There has been an increase in the number of chambers in heart during evolution of vertebrates. Give the names of the class of vertebrates having two, three or four chambered heart. 

Explanation

(a) Two chambered heart : It is present in animals like fishes. Only one atria and one ventricle is present which are not separated and hence mixing of oxygenated and deoxygenated blood occur.
(b) Three chambered heart : It develops after the division of auricle into right and left halves as in amphibian. Mixing of oxygenated and deoxygenated blood occurs in ventricles. 
(c) An intermidiary heart : It is present in reptiles in which ventricle get partially divided through a septum which is incomplete e.g. crocodiles, thus having four chambered heart.
(d) In four chambered heart: Both the auricle and ventricle are divided into two halves and thus no mixing of oxygenated and deoxygenated blood occurs e.g. birds and mammals.