The ability of an organism to move in a particular direction in its environment, which requires a propulsive force acting against a supporting structure. Most animals and many single-celled organisms have powers of locomotion. Some protists possess contractile fibres that exert force on the plasma membrane to change the shape of the cell; this may be combined with cytoplasmic streaming to bring about locomotion (see amoeboid movement). In many other protists and bacteria the propulsive force is provided by the action of undulipodia or flagella. In animals the force required to initiate locomotion is generated by voluntary muscles, which act against a supporting framework provided by a skeleton. See also fins; flight.
Subjects: Zoology and Animal Sciences.