Frogs

The frog is the vertebrate used to show how cells, tissues, organs and organ systems work together in an animal body. This page covers the levels of organisation and then the morphology and anatomy of the frog: its external features and its digestive, respiratory, circulatory, excretory, neural, sensory and reproductive systems. The order follows the NCERT Class 11 chapter on structural organisation in animals. NEET tests frog facts through match-the-list, statement and sequence questions, so learn each organ with its exact feature and number.

1. Levels of Organisation in Animals

1.1 From cells to organ systems

• In unicellular organisms, a single cell performs all functions, such as digestion, respiration and reproduction.
• In multicellular animals, the same basic functions are carried out by different groups of cells in a well-organised manner.
• The body of a simple organism like Hydra has different types of cells; each type may number in thousands.
• The human body is made of billions of cells that perform various functions.

• Organ: tissues arranged in a specific proportion and pattern, for example the stomach, lung, heart and kidney.
• Organ system: two or more organs that perform a common function through physical and/or chemical interaction, for example the digestive and respiratory systems.
• ★ Exam imp Cells, tissues, organs and organ systems split up the work. This division of labour ensures the survival of the body as a whole.
• Such organisation makes the activities of millions of cells more efficient and better coordinated.
• Each organ is made of one or more types of tissues. The heart contains all four: epithelial, connective, muscular and neural.
• The complexity of organs and organ systems shows a discernible trend, called the evolutionary trend.

1. Cells: the basic units that carry out life functions.
2. Tissues: groups of similar cells with intercellular substances.
3. Organs: tissues in a specific proportion and pattern.
4. Organ systems: organs that work together for a common function.
5. Organism: all organ systems working as one body.

1.2 Epithelia

• Epithelia: sheet-like tissues that line the body's surface and its cavities, ducts and tubes.
• An epithelium has one free surface, which faces a body fluid or the outside environment.
• Its cells are structurally and functionally connected at junctions.

1.3 Morphology and anatomy

• The morphology and anatomy of the frog are studied here as a representative of the vertebrates.

2. The Frog: Classification and Habits

• Frogs live both on land and in freshwater.
• They belong to class Amphibia of phylum Chordata.
• ★ Exam imp The most common species of frog in India is Rana tigrina, the Indian bullfrog.
• Poikilotherms (cold-blooded animals): animals whose body temperature is not constant but varies with the temperature of the environment. Frogs are poikilotherms.
• Frogs change their colour in grass and on dry land, which hides them from enemies: camouflage.
• This protective coloration is called mimicry.
• Frogs are not seen during peak summer and peak winter. They shelter in deep burrows to escape extreme heat and cold.

3. Morphology: External Features of the Frog

3.1 Skin and colour

• The body is covered by skin, which is smooth and slippery because of mucus; it is always kept moist.
• The skin has mucous glands and is highly vascularised (richly supplied with blood vessels). It helps in respiration both in water and on land.
• The dorsal side is generally olive green with dark irregular spots.
• The ventral side is uniformly pale yellow.
• ★ Exam imp The frog never drinks water; it absorbs water through its skin.

3.2 Body regions and head

• The body is divisible into a head and a trunk.
• A neck and a tail are absent.
• A pair of nostrils is present above the mouth.
• The eyes are bulged and covered by a nictitating membrane, which protects them while the frog is in water.
• A membranous tympanum (ear) on either side of the eyes receives sound signals.

3.3 Limbs

• The fore limbs and hind limbs help in swimming, walking, leaping and burrowing.
• The hind limbs end in five digits; they are larger and more muscular than the fore limbs.
• The fore limbs end in four digits.
• The feet have webbed digits, which help in swimming.

3.4 Sexual dimorphism

• Sexual dimorphism: male and female frogs differ in their external features.
• ★ Exam imp Male frogs have sound-producing vocal sacs and a copulatory pad on the first digit of each fore limb.
• Both of these are absent in female frogs.

4. Anatomy: Digestive System

• The body cavity holds the digestive, circulatory, respiratory, nervous, excretory and reproductive systems, each with well-developed structures and functions.
• The digestive system consists of the alimentary canal and the digestive glands; the main digestive glands are the liver and the pancreas.
• ★ Exam imp The alimentary canal is short because frogs are carnivores; hence the intestine is reduced in length.

4.1 The alimentary canal

1. Mouth: opens into the buccal cavity.
2. Buccal cavity: leads through the pharynx.
3. Pharynx: leads to the oesophagus.
4. Oesophagus: a short tube that opens into the stomach.
5. Stomach: continues as the intestine.
6. Intestine: its first part is the duodenum; it continues as the rectum.
7. Rectum: opens into the cloaca.
8. Cloaca: opens to the outside.

4.2 Digestive glands

4.3 Digestion and absorption

1. Food is captured by the bilobed tongue, which is muscular and bilobed at the tip.
2. In the stomach, food is digested by HCl and gastric juices secreted from the stomach walls.
3. The partially digested food, called chyme, passes from the stomach to the duodenum, the first part of the small intestine.
4. The duodenum receives bile from the gall bladder and pancreatic juice from the pancreas through a common bile duct.
5. Bile emulsifies fat; pancreatic juices digest carbohydrates and proteins.
6. Final digestion takes place in the intestine.
7. Digested food is absorbed by numerous finger-like folds of the inner intestinal wall, the villi and microvilli.
8. Undigested solid waste moves into the rectum and passes out through the cloaca.

5. Respiration

• Frogs respire on land and in water by two different methods.

• The lungs are a pair of elongated, pink, sac-like structures in the upper part of the trunk region (thorax).
• Air enters through the nostrils into the buccal cavity and then into the lungs.
• ★ Exam imp During aestivation and hibernation, gaseous exchange takes place through the skin.

6. Circulatory System

• The vascular system of the frog is well developed and of the closed type.
• Frogs also have a lymphatic system.

6.1 Heart

• The heart is a muscular structure in the upper part of the body cavity.
• ★ Exam imp It has three chambers: two atria and one ventricle.
• It is covered by a membrane called the pericardium.
• ★ Exam imp Sinus venosus: a triangular structure that joins the right atrium. It receives blood through the major veins, the vena cava.
• Conus arteriosus: a sac-like structure on the ventral side of the heart, into which the ventricle opens.

6.2 Blood vessels and portal systems

• Arteries carry blood from the heart to all parts of the body: the arterial system.
• Veins collect blood from different parts of the body and bring it to the heart: the venous system.
• Frogs have special venous connections, called portal systems.

6.3 Blood and lymph

• Blood is composed of plasma and cells.
• The blood cells are RBCs (red blood cells, or erythrocytes), WBCs (white blood cells, or leucocytes) and platelets.
• ★ Exam imp Frog RBCs are nucleated and contain the red pigment haemoglobin.
• Lymph differs from blood: it lacks a few proteins and RBCs.
• Blood carries nutrients, gases and water to the respective sites during circulation.
• Circulation is achieved by the pumping action of the muscular heart.

6.4 Type of circulation

• The frog's circulatory system is closed, and its circulation is summarised as single circulation.

7. Excretory System

• Nitrogenous wastes are eliminated by a well-developed excretory system.
• It consists of a pair of kidneys, the ureters, the cloaca and the urinary bladder.
• The kidneys are compact, dark red and bean-like. They lie a little posteriorly in the body cavity, on both sides of the vertebral column.
• Each kidney is made of several structural and functional units called uriniferous tubules or nephrons.
• The urinary bladder is thin-walled and lies ventral to the rectum; it also opens into the cloaca.
• ★ Exam imp The frog excretes urea, so it is a ureotelic animal.
• Excretory wastes are carried by the blood into the kidney, where they are separated and excreted.

8. Control, Coordination and Sense Organs

• The system for control and coordination is highly evolved. It includes both the neural system and the endocrine glands.

8.1 Endocrine glands

• Chemical coordination of the organs is achieved by hormones, which are secreted by the endocrine glands.
• The prominent endocrine glands of the frog are the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.

8.2 Nervous system

• ★ Exam imp Ten pairs of cranial nerves arise from the brain.
• The brain is enclosed in a bony structure, the brain box (cranium).
• The brain is divided into the fore-brain, mid-brain and hind-brain.

• The medulla oblongata passes out through the foramen magnum and continues into the spinal cord.
• The spinal cord is enclosed in the vertebral column.

8.3 Sense organs

• ★ Exam imp Only the eyes and internal ears are well-organised structures; the rest are cellular aggregations around nerve endings.
• The eyes are a pair of spherical structures in the orbit of the skull.
• They are simple eyes, possessing only one unit.
• ★ Exam imp The external ear is absent; only the tympanum can be seen externally.
• The ear is an organ of hearing as well as balancing (equilibrium).

9. Reproduction, Development and Importance

• Frogs have well-organised male and female reproductive systems.

9.1 Male reproductive system

• The male organs are a pair of yellowish ovoid testes.
• The testes adhere to the upper part of the kidneys by a double fold of peritoneum called the mesorchium.
• ★ Exam imp Vasa efferentia, 10-12 in number, arise from the testes. They enter the kidneys on their side and open into Bidder's canal.
• Bidder's canal communicates with the urinogenital duct, which comes out of the kidney and opens into the cloaca.
• ★ Exam imp Cloaca: a small, median chamber that passes faecal matter, urine and sperms to the exterior.

1. Testis: produces sperms.
2. Vasa efferentia: carry sperms into the kidney.
3. Bidder's canal: inside the kidney, receives the vasa efferentia.
4. Urinogenital duct: leaves the kidney, carrying sperms and urine.
5. Cloaca: the common chamber.
6. Cloacal aperture: the opening to the exterior.

9.2 Female reproductive system

• The female organs are a pair of ovaries.
• ★ Exam imp The ovaries lie near the kidneys but have no functional connection with them.
• A pair of oviducts arises from the ovaries and opens into the cloaca separately.
• ★ Exam imp A mature female can lay 2500 to 3000 ova at a time.

9.3 Fertilisation and development

1. A mature female lays 2500-3000 ova at a time.
2. Fertilisation is external and takes place in water.
3. The eggs hatch into a larval stage called the tadpole; development is also external.
4. The tadpole undergoes metamorphosis to form the adult frog.

9.4 Importance of frogs

10. Exam Essentials

Pairs to Match

Numbers to Remember

• 4 basic types of tissues in all complex animals; the heart has all 4.
• Thousands of cells of each type in Hydra; billions of cells in the human body.
• 2 body regions (head and trunk); 4 digits on the fore limb, 5 on the hind limb.
• 2 methods of respiration; 3 respiratory organs on land (buccal cavity, skin, lungs).
• 3 heart chambers: 2 atria and 1 ventricle.
• 8 prominent endocrine glands; 3 divisions of the brain.
• 10 pairs of cranial nerves.
• 10-12 vasa efferentia arise from the testes.
• 2500-3000 ova laid at a time by a mature female.

11. Quick Revision

• Tissue: similar cells with intercellular substances; only four basic types in complex animals.
• Organ systems show division of labour, and their complexity follows an evolutionary trend.
• Epithelia line body surfaces, cavities, ducts and tubes, with one free surface.
• Frog: Amphibia, Chordata; Rana tigrina; poikilotherm; aestivation in summer, hibernation in winter.
• Moist mucous skin, olive green above and pale yellow below; the frog never drinks water.
• Head and trunk only; nictitating membrane, tympanum, 4 fore digits, 5 webbed hind digits.
• Males: vocal sacs and copulatory pad on the first fore digit.
• Gut: mouth, buccal cavity, pharynx, oesophagus, stomach, intestine, rectum, cloaca; short in a carnivore.
• Common bile duct brings bile and pancreatic juice to the duodenum; villi and microvilli absorb.
• Respiration: cutaneous in water; buccal cavity, skin and lungs on land; skin during aestivation and hibernation.
• Closed circulation; three-chambered heart, sinus venosus, conus arteriosus, hepatic and renal portal systems.
• Nucleated RBCs with haemoglobin; lymph lacks some proteins and RBCs.
• Ureotelic; kidneys of nephrons; male ureter is a urinogenital duct.
• 8 endocrine glands; 10 pairs of cranial nerves; optic lobes in the mid-brain; eyes and internal ears organised.
• Testes, mesorchium, 10-12 vasa efferentia, Bidder's canal; ovaries, separate oviducts, 2500-3000 ova, tadpole.

12. Solved Examples

13. Practice Questions

Common Mistakes to Avoid