Locomotion in the protist kingdom is varied and extremely versatile. Amoebas use cytoplasmic processes called pseudopods, while paramecia have rows of cilia that move in unison like tiny oars. Euglena and dinoflagellates use whip-like proteins called flagella to swim, and some protozoa, such as diatoms and plasmodium (the organism that causes malaria), are non-motile, meaning they do not move independently.
Cilia and flagella exhibit a great deal of similarity with structures observed in bacteria. This suggests that motor proteins arose very early in the course of evolution. Other eukaryotes, including humans, have retained these useful structures. In the human respiratory tract, ciliated epithelial cells sweep airborne bacteria and debris out of the airways back toward the throat where the cough reflex ejects them from the body. As for flagella, sperm contain similar proteins in their tails that allow them to swim.
Pseudopodia as a means of locomotion remain largely confined to amoebas; nonetheless, a few human cell types retain this feature. Phagocytic white blood cells, called macrophages, use a structure similar to a pseudopod to engulf bacteria and fungal spores. Megakaryocytes (giant cells in the bone marrow) extend multiple pseudopod-like processes, then cleave off cytoplasmic fragments to form platelets.