BiologyISC Class 11 · Structural Organisation in Animals: Frog

The Frog: External Morphology, Digestive System and Respiration

Updated 17 October 2026 · Fact-checked

The common frog (Rana tigrina) has a moist, smooth skin that changes colour, a head and trunk without a neck or tail, and webbed hind limbs. It is carnivorous with a short alimentary canal, and respires through the skin (cutaneous), the lining of the mouth (buccopharyngeal) and the lungs (pulmonary).

Follow the arrows from mouth to cloaca, and notice that the liver (bile) and pancreas (enzymes) both pour their juices into the intestine.

Illustration drawn for QuestionBuddy.

Understand Frog: Morphology, Digestion and Respiration

External features. Body divided into head and trunk (no neck, no tail in the adult). The skin is smooth, moist and slippery with mucus; it is olive green with dark irregular spots on the dorsal side and pale yellow on the ventral side. It does not drink water but absorbs it through the skin. Frogs change colour to match their surroundings (camouflage or mimicry). They live in water and on land; in summer they go into aestivation (summer sleep) and in winter into hibernation (winter sleep). The head bears a pair of nostrils, bulging eyes with a transparent nictitating membrane that protects them in water, and a tympanum (ear drum) that receives sound signals. Forelimbs have four digits and hind limbs five webbed digits for swimming. *Sexual dimorphism:* males have vocal sacs and a copulatory pad on the first digit of the forelimbs, absent in females.

Digestive system. The alimentary canal is short because frogs are carnivores (they eat insects). Mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca. The bifid tongue is attached at the front and can be flicked to catch prey; the jaws have maxillary and vomerine teeth. The liver secretes bile, stored in the gall bladder; the pancreas secretes pancreatic juice (enzymes). Food is mixed with gastric juice (HCl and pepsin) in the stomach to form chyme; the chyme passes to the duodenum, where bile and pancreatic juice act, forming chyle; digestion is completed in the intestine and nutrients are absorbed through villi and microvilli of the intestinal wall. Undigested waste passes through the rectum and is expelled via the cloaca.

Respiration. *Aquatic:* through the skin (cutaneous respiration), where dissolved oxygen diffuses across the moist skin; gas exchange in water happens only by this route. *On land:* through the buccal cavity lining (buccopharyngeal) and the lungs (pulmonary). Lungs are a pair of elongated, pink, sac-like structures in the upper trunk (thorax); air enters through the nostrils into the buccal cavity and then to the lungs. During hibernation the skin is the main respiratory organ. Sound is produced by the passage of air from lungs through the laryngo-tracheal chamber with the help of the vocal sacs.

Frog: Morphology, Digestion and Respiration: key facts

Digestive tract: Mouth
Buccal cavity
Pharynx
Oesophagus
Stomach
Intestine
Rectum
Cloaca; Short because the frog is a carnivore.
Modes of respiration: $C u t an eo u s (w a t er) ∣ B u cco p ha r y n g e a l (l an d) ∣ P u l m o na r y (l an d)$; The skin is the main organ in hibernation.
Sexual dimorphism: Male: vocal sacs + copulatory pad | Female: absent; Copulatory pad on the first digit of the forelimb.
Adaptations: Aestivation summer
Hibernation winter
Camouflage colour change; Survival under extreme temperatures.

Aestivation	summer
Hibernation	winter
Camouflage	colour change

Frog: Morphology, Digestion and Respiration in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

A frog kept in a dry box

Suppose a student keeps a frog in a dry box and finds that it becomes sluggish, while a frog kept in a damp container remains active.

Explain the observation using the physiology of the skin and respiration.

Try it first, then show the solutionSolution

The skin of a frog is moist and glandular; oxygen dissolves in the moisture and diffuses into the blood vessels (cutaneous respiration).
In a dry box the skin dries out and gas exchange through it falls.
The frog then relies more on its lungs, which are not enough to meet its needs, and it loses water through the skin.

Answer: A dry skin cannot support cutaneous respiration or water balance.

Memory hook: A frog needs a moist skin to breathe through it.

Tricks to remember Frog: Morphology, Digestion and Respiration

Short memory aids. Say each one aloud once, then use it on a question the same day.

Short gut for meat
Carnivores have a short alimentary canal; herbivores have a long one.
Use it when: Why the frog has a short intestine.
Vocal sacs: male, pad: male
Both sexual dimorphism features are in the male.
Use it when: Sexual dimorphism questions.

Frog: Morphology, Digestion and Respiration: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Respiration in water · 1 mark · MCQBoard level

In water, a frog respires mainly through:
(a) lungs
(b) buccal cavity
(c) skin
(d) gills

Try it first, then show the solutionSolution

Cutaneous respiration through the moist skin is the aquatic mode.

Answer: (c) skin

Example 2: Sexual dimorphism · 2 marks · Short answerBoard level

How can a male frog be distinguished from a female frog externally? Give two points.

Try it first, then show the solutionSolution

Male frogs have vocal sacs, used for croaking. (1 mark)
Male frogs have a copulatory pad on the first digit of the forelimbs; females do not. (1 mark)

Answer: Vocal sacs and copulatory pads in males.

Example 3: Digestive system · 4 marks · Short answerHarder board level

Describe the path of food in the alimentary canal of a frog and the role of the liver and pancreas.

Try it first, then show the solutionSolution

Food is caught by the bifid tongue and passes through mouth, buccal cavity, pharynx and oesophagus into the stomach. (1 mark)
In the stomach the gastric juice (HCl and pepsin) forms chyme. (1 mark)
In the duodenum bile (from the liver, stored in the gall bladder) and pancreatic juice complete digestion; the nutrients are absorbed in the intestine. (1.5 marks)
Wastes pass through the rectum and are expelled through the cloaca. (0.5 mark)

Answer: Mouth to cloaca with bile and pancreatic juice acting in the duodenum.

Example 4: Modes of respiration · 4 marks · Short answerHarder board level

Name and describe the three modes of respiration in a frog and state which is used (i) in water, (ii) during hibernation and (iii) when the frog is on land.

Try it first, then show the solutionSolution

Cutaneous respiration: gas exchange through the moist skin. (1 mark)
Buccopharyngeal respiration: gas exchange across the lining of the mouth and pharynx. (1 mark)
Pulmonary respiration: air is taken in through the nostrils and passes to the lungs. (1 mark)
(i) Water: cutaneous; (ii) hibernation: cutaneous; (iii) land: buccopharyngeal and pulmonary. (1 mark)

Answer: Cutaneous, buccopharyngeal and pulmonary; cutaneous in water and in hibernation.

Example 5: Adaptations of the frog · 5 marks · Long answerHOTS / competency

Explain how each is an adaptation in the frog: (i) webbed hind limbs, (ii) nictitating membrane, (iii) colour change, (iv) aestivation and hibernation, (v) short alimentary canal.

Try it first, then show the solutionSolution

Webbed digits enlarge the surface for swimming. (1 mark)
The transparent nictitating membrane protects the eyes in water. (1 mark)
Colour change helps camouflage from predators. (1 mark)
Aestivation (summer) and hibernation (winter) help the frog survive extremes of heat and cold by remaining inactive. (1 mark)
The short gut suits a carnivorous diet. (1 mark)

Answer: Swimming; eye protection; camouflage; survival in extremes; carnivorous diet.

Where students lose marks in Frog: Morphology, Digestion and Respiration

The slips examiners see most often, and what to write instead.

Mistake: Writing gills for the adult frog
Using gills as the adult organ of respiration.
Do this: Tadpoles breathe with gills; adults use skin, buccal cavity and lungs.
Usually costs: 1 mark.
Mistake: Mixing the digestive juices
Giving bile for the stomach.
Do this: Gastric juice acts in the stomach; bile and pancreatic juice act in the duodenum.
Usually costs: 1 mark.
Mistake: Giving the copulatory pad to females
Reversing sexual dimorphism.
Do this: Vocal sacs and copulatory pads belong to males.
Usually costs: 1 mark.

Exam tips

Draw a labelled diagram of the digestive system of a frog.
Mention both the organs and the secretions.
Link every adaptation to the habitat.
Keep the three modes of respiration and where they are used.

Practice questions on Frog: Morphology, Digestion and Respiration

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The frog's heart is three-chambered. Which of the following is also a feature of the frog?
(a) a long alimentary canal
(b) cloaca
(c) fully external ear
(d) scales on the skin
Hint
The cloaca is the common exit.
Answer
(b) cloaca
Q2 · 2 marks · Short answerBoard level
What is the function of the nictitating membrane of the frog?
Hint
Think of the aquatic habit.
Answer
It is a transparent membrane that covers the eye in water, protecting it without blocking sight.
Q3 · 3 marks · Short answerHarder board level
Why does a frog have a bifid tongue?
Hint
Think about how prey is captured.
Answer
The tongue is attached at the front and free at the back with a forked tip. It can be flicked out rapidly to catch insects on its sticky surface and bring them to the mouth.
Q4 · 3 marks · Short answerHarder board level
Name the digestive glands of the frog, the juices they secrete and the site of their action.
Hint
Stomach and duodenum.
Answer
Gastric glands secrete gastric juice (HCl and pepsin) acting in the stomach; the liver secretes bile, stored in the gall bladder, and the pancreas secretes pancreatic juice, both acting in the duodenum.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): A frog does not drink water. Reason (R): It absorbs water through its skin. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Think about the permeability of the skin.
Answer
(a) Both are true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a student observes a frog in a pond and in a garden.
(i) State the main mode of respiration in the pond. (1 mark)
(ii) State the organ for respiration in the garden. (1 mark)
(iii) Name the structure that receives sound. (1 mark)
(iv) State the use of the vocal sacs. (1 mark)
Hint
Recall the modes of respiration.
Answer
(i) Cutaneous (skin). (ii) Lungs and buccal cavity. (iii) Tympanum. (iv) To amplify the croaking sound of the male.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Common frog of India?Show answer
Rana tigrina.
2.Mode of respiration in water?Show answer
Cutaneous.
3.Frog's tongue?Show answer
Bifid and sticky.
4.Exit of the alimentary canal?Show answer
Cloaca.

Frog: Morphology, Digestion and Respiration: frequently asked questions

Why is the frog called an amphibian?

It can live both in water and on land; amphibian means "double life".

Why does the frog have a short intestine?

Because it is carnivorous, and its diet of insects is easily digested.

What is aestivation?

A period of inactivity in the heat of summer to avoid heat and dryness.