Structural Organisation in AnimalsNEET Zoology · Class 11 · NCERT Chapter 7

Introduction: Levels of Organisation

As you move from simple to complex animals, the body is organised at four increasing levels of complexity.

1. Cellular level: individual cells carry out all life functions. Found in the simplest animals (sponges, Porifera).
2. Tissue level: cells of the same type group together to carry out a shared function. Seen in Coelenterata (cnidarians).
3. Organ level: two or more tissue types combine to form an organ with a specific function. Seen in Platyhelminthes (flatworms).
4. Organ system level: multiple organs work together as a system. Found in all higher animals, including humans.

Expect 1 NEET question from this chapter each year. The most commonly tested areas are the epithelial subtypes, cell junctions, distinguishing features of the three model organisms (earthworm, cockroach, frog) and the type of excretory organ each one uses.

Animal Tissues Overview

All animal bodies are built from four basic tissue types. Each type has a distinct structure and function.

• Epithelial tissue: covers body surfaces, lines ducts and organs, forms glands. Acts as a barrier and selectively allows exchange of materials.
• Connective tissue: most abundant tissue type. Connects, binds, supports and fills spaces. Cells are scattered in an extracellular matrix. Includes blood, bone, cartilage, tendons and ligaments.
• Muscular tissue: capable of contraction. Responsible for all movement. Three types: skeletal, smooth and cardiac.
• Neural tissue: receives stimuli and transmits nerve impulses. Made of neurons and supporting cells called neuroglia.

Epithelial Tissue

Epithelial tissue covers all body surfaces (skin), lines all internal cavities and organs (gut, lung, kidney tubules), and forms glands. Key features of epithelium:

• Cells are tightly packed with very little extracellular matrix between them.
• Avascular (no blood vessels): gets nutrients by diffusion from the underlying connective tissue.
• Rests on a basement membrane (basal lamina) that anchors it to the connective tissue below.
• Has a free surface (apical surface) exposed to the exterior or a body cavity.
• High capacity for regeneration (cells divide frequently to replace worn-out cells).

Simple Epithelium

Simple epithelium is a single layer of cells. Used where thin barriers are needed for exchange, filtration or secretion.

• Squamous (pavement) epithelium: cells are flat and tile-like. Forms a smooth surface. Found in the lining of blood vessels (endothelium) and body cavities (mesothelium).
• Cuboidal epithelium: cells are cube-shaped. Found in kidney tubules, salivary gland ducts, and thyroid follicles. Involved in secretion and absorption.
• Columnar epithelium: cells are taller than they are wide (column-shaped). Lines the stomach, intestine and gall bladder. Involved in absorption and secretion. Cells may have microvilli on the free surface to increase the absorptive area (brush border in intestinal lining).
• Ciliated columnar epithelium: columnar cells with cilia on the free surface. Lines the trachea, bronchi, fallopian tubes and ventricles of the brain. Cilia move mucus or other materials in one direction.

Compound and Glandular Epithelium

Compound epithelium has two or more layers of cells. It does not perform much absorption or secretion; its main role is protection from physical and chemical stress. Found in the skin (stratified squamous), the oral cavity, pharynx, oesophagus and urethra.

Glandular epithelium is specialised for secretion. It may form unicellular glands (like goblet cells, which secrete mucus directly) or multicellular glands. Glands are of two types:

• Exocrine glands: secrete into ducts that open onto a surface. Examples: sweat glands, salivary glands, gastric glands.
• Endocrine glands: ductless; secrete hormones directly into the blood. Examples: thyroid, adrenal, pituitary.

Cell Junctions

Epithelial cells are held together by three types of junctions:

• Tight junctions (zonula occludens): seal adjacent cells so that nothing can leak between them. Important in the intestinal lining and kidney tubules to ensure selective absorption.
• Adhering junctions (zonula adherens or desmosomes): mechanically attach adjacent cells to each other. Provide strength against stretching forces. Common in skin and the uterus.
• Gap junctions: form small channels between adjacent cells that allow ions and small molecules to pass directly. Important for electrical coupling, as in cardiac muscle.

Connective Tissue

Connective tissue is the most abundant and widely distributed tissue in the body. It connects and supports all other tissues. Its cells are sparsely scattered in a large extracellular matrix that they secrete. The matrix has two components: ground substance (fluid, gel or solid) and fibres (collagen, elastin, reticular).

Loose Connective Tissue

• Areolar tissue: the most common loose connective tissue. Fibres (collagen and elastin) are loosely arranged. Fills spaces between organs; attaches skin to underlying muscles. Contains fibroblasts, mast cells and macrophages.
• Adipose tissue: fat cells (adipocytes) packed with lipid droplets. Found beneath the skin and around organs. Stores energy, insulates against heat loss and cushions organs. Very few fibres.

Dense Connective Tissue

• Dense regular: tightly packed parallel collagen fibres for great tensile strength in one direction. Found in tendons (muscle to bone) and ligaments (bone to bone).
• Dense irregular: collagen fibres arranged in various directions, resisting stress from multiple angles. Found in the dermis of the skin.

Specialised Connective Tissue

• Cartilage: semi-rigid matrix (chondrin). No blood vessels in the matrix. Made by chondrocytes. Found at the ends of bones (articular cartilage), the nose, ear pinna, intervertebral discs and tracheal rings. Types: hyaline (most common), fibrocartilage, elastic.
• Bone: hard matrix calcified with calcium salts (hydroxyapatite). Made by osteoblasts; maintained by osteocytes. Very strong and rigid. Forms the skeleton.
• Blood: fluid matrix (plasma) with red blood cells (erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes) suspended in it. Transports gases, nutrients, hormones and waste. Classified as connective tissue because it develops from mesoderm and has a matrix (plasma).

Muscular Tissue

Muscular tissue is made of elongated cells called muscle fibres. Muscle fibres can contract because they contain the proteins actin and myosin.

• Skeletal (striated) muscle: attached to bones, responsible for voluntary movement. Fibres are long, cylindrical and multinucleated. Striated (cross-striations visible) because actin and myosin are arranged in regular units (sarcomeres).
• Smooth (visceral) muscle: lines the walls of internal organs (intestine, uterus, blood vessels). Involuntary. Spindle-shaped, uninucleate cells. No striations.
• Cardiac muscle: found only in the heart. Involuntary but striated. Cells are branched and joined by intercalated discs, which contain gap junctions. This allows the heart to contract as one unit.

Neural Tissue

Neural tissue is the most highly specialised tissue. Its cells are excitable: they can generate and conduct electrical impulses (action potentials).

• Neurons: the functional units of neural tissue. Each neuron has a cell body (soma) with a nucleus, dendrites that receive signals, and a single long axon that transmits signals. Neurons do not divide after maturity.
• Neuroglia (glial cells): supporting cells that surround and protect neurons. They make up about half the volume of the brain. Types include astrocytes, oligodendrocytes (which form myelin in the CNS) and Schwann cells (which form myelin in the PNS).

Organs and Organ Systems

An organ is made of two or more tissue types working together for a specific function. For example, the stomach wall contains epithelial, muscular and connective tissue. When organs work together in a coordinated way, they form an organ system(digestive system, circulatory system, nervous system, etc.). Complex animals have all four tissue types contributing to multiple organ systems.

Earthworm (Pheretima)

Pheretima posthuma is the common Indian earthworm. It is the main NCERT model organism for annelids.

• Habitat: moist soil, often emerges at night or after rain.
• Body plan: cylindrical, bilaterally symmetrical. About 100 to 120 segments (metameres). Segment 1 (prostomium) is a small pre-oral lobe. Segment 14 to 16 forms the clitellum (thick glandular band, important for reproduction).
• Setae: each segment (except 1, 14 to 16 and the last) has four pairs of S-shaped chitinous setae on the ventral surface. Used for locomotion.
• Circulatory system: closed. Blood always stays inside blood vessels. The dorsal blood vessel contracts to drive blood forward. Four to five pairs of aortic arches (hearts) in segments 7 to 11 pump blood from the dorsal to the ventral vessel.
• Excretion: through nephridia (three types: pharyngeal, integumentary and septal). Earthworms are ureotelic.
• Reproduction: hermaphrodite (both testes and ovaries in the same individual). Cross-fertilisation occurs. Clitellum secretes a cocoon for developing embryos.
• Respiration: through moist skin (cutaneous respiration).
• Nervous system: two nerve cords fuse to form a bilobed cerebral ganglion (brain) in segment 3. The ventral nerve cord has paired ganglia in each segment.

Cockroach (Periplaneta)

Periplaneta americana is the American cockroach, the standard NCERT model organism for insects (Class Insecta, Phylum Arthropoda).

• Body divisions: head (fused segments), thorax (3 segments: pro-, meso-, metathorax) and abdomen (10 segments).
• Exoskeleton: hardened chitinous cuticle that covers the body. The thorax and abdomen have segmented plates: dorsal tergites, ventral sternites, and lateral pleurites.
• Wings: two pairs. Forewings (tegmina) are leathery and cover the hindwings at rest. Hindwings are membranous and used for flight.
• Circulatory system: open. Haemolymph (blood) flows freely through body cavities (sinuses). A muscular dorsal tubular heart pumps haemolymph forward. No respiratory pigment (blood does not carry oxygen).
• Respiratory system: tracheal system. Air enters through 10 pairs of spiracles (2 on thorax, 8 on abdomen). Tracheae branch into tracheoles that deliver oxygen directly to cells.
• Excretory system: Malpighian tubules (about 100 to 150) at the midgut-hindgut junction. Cockroaches are uricotelic.
• Sexual dimorphism: males have a pair of short anal cerci and a pair of anal styles. Females have a broader abdomen; no anal styles.
• Nervous system: supra-oesophageal ganglion (brain) in the head, connected by circum-oesophageal connectives to the sub-oesophageal ganglion. Ventral nerve cord has a ganglion in each thoracic and abdominal segment.

Frog (Rana)

Rana tigrina is the common Indian frog and the NCERT vertebrate model for Class 11. Frogs are amphibians (can live in water and on land).

• Body: divided into head and trunk (no neck or tail in adults). Skin is moist and smooth (no scales).
• Temperature: poikilotherm (cold-blooded). Body temperature varies with the environment. Hibernate in winter, aestivate in summer.
• Respiration: through moist skin (cutaneous), lungs (pulmonary) and the lining of the buccal cavity. Tadpoles breathe through gills.
• Circulatory system: closed. Three-chambered heart (two atria and one ventricle). Adults have a single circuit in adults except in the pulmocutaneous circuit.
• Excretion: through kidneys (mesonephros). Frogs are ureotelic.
• Reproduction: sexual; external fertilisation in water. The male calls to attract females; the eggs hatch into tadpoles (aquatic larval stage with gills).

Worked NEET Problems

Summary Cheat Sheet

• Levels of organisation: cellular (sponges) to tissue (cnidarians) to organ (flatworms) to organ system (higher animals).
• Four animal tissue types: Epithelial, Connective, Muscular, Neural.
• Epithelial features: tightly packed, avascular, rests on basement membrane, high regeneration rate.
• Simple epithelium subtypes: squamous (blood vessels), cuboidal (kidney tubules), columnar (intestine), ciliated columnar (trachea).
• Compound epithelium: two or more layers, mainly for protection (skin, oral cavity).
• Glandular epithelium: exocrine (ducts, e.g. sweat glands) vs endocrine (no ducts, secretes into blood).
• Cell junctions: tight (seal leaks), adhering (mechanical link), gap (ion/molecule channel).
• Loose connective tissue: areolar (most common), adipose (fat).
• Dense connective tissue: regular (tendons: muscle to bone; ligaments: bone to bone), irregular (dermis).
• Specialised connective tissue: cartilage (no blood vessels, chondrin matrix), bone (calcified, hard), blood (fluid plasma matrix).
• Muscular tissue: skeletal (voluntary, striated, multinucleate), smooth (involuntary, no striations, spindle cells), cardiac (involuntary, striated, intercalated discs with gap junctions).
• Neural tissue: neurons (excitable, long axon, do not divide) and neuroglia (support cells).
• Earthworm (Pheretima): clitellum on segments 14 to 16; setae for movement; closed circulation; nephridia for excretion; hermaphrodite; breathes through skin.
• Cockroach (Periplaneta): open circulation (haemolymph in sinuses); tracheal respiration via spiracles (10 pairs); Malpighian tubules for excretion (uricotelic); sexual dimorphism (anal styles in males).
• Frog (Rana): poikilotherm; breathes via skin, lungs and buccal cavity; closed circulation, 3-chambered heart; kidneys (mesonephros) for excretion; ureotelic; external fertilisation.

Use the interactive learning tools to explore the four tissue types, compare epithelial subtypes side by side, and see the earthworm, cockroach and frog compared across every organ system. Then work through the 14+ NEET PYQs with full solutions. To time yourself, take the free chapter mock test.