Flatworms, members of phylum Platyhelminthes, meaning "flat worm" in Greek, are relatively simple animals without a true coelom, or body cavity. Unlike cnidarians such as jellyfish, which possess only two germ layers and whose ancestors never had a coelom, flatworms are triploblastic (possess three germ layers) and evolved from more complex animals with coeloms. Initially, it was thought that flatworms were basal among protosomes (a large category of animals), but this been found to only be true of the groups Acoela and Nemertodermatida. These animals were then given their own phylum, Acoelomorpha.
Flatworms are primarily aquatic, being found in both marine and freshwater environments, as well as some moist land environments. In Ireland and Scotland, one flatworm, Arthurdendyus triangulatus, has been so successful that since its accidental introduction in the 1960s, it has largely replaced the indigenous earthworms. This is an interesting example of an acoelomate species outcompeting a coelomate species on land, a relatively rare occurrence. Typically acoelomates are more successful in water, where they are better cushioned from physical impact. Because flatworms are acoelomate, their bodies have no "give," meaning that external pressure can easily cause organ damage.
Flatworms are free-living or parasitic, and range from microscopic to over 90 ft (27 m) long, in the case of the tapeworm, which is found in the intestinal tract of vertebrates. There are four flatworm classes: Trematoda (flukes), Cestoda (tapeworms), Monogenea (small fish parasites), and Turbellaria (carnivorous free-living flatworms). Flukes are parasites of vertebrates and mollusks, while tapeworms are parasites of vertebrates. Essentially, three of four flatworm classes are parasites while one is free-living. Free-living flatworms are characterized by a slightly more complex morphology, such as ocelli (eye spots) that detect ambient light, and auricles (ear-like flaps) that sense water currents.
There are about 25,000 species of flatworms, making them the largest acoelomate phyla. The vast majority of animals by species (>99%) are coelomate, although some acoelomate animals, such as nematodes, are extremely numerous, found in every habitable environment. Like other acoelomates and some coelomates, flatworms move by means of an undulating motion. Parasitic flatworms attach to their host by means of a haptor, a specialized structure that both adheres to the host and allows feeding.