Solution

All vertebrates possess a muscular chambered heart. Fishes have a 2-chambered heart with an atrium and a ventricle.
Amphibians and the reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle, whereas crocodiles, birds and mammals possess a 4-chambered heart with two atria and two ventricles.
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).