Sinus venosus in mammals is believed to have merged with the wall of `:-`

Options

A. Right auricle
B. Left auricle
C. Right ventricle
D. Left ventricle

Answer

To answer the question "Sinus venosus in mammals is believed to have merged with the wall of:", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Sinus Venosus**: The sinus venosus is a structure found in the heart of vertebrates, which is responsible for collecting deoxygenated blood returning to the heart.

2. **Development in Mammals**: In mammals, the sinus venosus is not a distinct chamber as it is in some other vertebrates. Instead, it is believed to merge with the right atrium (also known as the right auricle) during embryonic development.

3. **Fetal Heart Structure**: In the fetal heart, there are four chambers: the right atrium, left atrium, right ventricle, and left ventricle. The sinus venosus contributes to the formation of the right atrium.

4. **Closure Mechanism**: During fetal development, there is a structure known as the fossa ovalis, which allows blood to bypass the lungs. After birth, this structure closes, and the sinus venosus is fully incorporated into the right atrium.

5. **Conclusion**: Therefore, the sinus venosus in mammals is believed to have merged with the wall of the right atrium.

### Final Answer:
The sinus venosus in mammals is believed to have merged with the wall of the **right atrium**.

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