Flatworms occupy a broad range of habitats in salt water, fresh water and wetlands. They exist as parasites within or upon host organisms, or as free-living organisms.
Flatworms include over 20,000 species of invertebrates with flattened bodies and distinguishable heads and tails. Because they have not developed circulatory or respiratory systems, their external membrane is the site of oxygen uptake. Their reliance on diffusion restricts their maximum size and shape.
Flatworms typically perform sexual reproduction and are hermaphroditic. They do engage in asexual reproduction under stress or due to the lack of partners. During fission, the flatworm's membrane constricts in its rear, and it splits into two individuals, regrowing any missing tissues. Some species perform asexual reproduction exclusively, using a process called paratomy in which clones grow in chains from the rear of the worm. When the clones develop into complete individuals, they separate from the chain.
Flatworm groups include tapeworms, planarians and flukes. Tapeworms are internal parasites that grow in the digestive tracts of animals, robbing their hosts of nutrients. Planarians are non-parasitic flatworms, using cilia for mobility. Digenic flukes are parasites of vertebrates and mollusks. Monogenic flukes are external parasites using hooks and suckers to attach to fish.