Torsion is the characteristic of:

Options

A. Gastropoda
B. Pelecypoda
C. Cephalopoda
D. Amphinura

Answer

To solve the question "Torsion is the characteristic of:", we will analyze the options provided and determine which one correctly describes the phenomenon of torsion.

### Step-by-Step Solution:

1. **Understanding Torsion**:
- Torsion refers to the twisting or rotation of visceral organs in an anti-clockwise direction. This process typically occurs during the larval development of certain organisms.

2. **Analyzing the Options**:
- **Option 1: Gastropoda**:
- Gastropods are a class of mollusks that exhibit torsion. During their development, their body undergoes a 180-degree twist, allowing for the retraction of the foot after the head, which provides better protection for the head.
- **Conclusion**: This option is correct.

- **Option 2: Polyplacophora**:
- Polyplacophorans, commonly known as chitons, are marine mollusks with a body that is flattened and covered by a shell made of eight overlapping plates. They do not exhibit torsion.
- **Conclusion**: This option is incorrect.

- **Option 3: Cephalopoda**:
- Cephalopods, such as squids and octopuses, have a complex body structure with arms and tentacles. They do not undergo torsion in the same manner as gastropods.
- **Conclusion**: This option is incorrect.

- **Option 4: Amphinura**:
- Amphinura is not a commonly recognized class in the context of torsion and is likely a misrepresentation. Generally, this category does not exhibit torsion.
- **Conclusion**: This option is incorrect.

3. **Final Answer**:
- Based on the analysis, the correct answer is **Option 1: Gastropoda**.