What is torsion? What is its effect?

Explanation

### Step-by-Step Solution:

1. **Definition of Torsion**: Torsion refers to the twisting of the visceral mass in certain organisms, particularly in snails which belong to the phylum Mollusca. This process involves a 180-degree rotation of the internal organs.

2. **Occurrence in Snails**: In snails, torsion results in the rotation of the stomach and intestine, which alters the arrangement of their internal structures.

3. **Asymmetry**: One of the primary effects of torsion is that it causes asymmetry in the body structure of the snail. This means that the left and right sides of the snail's body are not mirror images of each other.

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