Cestoda (Cestoidea) is a class of parasitic worms of the flatworm (Platyhelminthes) phylum. They are informally referred to as cestodes.[2][3] The best-known species are commonly called tapeworms. All cestodes are parasitic and their life histories vary, but typically they live in the digestive tracts of vertebrates as adults, and often in the bodies of other species of animals as juveniles.