Structural Organisation in Animals Class 11 Notes

Animal tissues are groups of cells with similar structure and function. They are classified into four types: epithelial, connective, muscular and nervous (neural). Unlike plants, animals do not have meristematic or permanent tissue categories. The levels of organisation in animals are: cell → tissue → organ → organ system → organism.

Epithelial tissue covers body surfaces, lines cavities and forms glands. Cells are closely packed with little intercellular matrix and rest on a basement membrane. Epithelia are classified by number of layers and cell shape.

Simple Epithelia (single layer)

• Simple squamous: single layer of flat cells — lines blood vessels (endothelium) and body cavities (mesothelium). Allows diffusion and filtration.
• Simple cuboidal: cube-shaped cells — lines kidney tubules, ducts of glands. Functions in secretion and absorption.
• Simple columnar: tall cells — lines stomach and intestine. May have microvilli (brush border for absorption) or cilia (e.g. fallopian tubes).
• Ciliated columnar: cilia on the free surface move mucus or ovum — e.g. trachea, fallopian tubes.
• Glandular columnar: cells modified for secretion — form glands (sweat, salivary, mucous glands).

Stratified Epithelia (multiple layers)

• Stratified squamous: many layers; outermost cells flat — skin (keratinised, dead outer layer) and lining of mouth and oesophagus (non-keratinised). Protective function.
• Stratified cuboidal and columnar: rare, found in ducts of large glands.
• Transitional epithelium (urothelium): cells change shape from cuboidal to squamous as organ stretches — lines urinary bladder.

Connective tissue is the most abundant and widely distributed tissue. It has cells separated by an abundant extracellular matrix (fibres + ground substance). The matrix determines the tissue type.

Loose Connective Tissue

• Areolar: cells (fibroblasts, macrophages, mast cells) loosely arranged in gel-like matrix with all three fibre types (collagen, elastic, reticular). Found under skin and between organs — packing material.
• Adipose: cells (adipocytes) with large fat droplets — found under skin and around kidneys. Functions: insulation, energy storage, shock absorption.

Dense Connective Tissue

• Dense regular: collagen fibres in parallel bundles — tendons (muscle to bone) and ligaments (bone to bone).
• Dense irregular: collagen in all directions — dermis of skin, organ capsules.

Specialised Connective Tissues

• Cartilage: cells (chondrocytes) in lacunae, firm matrix (chondroitin sulphate). Hyaline (most common — trachea, nose), elastic (external ear), fibrocartilage (intervertebral discs — strongest).
• Bone: hardest connective tissue; matrix mineralised with calcium phosphate. Osteocytes in lacunae, Haversian canals run through osteons. Functions: support, protection, mineral storage, blood cell production.
• Blood: fluid matrix (plasma) with cells — RBC (haemoglobin, gas transport), WBC (immunity), platelets (clotting). Blood is a connective tissue because it has cells in a non-living matrix.
• Lymph: similar to blood but without RBCs and platelets; less protein.

Muscular tissue is made of elongated cells (muscle fibres) specialised for contraction using actin and myosin filaments. It produces movement, maintains posture and generates heat.

• Skeletal (striated, voluntary): long, cylindrical, multinucleate fibres with clear cross-striations. Attached to bones by tendons — under voluntary control. Functions: locomotion, breathing, facial expression.
• Smooth (visceral, involuntary): spindle-shaped, uninucleate fibres, no striations. Found in walls of gut, blood vessels, uterus — involuntary control. Functions: peristalsis, vasoconstriction.
• Cardiac: branched, striated, uninucleate (sometimes binucleate) fibres joined by intercalated discs (gap junctions for rapid electrical coupling). Found only in the heart — involuntary, autorhythmic. The heart beats ~72 times/min throughout life without fatigue.

Neural tissue consists of neurons (nerve cells) and neuroglial cells (support cells). A neuron has a cell body (soma) with Nissl bodies, a long axon, and many short dendrites. Axons may be myelinated (schwann cells form myelin sheath in PNS) for fast conduction. Neuroglia outnumber neurons and provide nutrition, insulation and protection.

• Sensory (afferent) neurons: carry impulses from receptors to CNS.
• Motor (efferent) neurons: carry impulses from CNS to effectors (muscles, glands).
• Interneurons (association): connect sensory and motor neurons within the CNS.
• Synapse: junction between two neurons where neurotransmitters cross the synaptic cleft.

The frog (Rana tigrina) is a representative amphibian studied in this chapter. Body is streamlined, divided into head and trunk (no neck). Skin is moist, glandular, and used for cutaneous respiration. Hind limbs are adapted for jumping and swimming; forelimbs are smaller.

Organ Systems of the Frog

• Digestive: complete alimentary canal — mouth, buccal cavity, oesophagus, stomach, small intestine (duodenum, ileum), large intestine, cloaca. Liver and pancreas are digestive glands. Teeth (maxillary + vomerine) are for gripping, not chewing.
• Respiratory: both lungs (pulmonary) and skin (cutaneous). Buccal cavity lining also helps. No ribs — ventilation by buccal cavity pumping.
• Circulatory: double circulation. Heart is 3-chambered (2 atria + 1 ventricle) with a sinus venosus receiving deoxygenated blood. Mixing of blood occurs but is minimised by the truncus arteriosus.
• Excretory: kidneys (mesonephric), ureters, urinary bladder (not connected to kidneys in males). Nitrogenous waste: urea (uricotelic in tadpoles, ureotelic in adults).
• Nervous: CNS (brain + spinal cord), PNS (cranial + spinal nerves), ANS (sympathetic + parasympathetic). Eyes have nictitating membrane for underwater vision.
• Reproductive: unisexual. Males have vocal sacs for croaking and nuptial pads. Fertilisation is external (in water). Development: egg → tadpole → metamorphosis → adult frog.

Example: A patient's blood smear shows an abnormally high count of white blood cells. What tissue is affected and what function might be altered?

Solution: Blood is a specialised connective tissue. White blood cells (leucocytes) are part of the immune system. An abnormally high WBC count (leukocytosis) suggests an active immune response — infection, inflammation, or in severe cases, leukaemia. The affected function is the body's defence against pathogens. The physician would examine the differential count (neutrophils, lymphocytes, eosinophils etc.) to pinpoint the cause.

Example: Why is the frog's heart 3-chambered rather than 4-chambered, and what is the consequence?

Solution: A 3-chambered heart (2 atria + 1 ventricle) allows some mixing of oxygenated and deoxygenated blood in the single ventricle. Frogs are amphibians — they supplement pulmonary breathing with cutaneous respiration, so the slightly less efficient double circulation is adequate. In contrast, 4-chambered hearts (birds, mammals, crocodiles) completely separate the two circuits for full separation of oxygenated and deoxygenated blood, supporting higher metabolic rates.

Exercises (NCERT Q1–Q2)

• 1) Draw a neat diagram of digestive system of frog.
• 2) Mention the function of the Ureters in frog.

Answers at a glance

1. 1
   Q1 → Draw a simple labelled tube with the associated glands, in the order the food passes. Label, in sequence: the mouth and the buccal cavity, the wide chamber behind the mouth, with the muscular bilobed tongue at its floor used for capturing the prey; the pharynx, through which the buccal cavity leads to the oesophagus; the oesophagus, the short tube carrying the food to the stomach; the stomach, into which the oesophagus opens and from which the intestine continues; the duodenum, the first part of the small intestine immediately after the stomach; the small intestine, the coiled region where final digestion and absorption take place, its inner wall bearing the numerous finger-like folds called villi and microvilli; the rectum, into which the undigested solid waste moves; and the cloaca, the small median chamber used to pass faecal matter, urine and sperms to the exterior, opening outside by the cloacal aperture. Then label the associated glands and their ducts: the liver, which secretes bile; the gall bladder, which stores that bile; the pancreas, the digestive gland that produces the pancreatic juice; the duct from the gall bladder and liver; and the duct from the pancreas, and show that both enter the duodenum through a single common bile duct, which is the standard feature to be shown. In the caption state that the canal ends in the cloaca, which receives the faeces, the urine and the gametes, so that the digestive, excretory and reproductive tracts all share one opening.
2. 2
   Q2 → The ureters carry the urine from the kidneys to the cloaca. The excretory system of the frog consists of a pair of kidneys, the ureters, the cloaca and a urinary bladder, and each kidney is composed of uriniferous tubules or nephrons into which the excretory wastes are carried by the blood, separated and then excreted. The urine passes along the ureters into the cloaca, a small median chamber that is used to pass faecal matter, urine and sperms to the exterior, and the thin-walled urinary bladder lies ventral to the rectum and opens into the cloaca as well. Two points should be added. The first is that the ureters are not the vasa efferentia, for the vasa efferentia are the ten to twelve ducts of the testis that carry the sperm, they enter the kidney on their side and open into Bidder’s canal, from which the sperm communicates with the urinogenital duct that comes out of the kidney and opens into the cloaca, so the sperm passes through the kidney and the ureter rather than the ureter being joined by the vasa efferentia. The second is that the ureters behave differently in the two sexes: in the male the ureters act as the urinogenital duct and open into the cloaca, whereas in the female the ureters and the oviducts open separately into it.

MCQ Questions

1 Mark Questions

1. 1
   How many body segments does an earthworm have? (Ans: 100–120)
2. 2
   What is the tympanum in a frog? (Ans: the circular membrane behind the eye used for hearing)
3. 3
   Name the respiratory organ of the cockroach. (Ans: tracheal system (spiracles and tracheae))
4. 4
   Where are the testes of the earthworm located? (Ans: in the 10th and 11th segments)

2 Mark Questions

1. 1
   Differentiate between the open and closed circulatory systems, giving one example of each animal.
2. 2
   Describe the structure of the alimentary canal of the frog. (Ans: mouth → buccal cavity → oesophagus → stomach → intestine → cloaca)
3. 3
   Why is the frog called a cold-blooded animal? (Ans: its body temperature changes with the surroundings)

3 Mark Questions

1. 1
   Describe the reproductive system of the earthworm. (Ans: hermaphrodite; testes 10–11, ovaries 13; accessory glands; clitellum secreting cocoon)
2. 2
   Describe the nervous system of the cockroach. (Ans: ventral nerve cord, paired ganglia, triploblastic brain - supra-oesophageal, sub-oesophageal; sense organs: compound eyes, antennae, sensory hairs)
3. 3
   Explain the digestive system of the cockroach. (Ans: foregut (mouth, crop, gizzard), midgut (hepatic caeca), hindgut (colon, rectum), with malpighian tubules for excretion)

5 Mark Questions

1. 1
   Describe the morphology and anatomy of the earthworm, covering its digestive, circulatory and excretory systems. (Ans: segmented body, setae, hydrostatic skeleton, closed circulation with blood glands, nephridia as excretory organs)
2. 2
   Describe the morphology of the frog with a labelled description of its respiratory system, explaining how it breathes on land and in water. (Ans: moist skin (cutaneous), buccopharyngeal respiration, pulmonary (lungs) on land)