Single circuit heart occurs in

Options

A. Fishes
B. frog
C. Reptiles
D. Man

Answer

### Step-by-Step Solution:

1. **Understanding the Concept of Circulatory Systems**:
- There are different types of circulatory systems in animals, primarily classified into single and double circulatory systems.

2. **Defining Single Circuit Heart**:
- A single circuit heart means that blood passes through the heart only once during each complete circuit of the body. This is characteristic of certain animals.

3. **Identifying the Organisms**:
- The question asks where a single circuit heart occurs. The options provided are:
- Fishes
- Frogs
- Reptiles
- Men

4. **Analyzing the Heart Structure in Different Organisms**:
- **Fishes**: They possess a two-chambered heart consisting of one atrium and one ventricle. Blood flows from the heart to the gills for oxygenation and then to the rest of the body, completing a single circuit.
- **Frogs (Amphibians)**: They have a three-chambered heart with two atria and one ventricle, allowing for a double circuit.
- **Reptiles**: Most reptiles also have a three-chambered heart, except for crocodiles, which have a four-chambered heart, indicating a double circuit.
- **Men (Mammals)**: Mammals have a four-chambered heart, which allows for a double circuit of blood flow.

5. **Conclusion**:
- Based on the analysis, the only group that has a single circuit heart is fishes. Therefore, the correct answer to the question "Single circuit heart occurs in" is **fishes**.

### Final Answer:
**Single circuit heart occurs in fishes.**

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