All flatworms differ from all round worms in having

Options

A. Solid mesoderm
B. Triploblastic body
C. Bilateral symmetry
D. Metamorphosis in the life history

Answer

To solve the question "All flatworms differ from all round worms in having," we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Phyla**:
- Flatworms belong to the phylum **Platyhelminthes**.
- Roundworms belong to the phylum **Nematoda** (also known as Aschelminthes).

2. **Understand Body Structure**:
- Flatworms are characterized as **acoelomate**, meaning they lack a true body cavity.
- Roundworms are classified as **pseudocoelomate**, which means they have a false body cavity.

3. **Examine Mesoderm Characteristics**:
- Flatworms have a **solid mesoderm**. This means that the mesodermal tissue is solid and does not form a body cavity.
- In contrast, roundworms have a body cavity that is not completely lined by mesoderm, hence they are pseudocoelomates.

4. **Compare Other Characteristics**:
- Both flatworms and roundworms are **triploblastic**, meaning they have three germ layers (ectoderm, mesoderm, and endoderm).
- Both exhibit **bilateral symmetry**, where the organism can be divided into two mirror-image halves.
- Both groups undergo **metamorphosis** during their life cycle.

5. **Conclusion**:
- The primary difference that distinguishes all flatworms from all roundworms is the presence of a **solid mesoderm** in flatworms, while roundworms possess a pseudocoelom.

### Final Answer:
All flatworms differ from all round worms in having a **solid mesoderm**.

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