The adult ……... are radially symmetrical but larvae are bilaterally symmetrical .

Options

A. Asterias
B. Balanoglossus
C. Obelia
D. Ascaris

Answer

To solve the question, we need to identify which organism has adults that are radially symmetrical and larvae that are bilaterally symmetrical. Let's break down the options step by step.

### Step 1: Analyze the options

1. **Option A: Astrea**
- **Adult Symmetry:** Astrea, which is a type of sea urchin, has adults that are radially symmetrical.
- **Larval Symmetry:** The larvae of Astrea exhibit bilateral symmetry.
- **Conclusion:** This option fits the criteria of the question.

2. **Option B: Palano Glosses**
- **Adult Symmetry:** Palano Glosses (a type of marine organism) are known to be bilaterally symmetrical in both adults and larvae.
- **Conclusion:** This option does not fit the criteria.

3. **Option C: Obelia**
- **Adult Symmetry:** Obelia is a colonial organism that exhibits radial symmetry in its adult form.
- **Larval Symmetry:** However, the larvae of Obelia are also radially symmetrical.
- **Conclusion:** This option does not fit the criteria.

4. **Option D: Ascaris**
- **Adult Symmetry:** Ascaris is a type of roundworm, which exhibits bilateral symmetry in both adults and larvae.
- **Conclusion:** This option does not fit the criteria.

### Step 2: Final Conclusion
Based on the analysis of all the options, the only organism that has adults that are radially symmetrical and larvae that are bilaterally symmetrical is **Astrea**.

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

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