long and flexible neck
scales to prevent water loss
the shelled amniote egg
efficient lungs and circulatory system
further development of limbs and skeleton
1. Reptiles have tough, dry, scaly skin offering protection against desiccation and physical injury. The skin consists of a thin epidermis, shed 3'l periodically, and a much thicker, well-developed dermis (Figure 3a). The dermis is provided with chromatophores, the colour-bearing cells that give many lizards and snakes their colourful patterns.
The characteristic scales of reptiles are formed largely of keratin. Scales are derived mostly from the epidermis; they are not homologous to fish scales, which are bony, dermal structures. In some reptiles, such as alligators, the scales remain throughout life, growing gradually to replace wear. In others, such as snakes and lizards, new scales grow beneath the old, which are shed at intervals. Turtles add new layers of keratin under the old layers of the platelike scutes, which are modified scales. In snakes the old skin (epidermis and scales) is turned inside out when discarded; lizards split out of the old skin leaving it mostly intact and right side out or it may slough off in pieces.
2. The shelled (amniotic) egg of reptiles contains food and protective membranes for supporting embryonic development on land. Reptiles lay their eggs in sheltered locations on land. The young hatch as lung- breathing juveniles rather than as aquatic larvae. The appearance of the 34- shelled egg (Figure 3b) widened the division between the evolving amphibians and reptiles and, probably more than any other adaptation, contributed to the evolutionary establishment of reptiles.
3. Reptilian jaws are efficient for crushing or gripping prey. The jaws of fish and amphibians are designed for quick jaw closure, but once the prey is seized, little static force can be applied. In reptiles jaw muscles became larger, longer, and arranged for much better mechanical advantage.
4. Reptiles have some form of copulatory organ, permitting internal fertilization. Internal fertilization is obviously a requirement for a shelled egg, because the sperm must reach the egg before the egg is enclosed. The glandular walls of the oviducts secrete albumin (source of amino acids, minerals, and water for the embryo) and shells for the large eggs.
5. Reptiles have a more efficient circulatory system and higher blood pressure than amphibians. In all reptiles the right atrium, which receives unoxygenated blood from the body, is completely partitioned from the left atrium, which receives oxygenated blood from the lungs. Crocodilians have two completely separated ventricles (Figure ); in other reptiles the ventricle is incompletely separated. Even in reptiles with incomplete separation of the ventricles, flow patterns within the heart prevent admixture of pulmonary (oxygenated} and systemic (unoxygenated} blood; all reptiles therefore have two functionally separate circulations.
6. Reptilian lungs are better developed than those of amphibians. Reptiles depend almost exclusively on lungs for gas exchange, supplemented by respiration through the pharyngeal membranes in some aquatic turtles. Reptiles suck air into the lungs by enlarging the pleural cavity, either by expanding the rib cage (snakes and lizards} or by movement of internal organs (turtles and crocodilians}. Reptiles have no muscular diaphragm, a structure found only in mammals. Cutaneous respiration (gas exchange across the skin), so important to amphibians, has been completely abandoned by reptiles.
7. Reptiles have efficient water conservation. All amniotes have a metanephric kidney which is drained by its own passageway, the ureter. However, the nephrons of the reptilian metanephros lack the specialized intermediate section of the tubule, the loop of Henle that enables the kidney to concentrate solutes in the urine. To remove salts from the blood, many reptiles have salt glands located near the nose or eyes (in the tongue of saltwater crocodiles) which secrete a salty fluid that is strongly hyperosmotic to the body fluids. Nitrogenous wastes are excreted by the kidney as uric acid, rather than urea or ammonia. Uric acid has a low solubility and precipitates out of solution readily, allowing water to be conserved; the urine of many reptiles is a semisolid suspension.
8. All reptiles, except the limbless members, have better body support than the amphibians and more efficiently designed limbs for travel on land. Nevertheless, most mode~ reptiles walk with their legs splayed outward and their belly close to the ground. Most dinosaurs, however, (and some modern lizards) walked on upright legs held beneath the body, the best arrangement for rapid movement and for the support of body weight. Many dinosaurs walked on powerful hindlimbs alone.
9. The reptilian nervous system is considerably more complex than the amphibian system. Although the reptile's brain is small, the cerebrum is larger relative to the rest of the brain. Connections to the central nervous ( system are more advanced, permitting complex kinds of behaviour unknown in amphibians. With the exception of hearing, sense organs in general are well developed. Jacobson's organ, a specialized olfactory chamber present in many tetrapods, is highly developed in lizards and snakes. Odours are carried to Jacobson's organ by the tongue