Eustachian valve which is of no significance in the adult mammal, is a vestigial organ, a vestige of

Options

A. Spiral valve
B. Sinus venosus
C. Sino-auricular valve
D. Semilunar valve

Explanation

**Step-by-Step Solution:**

1. **Understanding the Eustachian Valve**: The Eustachian valve is a remnant structure found in the heart of adult mammals. It is a vestigial organ, meaning it has lost its original function during the course of evolution.

2. **Identifying the Origin**: To determine what the Eustachian valve is a vestige of, we need to look at the structures present in earlier stages of vertebrate evolution, particularly in amphibians and other lower vertebrates.

3. **Comparing Structures**: In amphibians, the heart has a different structure compared to mammals. The right atrium in amphibians receives deoxygenated blood from the body through a structure known as the sinus venosus.

4. **Connecting the Dots**: In adult mammals, the sinus venosus is reduced and incorporated into the right atrium, where it is represented by the Eustachian valve. Hence, the Eustachian valve is a remnant of the sinus venosus.

5. **Conclusion**: Therefore, the Eustachian valve, which is of no significance in adult mammals, is a vestigial organ that is a vestige of the sinus venosus.

**Final Answer**: The Eustachian valve is a vestigial organ, a vestige of the sinus venosus.

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