Chapter Introduction
Structural Organisation in Animals bridges the gap between cell biology and whole-animal physiology. In Chapter 7 of NCERT Class 11 Biology, you first study the four types of animal tissues, then apply that knowledge to understand the body organisation of three model organisms — earthworm (Pheretima), cockroach (Periplaneta), and frog (Rana). These organisms represent annelids, arthropods, and amphibians respectively. Diagrams, system-by-system comparison, and NCERT terminology are the key to scoring well in this chapter.
Key Definitions
| Term | Definition |
|---|---|
| Tissue | Group of structurally and functionally similar cells plus intercellular substance |
| Epithelium | Covering/lining tissue; cells tightly packed, no intercellular spaces, on basement membrane |
| Connective tissue | Tissue that links, supports, or binds other tissues; cells scattered in matrix |
| Muscular tissue | Contractile tissue; specialised for movement |
| Neural tissue | Tissue specialised for receiving, transmitting, and processing stimuli |
| Neuron | Unit of neural tissue; cell body, dendrites, axon |
| Setae | Chitinous bristles in earthworm; aid in locomotion |
| Clitellum | Glandular band in earthworm segments 14–16; helps in cocoon formation |
| Hepatic caecae | Finger-like glandular outgrowths at midgut-hindgut junction in cockroach |
| Malpighian tubules | Excretory organs in cockroach; 100–150 thin yellowish tubes |
| Spiracles | External openings for respiration in cockroach; 10 pairs |
| Typhlosole | Internal fold of intestine in earthworm; increases absorption surface |
| Ostia | Slit-like openings in cockroach heart; 13 pairs; allow blood entry |
| Schizocoelom | Body cavity formed by splitting of mesoderm (arthropods, annelids) |
| Haemocoel | Body cavity filled with blood; in cockroach |
Animal Tissues
1. Epithelial Tissue
Epithelial tissue covers body surfaces, lines body cavities and ducts, and forms glands. Cells are tightly packed on a basement membrane with no intercellular spaces.
| Type | Structure | Location | Function |
|---|---|---|---|
| Simple squamous | Single layer of flat, thin cells | Alveoli, blood vessels, Bowman's capsule | Filtration, diffusion |
| Stratified squamous | Many layers; outer flat, lower cuboidal | Skin, oesophagus | Protection against wear and tear |
| Simple cuboidal | Single layer of cube-shaped cells | Kidney tubules, ducts of salivary glands | Secretion, absorption |
| Simple columnar | Single layer of tall, pillar-like cells; may have microvilli | Intestine, stomach lining | Absorption, secretion |
| Ciliated columnar | Columnar cells with cilia on free surface | Respiratory tract, fallopian tubes | Push mucus/ova forward |
| Glandular epithelium | Columnar/cuboidal cells modified to secrete | Salivary glands, sweat glands, gastric glands | Secretion of mucus, enzymes, sweat |
2. Connective Tissue
Connective tissues have cells scattered in an abundant matrix (ground substance + fibres). They are the most widely distributed tissue type.
| Type | Structure | Location | Function |
|---|---|---|---|
| Areolar (loose connective) | Fibroblasts, macrophages, mast cells in loose matrix with collagen and elastin fibres | Below skin, between organs | Binding, packing, repair |
| Adipose | Fat-filled cells (adipocytes) in sparse matrix | Below skin, around heart, kidneys | Insulation, energy storage |
| Dense regular | Dense collagen fibres in parallel rows; fibroblasts | Tendons (muscle-to-bone), ligaments (bone-to-bone) | Strong attachment |
| Dense irregular | Collagen fibres in irregular pattern | Dermis of skin, around organs | Strength in multiple directions |
| Cartilage | Chondrocytes in lacunae; matrix of chondroitin sulphate; no blood vessels | Ear pinna, nasal septum, tracheal rings, between vertebrae | Flexibility, smooth surface at joints |
| Bone | Osteocytes in lacunae; matrix of collagen + calcium phosphorus salts (hydroxyapatite); Haversian system | Skeletal system | Support, protection, mineral storage, blood cell formation |
| Blood | Plasma (55%) + RBCs, WBCs, platelets (45%) | Blood vessels | Transport of O₂, CO₂, nutrients, hormones, antibodies |
Note: Blood and lymph are fluid connective tissues. Cartilage and bone are solid connective tissues.
3. Muscular Tissue
| Feature | Skeletal (Striated) | Smooth (Non-striated) | Cardiac |
|---|---|---|---|
| Location | Attached to bones | Walls of intestine, blood vessels, iris | Walls of heart |
| Shape | Long, cylindrical, unbranched | Spindle-shaped, pointed ends | Branched, cylindrical |
| Striations | Present (dark A-band, light I-band) | Absent | Present (faint) |
| Nuclei | Multinucleate, peripheral | Uninucleate, central | Uninucleate, central |
| Voluntary/Involuntary | Voluntary | Involuntary | Involuntary |
| Speed of contraction | Fast | Slow | Rhythmic, never fatigues |
| Intercalated discs | Absent | Absent | Present (gap junctions) |
| Control | Somatic nervous system | Autonomic nervous system | Autonomic + myogenic |
4. Neural Tissue
- Neuron (nerve cell): Structural and functional unit of nervous system
- Cell body (cyton/soma): Contains nucleus, Nissl's granules (granular bodies for protein synthesis)
- Dendrites: Short, branched processes; receive signals
- Axon: Long, unbranched process; transmits signals away from cell body; covered by myelin sheath (Schwann cells); ends in synaptic knobs
- Neuroglia (glial cells): Support, protect, and nourish neurons; 50% of neural tissue volume
- Synapse: Junction between two neurons or neuron and effector organ; neurotransmitters (acetylcholine) carry signals across
Earthworm (Pheretima posthuma)
Morphology
- Long, cylindrical, segmented (metameric) body; 100–150 segments
- Prostomium: Lobe above mouth; sensory; not a true segment
- Peristomium: First segment; contains mouth
- Clitellum: Glandular band on segments 14–16; forms cocoon; used in identification
- Setae: Chitinous bristles in each segment except first, last, and clitellum; S-shaped; aid locomotion
- Female genital pore: Single; on 14th segment
- Male genital pores: Paired; on 18th segment
- Dorsal pores: Mid-dorsal, from segment 12 onwards; release coelomic fluid
- Nephridiopores: On body surface; excretory openings
Digestive System
- Mouth (segment 1) → Buccal cavity (1–3) → Pharynx (3–4) → Oesophagus (5–7) → Gizzard (8–9; grinds food) → Stomach (9–14; calciferous glands neutralise humic acid) → Intestine (15 to last; typhlosole from segment 26 increases absorption) → Anus
Circulatory System
- Closed type: Blood flows in blood vessels
- Hearts: 4 pairs (lateral oesophageal hearts on 7, 9; lateral hearts on 12, 13); pump blood from dorsal to ventral vessel
- Blood vessels: Dorsal vessel (distributes anteriorly), ventral vessel (collects from hearts), sub-neural vessel, lateral oesophageal vessels
- Blood: Red due to dissolved haemoglobin; no RBCs
Excretory System
- Nephridia: Excretory organs; three types:
- Septal nephridia: On septa from segment 15 onwards; open into intestine (enteronephric)
- Integumentary nephridia: Attached to body wall; open on body surface (exonephric)
- Pharyngeal nephridia: In 4th, 5th, 6th segments; open into buccal cavity/phaerinx
- Nephridium structure: nephrostome (funnel) → neck → body of nephridium → terminal duct
Nervous System
- Nerve ring: Cerebral ganglia (supra-pharyngeal) + circum-pharyngeal connectives + sub-pharyngeal ganglia
- Ventral nerve cord: Mid-ventral, with segmental ganglia in each segment
- Sense organs: Epidermal receptors (tactile), buccal receptors (chemoreceptors)
Reproductive System
- Hermaphrodite (bisexual): Both male and female organs in same individual
- Male organs: 2 pairs testes (segments 10, 11); testis sacs; seminal vesicles (store sperm); vas deferens; prostate glands; accessory glands
- Female organs: 1 pair ovaries (segment 13); oviduct; female genital pore (segment 14)
- Cross-fertilisation: Sperm from one worm transferred to another during copulation; stored in spermathecae (segments 6–9)
- Cocoon: Formed by clitellum; contains eggs and sperm; development occurs inside
Cockroach (Periplaneta americana)
Morphology
- Brown/black body; 3 regions: head, thorax, abdomen
- Head: Triangular; compound eyes (2000 ommatidia each), antennae (sensory), mouthparts (biting and chewing type: labrum, mandibles, maxillae, labium, hypopharynx)
- Thorax: 3 segments — prothorax, mesothorax, metathorax; each bears a pair of legs; meso- and meta-thorax bear wings
- Abdomen: 10 segments in both sexes; in males, 7th sternum is boat-shaped; in females, 7th sternum has a large brood pouch
- Anal cerci: Paired; on 10th segment in both sexes
- Anal styles: Paired; on 9th segment in males only (absent in females)
Digestive System
- Foregut (stomodaeum): Mouth → pharynx → oesophagus → crop (stores food) → gizzard (grinding; 6 chitinous teeth, bristles)
- Midgut (mesenteron): True stomach; digestion and absorption; hepatic caecae (6–8 finger-like glandular outgrowths at anterior end secrete digestive juice)
- Hindgut (proctodaeum): Ileum → colon → rectum → anus; reabsorption of water
- Gastric caecae at junction of midgut and hindgut (sometimes confused with hepatic caecae)
Circulatory System
- Open type: Blood flows through haemocoel (body cavity)
- Heart: 13-chambered, elongated tube; opens into haemocoel anteriorly; ostia (13 pairs) allow blood entry
- Blood (haemolymph): Colourless; no RBCs; no haemoglobin; plasma + haemocytes; does NOT carry oxygen
- Respiration: Through tracheal system — spiracles (10 pairs: 2 thoracic, 8 abdominal) → tracheae → tracheoles (reach cells directly)
Excretory System
- Malpighian tubules: 100–150 fine, yellowish, thread-like tubes at junction of midgut and hindgut; extract nitrogenous waste from haemolymph; convert to uric acid
- Fat body nephrocytes: Accessory excretory structures
- Uricotelic: Excrete uric acid as nitrogenous waste
Nervous System
- Nerve ring: Supra-oesophageal ganglia (brain) + circum-oesophageal connectives + sub-oesophageal ganglia
- Ventral nerve cord: Double, solid; 3 thoracic ganglia + 6 abdominal ganglia
- Sense organs: Compound eyes, antennae (smell), maxillary palps (taste), labial palps, anal cerci (wind/ vibration)
Reproductive System
- Male: 1 pair testes (4th–6th abdominal segments); vas deferens; seminal vesicles; accessory glands (mushroom gland); ejaculatory duct; male genital pore (9th segment)
- Female: 2 large ovaries (2nd–6th abdominal segments); oviducts; spermatheca (stores sperm after mating); colleterial glands (secrete egg case/ootheca); female genital pore (8th segment)
- Ootheca: Dark reddish-brown egg case; contains 14–16 eggs; deposited in warm, humid places
Frog (Rana tigrina)
Morphology
- Body divisible into head and trunk (no neck); streamlined for swimming
- Skin: Moist, smooth, slippery; respiratory function (cutaneous respiration); mucous glands keep it moist; chromatophores give colour
- Head: Eyes (bulging, nictitating membrane for protection); tympanum (eardrum behind eye); nostrils; mouth (wide)
- Forelimbs: 4 digits each; Hindlimbs: 5 digits each, webbed for swimming
- Sexual dimorphism: Males have vocal sacs (croaking) and nuptial pads (on first digit for gripping female during amplexus)
Digestive System
- Alimentary canal: Mouth → buccal cavity (teeth on upper jaw only — maxillary + vomerine; lower jaw toothless) → oesophagus → stomach → intestine (duodenum + ileum) → rectum → cloaca
- Pancreas: Between duodenum and stomach; produces pancreatic juice
- Liver: Largest gland; 3 lobes; produces bile (stored in gall bladder)
- Bile duct: Common duct for bile and pancreatic juice enters duodenum
- Cloaca: Common chamber for digestive, excretory, and reproductive products
Circulatory System
- Closed type; 3-chambered heart (2 atria + 1 ventricle); double circulation (pulmonary + systemic)
- Arterial system: Aortic arches arise from truncus arteriosus — carotid (head), systemic (body), pulmocutaneous (lungs + skin)
- Venous system: Two precavals (anterior) + one postcaval (posterior); hepatic portal system and renal portal system present
- RBCs: Nucleated, oval, biconvex (unlike human RBCs)
Respiratory System
- Cutaneous respiration (skin): Occurs on land and in water; most important in hibernation
- Buccal respiration: Lining of buccal cavity; on land at rest
- Pulmonary respiration: Lungs (pair of thin-walled sacs); during active state on land
- Breathing mechanism: Positive pressure breathing (mouth floor pushed up forces air into lungs)
Excretory System
- Kidneys: Paired; mesonephric type (opisthonephros); in dorsal body wall
- Ureters: Carry urine from kidneys; in males, also carry sperm (urinogenital duct)
- Urinary bladder: Thin-walled sac; stores urine; opens into cloaca
- Uricotelic on land (conserves water), ammonotelic in water (ammonia diluted by water)
Nervous System
- Brain: Forebrain (olfactory lobes, cerebral hemispheres, diencephalon), Midbrain (optic lobes — well-developed for vision), Hindbrain (cerebellum, medulla oblongata)
- Cranial nerves: 10 pairs
- Spinal nerves: 10 pairs
- Sense organs: Eyes (binocular vision), ears (inner ear with semicircular canals), lateral line organs (in water)
Reproductive System
- Male: Paired testes (attached to kidneys by mesorchium); vasa efferentia enter Bidder's canal → vas deferens; seminal vesicle; male gonoduct opens into cloaca
- Female: Paired ovaries (attached to kidneys by mesovarium); oviducts (coiled, with ovisac at base); oviductal funnel (ostium); opens into cloaca
- Fertilisation: External; in water; during amplexus
- Development: Indirect; tadpole larva → metamorphosis (thyroid hormone thyroxine) → adult frog
Skeletal System
- Axial skeleton: Skull (9 bones + cartilages), vertebral column (9 vertebrae + urostyle), sternum
- Appendicular skeleton: Pectoral girdle (clavicle, coracoid, scapula), pelvic girdle (ilium, ischium, pubis), bones of forelimbs and hindlimbs
- Vertebral column: Cervical (1 atlas) + trunk (7) + sacral (1) + urostyle (fused caudal vertebrae) = 9 vertebrae + urostyle
Comparison: Earthworm vs Cockroach vs Frog
| Feature | Earthworm | Cockroach | Frog |
|---|---|---|---|
| Phylum | Annelida | Arthropoda | Chordata (Amphibia) |
| Body cavity | Coelom (schizocoel) | Haemocoel | Coelom (enterocoel) |
| Segmentation | Metameric (externally + internally) | Only external | Not segmented |
| Circulatory system | Closed | Open | Closed |
| Heart | 4 pairs of hearts (no true heart) | 13-chambered tube | 3-chambered |
| Blood pigment | Haemoglobin (dissolved) | None | Haemoglobin (in RBCs) |
| Respiration | Through skin (cutaneous) | Tracheal system (spiracles) | Cutaneous + buccal + pulmonary |
| Excretory organs | Nephridia | Malpighian tubules | Kidneys (mesonephric) |
| Nitrogenous waste | Ammonia (ammonotelic) | Uric acid (uricotelic) | Ammonia in water / urea on land |
| Sex | Hermaphrodite (bisexual) | Unisexual (dimorphic) | Unisexual (dimorphic) |
| Fertilisation | Cross-fertilisation (in cocoon) | Internal | External (in water) |
| Locomotion | Setae + muscle contraction | Legs + wings | Limbs (jumping + swimming) |
Important Diagrams to Practise
1. Types of epithelial tissue — squamous, cuboidal, columnar, stratified squamous (draw cell shape and nucleus position) 2. Three types of muscle fibres — skeletal (striated, multinucleate), smooth (spindle, uninucleate), cardiac (branched, intercalated discs) 3. Earthworm — external view (dorsal) — label: prostomium, peristomium, clitellum, setae, genital pores, anal segment 4. Cockroach — external view (dorsal) — label: head, pronotum, thorax, abdomen, wings, legs, antennae, cerci 5. Cockroach — digestive system — label: mouth, pharynx, oesophagus, crop, gizzard, hepatic caecae, midgut, Malpighian tubules, hindgut, anus 6. Frog — digestive system — label: mouth, oesophagus, stomach, liver, gall bladder, pancreas, intestine, rectum, cloaca 7. Neuron structure — label: cyton, dendrites, axon, myelin sheath, nodes of Ranvier, synaptic knobs
Common Mistakes to Avoid
1. Calling earthworm blood red due to RBCs — earthworm has haemoglobin dissolved in plasma; there are no red blood cells. Cockroach blood has no pigment at all. 2. Confusing hermaphrodite with self-fertilisation — earthworm is hermaphrodite but does NOT self-fertilise; cross-fertilisation occurs through copulation between two individuals. 3. Saying cockroach respires through lungs — cockroach respires through the tracheal system (spiracles → tracheae → tracheoles). No lungs. 4. Mixing up nephridia and Malpighian tubules — nephridia are in earthworm; Malpighian tubules are in cockroach; kidneys are in frog. Each organism has its own excretory organ. 5. Forgetting that frog teeth are only on the upper jaw — frog has maxillary and vomerine teeth on the upper jaw; the lower jaw is toothless. 6. Confusing anal cerci and anal styles in cockroach — anal cerci are present in both sexes on the 10th segment; anal styles are present only in males on the 9th segment.
Exam Tips
- 5-mark questions often ask: "Describe the digestive system of cockroach" or "Describe the reproductive system of earthworm." Draw a labelled diagram alongside the description.
- 3-mark questions frequently ask: "Differentiate between skeletal, smooth, and cardiac muscle" or "Compare open and closed circulatory system." Use tables.
- 1-mark questions target definitions and one-word answers: clitellum, hepatic caecae, Malpighian tubules, typhlosole, nuptial pad, uricotelic.
- Diagram questions: cockroach digestive system and earthworm external features are commonly asked. Practise labels carefully.
- Sexual dimorphism in cockroach: remember — anal styles (males only), broader abdomen (females), boat-shaped 7th sternum (males), brood pouch (females).
- For frog questions, always mention "cloaca" — the common chamber for digestive, excretory, and reproductive openings.
Quick Revision Notes
- Animal tissues: epithelial (covering), connective (support), muscular (movement), neural (impulse)
- Epithelial: squamous (flat, diffusion), cuboidal (cube, secretion), columnar (tall, absorption), stratified (many layers, protection)
- Connective: areolar (packing), adipose (fat), dense (tendons/ligaments), cartilage (flexible), bone (rigid), blood (fluid)
- Muscular: skeletal (voluntary, striated, multinucleate), smooth (involuntary, non-striated, uninucleate), cardiac (involuntary, faintly striated, intercalated discs)
- Earthworm: metameric, closed circulation, dissolved haemoglobin, nephridia, hermaphrodite, clitellum (segments 14–16), typhlosole
- Cockroach: 3 body regions, open circulation, tracheal respiration, Malpighian tubules (uricotelic), unisexual, ootheca
- Frog: 3-chambered heart, 3 modes of respiration, kidneys, external fertilisation, metamorphosis (thyroxine), cloaca
- Earthworm excretory: septal, integumentary, pharyngeal nephridia
- Cockroach excretory: Malpighian tubules + fat body nephrocytes
- Frog excretory: mesonephric kidneys + urinary bladder
- Cockroach sexual dimorphism: anal styles in males, brood pouch in females
- Frog sexual dimorphism: vocal sacs and nuptial pads in males
Frequently Asked Questions
The four types are: (1) Epithelial tissue — covering and lining (squamous, cuboidal, columnar, stratified); (2) Connective tissue — binding and support (areolar, adipose, dense, cartilage, bone, blood); (3) Muscular tissue — movement (skeletal, smooth, cardiac); (4) Neural tissue — impulse conduction (neurons and neuroglia).