Reptile Classification and Anatomy

The class Reptilia is divided into 4 orders:

I Testudines / Chelonia (turtles) ~ 300 species

The words turtle, tortoise, and terrapin all refer to members of this group. There are sea turtles spending their entire lives at sea only emerging on land to lay eggs. Land turtles, or tortoises, are strictly terrestrial, while terrapins are those turtles that spend the majority of their time in fresh or brackish water and emerge on land occasionally. Chelonians tend to be long lived, some reaching 100 years or more. The shell makes this group of animals unique. It is comprised of the dorsal (top) carapace and the ventral (bottom) plastron held together on the sides by bridges. Outer keratinized plates called scutes cover the underlying bone of the shell. The scutes are not present in the soft shell and leatherback varieties. Hinges may also be present in the plastron or carapace. The turtles possess a horny beak, or ramphotheca, to make up for the absence of teeth.

II Crocodylia (alligators, crocodiles, and caimans) ~ 20 species

Most are tropical and similar in appearance and habits. The majority are freshwater inhabitants with a few saltwater varieties. They are at home both on land and in the water. They can attain 20-30+ year life spans and have been reported to reach 125 years in captivity. These are the largest and probably the oldest of the living reptiles.

III Squamata (lizards – suborder Sauria) > 3000 species

This order contains the largest number of living members who occupy a wide range of habitats.

Sauria: This is the most diverse group of reptiles. There are 16 families of lizards that occupy every continent except Antarctica. Until 2005, the only venomous lizards were the Gila monster (Heloderma suspectum) and the Mexican beaded lizard (Heloderma horridum). It has recently been discovered that some monitors and the Komodo dragon (Varanus komodoensis) are somewhat toxic also. Most have claws but some have adhesive discs on the toes. In most lizards the parietal ‘eye’ is found under the skin on top of the head and communicates with the brain. It senses light and probably functions in reproduction. Life spans range and usually correlate to the size of the species from 2-3 years for anoles to 20-30 years for iguanas and monitors. The Komodo dragon is the largest at 12 feet and 300 pounds.

(snakes – suborder Serpentes) > 2500 species

Serpentes: The snakes are divided into over a dozen families. The boas and pythons belong to the Boidaes. The family Colubridae has the most members and includes the king and corn snakes. Viperidae contains the vipers and Elapidae includes the cobras, coral, and sea snakes. Snakes are limbless and lack eyelids. The eye is covered by a clear spectacle that is shed with the rest of the skin. A few days prior to ecdysis (shedding) this spectacle becomes cloudy or opaque thus signaling an impending shed. The tongue serves as a sense organ by transferring scent particles to the vomeronasal (jacobson’s) organ in the roof of the mouth. Some vipers and constrictors have heat sensing pits on the face. The jaw is unique in that there is no temperomandubular joint and no mandibular symphysis allowing the lower jaw to unhinge and separate. They have two rows of upper teeth and one row below on the mandible. In venomous species the outer row on the top in replaced by fangs. Snakes also lack external ears but do have an internal ear structure to detect vibrations.

IV Sphenodontia (beaked lizards = tuatara) 2 species

The tuatara is only found in New Zealand. They live underground and are unusual in that they tolerate colder temperatures. None of these animals are on display in the United States by request of the New Zealand government.

Anatomy and Physiology

Some of the unique anatomy has been mentioned. The skin of reptiles is comprised of scales as a general rule. There are very few glands in the skin, the notable ones being the femoral/pre-anal pores found in some lizards, located on the underside of the thighs or in front of the vent. These produce a waxy secretion and are more prominent in males. Snakes shed their skin in one piece similar to turning a sock inside out. Lizards and turtles shed patches and some eat the shed skin. Certain reptiles, most notably chameleons, have chromatophores in their skin enabling them to change color in response to light and temperature and mood. Healing of the skin is prolonged compared to mammals; incisions take six weeks to heal instead of two.

The gastrointestinal tract starts with the mouth which is used to prehend food and functions in defense. Tongues vary from not very movable in most lizards and turtles to a specialized sense organ in snakes and some lizards. Chameleon tongues are quite long and highly specialized as they dart out to capture prey. Reptiles do a minimum of chewing, and snakes don’t chew at all; swallowing their food whole. The esophagus of Reptilia is rather distensible to accommodate large food items. The stomach, small intestine, and large intestine of omnivorous and carnivorous reptiles are simpler but similar to mammals. Some herbivores have a cecum attached to the large intestine for digesting roughage. The cloaca is the terminus of the gastrointestinal tract but is also where the urinary and reproductive tracts empty. The cloaca may be an important structure for absorbing fluids to help maintain hydration. The vent is the opening to the cloaca.

The heart of reptiles is three chambered having two atria and one ventricle. The ventricle is divided by a septum allowing very little mixing of venous and arterial blood. The dynamics of blood flow through the heart allows it to function more like a 4 chambered heart. There is a renal portal system which takes blood from the caudal (back) half of the animal directly to the kidneys for filtering before going to the heart. This can be important when administering injectable medications. A lymphatic system is also present.

The opening in the mouth to the respiratory tract is the glottis. This circular structure opens only during respiration and is a closed slit if the animal is not taking a breath. The trachea connects the glottis to the lungs. The lungs are saclike and in the majority of snakes only the right lung is functional with a tiny vestige of the left lung. Reptiles lack a diaphram and rely on chest and abdominal muscles along with limb movement to achieve inspiration. In addition, the lung wall contains smooth muscle to aid in expansion. Crocodilians do possess a pseudodiaphram. Turtle lungs are attached dorsally to the carapace and they can suffocate if left on their back causing the abdominal organs to weigh down the lungs. Since there is no diaphram to separate the thorax from the abdomen, the terms coelom or coelomic cavity are used to describe the combined space.

Turtles and many lizards have urinary bladders. The urine first flows to the cloaca then backflows into the bladder for storage. Nitrogen waste is excreted primarily as uric acid. The ovaries and testes are within the coelomic cavity. The penis is a paired structure in snakes and lizards called hemipenes. In chelonians it is a single organ. Gender determination is accomplished in various ways. The femoral/pre-anal pores of lizards are more pronounced in males. Male turtles may have a concaved plastron and usually have thicker tails to house the penis. Male eastern box turtles (Terrapene carolina carolina ) have red pupils. Snakes are sexed by “probing” for the hemipene. Some species have sexual dimorphism such as jackson’s chameleon (Chamaeleo jacksonii) where the male has horn-like structures.

Reptiles are ectothermic, that is, they thermoregulate by behavioral means seeking out warm and cool areas suitable to their individual physiologic requirements. Lack of appropriate temperature and temperature gradients in captivity is a leading cause of disease. Each species has a Preferred Optimal Temperature Zone (POTZ) and rely on their care-givers to provide such an environment. This POTZ  is critical for all physiologic functions including digestion, reproduction, immune function, growth, and healing. Such an important part of reptile husbandry requires thermometers are used to assure proper temperatures and temperature gradients are maintained. Equally important is prevention of burns caused by access to heat sources.