Incomplete double circulation is found in reptiles/birds.

Explanation

### Step-by-Step Solution:

1. **Understanding Circulation Types**:
- Circulation in animals can be classified into two types: complete and incomplete double circulation. In complete double circulation, blood passes through the heart twice, while in incomplete double circulation, blood passes through the heart only once for oxygenation.

2. **Defining Incomplete Double Circulation**:
- In incomplete double circulation, blood enters the heart through two different routes (from the body and lungs) but exits through a single route. This means that oxygenated and deoxygenated blood can mix to some extent.

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