Flatworms are a group of worms characterized by a flattened body shape, including planarians and tapeworms.

Flatworms are a group of worms characterized by a flattened body shape, including planarians and tapeworms.

• A. Nematodes
• B. Annelids
• C. Flatworms
• D. Roundworms

Answer: (C)

Flatworms are identified by their flattened bodies, a form that maximizes surface area for diffusion and supports their relatively simple organ systems. Planarians are classic free-living flatworms, while tapeworms are parasitic and attach to host intestines with a scolex and grow long chains of reproductive units called proglottids. Together, they belong to the phylum Platyhelminthes, which are acoelomates—lacking a true body cavity—and typically have bilateral symmetry with three tissue layers. This contrasts with roundworms, which are elongated and have a pseudocoelom, and with segmented worms, which have a true coelom and segmented bodies. Because the description centers on a flattened body shape and includes planarians and tapeworms, the group being described is flatworms.

Flatworms are identified by their flattened bodies, a form that maximizes surface area for diffusion and supports their relatively simple organ systems. Planarians are classic free-living flatworms, while tapeworms are parasitic and attach to host intestines with a scolex and grow long chains of reproductive units called proglottids. Together, they belong to the phylum Platyhelminthes, which are acoelomates—lacking a true body cavity—and typically have bilateral symmetry with three tissue layers. This contrasts with roundworms, which are elongated and have a pseudocoelom, and with segmented worms, which have a true coelom and segmented bodies. Because the description centers on a flattened body shape and includes planarians and tapeworms, the group being described is flatworms.