Which organisms are called primitive and how are they different from the so-called advanced organisms?

Explanation

### Step-by-Step Text Solution:

1. **Definition of Primitive Organisms**:
- Primitive organisms, also referred to as lower organisms, are characterized by their simple body structure and ancient body design. They have not undergone significant changes over time.

2. **Characteristics of Primitive Organisms**:
- **Simple Body Structure**: These organisms typically have a less complex arrangement of cells and tissues.
- **Old Body Design**: Their anatomical features have remained relatively unchanged through evolutionary history.
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