Pearl is secreted when a foreign particle happens to enter between

Options

A. Ostracum and hypoostracum
B. Periostracum and hypoostracum
C. Shell and mantle
D. Inside mantle cavity.

Answer

To answer the question "Pearl is secreted when a foreign particle happens to enter between," we need to analyze the options provided and understand the process of pearl formation in mollusks.

### Step-by-Step Solution:

1. **Understanding Pearl Formation**:
- Pearls are formed when a foreign particle, such as a grain of sand or a parasite, enters the soft tissue of a mollusk. This process is a defense mechanism of the mollusk.

2. **Identifying the Layers**:
- The mollusk has several layers associated with its shell:
- **Periostracum**: The outermost layer of the shell.
- **Ostracum**: The middle layer, which is a calcified layer.
- **Hypostracum**: The innermost layer, also calcified.
- **Mantle**: The soft tissue that secretes the shell and nacre (the substance that forms pearls).

3. **Location of Pearl Formation**:
- When a foreign particle enters, it typically gets trapped between the mantle and the shell. The mantle secretes layers of nacre around the foreign particle, eventually forming a pearl.

4. **Choosing the Correct Option**:
- Among the options given:
- **Option A**: Ostracum and Hypostracum - These are layers of the shell but do not involve the mantle directly.
- **Option B**: Periostracum and Hypostracum - Again, these are shell layers and do not directly involve the mantle.
- **Option C**: Shell and Mantle - This option correctly identifies the relationship where the foreign particle is trapped between the shell and the mantle.
- **Option D**: Inside the Mantle Cavity - While pearls are formed in the mantle cavity, this option does not specify the involvement of the shell.

5. **Conclusion**:
- The correct answer is **Option C: Shell and Mantle**, as the pearl forms when a foreign particle is trapped between the mantle and the shell, leading to the secretion of nacre around it.