Discuss the circulatory system in various vertebrates and the gradual development of the heart.

(N/A) All vertebrates possess a muscular,chambered heart.
$(1)$ Fishes have a $2$-chambered heart with one atrium and one ventricle. In fishes,the heart pumps deoxygenated blood to the gills,where it is oxygenated and then supplied to the body parts. Deoxygenated blood is then returned to the heart. This is known as single circulation.
$(2)$ In amphibians and reptiles (except crocodiles),there is a $3$-chambered heart with two atria and one ventricle.
$(3)$ In these organisms,the left atrium receives oxygenated blood from the gills/lungs/skin,and the right atrium receives deoxygenated blood from other body parts. However,these blood streams mix in the single ventricle,which pumps out mixed blood. This is known as incomplete double circulation.
$(4)$ Crocodiles,birds,and mammals possess a $4$-chambered heart with two atria and two ventricles.
$(5)$ In birds and mammals,oxygenated and deoxygenated blood received by the left and right atria,respectively,pass into the ventricles of the same side. The ventricles pump the blood out without any mixing. Thus,two separate circulatory pathways are present,known as complete double circulation.