Platyhelminthes are best described as

• A. Segmented, laterally compressed worms with a coelom
• B. Flatworms with bilateral symmetry and an acoelomate body
• C. Segmented worms with annulated cross-sections
• D. Roundworms with a complete digestive system

Answer: (B)

Platyhelminthes are flatworms with bilateral symmetry and an acoelomate body, meaning they lack a true body cavity between the gut and the body wall. This combination is a defining feature of flatworms and explains why they aren’t segmented and don’t have a coelom. Their flattened shape increases surface area for diffusion, which supports their simple forms of digestion and respiration. They typically have a gastrovascular cavity with a single opening (in many free-living forms) or, in some parasites, a reduced or absent digestive tract. This set of traits matches the description of flatworms, not segmented worms with a coelom, nor roundworms with a complete digestive system.