All flatworms differ from all round worms in having

Options

A. triploblastic body
B. solid mesoderm
C. dilateral symmetry
D. metamorphosis in the life history

Answer

**Step-by-Step Solution:**

1. **Understanding the Organisms**: Flatworms (Phylum Platyhelminthes) and roundworms (Phylum Nematoda) are two distinct groups of worms. They have different body structures and characteristics.

2. **Body Structure**: Flatworms are flat in shape, while roundworms are cylindrical. This difference in shape is one of the most obvious distinctions.

3. **Body Cavity**: Flatworms do not possess a true body cavity (coelom), whereas roundworms have a pseudocoelom. This means that flatworms have a solid body structure without a fluid-filled cavity.

4. **Embryonic Layers**: Both flatworms and roundworms are triploblastic, meaning they develop from three embryonic layers: ectoderm, mesoderm, and endoderm. However, the key difference lies in the presence of a solid mesoderm in flatworms.

5. **Digestive System**: Flatworms have a single opening that serves as both mouth and anus, while roundworms have a complete digestive system with separate mouth and anus.

6. **Conclusion**: The primary difference between flatworms and roundworms is that flatworms have a solid mesoderm, while roundworms have a pseudocoelom and only two embryonic layers.

**Final Answer**: All flatworms differ from all roundworms in having a solid mesoderm.

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