Reptiles are vertebrate, or backboned, animals constituting the class Reptilia. They are characterized by a combination of features, none of which alone could separate all reptiles from all other animals. Among these features are (1) cold-bloodedness; (2) the presence of lungs; (3) direct development, without larval forms as in amphibians; (4) a dry skin with scales but not feathers (a characteristic of birds) or hair (a characteristic of mammals); (5) an amniote egg; (6) internal fertilization; (7) a three- or four-chambered heart; (8) two aortic arches (blood vessels) carrying blood from the heart to the body; mammals have only one aorta, the left; birds also have but one, the right; (9) a metanephric kidney; (10) twelve pairs of head (cranial) nerves; amphibians have ten; and (11) (skeletal features such as (a) limbs with usually five clawed fingers or toes, (b) at least two spinal bones (sacral vertebrae) associated with the pelvis; amphibians have but one, (c) a single ball-and-socket connection (condyle) at the head-neck joint instead of two, as in advanced amphibians and mammals, and (d) an incomplete or complete partition (the secondary palate) 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, are now thought to have been warm-blooded and covered with hair; and 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; and 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, and these issues are now receiving a great deal of attention from paleontologists and zoologists.
Reptiles are cold-blooded. That is, they lack the ability to regulate their metabolic heat (heat derived from the oxidation, or "burning," of food and from other processes) for the production of sustained body warmth and a constant body temperature. Cold-bloodedness, however, does not mean that a reptile is necessarily cold. A lizard basking in the Sun may have a higher body temperature than a mammal but must move into the shade to keep from overheating. Because cold-bloodedness is a misleading term, biologists employ two others instead, describing reptiles as poikilothermic and ectothermic. Poikilothermy refers to the condition in which body temperature varies with the temperature of the environment; it is contrasted with homeothermy, a characteristic of birds and mammals, in which body temperature remains essentially the same through a wide range of environmental temperatures. Ectothermy refers to the condition in which an animal depends on an external source, such as the Sun, rather than its own metabolism, for body warmth. Birds and mammals, which use their internal metabolic heat for body warmth, are referred to as endothermic.
All reptiles possess lungs, and none passes through an aquatic larval stage with gills, as do many of the amphibians. In snakes, presumably as an adaptation to their long, thin bodies, the left lung is reduced in size or entirely lacking. Although lungs are the primary means of respiration in all reptiles and the only means of respiration in most reptiles, a number of species are also able to utilize other parts of the body for the absorption of oxygen and the elimination of carbon dioxide. In aquatic turtles, for example, the tissues (mucous membranes) lining the insides of the mouth are capable of extracting oxygen from the water; some file snakes, family Acrochordidae, and sea snakes, family Hydrophiidae, as well as the soft-shelled turtle, Trionyx, can use their skin for respiration when submerged.
Skin and Scales
Part of the ability of the amphibians' descendants, the reptiles, to invade dry-land environments was the development of a dry skin that served as a barrier to moisture and greatly reduced the loss of body water. The reptile skin, like that of other vertebrate animals, consists of two main parts: an outer epidermis and an underlying dermis. The epidermis produces horny, or keratinized (like fingernails), scales on its upper surface. These scales are not the same as (that is, not homologous to) the scales of fishes, which are bony, are formed in the dermis, and lie beneath the epidermis. The reptile's scales increase the skin's resistance to water, further reducing moisture loss; some scales may be modified for specialized functions, such as protective spines. Reptile scales may be small and overlapping, as in many lizards, or large and adjoining, as in turtles, where they are commonly called scutes. Some reptiles also have bony plates or nodules formed and lying within the dermis. Called dermal scales or osteoderms, these bony plates are similar in origin to fish scales. When present in lizards and crocodilians, the dermal scales are separated from one another, with each usually lying beneath and supporting an epidermal scale above. In turtles the bony plates are fused together to form a bony shell beneath the epidermal scales.
In lizards and snakes the scales do not increase in size as the animal grows; consequently, the old scales must be periodically shed and replaced by a new set of somewhat larger scales. Shedding may also occur when the outer layer becomes worn or when much food is consumed, as well as for causes not yet fully understood. In the shedding, or molting, process, also called ecdysis, the older upper layer of the epidermis with its attached scales loosens and breaks away from a newer layer that has developed beneath it. In turtles and crocodilians the large epidermal scales, or scutes, are not molted but are retained and are enlarged and thickened by additional layers of keratin from beneath; the uppermost layers of the scutes, however, may be lost through wear or other factors.
The Amniote Egg
A necessary part of the invasion of dry-land environments by the early reptiles was the development of an egg that could be laid out of water without drying up and that could "breathe" air rather than water. This egg, developed by the first reptiles, was the amniote egg, so named because it contains a membrane called the amnion. The amniote egg is found not only in reptiles but also in birds and (ancestral) mammals, and all three groups are sometimes collectively referred to as amniotes.
The amniote egg is different from the fishlike egg of most amphibians. It is enclosed in a protective shell, which is either flexible and leathery or rigid and calcareous. Within the shell is the fertilized egg cell lying on top of a large mass of yolk. The yolk is surrounded by a membrane (the yolk sac) and provides nourishment for the developing embryo. As the fertilized egg cell divides and redivides, and the embryo begins to form and grow, a folded membranous tissue grows up around the embryo, enclosing it in a double-walled sac. The outer wall of the sac is called the chorion, the inner wall, the amnion. The embryo is surrounded by fluid held with the amnion. The fluid provides the embryo with the aquatic environment it obviously still requires but which in amphibians is supplied by the waters of a pond or stream.
Another sac, called the allantois, projects from the embryo's lower digestive tract (the hind gut) and acts as a bladder to receive the embryo's waste products. The allantois sac becomes quite large, expanding out until its wall joins that of the chorion to form the chorioallantoic membrane, which is pressed up against the inside of the shell. Not only does the allantois serve as a bladder, receiving and storing insoluble wastes, but it also acts as a sort of "lung," allowing oxygen and carbon dioxide to pass to and from the embryo through the slightly porous (permeable) egg shell.
Because the egg cell reaches the outside environment surrounded by the shell of the egg, it is necessary that it be fertilized before it leaves the female's body; thus, in all reptiles fertilization is internal, with males depositing sperm within the females' genital tracts. In snakes and lizards the male organ, called the hemipenes, is actually a pair of structures, with one of the pair, or hemipenis, situated internally on each side of the male's vent. Each hemipenis, which itself may be forked, is a functional structure: either one may be protruded from the vent and used in mating, the choice usually depending upon the placement of the male's mate. In turtles and crocodilians there is a single penis, which serves only for the transmission of sperm and not also for the elimination of excretory products, as in mammals. In the lizardlike tuatara, Sphenodon, the only living species in its order, the male lacks a copulatory organ, and mating is accomplished by the pressing together of the male's and female's cloacae, as in most birds.
Except for crocodilians, which have a four-chambered heart, all reptiles have a three-chambered heart consisting of two atria and one ventricle. The chamber called the right atrium receives deoxygenated, or "spent," blood returning from the body tissues. It passes this blood into the ventricle, from where it is pumped to the lungs for oxygenation. The oxygenated blood from the lungs returns to the left atrium and once again enters the same ventricle, from which it is pumped to the body tissues.
Even in the three-chambered heart, however, as recent research has shown in contrast to earlier beliefs, there is little mixing of oxygenated and deoxygenated blood. This has been achieved by the development within the ventricle of interconnected "subchambers" within which a sequence of changes in blood pressure takes place. Several anatomical variations occur among the reptiles, but a simplified description of the lizard heart will serve to illustrate one way in which this circulatory efficiency was accomplished.
The ventricle of the lizard heart is incompletely partitioned into two subchambers by a muscular ridge that descends from the roof of the heart almost to the floor. The right subchamber is called the right ventricle, or cavum pulmonale; it leads to the lungs. The left subchamber is called the left ventricle, or cavum venosum; it receives blood from the right atrium and leads to the body. The two subchambers are connected not only beneath the partition but also across its incomplete rearward end.
A third subchamber, called the cavum arteriosum, is situated in the upper wall of the right ventricle; it receives blood from the left atrium and is connected through a valve-controlled opening to the left ventricle.
When the two atria contract, oxygenated blood from the left atrium enters the third subchamber, or cavum arteriosum, pressing against and shutting the valves controlling the opening into the left ventricle; this closes off the third subchamber and temporarily holds its contained blood in storage. At the same time the deoxygenated blood in the right atrium has been pumped into the left ventricle, filling it to overflowing, the excess blood moving across the open posterior end of the muscular partition into the right ventricle. The three ventricular subchambers are now filled with blood; oxygenated blood in the third subchamber (cavum arteriosum) and deoxygenated blood in the two ventricles.
Resistance to blood flow is lower in the pulmonary (heart-lung) circuit than in the systemic (heart-body) circuit, so that when the ventricles contract, the deoxygenated blood in the right ventricle follows the path of least resistance and proceeds through the pulmonary artery into the lungs. The emptying of the right ventricle causes more deoxygenated blood from the left ventricle to move around the open end of the partition into the right ventricle and to continue on to the lungs. The contraction of the ventricle also brings the partitioning muscular ridge into full contact with the floor of the heart, sealing off any flow from the right ventricle back into the left ventricle.
As the ventricles continue their contraction, the pressure on the blood held within the third subchamber (cavum arteriosum) exerts a reverse force on the valves, opening the passageway between the third subchamber and the left ventricle. The oxygenated blood in the third subchamber now moves into the left ventricle and from there into the aortas leading to the body circulation.
Adult amphibians have an opisthonephric kidney, that is, one that was developed from the main mass (except for the foremost end) of kidney-forming tissue in the embryo; the collecting tubules of the amphibian's opisthonephric kidney are connected to a drainage tube called the archinephric, or Wolffian, duct. In contrast, the metanephric kidney of adult reptiles (and birds and mammals) arises from the rearmost part of the kidney-forming tissue of the embryoÑthe front and middle portions having given rise to the pronephric and mesonephric kidneys, which eventually degenerate. The collecting tubules of the metanephric kidney are connected to a "newly evolved" drainage tube, the ureter. The Wolffian duct, having lost its excretory function, becomes partly involved in the transmission of sperm in males but is a degenerate vestige in females. In mammals the ureter drains into the bladder, which empties to the outside through another tube called the urethra. Crocodilians and snakes lack bladders, but even where one is present, as in most lizards and turtles, it is formed simply by an outpocketing of the cloaca and has no connection with the ureter. The cloaca, whose name comes from the Latin word for sewer, is a body chamber leading to the outside and into which empty the excretory products of the kidneys, the waste products of the intestines, and the reproductive products of the testes and ovaries. The reptilian ureter empties into the cloaca, and if a bladder is present it receives and holds the urine overflow.