Essay on Reptiles Reptiles are vertebrate, or backboned animals constituting the class Reptilia and are characterized by a combination of features, none of which alone could separate all reptiles from all other animals. The characteristics of reptiles are numerous, therefore can not be explained in great detail in this report. In no special order, the characteristics of reptiles are: cold-bloodedness; the presence of lungs; direct development, without larval forms as in amphibians; a dry skin with scales but not feathers or hair; an amniote egg; internal fertilization; a three or four-chambered heart; two aortic arches (blood vessels) carrying blood from the heart to the body, unlike mammals and birds that only have one; a metanephric kidney; twelve pairs of cranial nerves; and skeletal features such as limbs with usually five clawed fingers or toes, at least two spinal bones associated with the pelvis, a single ball-and-socket connection at the head-neck joint instead of two, as in advanced amphibians and mammals, and an incomplete or complete partition along the roof of the mouth, separating the food and air passageways so that breathing can continue while food is being chewed. These and other traditional defining characteristics of reptiles have been subjected to considerable modification in recent times. The extinct flying reptiles, called pterosaurs or pterodactyls, are now thought to have been warm-blooded and covered with hair. Also, the dinosaurs are also now considered by many authorities to have been warm-blooded. The earliest known bird, archaeopteryx, is now regarded by many to have been a small dinosaur, despite its covering of feathers The extinct ancestors of the mammals, the therapsids, or mammallike reptiles, are also believed to have been warm-blooded and haired. Proposals have been made to reclassify the pterosaurs, dinosaurs, and certain other groups out of the class Reptilia into one or more classes of their own. The class Reptilia is divided into 6 to 12 subclasses by different authorities. This includes living and extinct species. In addition, a number of these subclasses are completely extinct. The subclasses contain about 24 orders, but only 4 of these are still represented by living animals. Of the living orders of reptiles, two arose earlier than the age of reptiles, when dinosaurs were dominant. Tuataras, of the order Rhynchocephalia, are found only on New Zealand islands, whereas the equally ancient turtles, order Chelonia, occur nearly worldwide. The order Crocodilia emerged along with the dinosaurs. Snakes and lizards, order Squamata, are today the most numerous reptile species. The Rhynchocephalia constitute the oldest order of living reptiles; the only surviving representative of the group is the tuatara, or sphenodon (Sphenodon punctatus). Structurally, the tuatara is not much different from related forms, also assigned to the order Rhynchocephalia, that may have appeared as early as the Lower Triassic Period (over 2 000 000 000 years ago). The tuatara has two pairs of well-developed limbs, a strong tail, and a scaly crest down the neck and back. The scales, which cover the entire animal, vary in size. The tuatara also has a bony arch, low on the skull behind the eye, that is not found in lizards. Finally, the teeth of the tuatara are acrodont - i.e., attached to the rim of the jaw rather than inserted in sockets. Chelonia, another ancient order of reptiles, is chiefly characterised by a shell that encloses the vital organs of the body and more or less protects the head and limbs. The protective shell, to which the evolutionary success of turtles is largely attributed, is a casing of bone covered by horny shields. Plates of bone are fused with ribs, vertebrae, and elements of shoulder and hip girdles. There are many shell variations and modifications from family to family, some of them extreme. At its highest development, the shell is not only surprisingly strong but also completely protective. The lower shell (plastron) can be closed so snuggly against the upper (carapace) that a thin knife blade could not be inserted between them. A third order of the class Reptilia is Crocodilia. Crocodiles are generally large, ponderous, amphibious animals, somewhat lizardlike in appearance, and carnivorous. They have powerful jaws with conical teeth and short legs and clawed, webbed toes. The tail is long and