Which one of the following has mesoderm but no coelom-

Options

A. Flatworm
B. Earthworm
C. Roundworm
D. Hirudinaria

Answer

To solve the question "Which one of the following has mesoderm but no coelom?", we need to analyze the options provided and understand the definitions of mesoderm, coelom, and the types of organisms mentioned.

### Step-by-Step Solution:

1. **Understand the Terms**:
- **Mesoderm**: One of the three primary germ layers in the early embryo. It gives rise to muscles, the circulatory system, and other internal organs.
- **Coelom**: A fluid-filled body cavity that is completely surrounded by mesoderm. It allows for the development of complex organs and systems.
- **Acoelomate**: Organisms that have mesoderm but lack a coelom (e.g., flatworms).
- **Pseudocoelomate**: Organisms that have a body cavity (pseudocoel) that is not completely lined by mesoderm (e.g., roundworms).
- **Coelomate**: Organisms that have a true coelom (e.g., earthworms).

2. **Evaluate Each Option**:
- **Flatworm**: Flatworms are classified as acoelomates. They possess mesoderm but do not have a coelom. Therefore, this option is correct.
- **Earthworm**: Earthworms are coelomates, meaning they have a true coelom formed by the splitting of the mesoderm. This option is incorrect.
- **Roundworm**: Roundworms are pseudocoelomates, which means they have a pseudocoel that is not entirely lined by mesoderm. This option is also incorrect.
- **Hirudinea (Leeches)**: Leeches are coelomates and possess a true coelom. This option is incorrect as well.

3. **Conclusion**:
- The only organism from the options provided that has mesoderm but no coelom is the **flatworm**.

### Final Answer:
**Flatworm** is the correct answer as it has mesoderm but no coelom.