The Platyhelminthes are a phylum of 10,000 species 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 one opening, but in particularly long worms or those with highly branched guts, there may be one or more anuses. In acoel flatworms, now thought to be unrelated to the Platyhelminthes, the gut is absent or non-permanent.
Flatworms used to be considered basal among the protostomes. On the basis of molecular evidence it is now thought that they belong in the Lophotrochozoa, except for the orders Acoela and Nemertodermatida, which are considered basal bilaterians and together form the phylum Acoelomorpha. Within the true flatworms, the following three classes, grouped together based on some characteristics of the skin, probably form a monophyletic group:
- Trematoda (flukes, probably paraphyletic to Cestoda)
- Cestoda (tapeworms)
- Nemertinodermatida (now in phylum Acoelomorpha)
- Acoela (now in phylum Acoelomorpha)