Solution

All vertebrates possess a muscular chambered heart. Fishes have a 2-chambered heart with an atrium and a ventricle.
In fishes, the heart pumps out deoxygenated blood, which is oxygenated by the gills and supplied to the body parts from where deoxygenated blood is returned to the heart (single circulation). Because the heart pumps deoxygenated blood in fishes, it is called as a venous heart.