Advantages of being multicellular include an overall larger body size, cell differentiation and greater complexity as an organism. Cell differentiation refers to the designation of different cells to different functions, as opposed to all of an organism's biological processes taking place inside the same cell.
Flatworms are an example of a relatively simple multicellular organism that exhibits cell differentiation but not the amount of complexity seen in species that possess organ systems. The cells on the outer surface of a flatworm's body serve the specialized functions of absorbing food from the environment and protecting the cells that make up the inside of the organism. Because of this outer protective layer, a state of balance, or homeostasis, is able to be maintained on both the cellular and organismal levels. However, because the flatworm lacks organ systems, its body must be flat to ensure that oxygen diffuses to all areas of the body, while carbon dioxide is able to exit all areas.
More developmentally advanced multicellular organisms are able to have more complex body shapes and exhibit more behaviors due to the existence of organ systems, such as the digestive, nervous, circulatory and respiratory systems. Organisms such as humans and dogs do not need flat bodies because their circulatory and respiratory systems make it possible for important molecules to reach all necessary areas of the body without relying solely on diffusion, for example.