Animals, even those who seem rooted like plants, have special structures that move. Sessile animals include anemonies, sea lilies, and corals. Shell fish, such as clams and oysters, pump the water through their bodies and strain out little bits of food. Most animals, however, are mobile. This mobility, however, is costly.
Movement requires energy and good coordination.
Animals require nervous systems and efficient ways to breathe and nourish working cells. Body Form Plants grow to fit into their environments, and they differ in the arangement and number of their branches, flowers, and leaves. Mobile animals cannot afford such structural freedom. You can see that members of every species follow a basic body pattern. Staying alive is a challenge for every organism.