Being a sessile animal, unable to move from place to place, makes life difficult for a wide variety of reasons. For example, if food is not available in a very limited area, death may come very quickly. As a result, a great many living animals, from unicellular microbes to multicellular higher organisms, have developed the ability to move from one place to another as needed. This ability for self-directed movement is called locomotion.

Salem Press Encyclopedia of Science, 2023. 3p.