Which statement best contrasts roundworms and tapeworms?

• A. Tapeworms have a segmented body with a true coelom, while roundworms are unsegmented and acoelomate.
• B. Tapeworms possess a body cavity, whereas roundworms do not.
• C. Roundworms are exclusively parasitic, while tapeworms are always free-living.
• D. Roundworms have a complete digestive tract, while tapeworms absorb nutrients directly through their body surface.

Answer: (D)

Verified step by step guidance

1. Step 1: Begin by understanding the key biological characteristics of roundworms and tapeworms. Roundworms (phylum Nematoda) are unsegmented worms with a complete digestive tract, meaning they have both a mouth and an anus for food processing. Tapeworms (phylum Platyhelminthes, class Cestoda), on the other hand, lack a digestive system entirely and absorb nutrients directly through their body surface.
2. Step 2: Analyze the structural differences between the two organisms. Roundworms are cylindrical and possess a pseudocoelom, which is a fluid-filled body cavity that is not completely lined by mesoderm. Tapeworms are flat, segmented, and lack a true body cavity (acoelomate).
3. Step 3: Consider the mode of nutrient acquisition. Roundworms ingest food through their mouth, process it in their digestive tract, and expel waste through the anus. Tapeworms, being parasitic, attach to the host's intestinal lining using specialized structures like hooks and suckers, absorbing pre-digested nutrients directly through their tegument (body surface).
4. Step 4: Evaluate the ecological roles and lifestyles. Roundworms can be free-living or parasitic, depending on the species. Tapeworms are exclusively parasitic and rely on a host for survival, often causing harm to the host organism.
5. Step 5: Based on the above analysis, identify the correct statement that contrasts roundworms and tapeworms. The correct answer is: 'Roundworms have a complete digestive tract, while tapeworms absorb nutrients directly through their body surface.' This highlights the fundamental difference in their digestive systems and nutrient acquisition methods.