Frog Anatomy: Labeled Diagrams and Interactive 3D Model

A frog is an amphibian with a short, tailless body, long hind legs, moist skin and big eyes on top of its head. Its body has two regions, the head and the trunk; frogs have no neck and, as adults, no tail. Inside, it has the same organ systems as a human, a brain, heart, lungs, liver, stomach, intestines, kidneys and gonads, but with a three-chambered heart, no ribs and no diaphragm, and a single opening, the cloaca, for waste and reproduction.

This page uses the northern leopard frog, Lithobates pipiens (formerly Rana pipiens), as the reference frog. It is a medium-sized frog of ponds and meadows across much of northern North America, and the standard frog in North American biology labs. The other common lab frog, the American bullfrog (Lithobates catesbeianus), has the same internal anatomy; the differences are listed further down.

Directions: anterior means toward the snout, posterior toward the cloaca, dorsal the back and ventral the belly. Left and right always mean the frog's own left and right, so when a frog lies on its back in a dissecting tray, its left side is on your right.

The external anatomy of a frog includes the snout with two nostrils (external nares), two large eyes with eyelids and a nictitating membrane, a round eardrum (tympanum) behind each eye, a wide mouth, smooth moist skin, two short forelimbs with four fingers, two long hind limbs with five webbed toes, and the cloacal opening at the back.

Three features identify a leopard frog: rows of rounded dark spots with pale borders, a pale dorsolateral fold running from behind each eye to the hip, and a pale stripe along the upper lip. The sharp hump in the middle of the back of a sitting frog is not a vertebra sticking up; it is the joint between the hip bones (ilia) and the sacral vertebra.

Inside a frog, the first thing you see is the large, three-lobed, reddish-brown liver, with the heart in front of it and the two spongy lungs on either side. Under the liver are the stomach on the frog's left, the coiled small intestine, the yellow fat bodies, and at the back the large intestine, urinary bladder and cloaca. The kidneys lie flat against the back wall, and the gonads sit on or beside them.

For a step-by-step guide to finding these organs in a real specimen, with a worksheet and answer key, see our frog dissection guide.

This is the text version of the 3D model: every labeled structure of the leopard frog, grouped by body system, with what it does. Use it as a study list or to check a frog anatomy labeling worksheet.

A frog's mouth contains teeth on the upper jaw only, two patches of vomerine teeth on the roof, two internal nares that connect to the nostrils, two Eustachian tube openings that lead to the ears, a tongue attached at the front, the slit-like glottis that leads to the lungs, and the wide opening of the gullet. Males also have two vocal sac openings.

• Maxillary teeth: Grip slippery prey so it cannot escape. Frogs do not chew; they swallow food whole.
• Vomerine teeth: Hold struggling prey against the roof of the mouth while it is swallowed.
• Internal nares: Carry air between the nostrils and the mouth cavity for breathing.
• Eustachian tube openings: Connect the middle ear with the mouth and equalise air pressure on both sides of the eardrum.
• Tongue: Flips out over the lower jaw to catch insects and other small prey, then flips back to bring the prey into the mouth.
• Glottis: Opens to let air pass between the mouth and lungs, and closes to hold air in the lungs.
• Vocal sac openings: Let air pass from the mouth into the vocal sacs when the frog calls.

The tongue is the most famous part. Because it is attached at the front and folded back, it can flip out over the lower jaw like a catapult, hit an insect, and fold back in. The research section below explains why it is so sticky and how the frog uses its eyes to swallow.

Food passes from the mouth into the short esophagus, then the J-shaped stomach, through the pyloric sphincter into the duodenum and the coiled ileum (small intestine), then into the wide large intestine and out through the cloaca. The liver, gallbladder and pancreas add digestive juices to the duodenum.

• Esophagus: Carries whole prey from the mouth to the stomach.
• Stomach: Stores swallowed prey and starts digestion with acid and the enzyme pepsin.
• Pyloric sphincter: Controls the passage of food from the stomach into the duodenum.
• Duodenum: Receives bile and pancreatic juice and continues digestion.
• Ileum: Finishes digestion and absorbs nutrients into the blood.
• Mesentery: Suspends the gut and carries its blood vessels and nerves.
• Large intestine: Absorbs water and stores feces.
• Cloaca: Collects feces, urine, and eggs or sperm before they leave the body.
• Liver: Makes bile, stores glycogen and fat, and processes nutrients absorbed by the gut.
• Gallbladder: Stores bile from the liver and releases it into the duodenum through the bile duct.
• Pancreas: Makes digestive enzymes for the duodenum and the hormone insulin.
• Spleen: Part of the lymphatic and circulatory systems, not digestion: it filters blood, stores blood cells and makes white blood cells. It is listed here because it lies among the intestines.

Adult frogs are carnivores that swallow prey whole, so their gut is short compared with that of a plant-eating tadpole. The spleen is listed here because it sits in the mesentery among the intestines, but it belongs to the circulatory and immune systems.

The frog heart has three chambers: a right atrium, a left atrium and one ventricle. Blood from the body enters the right atrium through the thin sinus venosus; blood from the lungs enters the left atrium through the pulmonary veins. The ventricle pumps both out through the conus arteriosus into three pairs of arches: carotid (head), systemic (body) and pulmocutaneous (lungs and skin).

This is a double circulation, like ours, because blood passes through the heart twice per lap, once on the way to the lungs and skin and once on the way to the body. Unlike ours, it is incompletely divided: with one ventricle, some oxygen-rich and oxygen-poor blood mixes.

The frog skeleton is light and built for jumping. A leopard frog has nine vertebrae (eight presacral and one sacral), a rod-shaped urostyle instead of a tail, no ribs, very long hip bones (ilia), a single forearm bone (radio-ulna), a single shin bone (tibiofibula) and two long ankle bones (astragalus and calcaneum) that add an extra segment to the leg.

• Cranium (braincase): Protects the brain and inner ears. The sphenethmoid forms its front, the prootics hold the inner ears, and the exoccipitals form two condyles that join the first vertebra.
• Frontoparietals: Roof over and protect the brain.
• Upper jaw: Carries the maxillary teeth and forms the outer frame of the skull.
• Presacral vertebrae: Protect the spinal cord and form a short, stiff column that transmits the push of the hind legs to the body.
• Sacral vertebra: Connects the backbone to the pelvic girdle so jumping forces pass from the legs to the body.
• Urostyle: Stiffens the back of the body and helps transmit thrust from the hind limbs to the spine during jumping and swimming.
• Pectoral girdle: Anchors the forelimbs and absorbs landing forces. It also protects the heart.
• Radio-ulna: Supports the forearm; a single bone is stronger for absorbing the shock of landing.
• Ilium: Connects the hind limbs to the backbone and acts as a lever in jumping.
• Femur: Supports the thigh and anchors the big jumping muscles.
• Tibiofibula: Supports the shank and anchors the calf and shin muscles.
• Astragalus and calcaneum: Give the hind leg an extra long segment, so the foot works as a long lever for jumping and swimming.

Most of a frog's muscle is in its hind legs. The thigh holds the large triceps femoris on the front, which straightens the knee, and the sartorius, adductor magnus, gracilis, semimembranosus and iliofibularis on the underside and back. The calf is the spindle-shaped gastrocnemius, which ends in the Achilles tendon and gives the final push of a jump.

• Triceps femoris: Straightens the knee: the main muscle that powers a jump.
• Sartorius: Flexes the knee and pulls the thigh forward.
• Adductor magnus: Pulls the thigh toward the midline (adduction).
• Gracilis major and minor: Bend the knee and pull the thigh back and in.
• Semimembranosus: Bends the knee and extends the hip.
• Iliofibularis: Bends the knee.
• Gastrocnemius: Extends the ankle (points the foot) for the final push of a jump; also bends the knee.
• Achilles tendon: Transmits the pull of the calf muscle to the foot.
• Tibialis anticus longus: Flexes the ankle and straightens the knee.
• Peroneus: Extends the ankle and helps rotate the foot.

The body wall has a rectus abdominis down the belly with the white linea alba in the middle, external obliques on the sides, a fan-shaped pectoralis on the chest and a thin mylohyoid under the throat, which raises the floor of the mouth for breathing. The sciatic nerve runs down the back of the thigh between the iliofibularis and semimembranosus.

The frog brain has six main parts from front to back: the olfactory lobes (smell), two cerebral hemispheres, the diencephalon with the pineal and pituitary, two large optic lobes (vision), a narrow cerebellum (balance and coordination) and the medulla oblongata, which joins the spinal cord. Frogs have ten pairs of cranial nerves and ten pairs of spinal nerves.

• Olfactory lobes: Process smell signals from the nose.
• Cerebral hemispheres: Process smell and control behaviour such as feeding and mating; much simpler than the folded human cerebrum.
• Diencephalon: Relays signals and controls hormones. The pituitary controls growth and breeding; the pineal responds to light.
• Optic lobes: Process vision. They control the fast strike at moving prey.
• Cerebellum: Coordinates movement and balance.
• Medulla oblongata: Controls breathing, heart rate and other automatic functions. Most cranial nerves leave from here.
• Spinal cord: Carries signals between the brain and body and controls reflexes such as the jump.
• Sciatic plexus: Combines nerve fibres that then run into the hind limb as the sciatic nerve.

Frog eyes sit high on the head so the frog can watch above the water while hiding below it. Each eye has a thick upper lid that hardly moves and a thin, movable lower lid whose transparent part, the nictitating membrane, sweeps up to clean and protect the eye. The pupil is horizontal in leopard frogs and bullfrogs. With no bony floor under the eye, the eyeball can be pulled down into the mouth.

The ear has three parts. The tympanum on the outside vibrates with sound; a rod-shaped bone, the columella, carries the vibration across the middle ear; and the inner ear in the prootic bone senses sound and balance. The middle ear is connected to the mouth by the Eustachian tube, just as in humans.

You can sex a mature leopard frog from the outside: males have a swollen, dark nuptial pad on each thumb and paired vocal sacs, and females are usually larger. Inside, males have two small yellowish testes on the kidneys; females have two ovaries full of eggs and two long, coiled oviducts. Fertilization is external: the male clasps the female (amplexus) and releases sperm over the eggs as she lays them in water.

A female leopard frog lays one clump of 300 to 6,500 eggs per year, usually in spring. Switch the 3D model to Male or Female to compare the two sets of organs; the yellow fat bodies, which fuel egg and sperm production, are present in both.

Frogs and humans share the same basic vertebrate plan: a skull and backbone, a brain and spinal cord, a heart and blood vessels, lungs, a liver, stomach, intestines, pancreas, kidneys and gonads. That is why frog dissection is used to teach human biology. The differences show how frogs are adapted to life in water and on land.

All true frogs share the anatomy on this page. These are the main differences between the frogs students usually meet, and between frogs and toads.

Every adult frog and toad shares the same basic body plan, and almost all of it is about jumping. Compared with a salamander or a lizard, a frog has lost its tail, has a very short backbone, very long hind limbs, and a pelvis that is tilted backward and joined to a rod-shaped bone, the urostyle, which lies between the two long hip bones (ilia). Researchers call this combination the anuran bauplan. (Senevirathne et al. (2020): ontogeny of the anuran urostyle, PNAS.)

The urostyle is the most unusual piece. In other animals, fused tail bones form structures such as the human coccyx or the bird pygostyle. The frog urostyle is different because it also contains an ossified hypochord, a rod of tissue that runs under the notochord in the embryo and turns into bone at the end of metamorphosis. The urostyle helps pass the thrust of the hind legs into the backbone, and it has hardly changed since frogs first appear in the fossil record in the Early Jurassic, about 190 million years ago. (Senevirathne et al. (2020): ontogeny of the anuran urostyle, PNAS.)

You can see the same plan in a lab specimen. The leopard frog's backbone has just nine vertebrae in front of the urostyle, the hind leg is longer than the head and body together, and each leg segment is a lever: thigh, shank, a long ankle made of two rod-like bones, and the foot. Each extra segment adds reach, which is why frogs can leap many times their own length.

A frog's tongue is attached at the front of the mouth and folded back, so it can flip forward over the lower jaw. It is unusually soft, and the saliva on it behaves in a surprising way. High-speed films and material tests showed that the saliva gets thinner when the tongue slams into an insect, spreading into every gap, and then thickens to grip as the tongue pulls back. Together, the soft tongue and this reversible saliva gave about 50 times the work of adhesion of a sticky-hand toy. (Noel et al. (2017): frogs use a viscoelastic tongue and non-Newtonian saliva, Journal of the Royal Society Interface.)

Swallowing is helped by the eyes. Frogs have no bony floor under the eye sockets, so muscles can pull the eyeballs down into the roof of the mouth. X-ray films of northern leopard frogs eating crickets showed the eyes pressing on the prey and pushing it toward the gullet. When the eye-retracting muscles were denervated, the frogs could still swallow, but they needed an average of 4.0 swallows per cricket instead of 2.3. (Levine, Monroy & Brainerd (2004): eye retraction and swallowing in Rana pipiens, Journal of Experimental Biology.)

An old idea held that the same eye movements also pump air into the lungs. X-ray motion studies of bullfrogs did not support this: the eyes moved inward during prey transport, not in time with breathing. So when you see a frog blink while it eats, it is swallowing, not breathing. (Witzmann, Brainerd & Konow (2019): eye movements in frogs and salamanders, Integrative Organismal Biology.)

To find the tongue, teeth and openings in a real specimen, follow step 3 of the frog dissection.

Frogs have lungs, but no ribs and no diaphragm, so they cannot suck air in the way mammals do. Instead they use a buccal pump, an ancient method also used by air-breathing fish: they lower the floor of the mouth to draw air in, then raise it to push air into the lungs. This buccal pump is retained in frogs and toads, while reptiles, birds and mammals switched to aspiration breathing with the ribs or diaphragm. (Li & Wang (2021): hindbrain circuits for feeding and breathing across vertebrates, eNeuro.)

Two valves control the pump: the nostrils, which open and close the mouth to the outside air, and the glottis, which opens and closes the airway to the lungs. Between breaths, a frog makes long series of small throat movements with the glottis closed, which only refresh the air in the mouth. A recording study of cane toads found that the nostrils may not shut completely during a breath but instead increase their resistance. (Coelho, Baker & Smatresk (2003): ventilatory behaviors of the toad Bufo marinus, Brazilian Journal of Biology.)

The skin is a second breathing organ. In green frogs (Rana clamitans), over 80% of the carbon dioxide released left through the skin at every temperature tested. Oxygen was different: at 5 °C the skin took up more oxygen than the lungs, but at 25 °C the lungs took up most of it. Cold, inactive frogs can therefore rely heavily on their skin, which is why frogs can overwinter under water. (Vinegar & Hutchison (1965): pulmonary and cutaneous gas exchange in the green frog, Copeia.)

The frog heart has three main chambers: two thin-walled atria and one thick ventricle with a spongy inner wall. A thin-walled sinus venosus on the dorsal side collects blood from the three venae cavae, and the conus arteriosus (also called the truncus arteriosus) carries blood out of the ventricle. A classic dissection guide describes a spiral valve running along the inside of the conus, and three arches leaving each side of the truncus: the carotid, systemic and pulmocutaneous arches. (Marshall & Gamble, The Frog: An Introduction to Anatomy, Histology and Embryology, 11th ed. (Internet Archive).)

Because both atria empty into one ventricle, oxygen-rich blood from the lungs and oxygen-poor blood from the body could mix. In practice they mix only partly: the spongy ventricle wall, the timing of filling and the spiral valve tend to send more oxygen-poor blood into the pulmocutaneous arches and more oxygen-rich blood into the carotid and systemic arches. How strongly the spiral valve separates the two streams is still debated, so treat textbook diagrams with perfectly separated arrows as a simplification.

This system has an advantage for an animal that breathes through its skin. Blood returning from the skin is already partly oxygenated, and it enters the right atrium with the body blood. A single ventricle that can send blood either to the lungs or to the body lets a frog adjust when it is under water and not using its lungs at all.

Northern leopard frogs are medium-sized, about 5 to 11 cm from snout to vent, with a pale dorsolateral fold running down each side of the back and rounded, pale-edged dark spots. Females are larger than males. During the breeding season, males have thickened thumb pads, and their paired vocal sacs inflate over the shoulders when they call. (Animal Diversity Web: Lithobates pipiens (northern leopard frog).)

In Illinois specimens the spots are mostly larger than the eye, there is usually a dark spot on the snout and the tympanum is unspotted. The male's vocal pouches show at the angles of the jaws when he calls, producing a long, rattling snore. (Illinois Natural History Survey: northern leopard frog.)

American bullfrogs are much bigger, about 9 to 15 cm snout to vent. Their fold curves around the back of the eardrum but does not run down the body. Males have a single vocal pouch under the lower jaw, a yellow throat and a particularly large tympanum. (Illinois Natural History Survey: American bullfrog.)

In male bullfrogs the tympanum is much larger than the eye; in females it is about the same size as the eye or smaller. Bullfrogs are native to eastern North America but have been introduced to the western United States, Mexico, southern Europe, South America and Asia. (Animal Diversity Web: Lithobates catesbeianus (American bullfrog).)

Poison dart frogs have the same internal anatomy as other frogs. What is special is their skin glands. Many species do not make their own toxins: they take alkaloids from the ants, termites and mites they eat, pass them unchanged through the gut, and store them in glands in the skin. Frogs raised in zoos on crickets and fruit flies are not poisonous, because those insects contain no alkaloids. (Smithsonian's National Zoo: can once-poisonous frogs get their toxins back?.)

Leopard frogs and bullfrogs also have granular skin glands, but their secretions are only mildly distasteful. Their main defenses are camouflage, a quick jump into water and, in some cases, inflating the lungs to look bigger.

Use the 3D model and diagrams to answer these. Answers are in the lab answer key on the dissection page.

1. How many fingers and how many toes does a frog have?
2. Name the openings in the roof and floor of the frog's mouth.
3. Which organ is the largest in the body cavity, and how many lobes does it have?
4. Name the three chambers of the frog heart.
5. Why does a frog need to pump air with its mouth to fill its lungs?
6. What are the urostyle and the radio-ulna?
7. Name two features that tell a male leopard frog from a female.
8. What three systems empty into the cloaca?

Full lab questions with an answer key are in the frog dissection guide.