ABSTRACT
Amoebae, diatoms, and some types of cyanobacteria are all single-celled organisms that cannot swim, even though they live in water. Apparently they find it safer and more profitable to clamber over particles of rock or plants than to venture out into the third dimension. Most cells in an adult plant or animal are fixed in place as part of a tissue such as epidermis or muscle and do not move as individuals from one location to another. There are some cell types whose functions require active migration. Zoologists also use the terms 'pseudopodia' and 'pseudopods' rather generally to refer to a variety of cell-surface protrusions, even if they are not concerned primarily with locomotion. These include long and slender processes that act as feelers for the cell and others that extend for large distances away from the cell over surfaces. Eucaryotic flagella and cilia are used to drive cells over surfaces, as illustrated by the curious organisms known as hypotrichs.
