 For example, Iguana hearts, like the majority of the squamates hearts, are composed of three chambers with two aorta and one ventricle, cardiac involuntary muscles.The main structures of the heart are the sinus venosus, the pacemaker, the left atrium, the right atruim, the atrioventricular valve, the cavum venosum, cavum arteriosum, the cavum pulmonale, the muscular ridge, the ventricular ridge, pulmonary veins, and paired aortic arches.Some squamate species (e.g., pythons and monitor lizards) have three-chambered hearts that become functionally four-chambered hearts during contraction.