Any member of the diverse phylum Platyhelminthes. The majourity of these are free-living worms with a sac-like, often branching gut, with some flatworms bearing an eversible pharynx, and even a small yet well-organised and complex brain; though the phylum also contains parasitic and commensal flatworms, as well. The parasitic forms are often the most studied, due to their roles in disease.

• Examples:
• planarian
• polyclad
• tapeworm
• trematode
• Implies:
• marine
• worm
• invertebrate