Reptiles can change their skin color through a process of vasoconstriction and vasodilation. This is accomplished by a system of specialized cells called chromatophores that contain guanine crystals.
Like birds reptiles have no specific lymph nodes but there are accumulating lymph spaces within their major organs. They lack a diaphragm and inspiration is facilitated by the movement of intercostal muscles as well as elastic recoil of their lungs themselves.
Skull
The skull of a reptile is a solid structure with holes for the eyes, nose, and spinal cord. This structure is referred to as anapsid, and some scientists believe that turtles are surviving examples of the first true reptiles that had this primitive skull structure.
Snakes have a loose hinge between their lower jaw and skull, allowing them to swallow large prey items that are too big to fit through their nostrils. This modification is the result of convergent evolution. Mammal skulls have two articulating quadrate bones that articulate with the mandible. These bones have migrated into the middle ear, where they form malleus and incus.
Lungs
Reptiles use their lungs to exchange oxygen in the bloodstream. However, most do not have a diaphragm to lower the abdomen when it is time to breathe. Instead, they use their axial muscles to expand the rib cage. This creates negative pressure within the body cavity that is balanced by air rushing into the lungs.
The lungs of most reptiles are simple hollow sacs without bronchioles, but a few species have evolved more complicated lung structures. The trachea opens into the lungs through a narrow tube called a glottis. Some lizards open their nostrils and move the floor of their mouth up and down to “gulp” air, but this technique is not common in all reptiles.
Chest
Like crocodilians, lizards have a single coelomic cavity. They do not have a diaphragm, so their respiratory system relies on changing pressure within the body cavity to affect a change in internal volume. Contraction of two flank muscles enlarges the body cavity, causing inspiration; contraction of other muscles, coincident with relaxation of the first two, causes the viscera to be compressed and leads to exhalation.
The cardiac structure of reptiles differs from mammals and birds, although most have a two-chambered heart with two atria and one ventricle. The atria receive aerated blood from the lungs in the left atrium and non-aerated blood from the body in the right.
Stomach
The stomach is a tubular organ populated with compound glands secreting hydrochloric acid and pepsin. The oesophagus is distensible and passes into the stomach, which is joined to the cloaca caudal to the proctodeum.
The cloaca serves as the common emptying chamber for both the digestive and urinary systems. There are paired elongated kidneys, positioned in the distal half of the caudal third of the body. Each kidney has a single ureter that runs across the ventral surface to empty into the urodeum portion of the cloaca.
The reptilian cardiovascular system varies considerably between taxa. It is important to have normal radiographs of a matched conspecific for comparison when evaluating a snake’s heart and vascular anatomy.
Small Intestine
A thin membrane, the mesentery, suspends the small intestine from the back of the abdominal wall. It also supports blood vessels and nerves that supply the intestine.
The intestinal walls contain mucosa that secrete digestive juices and carries minute fingerlike projections, called villi, which increase the surface area of the intestinal wall to enhance absorption of digested foods. Brunner glands in the submucosa secrete secretions that protect the intestinal wall from gastric acid, and Lieberkuhn glands release enzymes into the mucus to aid digestion.
The small intestine (also known as the small bowel) extends from the stomach to the large intestine (colon). It is divided structurally into three regions, which are customarily named duodenum, jejunum and ileum.
Large Intestine
In the lizard and turtle, the large intestine contains leaf-like membranes which increase its surface area for vegetation digestion. It ends in a short tube, the cloaca, which is divided into three chambers; the coprodeum receives fecal matter and urogenital organs; the proctodeum collects other waste products, including those from the male intromittent organs; and the third chamber, the rectum, receives urine.
Reptiles lack a diaphragm, so they breathe through their skin and lungs. The reptile heart is a three-chambered structure with two atria and one ventricle, although it functions as a four-chambered organ. Radiographs of the reptilian heart can be useful in evaluating normal function.
Kidneys
Reptiles excrete nitrogenous waste primarily as uric acid. Stool from reptiles is usually brown and consists of both feces and clear liquid.
The kidneys are bean-shaped organs that lie in the lower back. They are part of the excretory system, along with the ureters and the urinary bladder. Like all vertebrates, the kidneys are made up of nephrons. However, reptiles lack a structure within the nephron known as the loop of Henle, which allows mammals and birds to concentrate urine. This ability to conserve water helps reptiles survive in environments that would overwhelm a mammal. The kidneys also regulate the blood’s osmotic concentration.
Cloaca
In reptiles, the cloaca functions as an all-purpose waste-elimination point for both urine and feces. It is also the location of the genital tract in female animals, and of the penis in males.
The paired testes of lizards lie intracoloemically (within the common body cavity, or coelomic cavity, since reptiles do not have a separate thorax and abdomen). Each testis drains into the cloaca via a solitary vas deferens that has a tightly coiled course over each kidney to empty into the urodeum portion of the cloaca.
Male lizards have paired penises, called hemipenes, located at the base of the tail structure. During mating, each hemipene guides sperm into the female lizard’s cloaca.
Teeth
Reptiles generally have fewer teeth than mammals. The ones they do have are designed for slicing and tearing flesh rather than grinding food like mammal teeth are. Most lizards have sharp cone-like teeth that are ideal for hunting and attacking prey.
Most lizard teeth are pleurodont, with a single row of teeth in the maxillae and dentary. Young green lizards have tricuspid teeth with a main central cusp and smaller mesial and distal cusps. As they grow older, the lizards gradually lose their tricuspid teeth and replace them with bicuspid teeth (fig. 6.40).
Some venomous snakes have a row of teeth with hollow or grooved surfaces for delivering venom. They also have enlarged teeth on the back of their maxillae for grasping prey.