The main characteristic of the lizards and snakes (Order Squamata) is that these reptiles have scaly skins. About 10,000 species exist today. They are the descendants of ancient terrestrial reptiles.
Three of the main groups of lizards are discussed here. The iguana relatives include types such as: iguanas, agamas, chameleons, casqueheaded (or helmeted) lizards, collared lizards, spiny lizards, anoles and neotropical ground lizards. Another group includes types such as: monitor lizards, spinytail lizards, spectacled lizards, wall lizards, skinks, tegus, and whiptail lizards. The latter group has been known previously by the long name “autarchoglossans” though ongoing studies suggest dividing this group further. These lizards differ from the iguana relatives by having long tongues that they use to detect prey. Except for chameleons, the iguana relatives usually have short tongues and capture prey with their jaws. Geckos are usually considered their own group, and they have the feature of the broad, padded toes that let them cling to smooth surfaces. Most of the lizards are terrestrial, though many swim very well. The marine iguanas found in the Galapagos Islands may be the only ones that feed underwater (on algae).
Snakes, or serpents, evolved from ancestors of the iguana relatives. One popular theory is that they descended from burrowing lizards. They evolved the long, slender shape to slither into burrows and crevices. Eventually their legs became smaller and vanished. While there are some exceptions, snakes usually have no eyelids or external ears like lizards. Some of the main types of the 3,600 snakes include: boas, colubrids, elapids, and vipers. Many of the elapids (cobras, kraits, etc.) and the vipers are very venomous.