Flatworms move in various ways. Some use muscular movements of their bodies, whereas others have soles underneath for locomotion. Some spit mucus and use it as a rope to pull themselves. Some aquatic species use their cilia, or small moving hairs, to swim. Some burrow, anchoring with their rear ends and moving their heads. Parasites within hosts use suckers, hooks or spines.
Flatworms are primitive life forms. They lack respiratory or circulatory systems and have no internal cavities except guts, and the smallest of them even lack guts. Simple diffusion enables them to absorb nutrition and oxygen and expel carbon dioxide. Many types have no anus and expel undigested material through the mouth, while others have multiple anuses. They are bilaterally symmetrical, which means the left and right sides of their bodies mirror each other. Most have specific tops and bottoms, as well as head and tail sections. Many are shaped like leaves or ribbons.
Parasitism by tapeworms is a serious cause of disease in humans and livestock. Apart from malaria, it is the main cause of parasitic disease in tropical countries. Though tapeworm infestation is not usually fatal in humans, it damages internal organs, impairs the development of children, and causes epilepsy, bladder cancer, vitamin B12 deficiency and anemia. Efforts to control tapeworm parasitism are hampered by modern trends towards organic farming and lightly cooked seafood and meat.