With thousands of species known to science, reptiles live almost everywhere in the world. Members of the class Reptilia share a few common characteristics; as a group, however, the four orders are much more different than they are alike. Common characteristics among reptiles include dry and scale-covered skin, internal fertilization and eggs laid on land.
The Squamata order includes lizards and snakes, and is the most diverse of all four orders of reptiles. While lizards and snakes share certain characteristics -- they shed their skins, they have similar cloacae and copulatory organs, and they have scales over their entire bodies -- lizards and snakes also have differences.
The primary difference between lizards and snakes is the presence of four limbs in most lizard species; it is important to note that certain lizard species lack limbs. Almost all lizards have movable eyelids and ear openings, whereas snakes lack these features.
The Crocodilia order contains crocodiles, alligators, caimans and gavials. These reptiles look alike, despite a few differences between the families. Members of order Crocodilia have thick bodies, long and powerful tails, strong limbs, and osteoderms -- heavy bone plates beneath the skin.
These aquatic reptiles have many characteristics that allow them to live most of their lives in the water. Their nostrils are located on the tops of their snouts, their feet are webbed, and they have special membranes that protect the eyes underwater.
Order Testudines includes the turtles and tortoises. This scientific order includes both extant and extinct species; the subgroup Chelonia contains today's living turtles.
Turtles, tortoises and terrapins have common characteristics. All species have have bony shells that cover their bodies. Most species have hard carapaces, but some have soft shells. No turtles have teeth on their jaws; instead, they have a horny surface inside their mouths.
Most but not all turtles are aquatic or semi-aquatic, but tortoises are land dwellers. Diets and habitats vary greatly among species.
While the order Rhynchocephalia has only one species today, this large, ancient group dates back to the time of the dinosaurs. Although tuataras look like lizards, they have a few key differences. One of the tuatara's distinctive features is a third eye on the top of the head. Although this third eye is visible during a tuatara's youth, it becomes covered with scales at a few months of age. Even though this eye has a retina, lens and nerve endings, it doesn't see; according to the San Diego Zoo, this third eye may be used for judging light or the time of year.