Which of the following is not a structural adaptation?

Options

A. Camels have long eyelashes to protect their eyes from dust storm.
B. Some animals migrate from less favourable to more favourable places in search of food and to escape harsh weather.
C. Stem of cactus becomes fleshy to store water and also performs photosynthesis.
D. Some aquatic plants have reduced root system and dissected ribbon like leaves.

Explanation

To solve the question "Which of the following is not a structural adaptation?", we will analyze each option provided and determine whether it describes a structural adaptation or not.

### Step-by-Step Solution:

1. **Understanding Structural Adaptation**:
- Structural adaptations are physical features of an organism that enhance its survival in its environment. This includes changes in body structure, such as the shape of leaves, body coverings, or specialized organs.

2. **Analyzing Option A**:
- **Camels have long eyelashes to protect their eyes from dust storms**.
- This is a structural adaptation because the long eyelashes are a physical feature that helps camels survive in their desert habitat by protecting their eyes from dust.

3. **Analyzing Option B**:
- **Some animals migrate from less favorable to more favorable places in search of food and to escape harsh weather**.
- This describes migration, which is a behavioral adaptation, not a structural adaptation. Migration is a response to environmental changes rather than a physical change in the organism's structure.

4. **Analyzing Option C**:
- **Stems of cactus become flaccid to store water**.
- This is a structural adaptation because the change in the stem structure allows the cactus to store water, which is crucial for survival in arid environments.

5. **Analyzing Option D**:
- **Aquatic plants have a reduced root system and dissected, ribbon-like leaves**.
- This is also a structural adaptation as it involves physical changes in the plant's structure that help it thrive in aquatic environments.

6. **Conclusion**:
- Based on the analysis, option B is the only one that does not describe a structural adaptation. Therefore, the answer to the question is **Option B**.

### Final Answer:
**Option B: Some animals migrate from less favorable to more favorable places in search of food and to escape harsh weather.**