flatworm flat·worm (flāt'wûrm')
Any of various worms of the phylum Platyhelminthes, including the parasitic tapeworms and flukes, characteristically having a soft, flat, bilaterally symmetrical body and no body cavity. Also called platyhelminth.
Any of various parasitic and nonparasitic worms of the phylum Platyhelminthes, characteristically having a soft, flat, bilaterally symmetrical body. Flatworms lack a coelom (body cavity), respiratory system, and circulatory system, but are the most primitive invertebrates to have a brain. The evolutionary history of flatworms is uncertain, but they share some basic characteristics with rotifers, nematodes, and a few other invertebrate phyla. Cestodes (tapeworms), planarians, and trematodes (flukes) are flatworms.