Structural Organisation in Animals
How animal bodies organise cells into tissues, organs and organ systems, illustrated through the morphology and anatomy of the frog (Rana tigrina).
Basic biology and animal physiology surface in Prelims general-science questions—classification, organ systems and the frog's signature features (nucleated RBCs, three-chambered heart, ureotelic excretion) are classic recall traps. For Mains, the chapter feeds GS-III Science & Technology and environment themes such as amphibians as bioindicators, natural pest control and biodiversity. Expect single-line factual MCQs rather than analytical questions.
Understand the chapter
Levels of Organisation & Division of Labour
In unicellular organisms a single cell performs all functions, but multicellular animals distribute work among groups of cells—this is division of labour. A group of similar cells with intercellular substances performing a specific function forms a tissue; tissues build organs, and organs combine into organ systems. All complex animals are built from only four basic tissue types arranged in specific proportions.
- Hierarchy: cells → tissues → organs → organ systems → organism.
- Four basic tissues: epithelial, connective, muscular, neural.
- The heart contains all four tissue types.
- Increasing organ-system complexity reflects an evolutionary trend.
The Frog: Classification & Adaptive Biology
The common Indian frog is Rana tigrina (Indian bullfrog), belonging to class Amphibia of phylum Chordata, able to live on land and in freshwater. Frogs are poikilotherms—body temperature varies with the surroundings. They survive extremes by sleeping in deep burrows and escape enemies through protective coloration.
- Poikilotherm (cold-blooded): body temperature tracks the environment.
- Aestivation = summer sleep; hibernation = winter sleep (both in burrows).
- Mimicry/camouflage: colour change for protection from enemies.
- Morphology = external features; anatomy = study of internal organs.
Morphology (External Features)
The skin is smooth, slippery and kept moist by mucus—dorsally olive-green with dark spots, ventrally pale yellow—and the frog absorbs water through skin rather than drinking. The body is divided into head and trunk only; neck and tail are absent. Limbs aid swimming, walking, leaping and burrowing, and webbed feet assist swimming.
- Forelimbs end in 4 digits; hind limbs end in 5 digits (larger, muscular, webbed).
- Eyes protected by a nictitating membrane; tympanum receives sound.
- Sexual dimorphism: males have vocal sacs and a copulatory pad on the first forelimb digit.
- Nostrils present above the mouth; external ear absent.
Digestive & Respiratory Systems
The alimentary canal is short because frogs are carnivores, reducing intestinal length, and runs mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca, served by liver and pancreas. Bile (stored in the gall bladder) emulsifies fat while pancreatic juice digests carbohydrates and proteins; villi and microvilli absorb food. Frogs respire by multiple routes depending on land or water.
- Bilobed tongue captures prey; HCl and gastric juices act in the stomach to form chyme.
- Duodenum receives bile and pancreatic juice via a common bile duct.
- Cutaneous respiration (skin) in water; pulmonary (lungs) and buccal on land.
- During aestivation/hibernation, gas exchange occurs only through skin.
Circulatory & Excretory Systems
Frogs have a well-developed closed, single circulation plus a lymphatic system. The heart has three chambers—two atria and one ventricle—enclosed in a pericardium, with a sinus venosus joining the right atrium and the ventricle opening into the conus arteriosus. Excretion is handled by paired bean-shaped kidneys made of nephrons, and the frog is ureotelic.
- RBCs are nucleated and contain haemoglobin; blood = plasma + RBC/WBC/platelets.
- Two portal systems: hepatic (liver–intestine) and renal (kidney–lower body).
- Excretory organs: kidneys, ureters, urinary bladder, cloaca; nephron = functional unit.
- Ureters act as urinogenital duct in males; lymph lacks some proteins and RBCs.
Nervous, Sensory, Endocrine & Reproductive Systems
Control and coordination is highly evolved, combining a nervous system (central, peripheral, autonomic) with endocrine glands secreting hormones. The brain, enclosed in the cranium, has fore-, mid- and hind-brain regions, and ten pairs of cranial nerves arise from it. Reproduction is sexual with external fertilisation in water and development through a tadpole stage.
- Brain: forebrain (olfactory lobes, cerebral hemispheres, diencephalon), midbrain (optic lobes), hindbrain (cerebellum, medulla oblongata).
- Endocrine glands: pituitary, thyroid, parathyroid, thymus, pineal, pancreatic islets, adrenals, gonads.
- Male: paired testes attached by mesorchium; vasa efferentia (10–12) → Bidder's canal.
- Female lays 2500–3000 ova; external fertilisation; tadpole undergoes metamorphosis.
Key terms
- Tissue
- A group of similar cells with intercellular substances performing a specific function.
- Poikilotherm
- A cold-blooded animal whose body temperature varies with the environment.
- Aestivation
- Summer sleep in deep burrows to escape extreme heat.
- Hibernation
- Winter sleep in deep burrows to escape extreme cold.
- Morphology
- Study of the external form and visible features of the body.
- Anatomy
- Study of the form and structure of internal organs.
- Cutaneous respiration
- Gaseous exchange through the moist skin, used especially in water.
- Ureotelic
- An animal that excretes nitrogenous waste mainly as urea.
- Sexual dimorphism
- Distinguishable external differences between the male and female of a species.
- Metamorphosis
- Transformation of the larval tadpole into the adult frog.
Must-know facts exam-ready
- Four basic tissues: epithelial, connective, muscular, neural; the heart contains all four.
- Common Indian frog: Rana tigrina (Indian bullfrog); class Amphibia, phylum Chordata.
- Frogs are poikilotherms; summer sleep = aestivation, winter sleep = hibernation.
- Forelimbs end in 4 digits; hind limbs end in 5 digits (larger, webbed, muscular).
- Male identifiers: vocal sacs + copulatory pad on first forelimb digit (sexual dimorphism).
- Heart = 3 chambers (2 atria + 1 ventricle); closed, single circulation; pericardium covers it.
- Sinus venosus joins the right atrium; ventricle opens into the conus arteriosus.
- Frog RBCs are nucleated and contain haemoglobin.
- Two portal systems: hepatic (liver–intestine) and renal (kidney–lower body).
- Frog is ureotelic; nephron (uriniferous tubule) is the functional unit of the kidney.
- Ten pairs of cranial nerves; medulla oblongata exits via the foramen magnum.
- Female lays 2500–3000 ova; external fertilisation in water; tadpole metamorphoses into adult.
Memory tricks remember it for good
Traps to avoid
- Aestivation (summer sleep) vs hibernation (winter sleep) are routinely swapped—'A for August' = summer.
- The frog heart has THREE chambers (2 atria, 1 ventricle) with single circulation, not four chambers/double circulation.
- Frog RBCs are NUCLEATED (unlike mammalian RBCs), and they do contain haemoglobin.
- Limb digits get reversed: forelimb = 4 digits, hind limb = 5 digits (the bigger hind limb has more).
- Frog is ureotelic (urea)—not ammonotelic or uricotelic; the tadpole is the larval, not adult, form.
- Frog never drinks water—it absorbs water through the skin; frog has ten (not twelve) pairs of cranial nerves.
Exam focus
🧠 Prelims angles
- Classification recall: Rana tigrina—class Amphibia, phylum Chordata, poikilotherm.
- Heart structure: three chambers, single closed circulation, sinus venosus and conus arteriosus.
- Excretion: ureotelic nature and nephron as the kidney's functional unit.
- Respiration modes: cutaneous, pulmonary and buccal—and which works during dormancy.
- Special features: nucleated RBCs, hepatic vs renal portal systems, ten pairs of cranial nerves.
- Reproduction: external fertilisation, 2500–3000 ova, tadpole metamorphosis.
✍️ Mains angles GS-III
- Ecological role of frogs/amphibians as pest controllers and bioindicators of ecosystem health.Link insect-eating/crop protection and food-web role to biodiversity conservation and amphibian decline.
- Division of labour and levels of biological organisation as a principle of efficiency in complex life.Trace cells→tissues→organs→systems and argue how specialisation aids survival and coordination.
- Cold-blooded adaptation (aestivation/hibernation) and climate sensitivity of amphibians.Connect poikilothermy and dormancy strategies to vulnerability under climate change.
Last-minute revision tick as you recall
- 4 tissues: Epithelial, Connective, Muscular, Neural.
- Rana tigrina; class Amphibia; phylum Chordata; poikilotherm.
- Heart: 3 chambers; single closed circulation; nucleated RBCs.
- Ureotelic; nephron = functional unit of kidney.
- Respiration: skin (cutaneous), lungs (pulmonary), buccal; only skin in dormancy.
- Hepatic + renal portal systems; sinus venosus → conus arteriosus.
- Ten pairs cranial nerves; brain = fore-, mid-, hind-brain.
- Male = vocal sacs + copulatory pad; forelimb 4 digits, hind limb 5.
- External fertilisation; 2500–3000 ova; tadpole → metamorphosis → frog.
Distilled from NCERT Class 11 · Biology (Class 11) for UPSC. Always cross-check facts with the original NCERT.