Torsion occurs in the visceral mass of

Options

A. cephalopods
B. gastropods
C. scaphopods
D. pelecypods

Answer

### Step-by-Step Text Solution:

1. **Understanding the Question**: The question asks about the occurrence of torsion in the visceral mass of certain groups of animals. Torsion is a specific phenomenon that occurs during the development of certain mollusks.

2. **Identifying the Groups**: The options given are cephalopods, gastropods, scaphopods, and pelecypods. We need to determine which of these groups exhibits torsion.

3. **Defining Torsion**: Torsion is the process of twisting or rotating the visceral mass, mantle, and shell of the organism. In gastropods, this rotation typically occurs by 180 degrees.

4. **Analyzing Gastropods**: Gastropods, which include snails and slugs, are known for this torsion process. During torsion, the position of the mantle cavity and the anus shifts to a position above the head, which is a distinctive feature of this group.

5. **Comparing Other Groups**:
- **Cephalopods** (like octopuses and squids) do not undergo torsion.
- **Scaphopods** (tusk shells) also do not exhibit torsion.
- **Pelecypods** (bivalves like oysters) are characterized by having two shells and do not undergo torsion either.

6. **Conclusion**: Based on the analysis, torsion occurs specifically in the visceral mass of **gastropods**.

### Final Answer: Torsion occurs in the visceral mass of **gastropods**.

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