1. simple invertebrate animals with flattened bodies, including planarians, tapeworms, and flukes
Usage: biology; plural form

Examples

• Flatworms are among the simplest animals with bilateral symmetry.
• The biology class studied flatworms under the microscope.
• Some flatworms are parasites that live inside other animals.
• Planarians are free-living flatworms found in freshwater streams.
• Tapeworms are flatworms that can infect the human digestive system.
• The aquarium had problems with flatworms growing on the glass.