Name a single-called organism (animal) which constantly changes its shape.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a single-celled organism (also referred to as a unicellular organism) that can constantly change its shape.

2. **Defining Single-Celled Organisms**: Single-celled organisms are those that consist of only one cell. They can perform all necessary life functions within that single cell.

3. **Identifying Examples**: Among various single-celled organisms, we need to find one that is known for its ability to change shape.

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