Planaria and tapeworms are examples of which group?

Planaria and tapeworms are examples of which group?

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

Answer: (A)

Understanding how body plans differentiate worm groups helps explain why planaria and tapeworms are flatworms. Both have flattened, ribbon-like bodies and lack a true body cavity (they are acoelomates), which sets them apart from other worm groups. This flattened form means diffusion is enough for getting gases in and nutrients around the body, so there aren’t specialized circulatory or respiratory systems. Planaria are free-living flatworms with a simple digestive system that has a single opening leading to a gastrovascular cavity. Tapeworms, on the other hand, are parasitic flatworms that completely lack a digestive tract and absorb nutrients directly through their outer surface, or tegument. These traits—flattened body plan, absence of a true coelom, and, in the case of tapeworms, no digestive system at all—define them as flatworms. In contrast, roundworms are pseudocoelomates with a complete digestive tract, and annelids are segmented coelomates with a true body cavity.

Understanding how body plans differentiate worm groups helps explain why planaria and tapeworms are flatworms. Both have flattened, ribbon-like bodies and lack a true body cavity (they are acoelomates), which sets them apart from other worm groups. This flattened form means diffusion is enough for getting gases in and nutrients around the body, so there aren’t specialized circulatory or respiratory systems.

Planaria are free-living flatworms with a simple digestive system that has a single opening leading to a gastrovascular cavity. Tapeworms, on the other hand, are parasitic flatworms that completely lack a digestive tract and absorb nutrients directly through their outer surface, or tegument. These traits—flattened body plan, absence of a true coelom, and, in the case of tapeworms, no digestive system at all—define them as flatworms.

In contrast, roundworms are pseudocoelomates with a complete digestive tract, and annelids are segmented coelomates with a true body cavity.