In biology, an organism is any organic, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fungi; or unicellular microorganisms such as protists, bacteria, and archaea. All types of organisms are capable of reproduction, growth and development, maintenance, and some degree of response to stimuli. beetles, squids, tetrapods, mushrooms, and vascular plants are examples of multicellular organisms that differentiate specialized tissues and organs during development.