op, and the blood is diverted from the former to the latter, there is a corresponding change in the carrying capacity of the blood-vessels, resulting in the final disappearance of the vessels connected with the gills. Moreover, while the blood was not returned directly from the gills to the heart, it is returned directly from the lungs, and a second auricle is developed. But the aerial respiration of the frog, with its mixed circulation, is more rapid than the aquatic respiration with the perfect circulation of the tadpole.
In the reptiles circulation is essentially the same as in the amphibians; but the ventricle is more or less divided by a partition into two chambers. This membranous partition is perfect only in the crocodile, where we find a right and left ventricle without communication, and the heart structurally like that of a bird or mammal. But the circulation is still the same as in the lower reptiles, for the pure and impure blood are somewhat mingled by a communication between the two arteries near their point of origin.
Although birds in their general organization are closely allied to reptiles, their circulation is similar to that of the mammals. In these
two highest classes of the animal kingdom, there are always two auricles and two ventricles, and the right and left sides of the heart are entirely distinct. Functionally these are two hearts: a systemic heart,
