Structural Organisation in Animals: Class 11 Biology · Chapter 7

An Amoeba in a village pond lives its whole life inside one cell. That cell takes in food, exchanges gases, throws out waste and splits in two to reproduce. The frog squatting at the edge of the same pond cannot work this way. Its body, like yours, contains an enormous number of cells,11A widely cited 2013 estimate puts an adult human body at about $3.7\times {10}^{13}$ cells, roughly 37 trillion. and no single cell could feed, protect or move a body that size. So cells specialise: each group takes on one job and relies on the others for everything else. This chapter asks how that teamwork is arranged, and then examines one vertebrate, the frog, from its skin inwards.

The first level of teamwork is the tissue (ऊतक). Cells of a similar kind sit together, held in material they lay down between themselves (the intercellular substance), and share one particular task. Animal bodies look wildly different, yet every complex animal, from an earthworm to an elephant, is assembled from just four basic kinds of tissue: epithelial, connective, muscular and neural (Table 7.1).

Tissues, in turn, build organs. In the heart, the liver or a kidney, each tissue is present in a fixed amount and arranged in a fixed pattern, and that arrangement is what makes the structure an organ. A set of organs that cooperate on one job, linked by physical contact, chemical messages or both, is an organ system: the kidneys, ureters and urinary bladder together form the excretory system. Climb the levels in Fig. 7.1 and the last step is the whole frog.

Why go to such trouble? Think of Mumbai's dabbawalas, who carry home-cooked lunches to offices every working day. One team collects the boxes, another sorts them at the station, a third rides the local trains and a fourth delivers. No team could do the whole job alone, yet together they rarely lose a box. A body runs on the same principle, division of labour. Cells, tissues, organs and organ systems each take a share of the work, and the body survives because every part depends on the rest and is coordinated with it.

7.1 Organ and Organ System

This layering is what allows the vast number of cells in a body to work efficiently and in step. A meal, a hormone or a nerve signal reaches the right cells at the right time because every organ belongs to a system with a defined role, and the systems are linked to one another.

An organ may use one kind of tissue or several. Slice through the wall of a frog's stomach (Fig. 7.2) and you cross all four. An epithelium on the inside lines the cavity and releases digestive juices. Beneath it, connective tissue carries the blood vessels. Next come layers of muscle that churn the food, and between them runs a network of nerve cells that tells the muscle when to squeeze. Your heart is another organ whose wall contains all four basic tissues.

Compare one organ system across several animal groups and you find that its complexity changes in a recognisable pattern. The heart is a good example: a fish's heart has two chambers, a frog's has three, and the hearts of birds and mammals have four. Such a discernible pattern is called an evolutionary trend; the Class 12 chapter on evolution looks at the evidence behind trends like this.

Describing the frog takes two vocabularies. Everything you can note from outside, such as its shape, colour, limbs and drum-like tympanum, belongs to morphology (आकारिकी), the study of form; for an animal the word covers the outward appearance of the body and its parts. Botanists and microbiologists use the word in its narrowest sense, for externally visible features alone. What you can find only by opening the body, the form and arrangement of the internal organs, belongs to anatomy (शारीर) (the comparison below). The rest of the chapter applies both words to the frog, our sample vertebrate.

7.2 Frogs

Walk past a paddy field after the first heavy monsoon shower and the night is loud with croaking. Frogs split their lives between land and fresh water, and zoologists place them in the class Amphibia (from the Greek for ‘double life’), within the phylum Chordata. The species you are most likely to meet in India is the Indian bullfrog, Rana tigrina,22Taxonomists now place the Indian bullfrog in the genus Hoplobatrachus, as Hoplobatrachus tigerinus. NCERT and exam papers use Rana tigrina. a big frog of ponds, ditches and flooded fields.

You keep your body near 37 °C in every season. A frog cannot: its body warms up on a sunny bank and cools down in a night-time pond, tracking the temperature around it (Fig. 7.3). Biologists call such an animal a poikilotherm (popularly, cold-blooded).33The everyday word cold-blooded misleads, since a poikilotherm basking in the sun can be quite warm. Many books now prefer ectotherm, an animal that gets its body heat from its surroundings.

A frog sitting still among water hyacinth can be almost invisible until it jumps. Many frogs can shift their skin to a darker or lighter shade that matches the background, so a hunting snake or heron overlooks them. Hiding by blending in is camouflage; NCERT calls this protective colouration mimicry, so expect that word in exam questions.44Strictly, biologists use mimicry for one species coming to resemble another species, and camouflage for blending into the background.

Frogs have two ways of riding out a harsh season, and both involve digging in. Aestivation (summer sleep) carries them through the hottest weeks, and hibernation (winter sleep) through the coldest. Either way the frog shelters deep in a burrow, where the soil buffers the heat or the cold, and its body processes slow right down. That is why you rarely see frogs at the height of summer or winter (the comparison below).

7.2.1 Morphology

A frog never drinks. All the water it needs soaks in through its skin, so the skin must stay moist at all times. Glands in the skin keep it coated with mucus, which is why a frog feels smooth and slippery. The colours tell top from bottom: the dorsal surface is usually olive green with dark, irregular blotches, while the ventral surface is a uniform pale yellow.

Seen from above (Fig. 7.4), the frog's head joins its trunk directly: an adult has no neck, and no tail either, so its body has just two regions, a head and a trunk. The most striking features of the head are the bulging eyes, each of which can be covered by a nictitating membrane, a see-through lid that sweeps across the eye and guards it under water. Close behind each eye is a round, drum-like membrane, the tympanum, which picks up sound. Nearer the tip of the snout, a pair of nostrils opens above the mouth.

The two pairs of limbs differ sharply. The hind limbs are long and powerfully muscled, the engines of a frog's leap, and each ends in five digits linked by webs of skin that act as paddles in water. The forelimbs are shorter, with four digits each. Together, the limbs let a frog leap, swim, walk and burrow.

Males and females also look different, a case of sexual dimorphism (Fig. 7.5). Two features give a male away. He has vocal sacs, pouches of skin that balloon out when he calls, and on the innermost (first) digit of each forelimb he carries a rough copulatory pad. A female has neither.

7.2.2 Anatomy

The body cavity houses well-developed digestive, respiratory, circulatory, excretory, nervous and reproductive systems (Fig. 7.6). We take them one at a time, each time asking what the system moves: food, air, blood, wastes, signals or gametes.

Digestive system

Compared with a plant-eater's, a frog's gut is strikingly short, and the intestine especially so. Frogs are carnivores, and animal food is digested far more readily than tough plant food, so a long gut is not needed.55A tadpole, which grazes on algae, has a much longer, tightly coiled intestine. The gut shortens during metamorphosis, when the diet changes to insects. The system has two parts: the alimentary canal, the tube that food passes along, and the digestive glands that pour juices into it.

Prey is captured by the muscular tongue, which is bilobed, split into two lobes at its tip. From the mouth, food enters the buccal cavity, passes through the pharynx into the short oesophagus and reaches the stomach. The stomach continues as the intestine, which leads to the rectum, and the rectum opens into the cloaca, a chamber that opens to the outside (Fig. 7.7). Two glands serve the canal. The liver makes bile, which is stored in the gall bladder, and the pancreas makes pancreatic juice, rich in digestive enzymes.

Think of digestion as three stations along the canal. Station one, the stomach. Its walls secrete hydrochloric acid ($\mathrm{H}\mathrm{C}\mathrm{l}$) and gastric juices, which turn the prey into a soupy, partly digested mass called chyme. Station two, the duodenum, the first part of the small intestine. Here the common bile duct delivers two juices at once: bile from the gall bladder, which emulsifies fat into tiny droplets, and pancreatic juice from the pancreas, which digests carbohydrates and proteins.66Pancreatic juice also carries a fat-digesting enzyme, lipase, which acts on the droplets that bile produces. Station three, the rest of the intestine. Digestion is completed here, and the products are absorbed through countless finger-like villi lining the wall, each covered in still finer microvilli. Anything left undigested moves on to the rectum and leaves through the cloaca.

Respiration

Where the frog is decides how it breathes (Fig. 7.8). Sitting on a bank, it uses three surfaces at once. Air drawn in through the nostrils collects in the buccal cavity, whose moist lining absorbs some oxygen, and is then pushed into the lungs; breathing with the lungs is called pulmonary respiration. Meanwhile the skin keeps exchanging gases too. Dive into the pond and the lungs fall idle: the skin alone now takes up oxygen dissolved in the water, by diffusion, a method called cutaneous respiration.

The lungs themselves are a pair of long, pink, bag-like sacs in the front part of the trunk. A frog buried in a burrow during aestivation or hibernation barely moves, and it then relies on its skin alone for gas exchange. That is one more reason the skin must never dry out.

Circulation

Follow one drop of blood back from a leg muscle (Fig. 7.9). Large veins, the venae cavae, deliver it to a thin, triangular chamber, the sinus venosus, which empties into the right atrium. From there it passes into the single ventricle, which also receives the blood that the left atrium has collected from the lungs. The ventricle's contraction drives blood into the conus arteriosus, a sac-like vessel on the heart's ventral side, and on into the arteries. So the muscular heart has three chambers (a pair of atria and a single ventricle), all wrapped in a protective membrane, the pericardium, and it sits high in the body cavity.

Arteries form the arterial system, which carries blood out to every organ; veins form the venous system, which brings it back. Because the blood never leaves this ring of heart and vessels, the frog's well-developed blood vascular system is described as closed. Running alongside it is a second network, the lymphatic system: lymph, the channels that carry it and the lymph nodes along those channels.

With only one ventricle, blood returning from the lungs and blood returning from the body partly mix before being pumped out. NCERT's summary of this chapter calls the frog's circulation single. In Chapter 15 the same arrangement is described more precisely as incomplete double circulation: blood does pass through the heart twice on a full circuit, but the two streams are not kept fully apart.

Frogs also have two special venous links called portal systems, in which a vein delivers blood from one organ into a second organ instead of returning it straight to the heart. In the hepatic portal system the link runs from the intestine to the liver, so blood carrying absorbed food passes through the liver first. In the renal portal system it runs from the lower parts of the body to the kidneys.

Put a drop of frog blood under a microscope beside a drop of yours and one difference jumps out: each of the frog's red blood cells (RBCs, or erythrocytes) contains a nucleus.77Your own RBCs lose the nucleus as they mature, which leaves more room for haemoglobin. Frog RBCs are also larger and oval. Otherwise the make-up is familiar: a fluid, plasma, carrying RBCs, white blood cells (WBCs, or leucocytes) and platelets, with the red pigment haemoglobin inside the RBCs. Driven by the heart, blood delivers nutrients, water and gases wherever they are needed. Lymph is a separate fluid: it has no RBCs and lacks some of the proteins found in blood (the comparison below).

Excretion

A frog's main nitrogenous waste is urea, which makes it a ureotelic animal.88A tadpole living in water excretes mostly ammonia; the switch to urea comes with metamorphosis. Blood carries the urea to a pair of well-developed kidneys, which separate it out. Each kidney is compact, dark red and bean-shaped, and sits towards the rear of the body cavity beside the vertebral column. Under a microscope a kidney turns out to be a bundle of many uriniferous tubules, or nephrons, its structural and functional units. Together with the ureters, the urinary bladder and the cloaca, the kidneys make up the excretory system.

A ureter leads from each kidney to the cloaca, but what it carries depends on the frog's sex (Fig. 7.11). In a male the ureter carries both urine and sperms, so it serves as a urinogenital duct. In a female the ureters carry urine only, and the oviducts open into the cloaca separately. A thin-walled urinary bladder lies ventral to the rectum and also opens into the cloaca.

Control and coordination

Two highly developed systems keep the frog's organs working in step. The neural system sends fast signals along nerves. The endocrine system coordinates organs chemically, using hormones that endocrine glands release. Listed roughly from head to tail, the frog's prominent endocrine glands are the pineal body, pituitary, thyroid, parathyroid, thymus, adrenals, pancreatic islets and gonads.

The nervous system has three divisions. The brain and spinal cord form the central nervous system. The nerves leaving them, ten pairs of cranial nerves from the brain plus the spinal nerves from the cord, form the peripheral nervous system, and the sympathetic and parasympathetic nerves form the autonomic nervous system. A bony brain box, the cranium, protects the brain.

Trace the central nervous system from the tail end forwards (Fig. 7.10). The spinal cord runs inside the vertebral column. At the skull it passes through an opening, the foramen magnum, and widens into the medulla oblongata. Just ahead of the medulla sits the cerebellum; these two make up the hindbrain. In front of them bulge two optic lobes, which form the midbrain. The forebrain comes last: an unpaired diencephalon, then a pair of long cerebral hemispheres, and at the very front the olfactory lobes.

Sense organs

Look at the side of a frog's head and you will find no ear flap: a frog has no external ear, and the tympanum is the only part of its ear visible from outside. Behind it lies the internal ear, which serves for hearing and also for balance (equilibrium). The internal ears and the eyes are the frog's two well-organised sense organs. The eyes are a pair of spherical organs set in the orbits of the skull. They are simple eyes: each works as a single unit, unlike an insect's compound eye, which is built from many units. Touch, taste and smell rely on much simpler organs, small clusters of cells around nerve endings (Table 7.3).

Reproduction

The male system is easiest to learn as a journey (Fig. 7.11). Sperms form in the two testes, yellowish oval bodies held against the upper part of the kidneys by a double fold of peritoneum, the mesorchium. They leave each testis through 10 to 12 fine tubes, the vasa efferentia, which run into the kidney beside it and open into Bidder's canal. From there they reach the urinogenital duct and travel with the urine to the cloaca, the small chamber in the midline through which faecal matter, urine and sperms all leave the body.

A female's reproductive system has no working link with the kidneys at all: her two ovaries merely lie close to them. Ova pass into a pair of oviducts, each with its own opening into the cloaca, so eggs never share a duct with urine. Spawning happens in water, where a mature female releases 2500 to 3000 ova in one go and they are fertilised outside her body, an external fertilisation. What hatches is not a small frog but a larva, the tadpole, which reaches adult form through metamorphosis (Fig. 7.12).

Frogs and people

In a paddy field at night, frogs sit in the middle of a busy food web. They eat insects, spiders and worms, and snakes, herons and egrets eat them, so frogs are an important link in food chains and webs and help hold the ecosystem in balance. Farmers gain directly too, because many of the insects frogs eat are pests of rice and other crops. In some countries people also eat the muscular legs of frogs.