Reptiles lack front limbs and have an extremely flexible backbone called the axial skeleton. Their skeletal system also has regional specializations of the cervical (neck), thoracic, lumbar, sacral and caudal vertebrae.
The stomach of a lizard is a simple, sac-like structure with combined hydrochloric acid and pepsin secreting glands. Its pylorus connects to the cloaca, a short, straight tube that acts as general collecting chamber for digestive and excretory wastes.
Reptiles, including snakes, lizards, turtles and crocodilians, have a dry, horny, plate-like outer covering called skin. It is made from a tough substance called keratin, also found in nails and claws. This specialized skin helps to protect the reptile’s body from injury and loss of water, especially when swimming. It is shed regularly throughout a reptile’s life.
Like other vertebrates, a reptile has a bony skeleton that supports the body. Most lizard species have two lungs and a three-chamber heart with two atria and one partially split ventricle. Blood flow can be redirected from the lungs to save energy that would otherwise be used to pump oxygenated blood to them, and this is important for aquatic animals.
There is no diaphragm in lizards, so inspiration is facilitated by outward movement of rib and intercostal muscles, as well as by elastic recoil of the lung structure itself. There are also paired kidneys, situated in the cranial half of the body, each with a single ureter that empties into the urodeum portion of the cloaca. The ureters in males are modified to provide seminal fluid for reproduction.
The skin of a reptile is covered in rough, horny scales made of keratin — the same substance that forms nails, hair and claws. Reptiles are ectothermic, and the keratin in their scales helps them retain heat and stay hydrated.
The underlying dermis of reptiles is composed of fibrous connective tissue and blood and lymph vessels. Pigment cells called melanophores produce black, brown, yellow and gray pigments, while carotenoid cells provide yellow to orange coloration in albino reptiles.
A reptile’s integument reflects their greater commitment to terrestrial life than amphibians. Their scales lack the bony under support of other vertebrates’ scales, but a flexible hinge joins adjacent epidermal plates. Some epidermal scales can be modified into crests, spines or horn-like processes that help the reptile grip and carry its prey. Bony skin structures called osteoderms are present in crocodilians and some lizards, and can fuse with vertebrae to form turtle shells.
Reptiles secrete a proteinaceous fluid during shedding that promotes separation between the old layer of skin and the new. If a snake cannot shed the outer skin, it may get “stuck.” A stuck shed is dangerous because it cuts off blood flow to the skin, which can lead to infection and gangrene.
In a lizard, the trachea bifurcates in the cranial thorax to supply two lungs. The lungs are dark pink and heavily veined. They are surrounded by the heart, which is a yellowish color, and a light pink organ known as the diaphragm. The lungs are separated by the body cavity, or coelom.
The lungs of reptiles are simple, compared with those of mammals, with partitions that contain alveoli, or small pockets. These increase the surface area available for gas exchange.
Like mammals, crocodiles have four-chambered hearts, but when they dive they compress their four-chamber hearts into three-chambered hearts. It is not known whether all modern reptiles have a three-chambered heart.
In addition to lungs, reptiles use their skin to bring oxygen into their bodies. Unlike mammals, which have a diaphragm to control their breathing, reptiles do not use a diaphragm, but use axial muscles attached to their ribs to expand and contract their lungs as needed. This enables them to breathe faster and at deeper levels than can be achieved with the mammalian respiratory system.
The digestive system is the last of the reptile’s internal organ systems to be fully developed. Carnivorous reptiles swallow their prey whole, and herbivorous ones only chew enough to break the plant matter into small pieces, making it easier for digestive enzymes to infiltrate it and pull out nutrients.
The gastrointestinal tract is lined with specialized cells that can sense chemicals from the food, such as smells or chemical changes in pH, and send signals to the nervous system about the food’s condition. These nerves control the gut muscles to speed up or slow down food movement and produce the requisite digestive juices.
The alimentary canal ends in the cloaca, which is common to all reptiles and serves as the shared emptying chamber for the digestive, urinary, and reproductive systems. There is a large, thin-walled stomach and intestines that are joined by a tube called the oesophagus that connects the throat to the cloaca. The stomach and intestines also have openings for bile, which is produced by the liver. The kidneys are paired and elongated, with each having a single ureter that travels to the cloaca for excretion of urine.
Reptiles are oviparous, with embryos developing inside the female’s body until they are ready to hatch. Some lizards and snakes are also viviparous (they give birth to live young).
Their eggs are fertilized by the single penis, or hemipenis, of the male reptile. The male tries to fertilize the egg by inserting his hemipenis into the female’s cloaca, in a process called copulation. This may be accompanied by ritualized courtship.
The paired testes of a male reptile are situated intracoloemically (within the common coelomic cavity of the abdomen and thorax, as they do not have a diaphragm or separate thoracic and abdominal cavities). They enlarge during the breeding season. Each testis has a solitary vas deferens leading into the urodeum portion of the cloaca, where seminal fluids are added.
Many lizards and snakes can be sexed by examining their cloaca. The cloaca is larger in males, and is typically positioned farther off the midline. A sex-identification probe can also be inserted into the cloaca to determine if it contains sperm or urine. If the cloaca is prolapsed, treatment includes analgesia and lubrication to help reduce the amount of the prolapsed tissue.