Members of the class Reptilia are cold-blooded animals that have backbones. Their skin is scaly and has two layers: the outer epidermis and the inner dermis.
Scales are found on the upper layer and are made of keratin. This is the same material that makes up human hair and fingernails. The scales protect the reptile from parasites and predators and help it retain water. Reptiles replace their scales and old skin through molting.
The reptilians have sturdy skeletons that support them mostly from the side, which gives the legged reptiles a sprawling walk.
Reptilian senses can be well-developed and are more acute than those of amphibians, with whom they share a common ancestor. Many types of reptilians have large and well-developed eyes, though some fossorial reptiles have poor vision or lack eyes. Most kinds of reptiles have eyelids, though snakes and some types of lizards lack them.
Some reptilians have senses that other types of animals lack. Pit vipers have heat-sensing pits in their heads, while tuataras have an organ at the top of their heads that can sense the amount of daylight.
Most reptilians lay eggs that have hard or leathery shells, though some reptilians give birth to live young.