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

Researchers have confirmed that invasive flatworms can hitch rides on your pet’s fur, clinging to their coat after time in the yard or garden. —Hanna Wickes, Kansas City Star, 9 Apr. 2026

Dogs and outdoor cats are acting as unwitting vehicles for an Australian flatworm species called Caenoplana variegata, carrying it between gardens and neighborhoods on their fur. —Hanna Wickes, Charlotte Observer, 9 Apr. 2026

The flatworm climbs aboard, rides along and drops off somewhere new — no harm done to the host. —Hanna Wickes, Miami Herald, 9 Apr. 2026

Ruptures, tears, and cracks are known to shape zebra fish nostrils, hydra mouths, fruit fly legs, and whole flatworms. —Clare Watson, Quanta Magazine, 27 Feb. 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