Reptile Anatomy

Reptiles do not have a diaphragm and thus have a common body cavity (the coelom). Unlike birds, the heart of most lizard species lies within the thoracic girdle and can be easily felt and palpated. In monitor lizards and some crocodilians, the heart is located farther back in the coelomic cavity.


Skeletal System

The skeleton supports and moves the body, stores minerals like calcium and phosphate, produces blood cells, and houses major organs like the brain and heart. It is also an important site for muscle attachment and movement and acts as the primary storage area for fat.

As vertebrates, reptiles have a skeletal system that is similar to the one of mammals. However, groups of reptiles that have adapted to a particular lifestyle – lizards and snakes as well as the shelled reptiles (turtles, tortoises) and crocodiles – have developed specializations in their skeletons that reflect these adaptations.

The basic skeleton is divided into two major subdivisions: the axial and the appendicular. The axial skeleton includes the spine and skull. It is attached to the appendicular skeleton, which comprises the bones of the limbs and shoulder and pectoral girdles. The appendicular skeleton is responsible for locomotion and other movements of the limbs, as well as protecting the major organs involved in digestion, excretion and reproduction. The lungs of reptiles are more simple than those of mammals, but reach a higher volume of air and have a larger surface area.

Muscular System

Muscle tissues are organized into cellular units called sarcomeres. Each sarcomere contains thin and thick protein filaments that overlap giving muscles their characteristic striated appearance. When muscles contract, they cause movement of bones and joints. Each muscle has a specific site of attachment to a bone known as its insertion point and another end which is fixed to the bone, called its origin. During movement, many muscles take part but there is one that initiates the action, it is called the prime mover.

Reptiles do not have a diaphragm, so inspiration is accomplished by the expansion and recoil of rib and intercostal muscles. They also have a tracheal lung, an outpouching of the trachea’s cartilage that can be expanded and compressed by contraction of abdominal and intercostal muscles to aid respiration.

Most reptiles have a complex integument with scales that are heavily keratinized. This enables them to shed their skin if they become injured or are caught by a predator, and it helps prevent excessive water loss. A unique feature of many reptiles is a parietal eye on the top of their head, similar to a bird’s or a mammal’s, but much simpler in structure.

Nervous System

The nervous system is the structure that controls movement and other body functions. It consists of a brain and spinal cord with nerves running from them to the organs of sense and other parts of the body. The structure of the nervous system varies widely among organisms, ranging from a diffuse nerve net in sponges and microscopic bloblike organisms called placozoans and mesozoans to complex brains and neural networks in humans.

The general character of the reptiles’ nervous system reveals a greater attachment to terrestrial life than amphibians. Their skin is thicker and keratinized, and the scales are widened ventrally, which helps them support locomotion.

The nerves of the reptiles have the same general appearance as those of other vertebrates; there is a brain supplying the sense organs, and a collar of nerves around the disk separating and encircling the esophagus. From these, two longitudinal nerves with ganglia are sent out, and from these the nerves of the limbs are also separated. A proper phrenic nerve, seen in mammals, birds, and reptiles, is lacking in fishes, who have a very small nerve without ganglia.

Digestive System

Reptiles have a shortened digestive tract, and they do not have the diaphragm that separates their thorax from their abdomen. The digestive tract starts with the mouth and salivary glands and ends in a cloaca.

The mouth opens into a pharynx that leads to the esophagus, a distensible muscular tube that can extend up to half the length of the body. Peristaltic movement within the esophagus moves food into the stomach, which is a j-shaped organ with cells secreting both hydrochloric acid and pepsin that break down proteins.

The trachea in lizards and snakes bifurcates in the cranial thorax to supply two lungs. The lungs are sac-like structures with large bullae-like divisions and alveoli. The lungs are vestigial in most reptiles, but they can be completely functional in some fish-eating snakes. In a snake, the trachea terminates in a short left lung, while in crocodiles and turtles it terminates in a complete right lung. The lungs are also used for resorption of water in some lizards and snakes. The lungs are protected by a thin layer of fat, which is important for the reptile’s terrestrial survival.

Respiratory System

Reptiles use behavioral adaptations to regulate their body temperature; they manage this through shifting their daily position. They also breathe through lungs.

The trachea in snakes and lizards bifurcates into two bronchi in the cranial thorax to supply two lungs. These lungs are sponge-like structures with large bulla-like divisions and alveoli for gas exchange. The lungs can be inflated and deflated through contractions of the intercostal muscles of the trunk and abdomen.

There is no diaphragm in lizards and snakes; air is passively drawn into the lungs by expansion of the abdominal wall. Breathing can also be accomplished by moving the floor of the mouth upward to force air through a small opening, called the glottis, which is closed except when the lizard or snake is taking a breath.

The renal system of a reptile is made up of nephrons that maintain normal concentrations of salt and water in the extracellular fluid, regulate acid-base balance, excrete waste products, produce hormones and vitamins. The kidneys contain glomeruli designed to filter the plasma, Bowman capsules that collect the filtrate, and tubules that resorb most of the filtered water and nutrients while excreting waste metabolites. The cloaca and urinary bladder (if present) provide additional fluid absorption.