Structural Organisation in Animals

In a unicellular organism one cell does everything: it digests, respires, excretes and reproduces. A multicellular animal divides the same jobs among groups of cells. Even a simple animal like Hydra is built of several types of cells, with thousands of cells of each type; the human body has billions. The question NCERT asks is how these cells work together, and the answer is the hierarchy you have known since Class 9.

A tissue is a group of similar cells, along with the intercellular substances, that performs a specific function. NCERT makes a point you should quote: all complex animals are built from only four basic types of tissues, epithelial, connective, muscular and neural. Tissues are organised in a specific proportion and pattern to form an organ such as the stomach, lung, heart or kidney. Our heart, for example, contains all four types of tissue. When two or more organs perform a common function through their physical and/or chemical interaction they form an organ system, for example the digestive system or the respiratory system. Cells, tissues, organs and organ systems split up the work in a way that shows division of labour, and together they keep the whole body alive.

NCERT also observes that the complexity of organs and organ systems shows a discernible trend across the animal kingdom, which it calls the evolutionary trend (studied in Class 12). Two terms are then defined, and the definitions are asked as 1-mark and 2-mark questions. Morphology is the study of form, that is, of the externally visible features. For plants and microbes the term means only this; for animals it refers to the external appearance of the organs or parts of the body. Anatomy is conventionally used for the study of the morphology of the internal organs of animals. The chapter then applies both words to a single animal, the frog, which stands as the representative of the vertebrates.

Frogs live both on land and in freshwater. They belong to class Amphibia of phylum Chordata, and the most common species in India is Rana tigrina, the Indian bullfrog. Every fact in this short section turns up as a 1-mark question, so learn the exact words.

Cold-blooded. Frogs do not have a constant body temperature; their body temperature varies with the temperature of the environment. Such animals are called cold-blooded or poikilotherms. This is the reason a frog is sluggish in cold weather and why it has to escape extreme temperatures altogether rather than regulate them.

Colour change. A frog looks different in grass and on dry land. It can change its colour to hide from enemies; this is camouflage. NCERT then says that this protective coloration is called mimicry. Strictly, in ecology, mimicry means resembling another organism, but in this chapter you should reproduce the NCERT sentence as it stands, because the examiner's key follows the textbook.

Aestivation and hibernation. Frogs are not seen during peak summer or peak winter. During these periods they take shelter in deep burrows to protect themselves from extreme heat and cold. The summer sleep is aestivation; the winter sleep is hibernation. In both states the frog is inactive, its metabolism is low, the lungs are not used and all gaseous exchange takes place through the moist skin (this link with cutaneous respiration is a favourite 2-mark and Assertion–Reason question).

Keep the three habits together in one line for the exam: poikilotherm (temperature), camouflage or mimicry (colour), aestivation and hibernation (seasonal dormancy). Two-mark questions ask you to distinguish aestivation from hibernation; the only difference the examiner wants is summer versus winter, plus the fact that both are spent in burrows and both are survival responses to extreme temperature.

This section is NCERT Figure 7.1 in words, and the labelled figure is worth drawing in a 3- or 5-mark answer.

Skin. The skin is smooth and slippery because of mucus and is always kept moist. The dorsal side is generally olive green with dark irregular spots; the ventral side is uniformly pale yellow. The frog never drinks water; it absorbs water through the skin. (The summary adds that the skin has mucous glands and is highly vascularised, which is what makes it a respiratory surface.)

Body divisions. The body is divisible into head and trunk only. A neck and tail are absent. This is a classic trap question: any option that gives the frog a neck or a tail is wrong.

Head. Above the mouth there is a pair of nostrils. The eyes are bulged and covered by a nictitating membrane that protects them while the frog is in water. On either side of the eyes is a membranous tympanum (ear) that receives sound signals; there is no external ear.

Limbs. The forelimbs and hind limbs help in swimming, walking, leaping and burrowing. The hind limbs end in five digits and are larger and more muscular than the forelimbs, which end in four digits. The feet have webbed digits that help in swimming. So the two numbers to remember are 4 (fore) and 5 (hind).

Sexual dimorphism. Frogs show sexual dimorphism, which means the male and female can be told apart externally. Male frogs are distinguished by (i) sound-producing vocal sacs and (ii) a copulatory pad on the first digit of the forelimbs. Both are absent in female frogs. The vocal sacs give the croaking call that attracts the female; the copulatory pad (nuptial pad) helps the male grip the female during amplexus, the mating clasp. NEET has asked the exact location of the copulatory pad more than once, and the answer is always the first digit of the forelimb, not the hind limb.

A typical 2-mark question asks for two differences between male and female frogs, and a 3-mark question asks you to describe the external features with a diagram. Draw a dorsal view: head with bulging eyes, nostrils and tympanum, the trunk, short forelimbs with four digits and long folded hind limbs with five webbed digits.

The body cavity of the frog holds the digestive, circulatory, respiratory, nervous, excretory and reproductive systems, and NCERT Figure 7.2 shows the internal organs with the complete digestive system. This is the most-asked diagram of the chapter and the first NCERT exercise, so draw it until you can do it in four minutes.

Parts. The digestive system consists of the alimentary canal and the digestive glands. The alimentary canal is short because frogs are carnivores, so the length of the intestine is reduced (an animal eating protein-rich prey needs less length for digestion than a herbivore). The mouth opens into the buccal cavity, which leads through the pharynx to the oesophagus. The oesophagus is a short tube that opens into the stomach, which continues as the intestine, then the rectum, and finally opens outside through the cloaca. The digestive glands are the liver, which secretes bile that is stored in the gall bladder, and the pancreas, which produces pancreatic juice containing digestive enzymes.

Process. Food is captured by the bilobed tongue, which is muscular, attached in front and free behind so that it can be flicked out at an insect. In the stomach, digestion takes place by the action of HCl and gastric juices secreted by the stomach wall. The partially digested food, called chyme, passes from the stomach into the first part of the small intestine, the duodenum. The duodenum receives bile from the gall bladder and pancreatic juice from the pancreas through a common bile duct. Bile emulsifies fat; pancreatic juice digests carbohydrates and proteins. Final digestion takes place in the intestine. The digested food is absorbed by the numerous finger-like folds of the inner wall of the intestine called villi and microvilli, which increase the absorptive surface. The undigested solid waste moves into the rectum and passes out through the cloaca.

For a 5-mark answer the marking scheme typically gives 2 or 3 marks for the labelled diagram and the rest for the sequence of organs and the roles of the two glands, chyme, bile and villi. Write the organs as a chain with arrows: mouth → buccal cavity → pharynx → oesophagus → stomach → duodenum → intestine → rectum → cloaca.

Frogs respire on land and in water by two different methods, and every question on this system is really asking which organ works where.

In water. The skin acts as the aquatic respiratory organ. This is cutaneous respiration. Dissolved oxygen in the water is exchanged through the skin by simple diffusion. The skin can do this because it is thin, always moist (mucus) and highly vascularised, so there is a short diffusion path between the water and the blood in the skin capillaries.

On land. Three organs work: the buccal cavity, the skin and the lungs. Respiration by the lungs is pulmonary respiration. The lungs are a pair of elongated, pink-coloured, sac-like structures present in the upper part of the trunk region (thorax). Air enters through the nostrils into the buccal cavity and from there into the lungs. (The frog has no ribs or diaphragm; it fills the lungs by lowering and raising the floor of the buccal cavity, a positive-pressure buccal pump. This mechanism is not in the rationalised text, so mention it only as an extra.) The moist lining of the buccal cavity also exchanges gases directly, which is buccal respiration.

During aestivation and hibernation. The frog is dormant in a burrow, the lungs are not ventilated, and gaseous exchange takes place through the skin alone. Because the metabolic rate is very low in these states, the small amount of oxygen taken in through the skin is enough.

A 3-mark question typically asks you to explain respiration in frog in water and on land; a 2-mark question asks for the difference between cutaneous and pulmonary respiration. Answer both in the same shape: organ, medium, mechanism. Cutaneous: skin, water (and land, and dormancy), diffusion of dissolved oxygen through the moist vascular skin. Pulmonary: lungs, land only, air drawn through nostrils into the buccal cavity and then the lungs.

The vascular system of the frog is a well-developed closed type, and the frog also has a lymphatic system. The blood vascular system involves the heart, blood vessels and blood; the lymphatic system consists of lymph, lymph channels and lymph nodes.

Heart. 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 chamber called the sinus venosus joins the right atrium; it receives blood from the body through the major veins, the venae cavae. The ventricle opens into a sac-like conus arteriosus on the ventral side of the heart, which carries blood out to the arteries. So the chain is: venae cavae → sinus venosus → right atrium → ventricle → conus arteriosus → arteries. The left atrium receives oxygenated blood from the lungs and skin. Because there is a single ventricle, oxygenated and deoxygenated blood mix partly; NCERT's summary calls it a closed system with single circulation.

Vessels and portal systems. Arteries carry blood from the heart to all parts of the body (arterial system). Veins collect blood from the body and return it to the heart (venous system). The frog has two special venous connections that are asked again and again. The hepatic portal system is the venous connection between the intestine and the liver: blood from the gut goes to the liver before reaching the heart. The renal portal system is the venous connection between the lower parts of the body and the kidneys: blood from the hind region passes through the kidneys before reaching the heart. A portal vein, by definition, begins in capillaries and ends in capillaries.

Blood. Blood is composed of plasma and cells. The cells are RBCs (erythrocytes), WBCs (leucocytes) and platelets. The frog's RBCs are nucleated and contain the red pigment haemoglobin. (Mammalian RBCs lose their nucleus; frog RBCs do not; this is the single most common MCQ from the chapter.) Blood carries nutrients, gases and water to the respective sites during circulation, which is driven by the pumping of the muscular heart.

Lymph. Lymph is different from blood: it lacks a few proteins and RBCs. A 3-mark question on the circulatory system should mention: closed type, three-chambered heart with sinus venosus and conus arteriosus, arterial and venous systems, the two portal systems, and nucleated RBCs. The heart of a frog is myogenic and auto-excitable, so it continues to beat for some time after being removed from the body; this is an NCERT Exemplar and NEET item.

Nitrogenous wastes are removed by a well-developed excretory system consisting of a pair of kidneys, ureters, cloaca and urinary bladder.

Kidneys. The kidneys are compact, dark red, bean-like structures situated a little posteriorly in the body cavity, one on each side of the vertebral column. Each kidney is composed of many structural and functional units called uriniferous tubules or nephrons. (Zoologically the frog kidney is a mesonephros, a term not needed for the school exam.)

Ducts, and the male–female difference. In the male frog two ureters emerge from the kidneys and act as urinogenital ducts, because they carry both urine and sperms, and they open into the cloaca. In the female, the ureters and the oviducts open separately into the cloaca; the ureters carry only urine. The thin-walled urinary bladder lies ventral to the rectum and also opens into the cloaca; urine collects in it before being voided. This is the answer to NCERT exercise 2 (function of ureters): they conduct urine from the kidneys to the cloaca, and in the male they also conduct sperms, hence the name urinogenital duct.

Nature of waste. The frog excretes urea and is therefore a ureotelic animal. Excretory wastes are carried by the blood into the kidney, where they are separated (filtered by the nephrons) and excreted. Tadpoles, which live in water, excrete mostly ammonia (ammonotelic); the switch to urea at metamorphosis is a classic link with the Class 11 chapter on excretion, and an examiner may use it in an Assertion–Reason.

For a 3-mark answer: name the four parts, describe the kidneys (position, shape, colour, nephrons), state the urinogenital arrangement in male and female, state the position of the bladder and say the frog is ureotelic. A neat labelled diagram of the male urinogenital system (Figure 7.3) covers both this section and the reproductive one.

NCERT says the system for control and coordination is highly evolved in the frog and includes both the neural system and the endocrine glands.

Endocrine glands. Chemical coordination is achieved by hormones 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 (eight names; a 2-mark question asks you to list them, and an MCQ asks which of a list is not one of them, where the wrong entries are usually liver, salivary gland or kidney).

Divisions of the nervous system. The nervous system is organised into (i) a central nervous system, the brain and the spinal cord; (ii) a peripheral nervous system, the cranial and spinal nerves; and (iii) an autonomic nervous system, sympathetic and parasympathetic. There are ten pairs of cranial nerves arising from the brain (mammals have twelve; the difference is a common MCQ).

Brain. The brain is enclosed in a bony structure called the brain box or cranium. It is divided into fore-brain, mid-brain and hind-brain. The fore-brain includes the olfactory lobes, the paired cerebral hemispheres and the unpaired diencephalon. The mid-brain is characterised by a pair of optic lobes. The hind-brain consists of the cerebellum and the medulla oblongata. The medulla oblongata passes out through the foramen magnum and continues as the spinal cord, which is enclosed in the vertebral column. Draw the brain in dorsal view as a row of structures from front to back: olfactory lobes, cerebral hemispheres, diencephalon (with the pineal body on top), optic lobes, cerebellum (a small transverse band), medulla oblongata, spinal cord.

A 3-mark question on the nervous system expects the three divisions with their components, the number of cranial nerves, and the three parts of the brain with their lobes. Note the two adjectives NCERT uses: paired cerebral hemispheres and unpaired diencephalon; examiners look for them.

The frog has five kinds of sense organs, each named with its structure in NCERT: organs of touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes) and hearing (tympanum with internal ears). Of these, only the eyes and the internal ears are well-organised structures; the rest are cellular aggregations around nerve endings. This last sentence is a frequent 1-mark question, so remember which two are organised organs.

Eyes. The eyes are a pair of spherical structures situated in the orbit of the skull. They are simple eyes, possessing only one unit each; contrast this with the compound eyes of insects, which are made of many ommatidia. From morphology, recall that the eyes bulge and are protected by the nictitating membrane in water.

Ears. An external ear is absent; only the tympanum can be seen from outside. The ear is an organ of hearing as well as balance (equilibrium). The tympanum receives sound signals and passes the vibrations to the internal ear.

Questions here are short: list the sense organs with the structure for each (3 marks), or a 2-mark question on why the eyes of a frog are called simple eyes, or on the two functions of the ear. Answer in the NCERT pattern, organ then structure then function.

Frogs have well-organised male and female reproductive systems, drawn in NCERT Figures 7.3 and 7.4. Both figures are asked as 5-mark diagram questions, and the male system is also the source of the well-known sperm-route MCQ.

Male. The male reproductive organs are a pair of yellowish, ovoid testes, found 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. They enter the kidney on their side and open into Bidder's canal, which runs along the kidney. Bidder's canal finally communicates with the urinogenital duct (the ureter) that comes out of the kidney and opens into the cloaca. The cloaca is a small median chamber used to pass faecal matter, urine and sperms to the exterior. So the route of sperms is: testis → vasa efferentia → kidney → Bidder's canal → urinogenital duct → cloaca. Notice that the sperms travel through the kidney, which is why the male frog's ureter is called a urinogenital duct.

Female. The female organs are a pair of ovaries situated near the kidneys, but with no functional connection with the kidneys. A pair of oviducts arising from (near) the ovaries opens into the cloaca separately from the ureters. A mature female can lay 2500 to 3000 ova at a time. The eggs are released into the body cavity and collected by the funnels of the oviducts; the ova shown in Figure 7.4 are the black-and-white eggs that fill the ovary in the breeding season.

Fertilisation and development. During the monsoon the male clasps the female (amplexus) using the copulatory pads and both shed gametes into water, so fertilisation is external and takes place in water. Development involves a larval stage called the tadpole, which is aquatic, has a tail and gills and feeds on plant matter. The tadpole undergoes metamorphosis, controlled by thyroid hormone, to form the adult frog: the tail is resorbed, limbs and lungs develop and the animal becomes a carnivore.

Importance. Frogs are beneficial to mankind because they eat insects and thus protect crops. They maintain ecological balance as an important link in food chains and food webs. In some countries the muscular legs of the frog are used as food. A 2-mark question on the importance of frogs needs any two of these three points.

The rationalised NCERT chapter mentions the four tissues only in its introduction and summary, and CBSE school exams follow the rationalised book. NEET, however, still lists 'animal tissues' under this unit, and the older NCERT text on them is the source of one question most years. This section is a compact reference; skip it if you are preparing only for the school exam.

Epithelial tissue is a sheet of closely packed cells with one free surface, resting on a basement membrane, with little intercellular matrix. Simple epithelium is one layer: squamous (flat cells; walls of blood vessels, air sacs of lungs, diffusion), cuboidal (cube-like; ducts of glands, tubular parts of nephrons, secretion and absorption; the brush border of the proximal convoluted tubule), columnar (tall; lining of stomach and intestine, secretion and absorption) and ciliated (cuboidal or columnar cells with cilia; bronchioles and fallopian tubes, moving particles or mucus). Glandular epithelium is unicellular (goblet cells) or multicellular (salivary gland); exocrine glands pour secretions through ducts, endocrine glands are ductless. Compound epithelium is many-layered and protective, with little role in secretion or absorption: dry surface of skin, moist lining of buccal cavity, pharynx, inner lining of ducts of salivary and pancreatic glands. Cells are held by junctions: tight junctions stop leakage across a tissue, adhering junctions cement neighbours together, gap junctions allow rapid transfer of ions and small molecules between cells.

Connective tissue is the most abundant and widely distributed tissue; except blood, its cells secrete fibres of the structural proteins collagen and elastin, plus a modified polysaccharide matrix. Loose connective tissue: areolar (beneath skin; fibroblasts, macrophages and mast cells) and adipose (fat storage). Dense regular: tendons (muscle to bone) and ligaments (bone to bone), with parallel fibres. Dense irregular: skin, with fibres in many directions. Specialised: cartilage (solid, pliable, resists compression; chondrocytes in lacunae; tips of long bones, nose, ear, vertebrae, between adjacent bones of the vertebral column), bone (hard, non-pliable calcium-rich matrix, osteocytes in lacunae; the bone marrow of some bones makes blood cells) and blood (fluid matrix, plasma, with RBC, WBC and platelets).

Muscle tissue: skeletal (striated, voluntary, multinucleate fibres bundled in fascicles), smooth (non-striated, spindle-shaped, uninucleate, involuntary; blood vessels, gut wall), cardiac (striated, involuntary, branched, joined by intercalated discs which allow the cells to contract as a unit). Neural tissue: neurons are the excitable cells; neuroglia make up more than half the volume of neural tissue in our body and support and protect neurons.