Chordates show a closed circulatory system i.e. proper system of heart and blood vessels. The heart has evolved from two chambers to four chambers. Pisces shows the presence of two chambers: one ventricle and one auricle. Amphibians show three chambers: two auricles and one ventricle due to which mixed blood flows in the heart. Reptiles like crocodiles, birds and mammals show four chambers (two auricles and two ventricles) and thus proper closed circulation is seen. Except for crocodilians, which have a four-chambered heart, all reptiles have a three-chambered heart consisting of two atria and one ventricle.