A jawless fish, which layes eggs in fresh water and whose ammocoetes larvae after metamorphosis return to the ocean is:

Options

A. Petromyzon
B. Exocoetus
C. Scoliodon
D. Trygon

Answer

To solve the question, we need to identify the jawless fish that lays eggs in freshwater and whose larvae return to the ocean after metamorphosis. Let's break down the solution step by step.

### Step-by-Step Solution:

1. **Identify the Characteristics of the Fish**:
- The fish in question is jawless.
- It lays eggs in freshwater.
- The larvae (ammocoetes) return to the ocean after metamorphosis.

2. **Recall the Class of Jawless Fish**:
- Jawless fish belong to the class Cyclostomata. This class includes species like lampreys and hagfish.

3. **Consider the Examples Provided**:
- The options given are:
A. Petromyson (Lamprey)
B. Exocetus (Flying fish)
C. Scolodon (Shark)
D. Trigon (Stingray)

4. **Analyze Each Option**:
- **A. Petromyson (Lamprey)**: This is a jawless fish that lays eggs in freshwater and has larvae that migrate to the ocean after metamorphosis.
- **B. Exocetus**: This is a bony fish (not jawless) and does not fit the description.
- **C. Scolodon**: This is a shark (cartilaginous fish) and does not fit the description.
- **D. Trigon**: This is a stingray (also cartilaginous) and does not fit the description.

5. **Conclusion**:
- Based on the characteristics and the analysis of the options, the correct answer is **A. Petromyson (Lamprey)**.

### Final Answer:
The jawless fish that lays eggs in freshwater and whose larvae return to the ocean after metamorphosis is **A. Petromyson**.

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