Heart of frog differs from that of man by presence of
or
Heart of rat differs from that of frog by the absence of

Options

A. Sinus venosus
B. Four chambers
C. Mitral valve
D. Two aorta

Answer

### Step-by-Step Solution:

1. **Identify the Heart Structure of Frogs and Humans**:
- The heart of a frog is classified as a three-chambered heart, consisting of two atria and one ventricle.
- In contrast, the heart of a human (a mammal) is a four-chambered heart, consisting of two atria and two ventricles.

2. **Determine the Key Differences**:
- The primary difference between the frog's heart and the human heart is that the frog's heart has an additional structure called the **sinus venosus**. This structure is responsible for collecting deoxygenated blood before it enters the atria.

3. **Conclusion for the First Part of the Question**:
- Therefore, the heart of a frog differs from that of a man by the presence of the **sinus venosus**.

4. **Identify the Heart Structure of Rats**:
- The heart of a rat, like that of humans, is a four-chambered heart.

5. **Determine the Key Differences Between Rat and Frog Hearts**:
- The rat's heart does not have the **sinus venosus**, which is present in the frog's heart.

6. **Conclusion for the Second Part of the Question**:
- Hence, the heart of a rat differs from that of a frog by the absence of the **sinus venosus**.

### Final Answers:
- The heart of a frog differs from that of a man by the presence of **sinus venosus**.
- The heart of a rat differs from that of a frog by the absence of **sinus venosus**.