Describe the evolutionary change in the pattern of heart among the vertebrates.

Explanation

### Step-by-Step Solution:

1. **Introduction to Evolutionary Change in Heart Structure**:
- The evolutionary changes in the heart structure among vertebrates primarily revolve around the separation of oxygenated and deoxygenated blood. This separation is crucial for efficient oxygen transport and overall metabolic function.

2. **Heart Structure in Fishes**:
- Fishes possess a **two-chambered heart**, which consists of one atrium (or auricle) and one ventricle.
- In this system, blood flows in a single circuit: deoxygenated blood enters the atrium, moves to the ventricle, and is then pumped to the gills for oxygenation. After oxygenation, the blood is distributed to the rest of the body.
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