Roundworms, hookworms, and Caenorhabditis elegans are examples of which phylum?

• A. Nematoda
• B. Platyhelminthes
• C. Annelida
• D. Arthropoda

Answer: (A)

Recognizing nematodes by their shared body plan helps explain why roundworms, hookworms, and Caenorhabditis elegans all belong to the same phylum. These organisms have unsegmented, slender, cylindrical bodies with a tough cuticle that covers a pseudo­coelomic cavity. They possess a complete digestive tract and a simple nervous system, but they lack segmentation and the jointed limbs of arthropods, the true body cavity and segmented segments of annelids, and the flattened, often leaflike form of flatworms in platyhelminthes. Caenorhabditis elegans is a classic model organism used to study development and genetics, and it is a quintessential example of a nematode, just like other roundworms and hookworms. So, the correct phylum is Nematoda.