Which is not true about an amphibian heart ?

Options

A. It is a myogenic heart
B. Two atria open in single ventricle
C. It is an arterovenous heart
D. Sinus venosus is replaced by SA node

Answer

To determine which statement is not true about an amphibian heart, let's analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding the Structure of the Amphibian Heart:**
- Amphibians, such as frogs, have a three-chambered heart consisting of two atria and one ventricle. This structure allows for some mixing of oxygenated and deoxygenated blood.

2. **Evaluating the Options:**
- **Option 1: "It is a myogenic heart."**
- This statement is true. The contraction of the heart in amphibians originates from the heart muscle itself, which is characteristic of a myogenic heart.

- **Option 2: "Two arteries are open in the single ventricle."**
- This statement is also true. In amphibians, both the pulmonary artery and the systemic artery open into the single ventricle.

- **Option 3: "It is an Arteriovenous heart."**
- This statement is true as well. An arteriovenous heart indicates a connection between the arteries and veins, which is consistent with the amphibian heart structure.

- **Option 4: "Sinus venosus is replaced by SA node."**
- This statement is not true. In amphibians, the sinus venosus is present and is not replaced by the SA node, which is a feature of the mammalian heart. The SA node serves as the pacemaker in mammals, but amphibians do not have an SA node in the same way.

3. **Conclusion:**
- The statement that is not true about the amphibian heart is **Option 4: "Sinus venosus is replaced by SA node."**

### Final Answer:
- The incorrect statement about the amphibian heart is **Option 4**.