Which statement best describes Nematoda?

• A. Unsegmented roundworms with a pseudocoelom and a tough, molted cuticle
• B. Segmented worms with a true coelom
• C. Radially symmetric jellylike parasites with no digestive system
• D. Jointed-legged with an exoskeleton

Answer: (A)

Nematoda are unsegmented, cylindrical worms with a pseudocoelom and a tough outer cuticle that is shed as they grow. The unsegmented body means there are no repeating true segments like in some worms, and the pseudocoelom is a fluid-filled space between tissues that provides some internal support but isn’t a true body cavity lined by mesoderm. The cuticle is secreted by the epidermis and must be molted (shed) during growth, a feature shared by ecdysozoans. Together, these traits—unsegmented shape, a pseudocoelom, and a molted cuticle—most accurately describe nematodes.

The other descriptions point to different groups: segmented worms have a true coelom and body segments; jellylike organisms with radial symmetry and no digestive system belong to different phyla with very different body plans; organisms with jointed legs and an exoskeleton are arthropods.