According to Encyclopædia Britannica, flatworms are mostly parasitic, soft-bodied invertebrates that include tape worms, flukes and planarians. Lacking circulatory, respiratory and skeletal systems, flatworms feature unsegmented bodies and spongy connective tissue that provide structure and fill the spaces between organs.
Flatworms are cosmopolitan and can live almost anywhere there is moisture. Their habitats range from bodies of fresh water and saltwater to terrestrial areas that are tropical or sub-tropical. Flukes and tapeworms are known parasites of both animals and humans. Poor sanitation can lead to a flatworm infestation, although this is less an issue in developed countries, such as the United States.