- Elongated, soft bodied, dorsoventrally flattened worms, without true segmentation.
- No body cavity
- Suckers or hooks or both for attachment to the body of the host
- Sexes usually united, mostly sexual reproduction, with asexual reproduction in some.
- Alimentary canal has only one opening–the mouth. In some forms (e.g. tapeworms) there is no alimentary canal at all.
- A few are free-living but mostly parasites.
Examples: Planaria (free living), Fasciola (liver-fluke) is a parasite of sheep liver, Taenia (tapeworm) is a parasite of the human intestine.