Solution

Sinus venosus and conus arteriosus are not found in rabbit but they are present in the frog. In the rabbit, sinus venosus is formed in the embryo but later it becomes a part of the wall of the right auricle. So the impure (deoxygenated) blood collected by the precavals and postcavals directly comes into the right auricle. The pure blood brought by the pulmonary veins from the lungs directly come into left auricle.