Reptiles are divided into four orders:
1. Testudines (tes-tu'din-ez) or Chelonia (ki-lo'ne-ah) - which includes the turtles and tortises
2. Rhynchocephalia (rin"ko-se-fa'le-ah) which includes the tuataras
3. Squamata (skwa-ma'tah) which includes the snakes and lizards
4. Crocodilia (krok"o-dil'e-ah) which includes the crocodiles, alligators, caimans, and gavials.
Reptiles are vertebrate animals with dry, scaly skin (remember keratin?). They are cold blooded and have a closed circulatory system with blood that is pumped by a heart. Fertilization is sexual with copulation taking place during the reproductive season. They were able to invade
the dry land because of the development of the amniotic (hard-shelled) egg.
The amniotic egg has several membranes that protect the egg from
desiccation (drying out). These membranes cushion and nourish the developing embryo, while also storing waste and allowing for Oxygen
and Carbon dioxide exchange. Look at the picture to the right. The outer membrane known as the chorion is hard and prevents water loss,
but it is porous enough to allow gases to pass in and out of the egg. The yolk sac contains the yolk, which is a protein rich food supply for the developing embryo. The amnion is the membrane that surrounds the developing embryo. It is filled with amniotic fluid that cushions the developing organism. Finally the allantois is a membranous sac that holds waste products produced by the developing embryo. It is the amniotic egg that has allowed the reptiles to flourish on land.
There are two different members of the order Chelonia; turtles and tortoises. Turtles are more aquatic and are sometimes known
as terrapins, whereas the tortoises are primarily terrestrial. Turtles are reptiles whose protection comes from a shell. The shell is composed of two hard, bone plates covered by scutes. Look at the pictures to the right, the dorsal (top) plate is called the carapace, and the ventral (bottom) plate is known as the plastron. Scutes cover the plates and are made of keratin, the primary substance in hair, nails and hooves of other animals. The pigment melanin, present in the scutes, may form intricate designs and brightly colored patterns in some species. 
The scutes of most turtles are arranged somewhat like the example to the left. A turtle shell is quite a defense for the animal, in fact military helments used in battle are designed like a turtle shell. Turtles have long life spans, some like the Galapagos turtles can live over 100 years. All turtles are oviparous which means
they lay their eggs outside of their bodies. Many turtles choose to bury their 5 to 100 eggs in the soil. Gestation takes between 4 weeks to 1 year, and the parent does not attend to the egg during this time.
Others are burrowers or tree dwellers. Reproduction is primarily oviparous and legs are deposited under rocks, debris or in burrows. Geckos are short and stout and can be found near human dwellings. They are nocturnal and unlike
most lizards, they can vocalize. They have adhesive disk on their digits that allow them to cling to trees and walls. Iquana are lizards with robust
bodies and distinct heads. Another form of the Iguana is the chameleon. Chameleons are adapted to an arboreal lifestyle and use a long sticky tongue to capture insects. Chameleons are known for their ability to change color in response to illumination, temperature, or their behavioral state.
There are 3 venomous species of lizards; the Komodo dragon, 
the Gila monster, and the Mexican beaded lizard. The Komodo dragon is the world's largest carnivorous lizard. It can weigh well over a hundred pounds and can grow to over nine feet in length. The most powerful weapon of the Komodo dragon is it's teeth. They are large, curved and serrated and tear flesh with the efficiency of a plow parting soil. The bacteria that live in the dragon's saliva are so virulent that wounds often will not heal. Even if the victim gets away, it usually dies from infection in a few days. The dragon's reputation as a human eater is well deserved; it does not seem to fear humans and many attacks and deaths have been reported. Vision and hearing are useful, but the Komodo's sense of smell is its primary food detector. Its long, yellow forked tongue flicks out and samples the air, after which the two tongue tips retreat to the roof of the mouth, where they make contact with the Jacobson's organs. These chemical analyzers "smell" the prey by recognizing (tasting) airborne molecules. 
Snakes also belong to the order Squamata but are classified in the suborder Serpentes (ser-pen'tez). Although the fast majority of snakes are not dangerous to humans, about 300 species are venomous. Worldwide about 30 to 40 thousand people die each year from snake bites. Snakes have no limbs, but have numerous vertebrae that allow for tremendous flexibility. Snakes move by later undaltion which is commonly
known as S-shaped movement (where the head goes the rest of the body follows). Snakes kill by one of three methods:
There are 21 members of the order Crocodilia. Along with crocodiles, other
members include the alligators, gavials, and caimans. The biggest difference between crocodiles, alligators and the other members of this order are easily distinguishable by the size and shape of
their mouths. Crocodilians are carnivorous and generally hunt by stealth (lying in wait). Their eyes are on top of their head and the nostrils are on top of their snout. This arrangement allows them to see and breathe while lying quietly submerged in water. A valve in the back of the mouth
prevents water from entering the air passage when a crocodilian feeds under water.
