Single circulation is found in fish and amphibians.

Explanation

**Step-by-Step Solution:**

1. **Understanding Single Circulation:**
- Single circulation refers to the process where blood passes through the heart only once during each complete circuit of the body. This means that the blood is pumped from the heart to the gills (in fish) or lungs (in amphibians) and then directly to the rest of the body without returning to the heart in between.

2. **Circulation in Fish:**
- Fish exhibit single circulation. They have a heart with two main chambers: one atrium and one ventricle.
- Blood flows from the heart to the gills through an artery, where it gets oxygenated.
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