Reptile Anatomy

Reptiles have a general anatomy similar to that of mammals and birds. The body is divided into a head, neck, trunk and tail.

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Snake scales, or gastropeges, are a distinct kind of dermal armor made of durable keratin. During molting they are ejected as one continuous contiguous outer layer, rather than individually shed.

Skin

Reptile skin is a tough, waterproof covering that helps protect the animal from physical trauma, desiccation and infection. It also serves to regulate body temperature. Scales (in lizards) or dermal scutes (in turtles and crocodiles) form the armor-like outer layer of skin. Each scale has an outer surface, an inner surface and a hinge region that is continuous with the surface of the next scale over it.

Inside the skin, a layer of horny epidermis has a thick stratum corneum that contains waxes arranged in membrane-like layers between keratinized cells. Shedding of this skin is called ecdysis or sloughing.

Shedding occurs when cells in the outermost layer of the epidermis, the stratum germinativum, go through mitosis and replace the old cell layer. Shedding is important because it helps remove dead skin, dirt and bacteria from the animal’s body.

Scales

Reptiles have a thick layer of scales that cover their bodies. These scales are keratinized and help protect them from injuries and dehydration. They also help them move freely from water to land.

The scales of reptiles are shed several times a year. This process is called ecdysis, or molting. This serves a few important functions: It removes old, worn out skin, gets rid of parasites and allows new skin to grow.

In addition, the overlapping structure of the scales helps snakes slide across the ground more easily. For example, a rattlesnake can use its belly scales to catch small bumps on the ground as it moves along, creating a rattling sound that warns predators to stay away. It’s interesting to note that the same type of overlapping skin found in reptiles is also present in mammalian hair and avian feathers, suggesting that they may have evolved from similar structures in earlier amniote groups.

Eyes

A reptile can see light and dark environments through its eyes. Snakes, amphibians and lizards have a third eye called the parietal eye on top of the head. This isn’t a complete eye, but it contains a rudimentary lens and cornea to detect changes in light. It also helps these creatures moderate hormone production and regulate their body temperature.

A ring of bones support the eyeball, which is covered by a transparent cornea and lens. This lens allows the eye to focus. The iris is a pigmented circle that controls the size of the pupil, and the retina is a sensory layer that interprets light signals.

Most reptilian species have moveable eyelids that protect the eye and enable them to blink. However, a number of snakes and geckos lack these features. These reptiles have adapted to this by developing a transparent scale that covers the eye and is shed when the snake sloughs it.

Lungs

In contrast to birds and mammals reptiles have single-chambered (unicameral) lungs. This arrangement may limit activity because pumping deoxygenated blood through the lungs consumes energy.

Reptiles have a large intestine that ends in the coprodeum portion of the cloaca. The cloaca is the common emptying chamber for the digestive, urinary and reproductive systems.

A tracheal lung is present in boids and colubrids but vestigial in viperids. It consists of a thin-walled, nonrespiratory portion that is connected to the respiratory portion by a narrow channel. This’splinter’ lung allows a snake to breathe during prey swallowing. Inspiratory movement occurs mainly through contraction of intercostal and abdominal muscles and elastic recoil of the main pulmonary sac. V/Q distributions in resting reptiles are similar to those of birds and mammals, suggesting that lung complexity has not influenced gas exchange.

Heart

All ectothermic reptiles, including crocodilians, turtles (Lepidopoda), lizards and snakes (Squamata) and tuataras (lizard-like nonlizards) have a three-chamber heart with two atria and a partially divided ventricle. This allows for some mixing of oxygenated and deoxygenated blood, but the heart valves and movement of the ventricle walls control this pattern of circulation. This helps to conserve energy by avoiding pumping deoxygenated blood to the lungs, and it also allows for longer diving times in aquatic species.

The hearts of ectothermic reptiles also allow for intercardiac shunting of blood. In chelonians, such as turtles and tortoises, and lepidosaurs, such as lizards and snakes, this can occur in response to a decrease or cessation of lung ventilation. The shunting is caused by the incompleteness of the ventricle septation, with blood ejected to either the systemic or pulmonary circuit depending on vascular resistance at after ventricular systole.

Stomach

In reptiles, the stomach is a j-shaped organ that does most of the digestive processing. Cells secrete enzymes to break down proteins in food and gastric acid helps to dissolve carbohydrates. The food then moves downward into the intestines through peristalsis.

Like snakes, lizards lack a diaphragm and respiration is facilitated by contraction of abdominal and intercostal muscles. An additional feature of lizards is a tracheal lung, which allows for aeration when the main lungs are compressed.

The large intestine of a lizard ends in the cloaca which acts as a common chamber for products of the digestive, urinary and reproductive systems. Male lizards have paired penises, known as hemipenes, that guide sperm and ejaculate into the cloaca during copulation. A cloaca can also discharge feces into the coprodeum and urodeum, which act as general waste storage compartments.

Cloaca

The cloaca (sewer) is the common passageway for feces, urine and reproduction. It is present in birds, reptiles, amphibians, some fish and some mammals.

During mating, a male bird will mount the female so their cloacas can touch, known as the ‘cloacal kiss’. This allows sperm to enter the reproductive tract to fertilize eggs.

The cloaca is a short simple tube that receives the rectum, the allantoic bladder and the genital ducts. It is the only excretory opening in reptiles, amphibians and birds. In fish, a true cloaca is only present in some elasmobranchs and some lobe-finned species. It is an integral part of the complex four-layer ventrolateral muscle patterning of the trunk in basal vertebrates.