Flatworms are a type of invertebrate that belong to the phylum Platyhelminthes. They are characterized by their flattened bodies and lack of a true skeleton or circulatory system. Planaria, which have 8 letters, are a type of flatworm that are commonly used in scientific research due to their regenerative capabilities. Trematode, with 9 letters, is another type of flatworm that is known for its parasitic lifestyle. FLUKE, with 5 letters, is a type of trematode that is known for its ability to cause diseases in humans and animals. TAPEWORM, with 8 letters, is a type of flatworm that is also known for its parasitic lifestyle. All of these terms are related to the term flatworm, which is a type of invertebrate that belongs to the phylum Platyhelminthes.