Platyhelminthes, better known as flatworms, play important roles in marine, freshwater and terrestrial ecosystems, and several species are dangerous parasites of humans. Flatworms fill a variety of niches in addition to parasitic, including a variety of nutrient sources. Flatworms have been an important part of biological communities since well before any animals emerged on land.
Flatworms have multiple parasitic forms that can be devastating for humans and other animals. Any fluke, including the human liver and lung flukes, is a type of flatworm. The infamous tapeworm is another very specialized form of parasitic flatworm.
Flatworms are distinguished by their very flat shape, which is necessitated by their lack of any respiratory or circulatory systems. They must absorb all oxygen via diffusion from the surrounding air, so no cell can be too far from the surface.
Flatworms are a very ancient group of animals, although, according to the University of California at Berkeley, they may actually be two groups. One of these groups is part of a larger group including molluscs and annelids, while the other originates from a time closer to the beginning of animal life. Because their fossil record is very poor, their presence so long ago is established through genetic studies, as well as telltale signs in the remains of other animals.