Correct Option: D. they have no backbones Detailed Explanation:

1. Definition of Invertebrates: Invertebrates are animals that lack a backbone or spinal column. This classification includes a wide variety of organisms, such as flatworms, roundworms, insects, mollusks, and many others. The absence of a backbone is a key characteristic that distinguishes invertebrates from vertebrates, which do possess a backbone.
2. Flatworms and Roundworms:
3. Flatworms (Phylum Platyhelminthes) are characterized by their flat, soft bodies. They include species such as planarians, flukes, and tapeworms.
4. Roundworms (Phylum Nematoda) have a more cylindrical body shape and are often found in various environments, including soil and aquatic habitats. Examples include hookworms and pinworms.
5. Why Option D is Correct:
6. Both flatworms and roundworms are classified as invertebrates because they do not have a backbone. This absence of a skeletal structure made of vertebrae is the primary reason they fall into the invertebrate category. Instead of a backbone, these organisms have other forms of support, such as a hydrostatic skeleton in the case of roundworms, which helps maintain their shape and allows for movement.

Why the Other Options are Incorrect:

• Option A: they are small animals:
• While many invertebrates, including flatworms and roundworms, tend to be small, size alone does not define whether an organism is an invertebrate. There are large invertebrates, such as giant squids and some jellyfish. Therefore, this option is misleading and does not address the defining characteristic of invertebrates.
• Option B: they can live inside the vertebrates:
• Some invertebrates, including certain flatworms and roundworms, are parasitic and can live inside vertebrate hosts. However, this trait does not define them as invertebrates. Many vertebrates can also be found living in or on other vertebrates, but this does not change their classification. Thus, this option is irrelevant to the definition of invertebrates.
• Option C: some of them are unicellular:
• This statement is inaccurate as both flatworms and roundworms are multicellular organisms. Invertebrates can include unicellular organisms (like protozoa), but flatworms and roundworms are not among them. This option confuses the classification of invertebrates with characteristics of other types of organisms.

Summary of Key Points:

• Invertebrates are defined by the absence of a backbone.
• Flatworms (Platyhelminthes) and roundworms (Nematoda) are both invertebrates due to this lack of a spinal column.
• Size, habitat, and cellular structure do not determine whether an organism is an invertebrate.
• Understanding the characteristics that define invertebrates is crucial for distinguishing them from vertebrates.

This thorough understanding of invertebrates, particularly flatworms and roundworms, will aid in recognizing their biological classification and characteristics in future studies.