The correct option is Two.
Explanation
The vertebrate heart has evolved in complexity across different classes to optimize the separation and transport of oxygenated and deoxygenated blood. The number of chambers is a defining characteristic of the circulatory system in each class.
- Fishes: Fishes possess a two-chambered heart, consisting of one atrium (auricle) and one ventricle. The heart pumps deoxygenated blood to the gills, where it is oxygenated before circulating to the rest of the body. This is known as single circulation.
- Amphibians: These animals typically have a three-chambered heart (two atria and one ventricle), allowing for some mixing of oxygenated and deoxygenated blood.
- Reptiles: Most reptiles have a three-chambered heart with a partially divided ventricle, except for crocodiles, which have a four-chambered heart.
- Birds and Mammals: These classes possess a four-chambered heart (two atria and two ventricles), ensuring complete separation of oxygenated and deoxygenated blood (double circulation).
Key Takeaway: Fishes are characterized by a two-chambered "venous heart" that pumps only deoxygenated blood in a single circulatory loop.