Structural Organisation in Animals: Frog | ISC Class 11 Biology Notes

This note covers the frog's body organisation, habitat, external features, feeding and digestion, breathing, circulation, waste removal, nervous coordination, sense organs, reproduction and development.

How is the frog's body organised for life on land and in water?

What do morphology and anatomy mean?

Morphology is the study of externally visible form and features. Anatomy examines the structure of internal organs. In the frog, the shape of the limbs belongs to morphology, while the arrangement of the digestive organs belongs to anatomy.

A tissue is a group of similar cells, together with substances between them, performing a specific function. Tissues combine in particular patterns to form an organ. Organs working together through physical or chemical interactions form an organ system.

This organisation produces division of labour, meaning that different parts share the body's work. Digestion processes food; respiration exchanges gases; circulation transports materials; excretion removes metabolic wastes, meaning wastes from body processes. Nervous coordination controls activities, while reproduction produces offspring. These systems contribute to the functioning of the whole frog.

How does the frog respond to its surroundings?

Frogs live on land and in freshwater. They belong to the class Amphibia, an animal classification group within the larger group, or phylum, Chordata. The most common species of frog found in India is Rana tigrina, the Indian bullfrog.

A frog is a poikilotherm: its body temperature varies with the temperature of its surroundings. The alternative expression, cold-blooded animal, describes this variability; it does not mean that the blood remains at one low temperature.

Frogs can change their colour to hide from enemies. Such concealment is called camouflage. In extreme seasonal conditions, shelter in deep burrows protects them from heat and cold.

Aestivation means summer sleep, whereas hibernation means winter sleep. These terms distinguish the two seasonal conditions. During both, gaseous exchange takes place through the skin, linking the frog's response to its surroundings with the functioning of its respiratory surface.

Which external features distinguish the adult frog?

How are the skin and body regions arranged?

The adult body has a head and a trunk, the main body region behind the head. A neck and tail are absent. The dorsal, or back, surface is generally olive green with dark irregular spots; the ventral, or underside, is uniformly pale yellow.

The skin is smooth and slippery because of mucus, its slippery secretion. It is always maintained in a moist condition. The frog absorbs water through the skin and does not drink it. Skin therefore has roles beyond merely covering the body.

A pair of nostrils, the external nasal openings, lies above the mouth. The bulging eyes have a nictitating membrane, a protective covering that protects them in water. A tympanum, the membranous structure receiving sound signals, is visible beside each eye.

The drawing shows a green, spotted frog with a folded hind limb. Its labels identify the head, trunk, eye, fore limb and hind limb. Observe the body outline and the relative size of the limbs.

See Fig. 7.1 in your NCERT textbook

How do the limbs and sexes differ?

The forelimbs are the front limbs and the hind limbs are the rear limbs. They help in swimming, walking, leaping and burrowing. The hind limbs are larger and more muscular than the forelimbs. Digits are the finger-like or toe-like ends of the limbs.

Each forelimb ends in four digits, while each hind limb ends in five. The feet have webbed digits, with skin joining neighbouring digits, which help in swimming. Keep the digit counts attached to the correct pair of limbs.

Sexual dimorphism means visible differences between males and females. Male frogs have sound-producing vocal sacs and a copulatory pad, a specialised pad on the first digit of the forelimbs. Female frogs lack these features.

How does the digestive system capture and process food?

What is the route through the alimentary canal?

Frogs are carnivores, animals that eat other animals. Their alimentary canal, the passage through which food travels, is short, with a reduced intestinal length. The digestive system consists of this canal and digestive glands, organs that produce secretions used in processing food.

The mouth opens into the buccal cavity, or mouth cavity. Food passes through the pharynx, the passage leading towards the food pipe, into the oesophagus, a short tube opening into the stomach. The stomach continues into the intestine and then the rectum.

The rectum is the terminal part of the alimentary canal before the cloaca, a common chamber through which digestive and other wastes pass outside. The external opening of this chamber is the cloacal aperture. The chamber and its aperture are distinct structures.

What happens to a captured meal?

1. The muscular, bilobed tongue, meaning a tongue divided into two lobes at its tip, captures food. The food enters the mouth and travels towards the stomach through the oesophagus.
2. The stomach wall secretes hydrochloric acid and gastric juice, the digestive secretion of the stomach. Their action digests the food in the stomach.
3. The partly digested food, called chyme, passes into the duodenum, the first part of the small intestine. It receives bile and pancreatic juice through a common bile duct.
4. Bile, a secretion produced by the liver and stored in the gall bladder, emulsifies fat. Emulsification breaks fat into small droplets. Pancreatic juice digests carbohydrates and proteins.
5. Final digestion occurs in the intestine. Villi and microvilli, the numerous finger-like folds and projections on its inner surface, absorb the digested food.
6. Undigested solid material enters the rectum and leaves through the cloaca. This removes the material that remains after digestion and absorption.

The pancreas is the digestive gland producing pancreatic juice. This juice contains digestive enzymes, substances that speed up the chemical breakdown of food. The gall bladder stores bile; it is not the organ that produces bile.

The drawing labels the oesophagus, liver, gall bladder, stomach, intestine, rectum and cloaca. It also labels the heart and lung alongside the digestive organs. The cloacal aperture is labelled at the lower end.

See Fig. 7.2 in your NCERT textbook

The fat bodies are the finger-like bodies labelled near the internal organs in the drawing. Several systems appear in one view, helping locate structures in relation to each other. The heart and lungs shown alongside the digestive organs are not parts of the alimentary canal.

Note: Keep three events separate: digestion breaks down food, absorption takes digested food through the intestinal surface, and removal of undigested matter clears the solid residue through the cloaca.

How does the frog exchange gases in different conditions?

What is cutaneous respiration?

Gaseous exchange is the exchange of respiratory gases between an animal and its surroundings. In water, the frog uses its skin as the respiratory organ. Breathing through the skin is called cutaneous respiration. Oxygen dissolved in water passes across the skin by diffusion.

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Here it explains movement of dissolved oxygen across the respiratory surface. The skin is moist and richly supplied with blood vessels.

On land, the buccal cavity, skin and lungs act as respiratory organs. Consequently, the frog's use of lungs on land does not mean that its skin stops functioning as a respiratory surface. The same skin also absorbs water.

What is pulmonary respiration?

Pulmonary respiration is respiration through the lungs. The lungs form a pair of elongated, pink, sac-like structures in the upper trunk region, called the thorax. Air enters through the nostrils, passes into the buccal cavity and then reaches the lungs.

Distinguish the place where the frog is living from the organ used for gas exchange. Water is the surrounding medium during aquatic respiration; skin is the exchanging surface. On land, lungs are one of the participating organs.

During summer and winter sleep, gaseous exchange takes place through skin. Thus the account of breathing connects directly with aestivation and hibernation: the frog can remain sheltered while using the skin for respiratory exchange.

How do the heart, vessels and blood provide circulation?

What are the heart's chambers and connections?

The frog has a closed blood vascular system, meaning blood circulates within the heart and blood vessels. Its components are the heart, vessels and blood. The muscular heart lies in the upper body cavity and is enclosed by the pericardium, a covering membrane.

The heart has three chambers: two atria, the receiving chambers, and one ventricle, the chamber opening into the outgoing conus arteriosus. The singular of atria is atrium. Do not count the associated vessels or sacs as additional atrial or ventricular chambers.

The sinus venosus is a triangular structure joining the right atrium. It receives blood through major veins called vena cava. The ventricle opens into the conus arteriosus, a sac-like structure on the ventral side of the heart.

Arteries carry blood from the heart to body parts. Veins collect blood from body parts and return it to the heart. The pumping action of the muscular heart maintains circulation, carrying nutrients, gases and water to the required sites.

What are the portal systems?

The frog's portal systems are special venous connections between particular body regions and the liver or kidneys. The hepatic portal system connects the intestine with the liver. The renal portal system connects the lower parts of the body with the kidneys.

How do blood and lymph differ?

Blood consists of plasma, its fluid component, and blood cells. Red blood cells, also called erythrocytes, contain the red pigment haemoglobin. Frog red blood cells are nucleated, meaning each contains a nucleus, the cell structure containing its genetic material.

Blood also contains white blood cells, also called leucocytes, and platelets, blood components involved in clotting. Lymph is the fluid of the lymphatic system, which accompanies the blood vascular system.

Compared with blood, lymph lacks red blood cells and a few proteins. Keep this limited comparison precise: it does not say that lymph lacks every protein. Blood composition and blood movement are separate aspects of the circulatory system.

How does the excretory system remove nitrogenous wastes?

Which structures form the system?

Excretion is the removal of wastes produced by the body's activities. The frog's excretory system removes nitrogenous wastes, waste substances containing nitrogen. It consists of a pair of kidneys, two ureters, the urinary bladder and the cloaca.

The kidneys are compact, dark red, bean-like organs situated a little towards the rear of the body cavity, on either side of the vertebral column, or backbone. Each kidney contains several nephrons, also called uriniferous tubules, its structural and functional units.

A ureter is a duct emerging from a kidney. A duct is a passage carrying material from an organ. The thin-walled urinary bladder, a sac storing urine, the liquid carrying excretory waste, lies ventral to the rectum and opens into the cloaca.

A frog excretes urea, a nitrogen-containing waste substance. It is therefore called ureotelic, meaning that urea is its principal nitrogenous excretory product. Blood brings excretory wastes to the kidneys, where they are separated and excreted.

How do the male and female ducts differ?

In a male frog, each ureter also serves as a urinogenital duct, a passage shared by urinary and reproductive products. In a female, the ureters and oviducts, the ducts carrying eggs, open separately into the cloaca.

1. Blood transports excretory wastes to the kidneys. This connects the transport role of circulation with the waste-removal role of excretion.
2. The kidneys separate the excretory wastes. Their numerous nephrons are the structural and functional units involved in kidney activity.
3. The ureters provide ducts from the kidneys towards the cloaca. In males these ducts also serve the reproductive system.
4. Urine is discharged through the cloaca to the exterior. The urinary bladder also opens into this shared chamber.

Do not confuse excretion with the removal of undigested food. Urine carries excretory waste, whereas faecal matter is the undigested solid material discharged from the digestive tract. Both use the cloaca, but they arrive there from different systems.

How are nervous coordination and the sense organs organised?

What are the divisions of the nervous system?

Coordination brings the activities of different body parts into an organised relationship. In the frog it involves the neural, or nervous, system and endocrine glands, glands that produce chemical messengers called hormones. Hormones provide chemical coordination between organs.

The testes are male reproductive organs producing sperms, the male reproductive cells. The ovaries are female reproductive organs producing eggs. The prominent endocrine glands are the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads. Pancreatic islets are the endocrine cell groups in the pancreas; gonads are the reproductive organs, testes in males and ovaries in females.

Ten pairs of cranial nerves arise from the brain. The brain is protected inside the cranium, or bony brain box. The spinal cord is the continuation of the hindbrain within the vertebral column.

Which parts make up the brain?

The forebrain, the front division, contains olfactory lobes, paired cerebral hemispheres and an unpaired diencephalon. Olfactory means related to smell; the cerebral hemispheres are the paired cerebral regions, and the diencephalon is the unpaired forebrain region.

The midbrain, the middle division, has a pair of optic lobes, regions associated with vision. The hindbrain, the rear division, consists of the cerebellum and medulla oblongata, its two named components.

The medulla oblongata passes through the foramen magnum, an opening in the skull, and continues into the spinal cord. Keep the brain's three divisions distinct from the three divisions of the entire nervous system.

Which organs receive sensory information?

The eyes and internal ears are well-organised structures. The other listed sense organs are groups of cells around nerve endings. Each eye lies in an orbit, a socket in the skull, and is a simple eye, possessing one unit.

An external ear is absent, although the tympanum is visible externally. The ear serves both hearing and equilibrium, meaning balance. Therefore, absence of an external ear does not mean absence of hearing or of the internal ear.

How do reproduction and development occur in the frog?

How are the male reproductive organs connected?

The male has a pair of yellowish, oval testes, organs that produce male reproductive cells called sperms. They adhere to the upper parts of the kidneys through the mesorchium, a double fold of the peritoneum, the membrane lining the body cavity and covering internal organs.

The vasa efferentia are small ducts arising from the testes. They are 10 to 12 in number, enter the kidneys on their respective sides, and open into Bidder's canal, a collecting passage within the kidney that communicates with the urinogenital duct.

The urinogenital duct emerges from the kidney and opens into the cloaca. In males, the cloaca passes faecal matter, urine and sperms to the exterior. Its shared role explains why reproductive anatomy and excretory anatomy must be studied together.

The drawing shows paired testes near the upper ends of elongated kidneys. Labels identify vasa efferentia, fat bodies, kidney, adrenal gland, urinogenital duct, rectum, urinary bladder, cloaca and cloacal aperture.

See Fig. 7.3 in your NCERT textbook

How does the female arrangement differ?

The female has a pair of ovaries, organs producing eggs. Eggs are called ova; the singular is ovum. The ovaries lie near the kidneys but have no functional connection with them. A pair of oviducts opens separately into the cloaca.

This differs from the male route through the kidneys and urinogenital ducts. In the female, proximity of ovaries to kidneys does not imply a shared functional passage. The ureters and oviducts retain separate openings into the cloaca.

The drawing shows coiled oviducts beside the paired reproductive structures. The labels identify oviduct, ovary, ova, ureter, urinary bladder, cloaca and cloacal aperture. Compare these labels with the male diagram's urinogenital duct.

See Fig. 7.4 in your NCERT textbook

What happens after eggs are released?

A mature female can lay 2500 to 3000 ova at a time. Fertilisation, the fusion of male and female reproductive cells, is external: it takes place in water outside the female's body. Development includes a larval stage, an immature stage differing from the adult.

1. A mature female releases ova. The stated capacity is 2500 to 3000 ova at a time; this is a range, not a fixed number.
2. External fertilisation takes place in water, where male and female reproductive cells unite.
3. The developing eggs hatch into tadpoles, the larval stage of the frog. The developmental sequence therefore includes an intermediate larval form.
4. The tadpole undergoes metamorphosis, the transformation from larva to adult, and becomes a frog.

Frogs also have ecological importance. They eat insects and help protect crops, and they form an important link in food chains and food webs. A food chain represents feeding relationships; a food web connects multiple feeding relationships within a community.

Glossary

• Morphology — Study of the external form and visible features of an organism or its parts.
• Anatomy — Study of the structure and arrangement of internal organs in an animal.
• Poikilotherm — Animal whose body temperature varies with the temperature of its surrounding environment.
• Aestivation — Summer sleep during which frogs shelter from extreme heat in deep burrows.
• Hibernation — Winter sleep during which frogs shelter from extreme cold in deep burrows.
• Sexual dimorphism — Visible differences between males and females, such as the male frog's vocal sacs.
• Chyme — Partially digested food passed from the stomach into the first part of the small intestine.
• Cutaneous respiration — Respiratory gas exchange through the skin, used by the frog in water.
• Pulmonary respiration — Respiration through the lungs, which function as respiratory organs on land.
• Pericardium — Membrane enclosing the muscular heart in the upper part of the frog's body cavity.
• Nephron — Structural and functional unit of the kidney, also called a uriniferous tubule.
• Ureotelic — Describes an animal that excretes urea as its principal nitrogenous waste product.
• Urinogenital duct — Shared duct carrying urinary and reproductive products towards the cloaca in a male frog.
• Cloaca — Common chamber through which faecal matter, urine and male reproductive products reach the exterior.
• Metamorphosis — Transformation of the frog's larval stage, the tadpole, into the adult form.

Common errors and misconceptions

• Misconception: A cold-blooded frog has a fixed low body temperature. Correct: Its body temperature varies with environmental temperature; the term refers to this variability.
• Misconception: The adult frog has a neck and tail. Correct: The adult body is divided into head and trunk, with neither a neck nor a tail.
• Misconception: Forelimbs and hind limbs have the same number of digits. Correct: Each forelimb has four digits; each hind limb has five.
• Misconception: The gall bladder produces bile. Correct: The liver produces bile, and the gall bladder stores it before delivery to the intestine.
• Misconception: Frogs use lungs to breathe under water. Correct: Their skin exchanges gases in water; lungs participate in respiration on land.
• Misconception: The heart has two ventricles. Correct: Its three chambers are two atria and one ventricle, enclosed by the pericardium.
• Misconception: Female oviducts and ureters form a shared urinogenital duct. Correct: They open separately into the cloaca; the male ureters serve as urinogenital ducts.
• Misconception: Fertilisation occurs inside the female and directly produces an adult. Correct: Fertilisation is external in water, and development includes a tadpole that undergoes metamorphosis.

Exam-style questions with model answers

Q1. State the number of digits on each forelimb and each hind limb of an adult frog. [2 marks]

1. Each forelimb of the adult frog ends in four digits.
2. Each hind limb of the adult frog ends in five digits.

Q2. Give two external features that distinguish a male frog from a female frog. [2 marks]

1. The male frog has sound-producing vocal sacs, which are absent in the female.
2. The male has a copulatory pad on the first digit of the forelimbs; the female lacks this pad.

Q3. Describe respiration in a frog under three conditions: in water, on land, and during aestivation or hibernation. [3 marks]

1. In water, the skin acts as the respiratory organ. Dissolved oxygen passes across it by diffusion; this is cutaneous respiration.
2. On land, the buccal cavity, skin and lungs act as respiratory organs. Respiration through the lungs is called pulmonary respiration.
3. During aestivation and hibernation, gaseous exchange takes place through the skin while the frog remains sheltered.

Q4. Describe the frog's heart under four headings: chambers, protective covering, sinus venosus connection, and ventricular outlet. [4 marks]

1. The heart has three chambers, consisting of two atria and a single ventricle.
2. A membrane called the pericardium encloses the muscular heart in the upper body cavity.
3. The triangular sinus venosus joins the right atrium and receives blood through major veins called vena cava.
4. The ventricle opens into a sac-like conus arteriosus on the ventral side of the heart.

Q5. Trace food processing in a frog through six stages: capture, stomach digestion, transfer to the duodenum, action of bile and pancreatic juice, absorption, and removal of undigested material. [6 marks]

1. The muscular tongue, bilobed at its tip, captures food. The food passes through the mouth cavity and oesophagus towards the stomach.
2. The stomach wall secretes hydrochloric acid and gastric juice. Their action digests food, producing partially digested material called chyme.
3. Chyme passes into the duodenum, the first part of the small intestine, which receives bile and pancreatic juice through a common bile duct.
4. Bile emulsifies fats, while pancreatic juice digests carbohydrates and proteins. Final digestion takes place in the intestine.
5. Digested food is absorbed through the numerous villi and microvilli on the inner surface of the intestine.
6. The remaining undigested solid material moves into the rectum and is discharged to the exterior through the cloaca.

Q6. Describe the male frog's reproductive arrangement in five points: testes, attachment to kidneys, vasa efferentia, Bidder's canal, and final exit route. [5 marks]

1. The male reproductive organs include a pair of yellowish, oval testes situated near the upper parts of the kidneys.
2. A double fold of peritoneum, called the mesorchium, attaches the testes to the upper parts of the kidneys.
3. The vasa efferentia are 10 to 12 ducts arising from the testes. They enter the kidneys on their respective sides.
4. Within the kidneys, the vasa efferentia open into Bidder's canal, which communicates with the urinogenital duct.
5. The urinogenital duct emerges from the kidney and opens into the cloaca, the chamber through which sperms pass to the exterior.

Q7. Compare male and female frogs in three respects: reproductive organs, their connection with the kidneys, and the ducts opening into the cloaca. [3 marks]

1. The male has a pair of testes, whereas the female has a pair of ovaries situated near the kidneys.
2. Male vasa efferentia enter the kidneys and communicate with Bidder's canal. Female ovaries have no functional connection with the kidneys.
3. Male ureters serve as urinogenital ducts opening into the cloaca. Female ureters and oviducts open separately into the cloaca.

Q8. In the frog, describe egg-laying capacity, the site of fertilisation, the larval stage, and the change leading to an adult. [4 marks]

1. A mature female frog can lay 2500 to 3000 ova at a time; this is a range rather than a fixed number.
2. Fertilisation is external, meaning it takes place outside the female's body, in water.
3. Development includes a larval stage called the tadpole, which hatches from the developing egg.
4. The tadpole undergoes metamorphosis, the transformation of the larval form into an adult frog.

Key takeaways

• The frog's body organisation links tissues, organs and organ systems through division of labour that supports the whole animal.
• Adult frogs have a head and trunk, moist skin, four digits on each forelimb and five on each hind limb.
• The liver produces bile, the gall bladder stores it, and pancreatic juice digests carbohydrates and proteins in the intestine.
• Skin provides gas exchange in water and during seasonal sleep; the buccal cavity, skin and lungs function on land.
• The closed circulatory system includes a heart with two atria and one ventricle; frog red blood cells contain nuclei.
• Kidneys remove nitrogenous wastes, chiefly urea, while male ureters also function as ducts carrying reproductive products.
• The nervous system has central, peripheral and autonomic divisions; the ear serves hearing as well as balance.
• External fertilisation occurs in water, and development includes a tadpole that transforms into an adult by metamorphosis.

Test yourself

What is the difference between aestivation and hibernation?

Aestivation is summer sleep associated with extreme heat; hibernation is winter sleep associated with extreme cold.

What protects a frog's eyes while it is in water?

The nictitating membrane covers and protects the bulging eyes while the frog is in water.

What is chyme, and where does it go after leaving the stomach?

Chyme is partly digested food. It passes from the stomach into the duodenum, the first part of the small intestine.

Which body regions are linked by the hepatic and renal portal systems?

The hepatic portal system links intestine and liver; the renal portal system links lower body parts and kidneys.

Why is a frog called ureotelic?

A frog is ureotelic because urea is its principal nitrogenous waste product.

How many pairs of cranial nerves arise from the frog's brain?

Ten pairs of cranial nerves arise from the frog's brain.

What is the relationship between a female frog's ovaries and kidneys?

The ovaries lie near the kidneys but have no functional connection with them.

What is the larval stage of the frog, and how does it become an adult?

The larval stage is the tadpole, which undergoes metamorphosis to become an adult frog.