Flatworms are simple animals without a true body cavity, but have three germ layers and evolved from more complex animals. They are primarily aquatic and range from microscopic to over 90 feet in length. There are four classes of flatworms, with three being parasitic and one being free-living. Flatworms lack any “gifts” and move in an undulating motion. They are the largest acoelomate phyla, with approximately 25,000 species.
Flatworms, members of the phylum Platyhelminthes, which means “flatworm” 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 (have three germ layers) and evolved from more complex animals with coeloms. Initially, flatworms were thought to be basal among the protosomes (a broad category of animals), but this has only been found for the Acoela and Nemertodermatida groups. These animals were then given their own phylum, Acoelomorpha.
Flatworms are primarily aquatic, found in both marine and freshwater environments, as well as some moist land environments. In Ireland and Scotland, a flatworm, Arthurdendyus triagulatus, 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 acelomate species outgrowing a coelomate species on the ground, a relatively rare occurrence. Typically acoelomates are most successful in water, where they are better cushioned from physical impact. Because flatworms are acoelomates, their bodies lack any “gifts,” meaning that external pressure can easily cause organ damage.
Flatworms are free-living or parasitic and range from microscopic to over 90 feet (27 m) in length, in the case of the tapeworm, which is found in the intestinal tract of vertebrates. There are four classes of flatworms: Trematoda (flukes), Cestoda (tapeworms), Monogenea (small fish parasites), and Turbellaria (free-living carnivorous flatworms). Flounders are parasites of vertebrates and molluscs, while tapeworms are parasites of vertebrates. Essentially, three of the four classes of flatworms are parasitic while one is free-living. Free-living flatworms are characterized by slightly more complex morphology, such as ocelli (eye spots) that sense ambient light and auricles (ear-like flaps) that sense water currents.
There are approximately 25,000 species of flatworms, making them the largest acoelomate phyla. The vast majority of animals by species (>99%) are coelomates, although some coelomatous animals, such as nematodes, are extremely numerous, present in every habitable environment. Like other acoelomates and some coelomates, flatworms move in an undulating motion. Parasitic flatworms attach to their host by means of a haptor, a specialized structure that adheres to the host and enables feeding.
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