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The circulatory system of beings of the class Reptilia is similar to the amphibian, closed and incomplete. Although the heart presents three chambers (two atria and one ventricle) in reptiles there is a beginning of ventricular septation and the mixture of arterial with venous blood is lessened.

Beings of the class Reptilia are abundant and more diverse in hot climate regions and they are rare in intensely cold regions like close to the earth poles. This is explained because these animals are heterothermic, i.e., they have “cold blood” and they need an external heat source to warm their bodies.

The heterothermic feature also explains why reptiles are more active during the day, a period when they can use the sun heat to warm their bodies. 

Snakes are carnivorous. Iguanas are herbivorous.

From the least to the most complex brain structure, it is evident that the brain, from fishes to beings of the class Reptilia, became larger and predominant in the central nervous system.

These beings do not have permeable skin so they do not make cutaneous respiration like amphibians do. Just like in birds and in mammals, their respiration is pulmonary. 

These beings excrete mainly uric acid. This substance is less toxic than ammonia and it can be kept stored for a longer time inside the individual, including within eggs. In addition uric acid is practically insoluble and it depends less on water to be eliminated.

Like fishes and amphibians, beings of the class Reptilia are heterothermic animals (also known as poikilothermic, or ectothermic), i.e., they are not able to control by themselves their body temperature and thus they depend on external warm sources (mainly the sun).

These beings reproduce sexually through internal fecundation by means of copulation between male and female individuals. They lay eggs with shell and extraembryonic membranes. The embryo thus develops within the egg and outside the mother’s body (there are also ovoviviparous reptiles that retain the egg within the body until hatching).

In beings of the class Reptilia the embryonic development is direct. So there is no larval stage.

The reptile skin is keratinized and impermeable to water while the amphibian skin is permeable. The skin impermeability made impossible the cutaneous gas exchange performed by amphibians and respiration became dependent on internal organs like airways and lungs.

The first entirely terrrestrial vertebrate class, totally independent from the aquatic habitat, is the class Reptilia.