Structural Organisation in Animals
Touch a frog and the first thing you notice is not scales or fur but slippery, moist skin — and that texture is not incidental, it is a second lung. Earlier chapters introduced the vast diversity of the animal kingdom, from single amoeba-like cells to elaborate invertebrates. This chapter asks a different question: once an animal's body is too complex for one cell to do everything, how does it organise the labour among billions of cells? The answer, common to every complex animal, is a hierarchy. A group of similar cells with intercellular substances performing a shared job is a tissue; complex animals are built of only four basic tissue types; tissues combine to build organs such as the stomach, lung, heart and kidney; and when organs cooperate to perform a common job, they form an organ system — digestive, respiratory, circulatory, and so on. To see this hierarchy at work, NCERT turns to one representative: the Indian frog, Rana tigrina. Frogs are amphibians of class Amphibia, phylum Chordata — at home in both pond and land, cold-blooded (poikilothermic), masters of camouflage, and capable of sleeping through summer heat (aestivation) and winter cold (hibernation). A frog's body is simple enough to study whole yet complete enough to display every major vertebrate organ system, making it the model on which morphology (external form) and anatomy (internal structure) are taught together. This article walks the frog from skin to system: external features and sexual dimorphism, the alimentary canal, the three-way respiratory strategy, the three-chambered heart and its portal circulations, ureotelic excretion, the nervous and endocrine systems, and the reproductive system ending in external fertilisation and a tadpole. By the end you should be able to describe every major frog organ system and explain why it is built the way it is.
- Complex bodies are organised in a hierarchy: cells combine into tissues, tissues into organs, organs into organ systems, giving division of labour.
- Morphology is external form; anatomy is internal structure — taught together here using the frog, Rana tigrina, as the model vertebrate.
- Frogs belong to class Amphibia, phylum Chordata. They are poikilotherms, show camouflage called mimicry, and undergo aestivation (summer sleep) and hibernation (winter sleep) in burrows.
- Digestion: mouth to buccal cavity to pharynx to oesophagus to stomach to duodenum (bile plus pancreatic juice) to intestine (villi, microvilli absorb) to rectum to cloaca; a carnivore's short intestine.
- Respiration is three-way: cutaneous (moist skin, dominant in water and during dormancy), buccal, and pulmonary (a pair of lungs, on land).
- Circulation is closed and single, via a three-chambered heart (two atria, one ventricle) with a sinus venosus and conus arteriosus, plus hepatic and renal portal systems; frog RBCs are nucleated.
- Excretion is by paired kidneys built of nephrons; the frog is ureotelic; in males the ureters double as urinogenital ducts, in females the ureter and oviduct open separately.
- Coordination uses a central, peripheral (ten pairs cranial nerves) and autonomic nervous system, plus endocrine glands such as the pituitary, thyroid and adrenals.
- Reproduction: separate sexes, external fertilisation in water, a female lays 2500 to 3000 ova at a time, and the egg develops via a larval tadpole that undergoes metamorphosis into the adult.
- Key idea: complex animal bodies split their work into a hierarchy of cells, tissues, organs and organ systems, and the frog is NCERT's model vertebrate for seeing how morphology and anatomy realise that organisation — from three-way respiration and a three-chambered heart to ureotelic excretion and a metamorphosing tadpole.
Complex animal bodies organise cells into tissues, tissues into organs and organs into organ systems for division of labour, and the frog Rana tigrina — a poikilothermic amphibian with cutaneous plus pulmonary respiration, a three-chambered heart with portal circulations, ureotelic excretion and external fertilisation ending in a metamorphosing tadpole — is NCERT's model vertebrate for studying how morphology and anatomy realise that organisation together.
What this chapter covers
This chapter has two connected parts. It opens with the general principle of organ and organ system: why tissues associate into organs and organs into organ systems, and the two ways biologists describe a body — morphology (external) and anatomy (internal). It then applies that principle to one representative vertebrate, the frog. You will meet its classification and lifestyle, its external morphology, and then, system by system, its anatomy: digestion, three-way respiration, the closed circulatory system built around a three-chambered heart, ureotelic excretion, the nervous and endocrine systems, the sense organs, and the reproductive systems that lead to external fertilisation, the tadpole, and metamorphosis. A short closing note places the frog in its ecological context.
The core ideas
From cells to organ systems: the logic of animal organisation
In a unicellular organism, one cell must digest, respire, excrete and reproduce by itself. Even a simple multicellular animal such as Hydra is built of thousands of cells of different types, and the human body of billions of cells performing enormously varied jobs. The organising principle that makes this workable is division of labour: a tissue is a group of similar cells, together with intercellular substances, performing one or more shared functions. All complex animals, however different they look outside, are built from only four basic tissue types, organised in specific proportion and pattern to form an organ — the stomach, lung, heart or kidney. Most organs are not made of one tissue: the human heart, NCERT's own example, is built of all four basic tissue types — epithelial, connective, muscular and neural — working together. When two or more organs cooperate, by physical or chemical interaction, to carry out one common job, they together form an organ system — the digestive system, the respiratory system, and so on. Cells, tissues, organs and organ systems thus split up the body's workload, exhibiting division of labour and contributing to the survival of the whole organism. Comparing organ systems across the animal kingdom also reveals a discernible trend of rising complexity — an evolutionary trend studied further in Class XII. Two words describe how biologists study a body. Morphology is the study of form — externally visible features. Anatomy, by convention, is the study of internal organs. This chapter covers both, using the frog to represent the vertebrates.
Meet the frog: classification, habitat and behaviour
Frogs can live both on land and in freshwater and belong to class Amphibia of phylum Chordata. The most common species of frog found in India is Rana tigrina, the Indian bullfrog. Frogs do not maintain a constant body temperature — their body temperature varies with that of the environment. Such animals are called cold-blooded, or poikilotherms. Frogs can also change the colour of their skin while sitting in grass or on dry land, hiding them from predators; this protective colour change is called camouflage, and the specific protective coloration is termed mimicry. Frogs are not seen during the peak of summer or winter: during these periods they take shelter in deep burrows to escape extreme heat and cold — summer sleep (aestivation) and winter sleep (hibernation) respectively.
External morphology of the frog
The skin of a frog is smooth and slippery because of the mucus secreted by mucous glands, and it is always kept moist. The dorsal (upper) side of the body is generally olive green with dark irregular spots; the ventral (lower) side is uniformly pale yellow. A frog never drinks water with its mouth — it absorbs water directly through its skin. The body is divisible into just two regions, head and trunk; a neck and a tail are both absent in the adult. A pair of nostrils sits above the mouth. The eyes are bulged and are protected, especially while the frog is in water, by a nictitating membrane. On either side of the eyes sits a membranous tympanum, the external hearing structure, which receives sound signals. The forelimbs and hind limbs together help the frog swim, walk, leap and burrow. The hind limbs, ending in five digits, are larger and more muscular than the forelimbs, which end in only four digits; the feet carry webbed digits that aid swimming. Frogs show sexual dimorphism: males can be told apart by sound-producing vocal sacs and by a copulatory pad on the first digit of the forelimb, both of which are absent in females.
The digestive system
The digestive system of the frog consists of an alimentary canal and its digestive glands. Because frogs are carnivores, the alimentary canal is comparatively short, with a reduced length of intestine. Food is captured with the help of a bilobed tongue. The mouth opens into a buccal cavity, which leads through the pharynx into the oesophagus, a short tube that opens into the stomach; the stomach continues as the intestine, then the rectum, and finally opens to the exterior through the cloaca. Digestion in the stomach is carried out by HCl and gastric juices secreted from its walls. The partially digested food, now called chyme, passes from the stomach into the first part of the small intestine, the duodenum. The duodenum receives bile from the gall bladder (secreted by the liver, which stores it there) and pancreatic juice from the pancreas, both delivered through a common bile duct. Bile emulsifies fat, while the pancreatic juice digests carbohydrates and proteins; final digestion is completed in the intestine. Digested food is absorbed through the numerous finger-like folds of the intestine's inner wall, the villi and microvilli, which increase the absorptive surface area. Undigested solid waste passes into the rectum and out through the cloaca.
Respiratory system: three ways to breathe
Frogs respire on land and in water by different methods, which is why the respiratory strategy is often called three-way. In water, the skin acts as an aquatic respiratory organ — dissolved oxygen in the water is exchanged through the skin by diffusion; this is cutaneous respiration. On land, the buccal cavity, the skin, and the lungs act as respiratory organs. Respiration by the lungs is called pulmonary respiration. The lungs are a pair of elongated, pink-coloured, sac-like structures present in the upper part of the trunk (thorax); air enters through the nostrils into the buccal cavity and then passes into the lungs. During aestivation and hibernation, when the frog is inactive in its burrow, gaseous exchange takes place entirely through the skin.
Circulatory system: a closed, single circulation
The vascular system of the frog is a well-developed closed type, and frogs also possess a lymphatic system. The blood vascular system involves the heart, blood vessels and blood, while the lymphatic system consists of lymph, lymph channels and lymph nodes. The heart is a muscular structure in the upper part of the body cavity, covered by a membrane called the pericardium. It has three chambers — two atria and one ventricle. A triangular structure, the sinus venosus, joins the right atrium and receives blood through the major veins, the vena cava. The ventricle opens into a sac-like conus arteriosus on the ventral side of the heart. From here, blood is carried to every part of the body by the arteries, forming the arterial system; veins collect blood from the body back to the heart, forming the venous system. Frogs also have special venous connections: one between the liver and intestine, called the hepatic portal system, and one between the kidney and the lower parts of the body, called the renal portal system. Blood is composed of plasma and cells — RBCs (erythrocytes), WBCs (leucocytes) and platelets. Unlike human RBCs, frog RBCs are nucleated and contain the red pigment haemoglobin. Lymph differs from blood in lacking a few proteins and RBCs. Blood carries nutrients, gases and water to their respective sites during circulation, driven entirely by the pumping action of the muscular heart.
Excretory system: kidneys and ureotelic excretion
The elimination of nitrogenous wastes is carried out by a well-developed excretory system consisting of a pair of kidneys, ureters, the cloaca and a urinary bladder. The kidneys are compact, dark red, bean-like structures positioned a little posteriorly in the body cavity on either side of the vertebral column. Each kidney is built of numerous structural and functional units called uriniferous tubules, or nephrons. In male frogs, two ureters emerge from the kidneys and act as a combined urinogenital duct, opening into the cloaca — they carry both urine and sperm. In females, the ureters and the oviduct open separately into the cloaca. A thin-walled urinary bladder lies ventral to the rectum and also opens into the cloaca. The frog excretes urea as its main nitrogenous waste and is therefore described as ureotelic. Excretory wastes are carried by the blood into the kidney, where they are separated and excreted.
Nervous system, endocrine glands and sense organs
The system for control and coordination in the frog is highly evolved, combining a neural system with endocrine glands. The endocrine glands achieve chemical coordination by secreting hormones; the prominent ones found in the frog are the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads. The nervous system has a central nervous system (brain and spinal cord), a peripheral nervous system (cranial and spinal nerves), and an autonomic nervous system (sympathetic and parasympathetic). Ten pairs of cranial nerves arise from the brain, enclosed in a bony brain box, the cranium. The brain is divided into a fore-brain (olfactory lobes, paired cerebral hemispheres, an unpaired diencephalon), a mid-brain (a pair of optic lobes), and a hind-brain (cerebellum and medulla oblongata). The medulla oblongata passes through the foramen magnum and continues as the spinal cord, enclosed within the vertebral column. The frog's sense organs are touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes), and hearing (tympanum with internal ears). 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, simple eyes (one unit each) in the skull's orbit. An external ear is absent — only the tympanum shows externally — and the ear also serves balance (equilibrium), not hearing alone.
Reproductive system and life cycle
Frogs have well-organised, separate male and female reproductive systems. The male reproductive organs consist of a pair of yellowish, ovoid testes, adhered to the upper part of the kidneys by a double fold of peritoneum called the mesorchium. From each testis arise 10 to 12 vasa efferentia, which enter the kidney on their side and open into Bidder's canal. This canal finally communicates with the urinogenital duct emerging from the kidney, which opens into the cloaca — a small, median chamber used to pass faecal matter, urine and sperm all to the exterior. The female reproductive organs include a pair of ovaries, situated near the kidneys but with no functional connection to them. A pair of oviducts, arising from the ovaries, opens into the cloaca separately from the ureters. A mature female can lay 2500 to 3000 ova at a time. Fertilisation is external, taking place in water, and development proceeds through a larval stage called the tadpole, which undergoes metamorphosis to become the adult frog.
Frogs in the ecosystem
Frogs are beneficial to humans because they eat large numbers of insects, protecting crops, and they maintain ecological balance by serving as an important link in food chains and food webs. In some countries, the muscular legs of frogs are also used as food.
Tracing three pathways through the frog's body
The three sequences below turn the organ-system descriptions above into step-by-step pathways, exactly the kind of question NCERT and NEET like to ask: trace a substance from its point of entry to its point of exit through named structures, in the correct order, without skipping a step.
The goal is to trace a single mouthful of prey, captured on the bilobed tongue, all the way through the frog's alimentary canal to the point where undigested waste leaves the body, naming every organ and gland it passes and what happens to it there. This matters because board and competitive questions routinely ask students to sequence the canal correctly and to attach the right secretion to the right step — a jumbled order or a misplaced gland is the single commonest way marks are lost on this topic.
Step 1. Capture and entry. The bilobed tongue flicks out and captures the prey, drawing it into the mouth, which opens into the buccal cavity. From here food is pushed through the pharynx into the oesophagus, a short muscular tube.
Step 2. Gastric digestion. The oesophagus opens into the stomach, where the walls secrete hydrochloric acid and gastric juices. This acidic digestion converts the food into a semi-liquid, partially digested mass called chyme.
Step 3. The duodenum receives two secretions at once. Chyme passes into the duodenum, the first part of the small intestine, which receives bile from the gall bladder (made by the liver) and pancreatic juice from the pancreas, delivered through a common bile duct. Bile emulsifies fat into smaller droplets that enzymes can act on efficiently; pancreatic juice supplies the enzymes that digest carbohydrates and proteins.
Step 4. Final digestion and absorption. As the food moves on into the rest of the intestine, digestion is completed. The digested nutrients are absorbed across the intestinal wall through its numerous finger-like folds, the villi and microvilli, which multiply the absorptive surface area available for a short, carnivore-adapted gut.
Step 5. Egestion. Whatever remains undigested moves into the rectum and is finally expelled through the cloaca, the single common opening that also serves the excretory and reproductive systems. Sequence to remember: mouth, buccal cavity, pharynx, oesophagus, stomach (chyme), duodenum (bile + pancreatic juice), intestine (villi and microvilli absorb), rectum, cloaca.
Go deeper · Derivation B · 1 minThe path of blood through the three chambered frog heart in one cardiac cycle
The goal is to trace one cycle of blood flow through the frog's heart, showing exactly where the two separate bloodstreams enter, where they meet, and why a three-chambered heart with a single ventricle still keeps the frog alive despite mixing that a mammalian heart is built to avoid. This is the pathway students most often garble, usually by forgetting that two atria fill separately before both empty into one shared ventricle.
Step 1. Deoxygenated blood returns from the body. Venous blood collected from the whole body arrives through the major veins, the vena cava, into the triangular sinus venosus, which channels it into the right atrium.
Step 2. Oxygenated blood returns from the lungs. At the same time, blood that has picked up oxygen in the lungs during pulmonary respiration returns and fills the left atrium separately, so the two bloodstreams are kept apart as far as the atria.
Step 3. Both atria contract together into the single ventricle. Because the frog's heart has only one ventricle, contraction of the two atria empties both bloodstreams into this shared chamber, where some mixing of oxygenated and deoxygenated blood necessarily occurs — the defining limitation of a three-chambered heart compared with the fully separated four-chambered mammalian heart.
Step 4. The ventricle pumps outward through the conus arteriosus. Ventricular contraction forces the mixed blood into the sac-like conus arteriosus on the ventral side of the heart, from which it is distributed through the arterial system to the body and the lungs.
Step 5. Blood returns via two special detours before completing the circuit. On its way back to the heart, blood from the intestine passes through the liver first via the hepatic portal system, and blood from the lower body passes through the kidney first via the renal portal system, before rejoining the general venous return to the sinus venosus. Why it still works: frogs are poikilotherms with modest, intermittent oxygen demand, unlike constantly warm mammals, so partial mixing in a single ventricle is a workable, not a fatal, compromise.
Go deeper · Derivation C · 1 minThe path of sperm from the testis to the exterior in a male frog
The goal is to trace the male urinogenital pathway precisely, because this is the structure NCERT explicitly tests by asking for the function of the ureter in a frog — a question that only makes sense once you see that, in the male, the excretory and reproductive routes are physically the same duct. The chain from testis to cloaca has four named stops, and skipping any one of them is the most common error.
Step 1. Sperm are produced in the testes. The paired, yellowish ovoid testes are held against the upper part of the kidneys by a double fold of peritoneum, the mesorchium, keeping them anatomically attached to the excretory organs from the start.
Step 2. Sperm leave the testis through the vasa efferentia. From each testis, 10 to 12 fine tubules called vasa efferentia carry sperm away and enter the kidney on the same side.
Step 3. Sperm cross the kidney via Bidder's canal. Inside the kidney, the vasa efferentia open into a canal called Bidder's canal, which threads sperm through kidney tissue to connect with the duct that will carry them out.
Step 4. Sperm exit through the shared urinogenital duct into the cloaca. Bidder's canal communicates with the urinogenital duct — the same duct that carries urine out of the kidney — and this single duct opens into the cloaca, the common chamber that releases faecal matter, urine and sperm together to the exterior. Contrast with the female: in a female frog the ureter carries only urine to the cloaca, while a separate pair of oviducts, arising from the ovaries, carries ova to the cloaca independently — there is no shared urinogenital duct in females.
Worked examples
Board level - why does a frog switch its mode of respiration with habitat and season
Explain why a frog uses cutaneous respiration in water, pulmonary respiration on land, and cutaneous respiration again during aestivation and hibernation, referring to the structures involved in each case.
Step 1. In water, oxygen is dissolved at low concentration in the surrounding medium; the frog is fully submerged, so its moist, richly vascularised skin stays in direct, continuous contact with that water.
Step 2. Because the skin is thin, permanently moist, and well supplied with blood vessels, dissolved oxygen can diffuse directly across it — this is cutaneous respiration, and it needs no surfacing or active air movement.
Step 3. On land, the skin alone cannot supply an active frog's higher oxygen demand, so it relies additionally on the buccal cavity and, mainly, its pair of lungs: air is drawn in through the nostrils, into the buccal cavity, then the lungs, where pulmonary respiration takes over.
Step 4. During aestivation and hibernation the frog is inactive in its burrow, oxygen demand falls sharply, and it is not breathing air through its nostrils, so gaseous exchange reverts entirely to the skin. Answer: cutaneous respiration dominates in water and during dormancy, since the moist vascular skin exchanges gases passively; pulmonary respiration through the lungs takes over on land, where activity and oxygen demand are higher.
Board level - tracing the ureter and explaining why its function differs between the sexes
State the function of the ureter in a male frog and in a female frog, and explain why this function differs between the two sexes.
Step 1. In both sexes, the ureter is a duct from each kidney, carrying away the nitrogenous waste (mainly urea) filtered out by that kidney's uriniferous tubules.
Step 2. In the male, sperm from the testis travel through the vasa efferentia into the kidney and then Bidder's canal, joining the same duct that carries urine. So the male's ureter functions as a combined urinogenital duct, carrying urine and sperm together to the cloaca.
Step 3. In the female, the ovaries have no functional link to the kidneys; a separate pair of oviducts carries ova to the cloaca instead. The female's ureters therefore carry only urine. Answer: in the male frog, the ureter is a urinogenital duct carrying both urine and sperm; in the female frog, the ureter carries only urine, since ova travel via a separate, unconnected pair of oviducts.
Competitive level (NEET) - identifying the correct statement about the frog's circulatory system
Which one of the following statements about the frog's circulatory and excretory systems is correct? *(a) The frog's heart is four-chambered, with complete separation of oxygenated and deoxygenated blood. (b) The sinus venosus opens into the ventricle, and the conus arteriosus opens into the right atrium. (c) The hepatic portal system connects the liver and the intestine, while the renal portal system connects the kidney and the lower parts of the body. (d) In both male and female frogs, the ureter carries only urine, with reproductive cells using a completely separate duct.*
Step 1. Eliminate option (a). The frog's heart has three chambers — two atria and one ventricle — not four; complete separation of oxygenated and deoxygenated blood is a mammalian and avian feature, not amphibian.
Step 2. Eliminate option (b). The sinus venosus joins the right atrium, not the ventricle, and the conus arteriosus arises from the ventricle, not the right atrium — the positions are swapped.
Step 3. Test option (c). NCERT states directly that a hepatic portal system links the liver and intestine, and a renal portal system links the kidney and the lower body. This matches exactly.
Step 4. Eliminate option (d). This is true only for the female; in the male the ureter doubles as the urinogenital duct, carrying both urine and sperm, so "in both male and female frogs" is false. Answer: (c) — the hepatic portal system connects the liver and the intestine, and the renal portal system connects the kidney and the lower parts of the body. Timing note: about 90 seconds. Check each option against one specific, memorised fact — heart chamber count, sinus venosus and conus arteriosus positions, portal system connections, or the male-versus-female ureter difference — rather than judging all four options by feel.
Common traps
Students often call the frog's heart "two-chambered like a fish" or "four-chambered like a human" — it is neither; it has exactly three chambers, two atria and one ventricle, kept separate at the top but emptying into a single shared ventricle. A common slip reverses the sinus venosus and conus arteriosus: the sinus venosus receives blood into the right atrium, while the conus arteriosus is where blood leaves the ventricle. Others assume a frog "breathes only through its skin," forgetting that pulmonary respiration dominates on land, with cutaneous respiration taking over only in water and during dormancy. "Morphology" and "anatomy" are also wrongly used interchangeably — external form versus internal structure. Finally, aestivation and hibernation are opposite responses to opposite extremes, summer heat versus winter cold, not two names for one thing.
You should be able to…
- Explain the hierarchy from tissue to organ to organ system, and define both terms in your own words.
- State the difference between morphology and anatomy, and give one frog example of each.
- Classify the frog (Amphibia, Chordata) and explain poikilothermy, mimicry, aestivation and hibernation.
- List the frog's external features, including sexual dimorphism, from head to hind limb.
- Sequence the alimentary canal from mouth to cloaca, naming where bile and pancreatic juice enter.
- Contrast cutaneous, buccal and pulmonary respiration, and say when each dominates.
- Trace one cardiac cycle through the sinus venosus, both atria, the single ventricle and the conus arteriosus.
- Explain why the male frog's ureter is a urinogenital duct while the female's carries only urine.
- Name the three divisions of the frog's nervous system and the three regions of its brain.
- Describe the male and female reproductive pathways and the tadpole-to-adult life cycle.
Ask the tutor: Want a step-by-step comparison of how the frog's three-chambered heart and portal circulations would need to change to become a mammalian four-chambered, fully separated system?
Why does the frog's heart have only three chambers while ours has four, and doesn't the mixing hurt it?
The frog's heart has two atria and one shared ventricle. Deoxygenated blood enters the right atrium via the sinus venosus, while oxygenated blood from the lungs enters the left atrium separately, so the two streams stay apart until both atria empty into the single ventricle, where some mixing occurs. This works because the frog is a poikilotherm with modest, intermittent oxygen demand, unlike a warm-blooded mammal that needs fully separated, maximally oxygenated blood for continuous heat production. A three-chambered heart is a workable trade-off, not a defect.
What exactly is the difference between aestivation and hibernation in a frog?
Both are dormancy spent sheltered in a deep burrow, but triggered by opposite conditions. Aestivation is summer sleep, escaping extreme heat and drying out. Hibernation is winter sleep, escaping extreme cold. In both states metabolic activity drops sharply, the frog becomes inactive, and gaseous exchange happens entirely through the skin by cutaneous respiration, since it is not actively drawing air through its nostrils and lungs.
Why does the male frog's ureter carry sperm as well as urine, but the female frog's ureter does not?
It depends on how the reproductive ducts connect to the kidney in each sex. In the male, sperm from the testis travel through 10 to 12 vasa efferentia into the kidney, then Bidder's canal, and join the same duct that carries urine out — a combined urinogenital duct opening into the cloaca. In the female, the ovaries have no functional connection to the kidneys; a separate pair of oviducts carries ova to the cloaca directly. So the female's ureter carries only urine, while the male's does double duty.
What is the actual difference between morphology and anatomy, as this chapter uses the terms?
Morphology is the study of an organism's form, its externally visible features — in the frog, skin colour, the shape of head and trunk, eyes, tympanum and limbs, all observable without dissection. Anatomy, by convention, is the study of internal organs — the digestive, respiratory, circulatory, excretory, nervous and reproductive systems this chapter covers system by system. The terms are not interchangeable: a morphology question wants external features only; an anatomy question wants internal structure.
Why does a mature female frog lay thousands of ova at once if fertilisation and development happen outside the body?
External fertilisation, in water, is far less reliable than internal fertilisation because sperm and ova must meet by chance, and the eggs and tadpole larvae that follow face heavy losses to predation, disease and unfavourable conditions before any survive to adulthood. Laying 2500 to 3000 ova at a time compensates for this high loss rate by sheer numbers, improving the odds that some survive through the tadpole stage, metamorphosis, and into an adult frog.