Snakes are clearly derived from some ancient kind of lizard, and the two are put together in the order Squamata. One of the features distinguishing the lizards and snakes from other reptiles is a drastic reduction of bones in the temporal region of the skull, which reaches its extreme among the snakes. Another is that the anal opening in lizards and snakes is transverse, instead of longitudinal as in crocodilians and turtles. Finally, both snakes and lizards have paired copulatory organs, and both have distinctive sets of sensory cells in their mouths, called Jacobson’s organs.
As to differences between snakes and lizards themselves, most lizards can close their eyes, but a snake’s eyes remain permanently open behind a clear covering called the spectacle. The unblinking stare of snakes may account for some of the superstitious fears people have about them. Snakes also generally have a single row of widened scales under the belly, while the scales of lizards tend to be more nearly the same size above and below. Lizards typically have some sort or external ear; snakes have none. In most lizards the tail can be readily shed, evidently as an escape mechanism. In some, the broken-off section snaps and jumps about in an irresponsible way. It is easy to imagine that this allows the rest of the lizard to slip quietly away from the scene while its attacker is preoccupied with the twitching tail. Later, a new tail generally grows again, sometimes lighter in color, with a different scale pattern and shorter than the one that was left behind.
The most obvious difference between typical lizards and snakes, however, is the leglessness of the latter. Although there are lizards that have no legs and that superficially resemble snakes, it is still generally easy to draw the line between the two groups. At the same time, it also is helpful to keep in mind that snakes are really a specialized and quite successful sort of lizard.
Of the two groups of the Squamata, the lizards are of course the older. They have the conventional body plan of a typical land vertebrate: four legs, five toes to a foot, and the sprawling gait of the earliest reptiles. Most of the adaptations that have allowed them to spread and prosper are relatively unspectacular changes in the old four-legged look – exceptions being the various groups in which the legs have been lost completely. As vertebrates, lizards are a fairly representative group and it has been suggested that the lizard would be more suitable as a type with which to introduce freshman biology students to vertebrate anatomy than the universally used frog. Perhaps it sounds cynical to say so, but I think the answer there is that the frog, being tailless, fits dissection pans more gracefully.
In spite of their fundamentally conventional body plan, modern lizards are a diverse lot. They range in length from two inches to 10 feet. They may look like dragons and they may look like worms, and they show a complex adaptive range through terrestrial, arboreal, subterraneous and aquatic environments.
Out at my farm lizards are all over the place on warm days. The large family of the Iguanidae is there, represented by the slender anole that stalks insects on the screens, and by the scaly-backed fence lizards that bask on almost every log or stump. This is, as the name suggests, the group to which the big tropical arboreal and marine iguanas belong, and it includes a host of smaller forms. Its counterpart in the Old World is the family Agamidae, which has a curiously similar structural and ecological spread.
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