Lizard Anatomy: Labeled Diagrams and Interactive 3D Model

A lizard is a scaly reptile with, in most species, four sprawling legs, a long tail, movable eyelids and external ear openings. Its body has three regions: the head, the trunk (which holds the body cavity) and the tail, which starts at the vent. Inside, it has a three-chambered heart, two simple lungs, kidneys that excrete uric acid, a cloaca shared by the gut, urinary and reproductive tracts, and, in males, a pair of hemipenes.

Lizards are not one natural group. There are more than 7,000 species, and snakes evolved from within them, so "lizard" is a paraphyletic name for all squamates (scaled reptiles) that are not snakes or worm lizards. Limbs have been lost or reduced more than two dozen times in different lizard lineages.

This page uses the green anole, Anolis carolinensis, as the reference lizard. It is a small, tree-living iguanian lizard from the south-eastern United States, widely kept in schools and used as a laboratory model; in 2011 it became the first non-bird reptile to have its genome sequenced. Adult males are 12.5–20 cm long including the tail. Where other lizards differ, such as bearded dragons, monitors and geckos, the text says so.

Directions: anterior means towards the snout, posterior towards the tail, dorsal is the back and ventral the belly. Left and right are always the lizard's own. Body length is given as snout–vent length (SVL) because so many lizards have broken or regrown tails.

The external anatomy of a lizard includes the snout with two nostrils, two eyes with movable eyelids, the parietal (third) eye on top of the head, an ear opening with the eardrum on each side, the mouth with its lip scales, four limbs with five clawed digits each, the vent (cloacal opening) and the tail. Male anoles add a large throat fan (dewlap) and enlarged scales behind the vent.

• Dewlap: A visual signal used in courtship, in territorial contests between males and to deter predators, together with head-bobs and push-ups.
• Ear opening and tympanum: Sound vibrates the eardrum, which passes the vibration through a single middle-ear bone (columella) to the inner ear.
• Eyelids: Close to protect and clean the eye. A thin third eyelid (nictitating membrane) can also sweep across it.
• Toe pads: Each lamella is covered with microscopic hair-like setae that grip surfaces, so anoles can climb smooth leaves and walls.
• Vent: The single external opening of the cloaca, through which faeces, urine (as uric acid), eggs and sperm leave the body, and through which the hemipenes are everted.

The skin is covered by small overlapping scales made of keratin. Unlike fish scales, these are thickened folds of the skin itself, and they are shed regularly, in patches in lizards rather than in one piece as in snakes. Some lizards, such as skinks and monitors, have small bony plates (osteoderms) under the scales; anoles do not.

Inside a lizard, the heart sits in the chest between the shoulders; the paired, sac-like lungs fill the upper part of the front half of the body cavity; the large, two-lobed liver lies on the floor of the cavity behind the heart; the tubular stomach runs along the left side; and the small intestine coils behind it. At the back of the cavity are the fat bodies, the gonads, the kidneys inside the pelvis, the urinary bladder and the cloaca. Lizards have no diaphragm, so the chest and abdomen form one body cavity (coelom).

External anatomy is everything you can see on the outside of the lizard without cutting: scales, eyes, ears, nostrils, dewlap, limbs, toe pads, claws, vent and tail. Internal anatomy covers the organs and skeleton inside. The two are linked: the dewlap is moved by the hyoid bones inside the throat, the ear opening leads to a single middle-ear bone, the parietal eye sits over a hole in the skull, and the swollen tail base of a male hides the hemipenes. Use the skin slider in the 3D model to move between the two views.

This is the text version of the 3D model: every labeled structure of the green anole, grouped by body system, with what it does.

Worksheets built around the green anole usually ask students to measure the lizard, find the external features, explain colour change and the dewlap, and compare the anole with other reptiles. Typical answers: an adult male is about 12.5–20 cm long in total but at most about 7.5 cm from snout to vent, so the tail makes up roughly 60–70% of its length. Males are about 15% larger than females and weigh only 3–7 g. (Wikipedia: Carolina anole.)

How does an anole change colour? It has three layers of pigment cells in the skin: yellow xanthophores on top, blue-reflecting cells below them and brown-black melanophores at the bottom. When the melanophores spread their pigment upward, the green is masked and the lizard turns brown. The change follows stress, activity, temperature and social signals; the evidence does not support the popular idea that anoles match their background. Darker browns and black often signal a cold or stressed animal. (Animal Diversity Web: Anolis carolinensis.)

How do you tell the sexes apart? Males have a large pink-red dewlap, about three times the size of the female's, and can raise a crest along the neck and back when displaying. Females have a small, pale dewlap or none at all, and usually a pale stripe down the middle of the back. Males also have a pair of enlarged scales just behind the vent, a less obvious but reliable difference. (Wikipedia: Dactyloidae (anoles).)

How does an anole climb glass? Under each finger and toe is a pad made of a row of overlapping flaps (lamellae) covered in microscopic hair-like setae; the number of lamellae varies between species and with body size. They grip smooth surfaces, although less strongly than a gecko's toe pads. Anoles that live high in the canopy tend to have better-developed pads than those living near the ground. (Wikipedia: Dactyloidae (anoles).)

How do anoles reproduce? Breeding runs roughly from April to August. A female matures one egg at a time, alternating between her two ovaries, and lays a single egg (sometimes two) about every two weeks, around 10 or more in a season. Eggs are about 12.5 × 9.3 mm, are buried in moist soil or leaf litter, and hatch after 30–45 days at 27–29 °C. Females can store sperm for up to eight months. (Wikipedia: Carolina anole.)

A lizard skeleton has a kinetic skull, a long backbone of procoelous vertebrae (ball-and-socket joints) divided into cervical, dorsal, two sacral and many caudal vertebrae, ribs on almost every trunk vertebra, a sternum, pectoral and pelvic girdles, and four limbs with five digits each. The tail vertebrae of most lizards carry autotomy fracture planes.

• Cervical vertebrae: Support and turn the head. The atlas rotates on the single occipital condyle of the skull.
• Dorsal (trunk) vertebrae: Form the backbone that supports the trunk and bends from side to side during walking.
• Sacral vertebrae: Transfer the thrust of the hindlimbs to the backbone.
• Caudal vertebrae: Support the tail and anchor the tail muscles. The chevrons (haemal arches) protect the tail artery and vein.
• Ribs: Support the body wall and, moved by the intercostal muscles, ventilate the lungs. Lizards have no diaphragm.
• Sternum: Joins the front ribs and the coracoids, closing the rib cage below.
• Pectoral girdle: Attaches the forelimbs to the body. The humerus joins it at the shoulder socket (glenoid).
• Pelvic girdle: Attaches the hindlimbs to the backbone. The femur joins it at the hip socket (acetabulum).

Limb bones follow the standard four-legged plan: humerus, radius and ulna, carpals, metacarpals and phalanges in the forelimb; femur, tibia and fibula, tarsals, metatarsals and phalanges in the hindlimb. Two features are typical of lizards: the ankle bends between two rows of tarsal bones (a mesotarsal joint), and the fifth metatarsal is hooked, which sets the fifth toe apart.

A lizard skull is a light, open framework with large orbits and an upper temporal opening on each side, but no lower temporal bar. Losing that bar frees the quadrate bone, on which the lower jaw hinges, to swing forward and back (streptostyly). Many lizards can also flex the skull at the joint between the frontal and parietal bones (mesokinesis) or slide the outer skull against the braincase (metakinesis). Chameleons, agamids such as bearded dragons and phrynosomatids such as horned lizards have relatively rigid, akinetic skulls, and the purpose of skull kinesis in lizards is still debated.

• Premaxilla: Carries the front upper teeth and forms the front edge of the nostrils.
• Maxilla: Carries most of the upper teeth and forms the side wall of the snout.
• Nasal bones: Roof over the nasal cavity, which holds the olfactory epithelium.
• Frontal bone: Protects the front of the brain and the olfactory tracts and forms the upper rim of each orbit.
• Parietal bone and parietal foramen: Protects the brain, anchors jaw-closing muscles along its edges, and holds the parietal foramen, under which the parietal (third) eye sits.
• Postorbital, jugal and squamosal: Form the rear rim of the orbit and the upper temporal arch, and give attachment to the jaw-closing muscles.
• Quadrate: Forms the upper half of the jaw joint. In lizards it can swing at its top (streptostyly), which changes the gape and the position of the lower jaw.
• Palate: pterygoid, palatine and vomer: Separate the mouth from the nasal passages and braincase and link the snout to the quadrate, so the palate moves when the skull flexes.
• Braincase: Protects the brain and inner ears and joins the skull to the first vertebra through a single occipital condyle.
• Scleral ring: Supports the eyeball and gives attachment for the muscles that change its shape for focusing.
• Dentary: Carries all the lower teeth.
• Postdentary bones: The coronoid anchors the jaw-closing muscles; the articular forms the lower half of the jaw joint with the quadrate; the others brace the jaw.

A lizard's mouth runs back almost to the ear. Its teeth are simple, pointed and usually all alike; they grip and pierce prey rather than chew it, and lizards swallow food whole or after a few bites. In the floor of the mouth sit the fleshy tongue and, behind it, the slit-like glottis that leads to the windpipe. In the roof are the internal nostrils (choanae) and the openings of the vomeronasal organs.

Most lizards and snakes, including anoles and iguanas, have pleurodont teeth: each tooth is fused on one side to the inner (lingual) face of a high outer jaw wall. This leaves space beside the tooth for a replacement to develop, so pleurodont lizards replace their teeth throughout life. (Haridy (2018): tooth implantation in Pogona vitticeps, PeerJ.)

Agamids (such as the bearded dragon) and chameleons form the group Acrodonta. Most of their teeth appear fused to the top of the jaw (acrodont) and are not replaced; the tuatara is the only other living reptile like this. A bearded dragon also has a few pointed pleurodont teeth at the front of the jaw, which are replaced. Histology of Pogona vitticeps suggests its acrodont teeth start out pleurodont and only look acrodont after bone remodelling and wear, so the jaw edge itself becomes part of the cutting surface in old animals. (Haridy (2018): tooth implantation in Pogona vitticeps, PeerJ.)

Lizard tongues come in two main kinds. Iguanian lizards, including anoles, bearded dragons and horned lizards, have a thick, fleshy tongue with a slightly notched tip that they use to pick up prey (lingual prehension). Many other lizards, especially monitors and tegus, have long, deeply forked tongues used mainly for smelling: they flick the tongue to pick up scent particles and deliver them to the vomeronasal organ in the roof of the mouth. Chameleons shoot out an extremely long, sticky tongue, and geckos use the tongue to wipe their eye spectacles clean.

The tongue sits on the hyoid apparatus, the same set of bones and cartilages that, in anoles, extends the dewlap. A monitor's forked tongue lets it compare scent from the left and right tips, like smelling in stereo.

Most lizards have large eyes with movable upper and lower eyelids and a thin third eyelid (nictitating membrane). The wall of each eyeball is stiffened by a scleral ring, typically of 14 small overlapping bones, which supports the eye and anchors the muscles used for focusing. Anoles hunt by sight and are thought to see ultraviolet light, which may make dewlap colours stand out.

Geckos and some skinks are different: their eyelids are fused into a transparent, fixed scale (spectacle), as in snakes. Chameleons can turn each eye independently. On top of the head, many lizards also have a light-sensing parietal eye.

• Eye: Vision is the anole's main sense for finding prey, mates and rivals.
• Eyelids: Close to protect and clean the eye. A thin third eyelid (nictitating membrane) can also sweep across it.
• Scleral ring: Supports the eyeball and gives attachment for the muscles that change its shape for focusing.
• Parietal eye: Senses light and dark (not images). It helps regulate daily rhythms, hormones and basking.

The parietal eye sits in a hole in the skull roof, the parietal (or pineal) foramen, and is always covered by skin. It develops together with the pineal organ from the roof of the forebrain and is part of the same epithalamic complex. It occurs in the tuatara, most lizards, frogs and some fishes, but not in snakes, crocodilians, turtles, birds or mammals. (Wikipedia: parietal eye.)

The parietal eye has only a simple lens and retina and cannot form images. In bearded dragons it appears to be involved in sensing magnetic fields: dragons reacted to a weak low-frequency electromagnetic field, the reaction disappeared when the third eye was covered with a small aluminium cap, and it only occurred when the light reaching the eye had a wavelength shorter than 580 nm. (Nishimura (2020): the parietal eye of Pogona vitticeps, Animals.)

Lizards have no outer ear flap, but most have a visible ear opening on each side of the head, behind the jaw, with the eardrum (tympanic membrane) just inside. Sound vibrates the eardrum, which moves a single slender middle-ear bone, the columella, against the inner ear. The inner ear holds the cochlear duct for hearing and the semicircular canals for balance. In the green anole, the ear opening is a small oval behind the corner of the mouth.

Some lizards have lost the ear opening: earless lizards (Holbrookia) have it covered by scales, and many burrowing lizards have it reduced. Snakes have no ear opening, eardrum or middle-ear cavity at all, which is one of the clearest ways to tell a legless lizard from a snake.

A lizard heart has three chambers: two atria and one ventricle. The ventricle is not a single open space: two incomplete partitions, the vertical septum and the muscular ridge, divide it into three linked compartments, the cavum arteriosum, cavum venosum and cavum pulmonale. A thin-walled sinus venosus collects blood from the body before the right atrium.

Because the partitions are incomplete, a lizard can send some blood past the lungs (a right-to-left shunt), for example while it holds its breath. Monitor lizards have the most complete division and can keep a high-pressure body circuit and a low-pressure lung circuit, like a mammal.

A lizard heart has a sinus venosus, two fully separated atria and one ventricle. Blood flows in through three systemic veins to the right atrium and through one or two pulmonary veins to the left atrium, and leaves through three arteries: the left aorta, the right aorta and a single pulmonary artery. (Jensen et al. (2013): development of the hearts of lizards and snakes, PLoS ONE.)

The ventricle is divided by two incomplete partitions into three compartments. From the animal's left to right they are the cavum arteriosum, the cavum venosum and the cavum pulmonale. A sheet of trabeculated muscle, the vertical septum, partly separates the cavum arteriosum from the cavum venosum, and a spiralling muscular ridge (also called the horizontal septum) partly separates the cavum venosum from the cavum pulmonale. (Jensen et al. (2013): development of the hearts of lizards and snakes, PLoS ONE.)

During filling, oxygen-rich blood from the left atrium enters the cavum arteriosum, while oxygen-poor blood from the right atrium enters the cavum venosum and cavum pulmonale. When the ventricle contracts, the right-atrial blood is sent mainly to the pulmonary artery and the left-atrial blood to the two aortae. Mixing is small, but because resistance in the lung circuit is often higher, some blood can bypass the lungs (a right-to-left shunt), especially in resting or diving animals. (Jensen et al. (2013): development of the hearts of lizards and snakes, PLoS ONE.)

Monitor lizards are the exception among lizards: during contraction their ventricle works almost like two separate pumps, so they can keep a high blood pressure in the body and a low pressure in the lungs, much like mammals. This goes with the high metabolic rates and endurance of monitors. (Wikipedia: monitor lizard.)

Lizards breathe air through the nostrils, glottis and trachea into two lungs. Anole lungs are simple sacs whose inner walls form a honeycomb of small chambers (faveoli). Monitors and iguanas have larger, multi-chambered lungs, and in savannah monitors and green iguanas air moves one way through the lung, as in birds. With no diaphragm, lizards ventilate by moving the ribs with the body wall muscles. Because the trunk bends sideways when they run, they find it hard to breathe while running fast (Carrier's constraint).

A lizard brain is small and elongated. From front to back it has the olfactory bulbs (smell), the paired cerebral hemispheres, the optic lobes of the midbrain (vision), a small cerebellum (coordination) and the medulla oblongata, which continues as the spinal cord. The pineal organ and parietal eye grow from the roof of the forebrain.

• Olfactory bulbs: Process smell. A separate accessory olfactory bulb receives the vomeronasal nerve.
• Cerebral hemispheres: Integrate senses, learning and behaviour such as courtship and territorial display.
• Optic lobes: Process vision and guide prey-catching strikes; very large in sight-hunting lizards like anoles.
• Cerebellum: Coordinates movement and balance.
• Medulla oblongata: Controls breathing and heart rate and relays signals between the brain and the body.
• Spinal cord: Carries signals between the brain and the body and controls simple reflexes such as the writhing of a shed tail.

The popular phrase "lizard brain" for the instinctive parts of the human brain is a simplification; real lizard brains have well-developed forebrains that support learning, social behaviour and complex displays.

The cloaca is the chamber at the end of the gut into which the intestine, the urinary ducts and the reproductive ducts all open. It has three parts: the coprodeum (faeces), the urodeum (urine, eggs or sperm) and the proctodeum, which opens to the outside through the transverse vent. Lizard kidneys lack the loop of Henle and cannot make concentrated urine; instead, nitrogen leaves mainly as solid uric acid, the white part of lizard droppings, and water is reabsorbed in the cloaca and the urinary bladder. Most lizards have a bladder; snakes and crocodilians do not.

• Kidneys: Filter the blood and remove nitrogen waste, mostly as uric acid.
• Urinary bladder: Stores urine and lets water be reabsorbed back into the body.
• Cloaca: Its three parts handle faeces (coprodeum), urine and reproductive products (urodeum), and form the exit (proctodeum). It also reabsorbs water.
• Large intestine: Absorbs water and forms faeces.

Male and female lizards share paired gonads at the back of the body cavity, but their ducts and external features differ. Male lizards have two hemipenes, copulatory organs that lie inside out in pockets in the base of the tail and are everted through the vent to mate. Only one is used at a time, and sperm runs along a groove on its surface. Females have two ovaries and two oviducts that add albumen and the shell to each egg.

Other lizards differ. Male bearded dragons have larger femoral and precloacal pores (waxy scent glands under the thighs and in front of the vent), a darker beard and visible hemipenal bulges. About 20% of lizard species give birth to live young, including blue-tongued skinks, and a few whiptail lizards reproduce by parthenogenesis, with all-female populations. Switch the 3D model between male and female to compare.

A lizard tail contains a long series of caudal vertebrae with chevron bones below them, segmented muscles, fat stores, the caudal artery and vein, and the end of the spinal cord. In anoles, geckos, skinks and many other lizards, most tail vertebrae are crossed by a pre-formed fracture plane, so the tail can be shed when a predator grabs it (caudal autotomy). The shed tail wriggles for minutes and distracts the predator while the lizard escapes.

• Caudal vertebrae: Support the tail and anchor the tail muscles. The chevrons (haemal arches) protect the tail artery and vein.
• Autotomy fracture planes: Let the lizard shed its tail at a chosen point when grabbed, then seal the wound with little blood loss.
• Tail muscles: Move the tail and, at autotomy, contract to tear the tail apart at a fracture plane.
• Caudofemoralis: Pulls the thigh backwards: the main power stroke of running.

Losing the tail has costs: fat reserves, balance and sometimes social rank. The regrown tail is supported by a cartilage tube rather than vertebrae, is usually shorter and differently coloured, and cannot break at the same place again.

Lizards shed the tail at pre-formed planes of weakness. In most species the plane runs straight across the middle of a tail vertebra (intravertebral autotomy); breaking between two vertebrae (intervertebral autotomy) is less common. The break follows the boundary between muscle segments, so the tail separates cleanly. The lizard helps by contracting its tail muscles, and the break usually happens just in front of where the tail was grabbed, or a few segments further forward. (Sanggaard et al. (2012): structural features of lizard tail autotomy, PLoS ONE.)

Studies of the Tokay gecko found that the muscle fibres at a fracture plane end in finger-like, interlocking tips that are held to the connective tissue septum by adhesion; no fibres or tendons cross the plane. During autotomy the tips change shape, which suggests the lizard can release the adhesion quickly. This design lets the tail stay strong in daily life but break easily on demand. (Sanggaard et al. (2012): structural features of lizard tail autotomy, PLoS ONE.)

Not every tail vertebra can break. In the skink Egernia kingii, adults have 5–6 vertebrae at the base of the tail with no fracture plane, followed by 39–44 that can break. This keeps the base of the tail, where large leg muscles attach, intact. In some other lizards the fracture planes fuse with age, so old adults lose the ability. (Barr et al. (2022): caudal autotomy fracture planes in Egernia kingii, Scientific Reports.)

Agamid lizards vary a great deal. Some can shed and regrow the tail, some can shed it but not regrow it, and some cannot shed it at all. Museum specimens of bearded dragons (Pogona vitticeps and P. barbata) examined in one review all had intact tails, and agamids that run on their hind legs tend to keep their tails, which they need for balance. (Ananjeva et al. (2021): autotomy of agamid lizards, Journal of Developmental Biology.)

The new tail of a green anole is not a copy of the old one. A wound skin forms over the stump within about 10 days, and a tube of ependymal cells grows out from the spinal cord. By 15–20 days a central cartilage tube and new muscle start to form, and outgrowth largely stops after about 60 days. The regenerated tail has a single unsegmented cartilage tube instead of vertebrae, unsegmented muscle bundles and no segmental nerves or dorsal root ganglia. That is why a regrown tail is stiffer and cannot be shed again at the same place. (Hutchins et al. (2014): tail regeneration in Anolis carolinensis, PLoS ONE.)

Lizard legs stick out sideways from the body (sprawling posture), with the upper arm and thigh held nearly horizontal and the forearm and shank pointing down. Each foot has five clawed digits. The usual phalangeal formula is 2-3-4-5-3 in the hand and 2-3-4-5-4 in the foot, so the fourth toe is the longest. Anoles, like many geckos, add adhesive toe pads under the middle of each digit.

• Forelimbs: Climbing, gripping branches and, together with the hindlimbs, walking with the sprawling, side-to-side gait typical of lizards.
• Hindlimbs: Main source of thrust for running, climbing and jumping.
• Humerus: Swings forward and back in a near-horizontal plane in the sprawling lizard gait.
• Radius and ulna: Support the forearm and let the hand rotate slightly.
• Hand bones: Support the fingers and the toe pads.
• Femur: Pulled backwards by the caudofemoralis muscle from the tail, it gives the main thrust in running.
• Tibia and fibula: Support the shank. The ankle joint of lizards lies between two rows of tarsals (mesotarsal joint).
• Foot bones: Form the long, flexible foot used for pushing off.
• Claws: Grip rough bark and soil where adhesive pads work less well.

Leg length tracks habitat. Among Caribbean anoles, species that perch on broad tree trunks have long legs for running and jumping, while those living on thin twigs have short legs for careful clambering.

The main muscles of a lizard are the epaxial muscles along the back, the hypaxial and intercostal muscles of the body wall, the jaw muscles, the limb muscles and the segmented muscles of the tail. The largest single limb muscle is the caudofemoralis, which runs from the tail base to the femur and pulls the thigh back during running, so the base of the tail is part of the locomotor system as well as a balance organ.

• Epaxial muscles: Bend the body sideways and stiffen it during locomotion.
• Body wall muscles: Move the ribs to breathe and support the abdomen.
• Jaw-closing muscles: Bite down on prey.
• Depressor mandibulae: Opens the mouth by pulling up the back end of the lower jaw.
• Ceratohyoid muscles: Swing the second ceratobranchials forward to extend the dewlap.

The central bearded dragon (Pogona vitticeps) is an agamid from inland Australia and the most popular pet lizard. It reaches about 60 cm, more than half of it tail. Its organ systems match the anole's, but several features differ:

• A spiny throat pouch (beard) that inflates and darkens in threat and courtship displays; it is darkest in breeding males.
• Mostly acrodont teeth that are not replaced, with a few pleurodont front teeth that are.
• Femoral pores under the thighs, more pronounced in males, and no adhesive toe pads.
• A well-developed parietal eye, used in experiments on light-dependent magnetic sensing.
• Sex is set by ZW chromosomes, but incubation above about 32 °C can turn genetic males into functional females.
• Bearded dragons rarely, if ever, shed their tails.

Monitor lizards (Varanus), which include the Bengal monitor and the Komodo dragon, are active predators with the highest standard metabolic rates of any living reptile. Living species range from about 20 cm to over 3 m long. Their anatomy reflects this lifestyle:

• A long, deeply forked tongue that carries scent to the vomeronasal organ.
• A heart whose ventricle is functionally divided during contraction, giving mammal-like pressure differences between the body and lung circuits.
• Large, multi-chambered lungs with one-way airflow in the savannah monitor.
• Venom glands in the lower jaw in at least some species.
• No femoral pores: they are absent in all monitors, as in skinks and chameleons.

Many geckos and almost all anoles have toe pads: rows of widened scales under the digits that bear microscopic hair-like filaments (setae). Only the setae with flattened, spatula-shaped tips are true adhesive setae. Toe pads have evolved independently many times within geckos and separately in anoles, so the two are a textbook case of convergent evolution. (Russell, Delannoy & Garner (2026): toe pads of Gekko gecko, Journal of Anatomy.)

The numbers are large. In the Tokay gecko (Gekko gecko), the toe pads cover about 400 mm² in total, a single front toe carries about 262,000 setal stalks and a single hind toe about 353,000, and each seta branches into roughly 250–560 spatula tips. Earlier experiments cited in this study (Autumn et al., 2002) showed that these tips stick by weak molecular attraction (van der Waals forces), so the foot grips dry glass without any glue. (Russell, Delannoy & Garner (2026): toe pads of Gekko gecko, Journal of Anatomy.)

The frill of the frill-necked lizard (Chlamydosaurus kingii) is a large flap of skin around the neck, supported by long rod-like hyoid bones and linked to each side of the head by Grey's cartilage. When the lizard opens its mouth, the hyoid rods and lower jaw spread the frill into a disc about 30 cm across; at rest it folds back over the shoulders like a cape. It is the same hyoid apparatus that stiffens an anole's dewlap. Frilled lizards reach about 90 cm in total length and can run on their hind legs. (Wikipedia: frill-necked lizard.)

Blue-tongued skinks (Tiliqua) have long, heavy bodies, short legs and a broad blue tongue. Threatened, the skink opens its mouth and shows the tongue as a bluff warning. All blue-tongued skinks give birth to live young: the eggs are kept and hatch inside the mother, with litters ranging from about 1–4 young in some species to 5–24 in others. (Wikipedia: blue-tongued skink.)

The common basilisk (Basiliscus basiliscus) runs across water on its hind legs. Its hind feet are large, and its toes carry fringes of scales that fold flat on land and spread on water, increasing the foot's area. Each step has three phases: a slap that pushes water down and traps an air pocket around the foot, a stroke that pushes the lizard forward, and a recovery in which the foot is pulled out before the air pocket collapses. Young, light basilisks can run much farther on water than adults. (Wikipedia: common basilisk.)

Flying lizards of the genus Draco glide between trees on a membrane (patagium) along each side of the body, supported by five to seven greatly lengthened thoracic ribs. The ribs are spread by specialised iliocostalis and intercostal muscles. Field observations of Draco dussumieri showed that in flight the lizard grabs the front edge of the membrane with its hands, forming a composite wing; the arms then steer, and are free again for climbing after landing. (Dehling (2017): how lizards fly, PLoS ONE.)

Horned lizards (Phrynosoma) have flattened, round, toad-like bodies and a crown of true horns, which have a bony core, unlike the spiny scales on their back and sides. Several species can squirt blood from the corners of the eyes: they restrict blood flow leaving the head, the pressure rises, and small vessels around the eyelids burst, spraying blood up to about 1.5 m. The blood tastes foul to dogs, coyotes and cats but has no effect on birds. At least eight species are known to do it. (Wikipedia: horned lizard.)

Many horned lizards eat mainly harvester ants, which bite and sting. The Texas horned lizard has a factor in its blood plasma that neutralises the ants' venom, catches each ant with a flick of the tongue, and coats it in large amounts of mucus in the throat and oesophagus, so swallowed ants cannot sting. Like other iguanians, horned lizards catch food with a sticky tongue rather than with the jaws. (Fertschai et al. (2021): prey capture in the Texas horned lizard, Biology Open.)

Legless lizards look like snakes but keep lizard anatomy. Glass lizards (Ophisaurus) of North America, such as the eastern glass lizard, have no legs, yet they have movable eyelids, external ear openings, a groove along each side of the body, rigid jaws and a very long tail that breaks easily, hence the name "glass snake". The eastern glass lizard grows to 46–108 cm, but its head and body are at most about 30 cm; the rest is tail.

Snakes are highly modified lizards, but the two groups can be told apart by several features. These work even for legless lizards.

For the snake side of the comparison, see the snake anatomy and snake skeleton pages.

The tuatara (Sphenodon punctatus) of New Zealand looks like a large lizard, but it is the only living member of a separate order, Rhynchocephalia. Its closest living relatives are the squamates (lizards and snakes), and it has kept many features that lizards lost. In 1867 Albert Günther noted these features and proposed the separate order. (Wikipedia: tuatara.)

Anatomical differences students can look for: the tuatara has a complete lower temporal bar, so its skull is rigid and its quadrate cannot swing; its teeth are acrodont, with a second row on the palate so the lower teeth fit between two upper rows; it has no ear opening or eardrum; its ribs carry hooked uncinate processes and it has well-developed belly ribs (gastralia); and the male has no copulatory organ, whereas male lizards have paired hemipenes. Its parietal eye is the most developed of any living animal, but it is only visible in hatchlings. (Wikipedia: tuatara.)

A comparison often set in class: frogs are amphibians and lizards are reptiles. A lizard has dry, scaly skin, breathes only with lungs, lays shelled eggs on land (or bears live young), has claws, a long tail and a single middle-ear bone behind a recessed eardrum. A frog has moist, glandular skin that also exchanges gases, lays jelly-coated eggs in water that hatch into tadpoles, has no claws or tail as an adult, and pushes air into its lungs by throat pumping. Both have a three-chambered heart and a cloaca.

Use the 3D model and the diagrams above. Answers are in brackets.

1. Which external features show that a green anole is male? (Large dewlap, enlarged postcloacal scales, wide tail base.)
2. What supports the dewlap? (The second ceratobranchials of the hyoid apparatus.)
3. How many chambers does the lizard heart have, and what are the three parts of the ventricle? (Three; cavum arteriosum, cavum venosum, cavum pulmonale.)
4. Where does a lizard tail break during autotomy? (At fracture planes across the caudal vertebrae.)
5. What supports a regenerated tail? (A cartilage tube instead of vertebrae.)
6. Name two features that separate a glass lizard from a snake. (Movable eyelids, ear openings, rigid jaw, long breakable tail.)
7. What does the parietal eye detect? (Light levels, not images.)
8. Why do lizards produce white droppings? (Nitrogen is excreted as uric acid.)