Structural Organisation in Animals: NEET notes

1. Tissues, organs and organ systems

NCERT §7.1

• In a unicellular organism one cell does everything: digestion, respiration and reproduction. In a multicellular animal the same jobs are shared out among organised groups of cells.
• Even a simple animal like Hydra has several types of cells, with the number of each type running into thousands; the human body has billions of cells.
• A tissue is a group of similar cells, together with the intercellular substance around them, that carries out a particular function.
• All complex animals are built from only four basic types of tissue: epithelial, connective, muscular and neural.
• Tissues arranged in a definite proportion and pattern make an organ such as the stomach, lung, heart or kidney. Each organ is made of one or more types of tissue; the heart contains all four.
• When two or more organs perform a common function through physical and/or chemical interaction, they form an organ system, such as the digestive or respiratory system.
• Cells, tissues, organs and organ systems divide the work among themselves, which is division of labour, and this keeps the body as a whole alive. The organisation makes the activities of millions of cells more efficient and better coordinated.
• The complexity of organs and organ systems shows a discernible trend, called the evolutionary trend (studied in Class XII).
• Morphology is the study of form or externally visible features; in animals it means the external appearance of organs or body parts. Anatomy is conventionally the study of the morphology of internal organs in animals. The frog is studied as the representative vertebrate.

2. The frog and its way of life

NCERT §7.2

• Frogs live both on land and in freshwater. They belong to class Amphibia of phylum Chordata.
• The most common species of frog in India is Rana tigrina, the Indian bullfrog.
• A frog has no constant body temperature; it varies with the temperature of the surroundings. Such animals are cold-blooded, or poikilotherms.
• Frogs can change colour to hide from enemies (camouflage), for example on grass and on dry land. NCERT calls this protective colouration mimicry.
• Frogs are not seen in peak summer or peak winter, when they shelter in deep burrows against extreme heat and cold. Summer sleep is aestivation; winter sleep is hibernation.

3. External features of the frog

NCERT §7.2.1

• The skin is smooth and slippery because of mucus, and it is always kept moist. The frog never drinks water; it absorbs water through its skin.
• The dorsal side is generally olive green with dark irregular spots; the ventral side is uniformly pale yellow.
• The body is divided into head and trunk only. A neck and a tail are absent.
• A pair of nostrils lies above the mouth. The eyes are bulging and covered by a nictitating membrane that protects them in water.
• On either side of the eyes lies a membranous tympanum (ear) that receives sound signals.
• Fore limbs and hind limbs are used for swimming, walking, leaping and burrowing. Fore limbs end in four digits; hind limbs end in five digits and are larger and more muscular. The digits of the feet are webbed for swimming.
• Frogs show sexual dimorphism. A male has sound-producing vocal sacs and a copulatory pad on the first digit of each fore limb; both are absent in the female.

4. Digestive system

NCERT §7.2.2

• The digestive system is made of the alimentary canal and the digestive glands.
• The alimentary canal is short because the frog is a carnivore, so the length of the intestine is reduced.
• The path of food: mouth → buccal cavity → pharynx → oesophagus (a short tube) → stomach → intestine → rectum → cloaca, which opens to the outside.
• Food is caught with the bilobed tongue.
• In the stomach, HCl and gastric juices secreted by the stomach wall act on the food. The partly digested food, chyme, passes to the duodenum, the first part of the small intestine.
• The liver secretes bile, which is stored in the gall bladder. The pancreas produces pancreatic juice containing digestive enzymes.
• Bile and pancreatic juice reach the duodenum through a common bile duct. Bile breaks fat into fine droplets (emulsification), and pancreatic juice acts on carbohydrates and proteins; digestion is completed in the intestine.
• Digested food is absorbed by numerous finger-like folds of the inner intestinal wall, the villi and microvilli. Undigested solid waste passes into the rectum and out through the cloaca.

5. Respiration

NCERT §7.2.2

• A frog respires by two different methods, one in water and one on land.
• In water the skin alone serves for respiration: oxygen dissolved in the water diffuses in across the moist skin. This is cutaneous respiration.
• On land the buccal cavity, the skin and the lungs all act as respiratory organs. Breathing through the lungs is pulmonary respiration.
• The lungs are a pair of elongated, pink, sac-like structures in the upper part of the trunk (the thorax).
• Air is drawn in through the nostrils, passes into the buccal cavity and from there to the lungs.
• During aestivation and hibernation, gas exchange takes place through the skin.

6. Circulation, blood and lymph

NCERT §7.2.2

• The frog's blood vascular system is well developed and of the closed type. A lymphatic system is also present.
• The blood vascular system has the heart, blood vessels and blood. The lymphatic system has lymph, lymph channels and lymph nodes.
• The heart is muscular, lies in the upper part of the body cavity and is covered by a membrane, the pericardium. It has three chambers: two atria and one ventricle.
• The sinus venosus, a triangular chamber, collects blood from the major veins (the vena cava) and joins the right atrium.
• On the ventral side the ventricle leads into a sac-like conus arteriosus. Arteries carry blood from the heart to the body (arterial system); veins bring it back (venous system).
• Frogs have special venous connections: the hepatic portal system between the liver and the intestine, and the renal portal system between the kidneys and the lower parts of the body.
• Blood is plasma plus cells: RBCs (erythrocytes), WBCs (leucocytes) and platelets. The frog's RBCs are nucleated and contain the red pigment haemoglobin.
• Lymph differs from blood: it lacks some proteins and has no RBCs.
• Blood carries nutrients, gases and water to where they are needed, and the pumping of the muscular heart drives the circulation. The frog has a single circulation.

7. Excretory system

NCERT §7.2.2

• Nitrogenous wastes are removed by a well-developed excretory system made of a pair of kidneys, the ureters, the cloaca and the urinary bladder.
• The kidneys are compact, dark red, bean-like organs placed a little towards the back of the body cavity, one on each side of the vertebral column.
• Each kidney is made of many structural and functional units called uriniferous tubules, or nephrons.
• In the male, the two ureters coming out of the kidneys also carry sperms, so they act as urinogenital ducts and open into the cloaca.
• In the female, the ureters and the oviducts open separately into the cloaca.
• A thin-walled urinary bladder lies ventral to the rectum and also opens into the cloaca.
• Blood carries the excretory wastes to the kidneys, where they are separated and excreted. The frog excretes urea, so it is ureotelic.

8. Nervous and endocrine control

NCERT §7.2.2

• Control and coordination in the frog is highly evolved and uses both the neural system and the endocrine glands.
• Hormones secreted by the endocrine glands coordinate the organs chemically. The frog's prominent endocrine glands are the pituitary, thyroid, parathyroid, thymus and pineal body, together with the pancreatic islets, the adrenals and the gonads.
• The nervous system has three parts: the central nervous system (brain and spinal cord), the peripheral nervous system (cranial and spinal nerves) and the autonomic nervous system (sympathetic and parasympathetic).
• Ten pairs of cranial nerves arise from the brain.
• The brain lies inside a bony brain box, the cranium. It has three regions: fore-brain, mid-brain and hind-brain.
• The fore-brain has the olfactory lobes, a pair of cerebral hemispheres and the unpaired diencephalon. The mid-brain is marked by a pair of optic lobes. The hind-brain has the cerebellum and the medulla oblongata.
• The medulla oblongata leaves the cranium through the foramen magnum and continues as the spinal cord, which is enclosed in the vertebral column.

9. Sense organs

NCERT §7.2.2

• The frog senses touch through sensory papillae, taste through taste buds, smell through the nasal epithelium, sight through its eyes and sound through the tympanum and internal ears.
• Of these, only the eyes and the internal ears are well-organised structures. The others are cellular aggregations around nerve endings.
• The eyes are a pair of spherical structures in the orbits of the skull. They are simple eyes, each with only one unit.
• The frog has no external ear; only the tympanum is visible from outside.
• The ear serves both hearing and balance (equilibrium).

10. Reproduction and development

NCERT §7.2.2

• The male reproductive organs are a pair of yellowish, ovoid testes, attached to the upper part of the kidneys by a double fold of peritoneum called the mesorchium.
• Ten to twelve vasa efferentia leave the testes and pass into the kidney on the same side, where they open into Bidder's canal. Bidder's canal connects with the urinogenital duct coming out of the kidney, and this duct opens into the cloaca.
• The cloaca is a small median chamber through which faecal matter, urine and sperms all pass to the outside.
• The female reproductive organs are a pair of ovaries lying near the kidneys, with no functional connection to them. A pair of oviducts arising from the ovaries opens into the cloaca separately.
• A mature female lays 2500 to 3000 ova in one go.
• Fertilisation is external and happens in water. Development includes a larval stage, the tadpole, which undergoes metamorphosis to become the adult.
• Frogs are useful to people: they eat insects and so protect crops, and they are an important link in food chains and food webs, keeping the ecosystem in balance. In some countries their muscular legs are eaten.

Must-know facts

1. Only four basic tissue types build all complex animals: epithelial, connective, muscular and neural; the heart contains all four.
2. Organ system = two or more organs doing a common function by physical and/or chemical interaction; the sharing of work is division of labour.
3. Frog: class Amphibia, phylum Chordata; common Indian species Rana tigrina; poikilotherm.
4. Aestivation = summer sleep, hibernation = winter sleep, both in deep burrows; gas exchange is then only through the skin.
5. The frog never drinks water; it absorbs water through its moist, mucus-covered skin.
6. Body = head + trunk; no neck, no tail.
7. Fore limb 4 digits, hind limb 5 digits (larger, muscular, webbed).
8. Male frog: vocal sacs, and a copulatory pad on the first digit of each fore limb.
9. Short alimentary canal because the frog is carnivorous; tongue bilobed; bile and pancreatic juice reach the duodenum through a common bile duct.
10. In water: cutaneous respiration only. On land: buccal cavity, skin and lungs.
11. Heart: three chambers (two atria, one ventricle), pericardium, sinus venosus joining the right atrium, conus arteriosus on the ventral side.
12. Hepatic portal system (liver–intestine) and renal portal system (kidney–lower body) are both present in the frog.
13. Frog RBCs are nucleated; lymph lacks some proteins and RBCs; circulation is closed and single.
14. Frog is ureotelic; kidneys are made of uriniferous tubules (nephrons).
15. Male ureter = urinogenital duct; in the female, ureters and oviducts open separately into the cloaca.
16. Ten pairs of cranial nerves; optic lobes in the mid-brain; medulla leaves through the foramen magnum.
17. Eyes and internal ears are the only well-organised sense organs; no external ear, only a tympanum; the ear also balances.
18. 10–12 vasa efferentia → Bidder's canal → urinogenital duct → cloaca; testes held by the mesorchium.
19. A female lays 2500–3000 ova at a time; fertilisation external in water; tadpole larva metamorphoses into the adult.

Common traps

Key terms

Tissue

A group of similar cells, with their intercellular substance, doing a particular job.

Organ system

Two or more organs that work together, physically and/or chemically, for one common function.

Division of labour

The sharing of the body's work among cells, tissues, organs and organ systems.

Morphology

Study of the form of an organism; in animals, the external appearance of its organs and parts.

Anatomy

In animals, the study of the form of the internal organs.

Poikilotherm

A cold-blooded animal whose body temperature follows that of its surroundings.

Aestivation

Summer sleep in a burrow to escape extreme heat.

Hibernation

Winter sleep in a burrow to escape extreme cold.

Nictitating membrane

A membrane covering the frog's eye that protects it in water.

Tympanum

The membrane beside the frog's eye that receives sound; the only visible part of its ear.

Cutaneous respiration

Gas exchange through the moist skin by diffusion.

Pulmonary respiration

Breathing with the lungs.

Sinus venosus

Triangular chamber that receives blood from the vena cava and joins the right atrium.

Conus arteriosus

Sac-like structure on the ventral side of the heart into which the ventricle opens.

Portal system

A special venous link between two organs: hepatic portal (intestine–liver) and renal portal (lower body–kidney).

Ureotelic

Excreting nitrogenous waste mainly as urea.

Urinogenital duct

The male frog's ureter, which carries both urine and sperms to the cloaca.

Mesorchium

Double fold of peritoneum holding each testis to the kidney.

Bidder's canal

Canal in the kidney into which the vasa efferentia open, linking them to the urinogenital duct.

Metamorphosis

The change of the tadpole larva into the adult frog.