- Kingdom: Animalia
- Phylum: Platyhelminthes
Flatworms are unsegmented worms with a tail and a head end. They are bilaterally symmetrical with a definite upper and lower surface. They have no body cavity or and no specialized respiratory or circulatory organs. This is the reason behind the flat shape, common to all flatworms. They depend on a large surface to allow oxygen and nutrition to diffuse into their bodies. Most species are hermaphrodites, having both female and male reproduction organs.
Many flatworms are parasites, but most flatworms encountered by divers are members of the order Polycladida
, a group of free-living flatworms, mainly found in tropical waters. A few species though, are relatively common along the coast of Norway.