Concept:Invertebrates are animals that do not have a vertebral column, also called a backbone.
Explanation:Flatworms and roundworms belong to the groups Platyhelminthes and Nematoda.
These groups do not possess a backbone or vertebral column.
A backbone or vertebral column is derived from the notochord and is present only in vertebrates.
Since flatworms and roundworms lack this structure, they are classified as invertebrates.
Their small size, ability to live inside vertebrates, or unicellular nature are not the reasons for this classification.
Answer:They are said to be invertebrates because they have no backbones.
Correct option: D.