flatworm

noun

flat·​worm ˈflat-ˌwərm 

: any of a phylum (Platyhelminthes) of soft-bodied usually much flattened acoelomate worms (such as the planarians, flukes, and tapeworms)

called also platyhelminth

Examples of flatworm in a Sentence

Recent Examples on the Web

This is an infection caused by blood flukes (parasitic flatworms) that can accidentally penetrate the skin, leading to an allergic reaction, according to the CDC. —Justin Laube, EverydayHealth.com, 1 June 2026

And sure, animals such as starfish and flatworms can reproduce by cloning themselves—but at the end of the day, in most species, the survival of animals rests on their mothers. —Jackie Flynn Mogensen, Scientific American, 10 May 2026

The flatworm doesn’t feed on or live within your pet. —Hanna Wickes, Charlotte Observer, 9 Apr. 2026

In July 2025, the European Union listed three land flatworm species as invasive species of Union concern, reflecting growing awareness of their ecological impact. —Hanna Wickes, Kansas City Star, 9 Apr. 2026

Word History

First Known Use

1874, in the meaning defined above

Kids Definition

flatworm

noun

flat·​worm -ˌwərm 

: any of a phylum of simple worms (as planarians, flukes, or tapeworms) that have a soft flat body, lack a body cavity, and have bilateral symmetry

Medical Definition

flatworm

noun

flat·​worm ˈflat-ˌwərm 

: any worm of the phylum Platyhelminthes : platyhelminth

especially : turbellarian