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The Platyhelminthes are a phylum of relatively simple animals, called flatworms. The body is triploblastic and divided into distinct organs, but there is no respiratory or circulatory system, and there are no body cavities except the gut, which is absent in some highly reduced forms. Usually the digestive tract has two openings, but in acoel flatworms it only has one.

Flatworms are considered basal among the protostomes. As the phylum are defined mainly by a lack of characteristics, it is quite possible that it is polyphyletic. However, the following three classes of flatworms, grouped together based on some characteristics of the skin, probably form a monophyletic group:

The remaining orders included in the phylum, grouped together for convenience as the class Turbellaria, are the following:

Most of these groups include free-living forms. The flukes and tapeworms, though, are parasitic, and a few cause extreme damage to people and other animals.