The reptiles (from Latin reptare, 'crawling') cover about 7,000 known species.
They emerged about 300 million years ago, and probably evolved from certain amphibians. They were the first vertebrates effectively adapted to life in dry places, although some animals in this group, such as turtles, are aquatic.
The earth has already harbored gigantic forms of reptiles, such as the dinosaurs. Today this group is represented by relatively smaller animals such as alligators, turtles, snakes and lizards.
The skin of reptiles
Reptiles have their bodies covered by dry and practically impermeable skin. The most superficial cells of the epidermis are rich in keratin, which protects the animal against dehydration and represents an adaptation to life in terrestrial environments. The skin may have scales (snakes), plaques (alligators, crocodiles) or carapaces (turtles, jabutis).
Reptiles, like fish and amphibians, are animals pecyrothermic: Body temperature varies with ambient temperature.
Breathing and blood circulation
Reptile respiration is pulmonary; their lungs are more developed than those of amphibians, with internal folds that increase their breathing capacity.
The lungs supply the reptiles with a sufficient amount of oxygen gas, which makes breathing through the skin "amphibian" unnecessary. In fact, with the large amount of keratin it presents, the skin becomes virtually impermeable, which makes it impossible to acquire oxygen gas.
The heart of most reptiles has two atria and two partially divided ventricles. In the ventricles a mixture of oxygenated blood with non-oxygenated blood occurs. In crocodilian reptiles (crocodile, alligators), the two ventricles are completely separated, but oxygenated and non-oxygenated blood continue to mix, now outside the heart.
Food and digestion
Most reptiles are carnivorous animals; Some species are herbivorous and others are omnivorous. They have complete digestive system. The large intestine ends in the cloaca.
Reptiles have sense organs that allow them, for example, to taste and smell things. The eyes have eyelids and nictitating membrane, which help in the protection of these structures. They have tear glands, which are critical for keeping the moist eye surface out of the water.
We highlight here a structure existing between the eyes and the nostrils of snakes, called loreal pit (in detail). It enables the snake to perceive the presence of other living animals through the heat emitted by their bodies.
Although reptiles do not have an outer ear, some of them have an external ear canal and a cure, which lies below a fold of skin on either side of the head. At the end of each ear canal is the eardrum, which communicates with the middle ear and the inner ear. Several experiments prove that most reptiles can hear various sounds.