Flatworm
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 "Platodes" from Ernst Haeckel's Kunstformen der Natur, 1909
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Flatworms (Platyhelminthes, Greek "platy"': flat; "helminth": worm) are a phylum of invertebrates . They are relatively simple animals. They have soft bodies.
With about 25,000 known species, they are the largest phylum of animals without a body cavity. Flatworms can be found in marine, freshwater, and even damp terrestrial environments.
A troublesome terrestrial example is the New Zealand flatworm, Arthurdendyus triangulatus. It rapidly colonized large areas of Ireland and Scotland. It was brought there unintentionally in the 1960s. Since then, it has destroyed most of the indigenous earthworms.
Most flatworms are free-living forms, but many are parasitic on other animals. There are four classes:
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