The adults are readially symmetrical but larvae exhibit bilateral symmetry in

Options

A. Mollusca
B. Hemichordara
C. Echinodermata
D. Cephalochordata.

Answer

To solve the question regarding which group of organisms has adults that are radially symmetrical while their larvae exhibit bilateral symmetry, we can break down the analysis step by step.

### Step-by-Step Solution:

1. **Understanding Symmetry Types**:
- **Radial Symmetry**: Organisms with radial symmetry can be divided into similar halves by multiple planes passing through a central axis. This is typical in adult echinoderms.
- **Bilateral Symmetry**: Organisms with bilateral symmetry can only be divided into two similar halves by one plane. This is typical in many larval forms.

2. **Analyzing the Options**:
- **Mollusca**:
- Larvae: Bilaterally symmetrical.
- Adults: Asymmetrical or sometimes exhibit torsion leading to asymmetry.
- Conclusion: Incorrect option because adults are not radially symmetrical.

- **Hemichordata**:
- Both larvae and adults: Bilaterally symmetrical.
- Conclusion: Incorrect option as both stages are bilaterally symmetrical.

- **Echinodermata**:
- Larvae: Bilaterally symmetrical.
- Adults: Radially symmetrical.
- Conclusion: This is the correct option as it fits the criteria of the question.

- **Cephalochordata**:
- Both larvae and adults: Bilaterally symmetrical.
- Conclusion: Incorrect option as both stages are bilaterally symmetrical.

3. **Final Conclusion**:
- The only group that has adults that are radially symmetrical while their larvae exhibit bilateral symmetry is **Echinodermata**.

### Answer:
The correct answer is **Echinodermata**.