extending parts of their cells.
Ciliates form the largest group of protozoa. These organisms vary in size and often live in watery environments, including oceans, marshes, bays and streams. These organisms contain special structures, called cilia, which are essentially tiny hair-like strands that act as sensors and tiny limbs. Ciliates have several hundred cilia, which cover their bodies. Waterborne ciliates use their cilia to move through the water by beating them in rhythmical patterns in a motion resembling the strokes of oars. Flagellates also live primarily in water and use their long tail-like flagella to move through the water. These flagella act like rudders by helping to stabilize the flagella as they move. The movements of ciliates and flagellates are quite different from amoebas, which have flexible cell membranes that act as feet. Amoebas cross surfaces by stretching, bending and curving these cell structures, which bulge outward to help amoebas move about their environments.