Reptile Anatomy

크레스티드게코 Reptiles are cold-blooded, air-breathing vertebrate animals. Their skin is scaled, either smooth or bony plates.

크레스티드게코

Their hinged upper and lower jaws stretch to allow them to swallow large prey. Saliva begins digestion before the food reaches the stomach, which is an enlarged end of the oesophagus.

Most non-crocodilian reptiles have a three-chambered heart with some mixing and double circulation. They also have a paracloacal spur.

Skull

The skull is a rigid skeletal structure that protects the brain and sense organs. It consists of 22 bones that come together through cranial sutures. The skull also houses muscle and tendinous attachments, protects neurovascular structures, and contains sinuses that can expand to accommodate changes in pressure.

Skulls of reptiles vary in shape and size but they all have a common feature: an opening (fenestra) high on the frontal bone, which is closed in mammals. Euryapsids, like the prehistoric fish icthyosaur and the plesiosaurus, had two fenestrae but sealed one to strengthen their skulls for life underwater.

The lizard’s skull has four moveable joints, permitting a high degree of cranial kinesis, and this allows the animal to redistribute bite forces for improved prey capture. Mammals have two occipital condolytes at the 크레스티드게코 articulation between their skulls and neck vertebrae. This limits their head movements but it allows them to swallow larger prey. Snakes, on the other hand, have a single occipital condolyte at the articulation between their skulls. This permits more rapid and greater cranial kinesis.

Teeth

Reptiles differ from mammals in several ways, including the number of upper and lower teeth. While some species, like crocodiles and dinosaurs, have distinct bladelike incisors, tusklike canines, and flat-crowned molars, most have only one row of long, conical teeth.

The lower jaw bone in a reptile has only one tooth-bearing bone, the dentary, which connects to an adjacent small bone called the articular in the rear of the skull. A small tooth-bearing ridge on the lingual side of this bone is lined with enamel organs. A dental lamina runs across the lingual surface and links the tooth germs to oral epithelium. In most vertebrates, the replacement of incisors and canines occurs serially, while molars are replaced continuously.

However, this pattern of tooth replacement is less common in reptiles than in most other amniotes, including mammal lineages (Fig. 1A). This difference may have helped to promote the development of complex molars in lizards, as well as the evolution of the jaw structure that supports them. For example, the Cretaceous sphenodontian Priosphenodon has a unique form of prismatic teeth that look more like half-cones than full-fledged molars.

Scales

The external covering of reptiles is dry and composed of scales made of a hard substance called keratin. This is in contrast to amphibian and mammal skin which contains glands. Scales are a primitive type of skin appendage from which feathers and hair evolved. They seal the body to reduce water loss and allow for movement over land.

Each scale has an outer surface and an inner surface. The cells on the outer surface grow faster than those on the inner side. The resulting bump (papilla) forms the center of the scale and becomes lopsided and flat as it grows. Scales are shed to replace those that wear away.

Reptiles also have specialized scales for climbing and frictional movement. The ventral scales, known as scutes, are mobile and have a rough texture to increase friction when snakes move up an incline.

Like birds, reptiles have paired kidneys that empty into the coprodeum portion of the cloaca. In males there are a pair of elongated kidneys that are part of the reproductive system and enlarge during breeding season.

Skin

The skin of reptiles is dry and has fewer glands than in mammals and amphibians. It also is heavily keratinized, a feature that prevents water loss and supports their terrestrial lifestyle. This is particularly important for a reptile like the snake, which has no limbs and depends on its skin to support it over long distances.

The scales of a reptile are covered by epidermal skin that contains pigment cells and three types of chromatophores that control color patterns. Melanocytes produce black, brown, and gray pigments. The carotenoid cells produce yellow to orange pigments. These are the colors seen in a snake’s skin, for example, and can vary in their patterning between individuals of the same species.

The skin of a reptile goes through a shedding process, in which the reptile loses the outer layer of its scales. This enables it to shed a worn body covering in order to grow a new one. This process allows the lizard to keep itself cool and provides an opportunity for it to clean its body. In this process, macrophages and heterophils move into the defect under the scab layer to clean it of bacteria and other debris.

Egg

Reptiles are oviparous, and all except for some snakes and lizards lay eggs. Their eggs are giant single cells that contain stockpiles of all the materials needed to carry the embryo through a period of very rapid growth until it can begin feeding. This is a major advantage over the spore-like seeds of other kingdoms and allows reptile embryos to grow at an astounding rate.

Once fertilized by a male sperm cell, the egg begins to develop. It is surrounded by extra-embryonic membranes (amnion and allantois) to support its development and serve as a repository for wastes. It also has a protective calciferous shell.

Air-breathing reptiles are characterized by highly developed lungs, even in those species that have permeable skin. The lungs are situated in the first half of the thorax and share a common body cavity (coelom) with the heart. There is no diaphragm to aid respiration, and inspiration occurs by outward movement of the ribs and intercostal muscles. The lungs themselves are sponge-like structures with large bulla-like divisions and alveoli. A tracheal lung is present to help inhale when the main lungs are compressed during swallowing of large food items.