Lizards are four-legged reptiles, with ear holes directly on the head and movable eyelids. Lizards can grow to be anywhere from a few centimeters to three meters. Lizards are all in the order of Squamata, which is also the same order that some snakes are in. Most lizards lay eggs although there are a few species that can give live birth. Many lizards are also capable of regeneration of broken off limbs or tails.
In all lizards, the heart is separated into right and left chambers. There is also always at least one interventricular septum. Lizards are cold blooded animals, which means that they have to regulate their own temperature using the environment as a heating or as a cooling device. Some ways that they heat themselves up is by sitting out in the sun or on a warm rock on a sunny day. Some ways that they cool off is by jumping into a body of water or by laying in the shadows or under rocks.
All lizards have a cecum, which is the blind pouch at the beginning of the large intestine into which the ileum opens from one side and which is continuous with the colon , and a gall bladder, which is A small, pear-shaped muscular sac, located under the right lobe of the liver, in which bile secreted by the liver is stored until needed by the body for digestion. Contained inside the liver of the lizard is melanin. Melanin are various pigments that are responsible for the dark color of the skin, hair, scales, feathers, and eyes of animals and are also found in plants, fungi, and bacteria. Melanins are polymers, often bound to proteins, and in the animal kingdom are built from compounds produced by the oxidation of the amino acid tyrosine. Melanin causes the liver to have black spots or streaks across it.
Lizards have a thick outer skin layer which is mainly composed of dead, compacted layers of skin cells which have been filled with keratin. Keratin is a class of tough, fibrous proteins that are the main structural component of hair, nails, horns, feathers, and hooves. Keratins are rich in sulfur-containing amino acids, especially cysteine. Individual keratin molecules are entwined helically around each other in long filaments, which are cross-linked by bonds between sulfur atoms on different chains. The twining and cross-linking produce strength and toughness. The form of the outer layer of skin cells can vary between small and grain-like to being large and plate-like. The texture of them can be very unique too. The skin could feel very smooth or it could feel like a sandy-spiny feeling.
Lizards have a very acute sensory system. The can see and hear very well. 
Most of the lizards today have scent glands at the base of the tail and males normally have glands in front of the anus in the pelvic region that becomes filled with a hard, waxy, yellow substance. These glands apparently play a role in courtship. Males have two joined organs called hemipenes. There are two hemipenes on each side of the base of the tail. They usually have pleats and folds in them and are also very short but wide. These two hemipenes can both be used for internal fertilization and lizards tend to fluctuate between them. Lizards are born two ways. One way, the most common way, is that they are hatched from an egg that was deposited outside of the female's body after fertilization. Another way, the least common way, is that they are hatched from their egg while it was in their ovaries. Female lizards have sac-like ovaries and large-yolked hard shelled eggs which help aid in the protection of the embryo. The parental role for lizards is still unknown but female lizards have been known to sit upon her eggs until they hatched and also to care for the eggs in many other ways. The sexual peak in lizards depends on the size of the lizard. If it is a small lizard then they usually reach their sexual maturity in the first breeding season after their birth, but in the larger lizard species it can take several years for this to occur. Small lizards also live only 1 to 5 years were as larger ones live anywhere from 10 to 20 years or more.
Lizards can be found virtually anywhere in the world except in the polar areas. Most of the lizards live on the ground or in the trees and some even live in the water but that is a very limited number. Lizards, just like all other cold blooded animals, like to warm themselves up in the sun to increase their body tempturature.
Lizards as Pets
Many people enjoy lizards as pets. A few examples of lizards that are commonly pets are the iguanas, bearded dragon, leopard geckos, tegus, and monitor lizards are a few of them. Lizards require a consistently clean environment and a regular source of food. After handling lizards, you should be sure to wash your hands just like with every other pet. Just because it is your pet does not mean that it is completely germ free.
Many species of lizard are now sold as pets. A few of these include iguanas, bearded dragon, leopard geckos, tegus, and monitor lizards to name a few. In general, lizards require more maintenance than other exotic pets, particularly snakes and tarantulas. Their feces are usually more offensive, requiring frequent cage cleanings.
Green tree monitor lizard