Explanation:

Invertebrates are animals that lack a vertebral column (backbone). Both flatworms and roundworms fall into this category because they do not possess a backbone.


Key points about flatworms and roundworms:

- Flatworms (e.g., planarians, tapeworms) and roundworms (e.g., hookworms, Ascaris) are soft-bodied organisms and do not have an internal skeleton or backbone.
- They belong to invertebrate phyla, meaning they lack the vertebral column found in vertebrates (animals with backbones, such as mammals, birds, etc.).



Why the other options are incorrect:

- A. they are small animals – Size is not the defining feature of invertebrates; it’s the lack of a backbone.
- B. they can live inside the vertebrates – Some invertebrates are parasitic, but this does not define them as invertebrates.
- C. some of them are unicellular – While some invertebrates (like certain protists) are unicellular, flatworms and roundworms are multicellular.
- E. they are parasitic – While some invertebrates can be parasitic, this is not the characteristic that classifies them as invertebrates.


Summary:
Flatworms and roundworms are invertebrates because they lack a backbone.

Correct Answer: D. they have no backbones