Morphology & Anatomy of Frog (at a glance)


  • ovoid, slightly flattened body, generally 10-15 cm long, bilateral symmetry.
  • skin is naked, smooth, moist and slippery (due to mucus secreted by the cutaneous glands present in skin).
  • dorsal surface of skin is generally olive green and pigmented while ventral surface is uniformly pale yellow.
  • neck and tail are absent
  • body = head + trunk
  • Head = mouth + external nares + eyes + brow spot + ear-drums + throat
  • mouth = located below the snout (blunt apex, helps in digging the earth), extends from one ear drum to the other, upper jaw is armed with small, conical teeth while the lower jaw is toothless.
  • external nares = pair of small apertures, the nostrils or external nares, helps in breathing, situated dorsally at the tip of the snout.
  • eyes = pair of eyes one on either side of the median line, almost on the top of the head, position of the eyes enables to see in all directions, round and protected by three eye-lids (upper + lower + third), third eyelid is also called nictitating membrane. Nictitating membrane is thin transparent and freely movable eyelid that protect the eyes under water and on dry lands.
  • brow spot = represents a reduced third eye, sensitive to light of the lower wavelength, producing colour changes in the skin.
  • ear-drums = also called tympanic membrane or tympanum or ear-drum, receives sound waves.
  • Trunk = forelimbs + hindlimbs
  • forelimbs = brachium (upper arm) + antebrachium (fore arm) + manus (hand)
  • hands = wrist (carpal) + palm (metacarpal) + four, short, tapering fingers (digits)
  • the first digit (thumb or pollex) is rudimentary and invisible externally
  • nuptial or amplexusory or copulatory pads = present in males only at inner finger of each, become specially enlarged during the breeding season.
  • hind limbs = thigh + shank (crus) + foot (pes)
  • foot = ankle (tarsal) + instep (metatarsal) + five, long toes (digits)
  • toes are joined together by thin folds of skin called webs
  • Sexual dimorphism = separate sexes
  • male possess vocal sacs placed ventrolaterally on either side of the head behind the mouth, used to produce croaking sounds to call the females.


  • skin (naked) = epidermis + dermis
  • epidermis = outer thin, derived from ectoderm, keratinized, stratified squamous epithelium.
  • dermis = inner thick, developed from mesoderm, contain sac-like mucous glands and chromatophore.
  • true coelom = pericardial cavity (around the heart & very small) + pleuroperitoneal cavity (very large)
  • skeleton = axial skeleton (skull + vertebral column) + appendicular skeleton (limb-bones + girdles)

Digestive System

mouth –> buccopharyngeal cavity –> pharynx –> oesophagus –> stomach –> intestine (duodenum –> ileum –> rectum –> cloaca).

  • buccopharyngeal cavity contains numerous maxillary teeth arranged along the margin of the upper jaw and vomerine teeth, lower jaw is toothless
  • salivary glands are absent
  • muscular tongue = bilobed at the tip and free from behind, used to capture the prey.
  • small intestine = long coiled ileum + short straight duodenum
  • digestive glands = gastric glands + intestinal glands + liver + pancreas + gall bladder
  • gastric glands + intestinal glands = present in the walls of stomach and small intestine respectively, produce gastric juices
  • gastric juice = thin, strongly acidic (pH varying from 1 to 3), almost colorless liquid, essential constituents are the digestive enzymes pepsin, hydrochloric acid, and mucus.
  • gall bladder = stores bile secreted by liver, bile emulsifies fats, changes pH of food from acidic to alkaline and checks growth of bacteria.
  • pancreas = secretes pancreatic juice containing digestive enzymes (trypsin, amylase, lipases etc)
  • villi + microvilli = digested food is absorbed by the numerous finger-like folds in the inner wall of intestines called villi and microvilli.

Respiratory System

cutaneous + buccopharyngeal + pulmonary

  • cutaneous respiration = occurs through naked skin, takes place in water as well as on land. During hibernation and aestivation frogs respires through this method only.
  • buccopharyngeal respiration = occurs only on land through moist lining of buccopharyngeal cavity.
  • pulmonary respiration = lungs are a pair of oval, pinkish, sacs located in the thorax, pulmonary respiration has a maximum frequency of 20/minutes. Occurs only when more energy is required.

Nervous System

  • Central nervous system + Peripheral nervous system + Autonomic nervous system.
  • CNS = brain + spinal cord.
  • Brain is contained in a bony structure known as brain box or cranium and protects it from the external shocks. Brain of frog consists of a pair of occipital condyles.
  • Brain = Fore-brain + Mid-brain + Hind-brain
  • Fore-brain = olfactory lobes + a pair of cerebral hemispheres + unpaired diencephalon.
  • Cerebrum = response to environment
  • Olfactory lobe = controls the sense of smell.
  • diencephalon = relays sensory information between brain regions and controls many autonomic functions of the peripheral nervous system.
  • Mid-brain = consists of a pair of optic lobes, involved in the frog’s vision.
  • Hind-brain = cerebellum + medulla oblongata
  • cerebellum = helps to maintain the balance and equilibrium of the frog, also controls the muscular coordination and posture.
  • medulla oblongata = passes out via the foramen magnum and continues into the spinal cord of the frog up to the tip of its trunk, which is contained in the vertebral column, helps in the regulation of respiration, digestion, and other automatic functions.
  • Peripheral nervous system = cranial + spinal nerves
  • Ten pair of cranial nerves are present in the frog (as compared to twelve pairs present in human) and it arises from the brain and innervate to the different parts of body. Cranial nerves are involved in passing the information from outside to the brain.
  • Frog contains 10 pairs of spinal nerves (as compared to 30 pairs present in humans) that rise from the spinal cord and gets distributed to the different parts of body. Spinal nerves of frog appear as white in color and are thread-like structures that emerge between the vertebrae and are located along the dorsal wall of body cavity. Spinal nerves functions in passing information from the extremities to brain through spinal cord.
  • Autonomic nervous system = the part of the nervous system responsible for control of the bodily functions not consciously directed, such as breathing, the heartbeat, and digestive processes.


By Shayan Ghani

The author is a co-founder and director at 'FLAIR - Innovative Classes for Maths and Science'. He is a post-graduate in Biotechnology from Patna University and currently living in Bihar (India).

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