Question

what is the major characteristic of the phylum platyhelminthes?
they have complex life cycles with many hosts
they are small and round
they are all parasitic
they are dorsoventrally flattened

Explanation:

1. Analyze each option:

• Option 1: Complex life cycles with many hosts are more characteristic of some parasitic Platyhelminthes (like tapeworms), but not all members of the phylum. Free - living Platyhelminthes (e.g., planarians) do not have such complex life cycles with many hosts.
• Option 2: Platyhelminthes are flat, not round. Roundworms belong to the phylum Nematoda.
• Option 3: Not all Platyhelminthes are parasitic. There are free - living forms such as planarians.
• Option 4: The defining characteristic of Platyhelminthes (flatworms) is that they are dorsoventrally flattened (flat - bodied), which gives them their name.

Answer:

D. they are dorsoventrally flattened