Structural Organisation in Animals – Comprehensive Bullet-Point Notes

Hierarchical Levels: Cells → Tissues → Organs → Organ Systems

  • Living functions (digestion, respiration, reproduction, etc.) handled by a single cell in unicellular organisms but become compartmentalised in multicellular animals.
  • Definition of tissue: Group of similar cells + intercellular substance performing a specific function.
  • Only four basic tissue types build every complex animal: epithelial, connective, muscular, neural.
  • Organs (e.g.
    • stomach, lung, heart, kidney) are specific patterns & proportions of the four tissues.
  • Organ systems = 2 + organs that collaborate (physically/chemically) toward a common function (e.g. digestive, respiratory).
  • Overall principle: division of labour maximises survival efficiency.

Animal Tissues

Epithelial Tissue
  • General features
    • Free surface faces body fluid or exterior; forms coverings/linings.
    • Cells are compact, little matrix; linked by specialised cell junctions:
    • Tight (prevent leakage)
    • Adhering (cement neighbours)
    • Gap (cytoplasmic continuity → rapid signal/ion transfer).
  • Two organisational levels
    • Simple epithelium (single layer → lining/absorption/secretion)
    • Compound epithelium (multi-layered → protection from mechanical/chemical stress; skin, buccal cavity, pharynx, large ducts).
  • Simple sub-types (structural modification)
    • Squamous: flat, irregular; diffusion barriers (blood vessels, alveoli).
    • Cuboidal: cube-like; secretion/absorption (gland ducts, nephron tubules, PCT bears microvilli).
    • Columnar: tall, nuclei basal; secretion/absorption (stomach, intestine).
    • Ciliated (columnar/cuboidal): cilia propel mucus/particles (bronchioles, fallopian tubes).
    • Glandular (modified columnar/cuboidal)
    • Unicellular (goblet cell) or multicellular (salivary gland).
    • Classified by secretion route: exocrine (ducts: mucus, saliva, milk…) vs endocrine (duct-less, hormones → ECF/blood).
Connective Tissue
  • Ubiquitous; links, supports, binds, protects.
  • Common elements:
    • Cells secrete fibres (collagen/elastin) → strength & elasticity.
    • Ground substance (modified polysaccharides) = matrix.
  • Categories
    1. Loose connective tissue
    • Semi-fluid matrix; fibroblasts + macrophages + mast cells.
    • Areolar (beneath skin, supports epithelia)
    • Adipose (beneath skin; fat storage, energy reserve, insulation).
    1. Dense connective tissue
    • Fibres/fibroblasts compactly arranged.
    • Dense regular: parallel collagen rows (tendons →\rightarrow bone–muscle; ligaments →\rightarrow bone–bone).
    • Dense irregular: fibres oriented variously (dermis).
    1. Specialised connective tissue
    • Cartilage: solid, pliable matrix; chondrocytes in lacunae; embryonic skeleton, tip of nose, pinna, joints, intervertebral discs.
    • Bone: hard Ca2+^{2+} salt/ collagen matrix; osteocytes in lacunae; support, protection, blood-cell formation (marrow).
    • Blood: fluid matrix (plasma) + RBC, WBC, platelets; transport.
    • Adipose already covered but also specialised.
Muscle Tissue
  • Basic property: contractility (shorten on stimulus) →\rightarrow movement, posture, homeostatic adjustments.
  1. Skeletal (striated)
    • Long, cylindrical, multinucleate fibres with cross-striations.
    • Voluntary; anchored to bones (e.g. biceps).
  2. Smooth (unstriated/visceral)
    • Spindle-shaped, single nucleus, no striations; bundled via junctions.
    • Involuntary (vessels, stomach, intestine, bronchi).
  3. Cardiac
    • Striated, branched cells; intercalated discs = fused membranes + gap junctions for synchronised beat.
    • Exclusive to heart; myogenic rhythmicity.
Neural Tissue
  • Neurons (excitable) + Neuroglia (support, half tissue volume).
  • Stimulus →\rightarrow electro-chemical impulse along axon →\rightarrow synaptic output (triggers/inhibits next cell).
  • Highest regulatory control over body’s responsiveness.

Organ & Organ-System Concept

  • Multiple tissue types →\rightarrow organ (e.g. heart = epithelial + connective + muscle + neural).
  • Evolution shows increasing organ/system complexity across phyla; earthworm, cockroach, frog exemplify ascending trend (Annelida →\rightarrow Arthropoda →\rightarrow Amphibia/Chordata).

Earthworm (Pheretima / Lumbricus)

Morphology
  • Cylindrical, reddish-brown; >100>100 metameres (≈100–120\approx100\text{–}120).
  • Dorsal blood vessel visible as dark mid-dorsal line.
  • Prostomium (sensory lobe) + peristomium (1st segment with mouth).
  • Clitellum: glandular band on segments 14–1614\text{–}16 →\rightarrow cocoons.
  • External apertures
    • Spermathecal pores: 4 pairs (inter-segments 5/6→8/95/6 \rightarrow 8/9).
    • Female pore: mid-ventral 14th14^{th}.
    • Male pores: ventro-lateral 18th18^{th}.
    • Nephridiopores: numerous, all segments except 1st, last, clitellum.
  • Setae (S-shaped chitin) in all but 1st/last/clitellar segments →\rightarrow locomotion.
Anatomy & Physiology
  • Body wall: cuticle → epidermis (columnar + gland cells) → circular & longitudinal muscles → coelomic epithelium.
  • Alimentary canal (straight)
    • Mouth → buccal cavity (1-3) → muscular pharynx → oesophagus (5-7) → gizzard (8-9; grinding) → stomach (9-14; calciferous glands neutralise humic acid) → intestine (15-last) with typhlosole (after 26th seg.) for area expansion; ends at anus.
    • Pair of intestinal caecae (26th seg.).
  • Circulatory system: closed; dorsal/ventral vessels, hearts; blood glands (4-6) produce erythrocytes + plasma haemoglobin.
  • Respiration: cutaneous (moist skin).
  • Excretion: segmental nephridia
    1. Septal (15-last, to gut)
    2. Integumentary (3-last, to surface)
    3. Pharyngeal (paired tufts in 4-6).
  • Nervous system: ventral nerve cord + segmental ganglia; circumpharyngeal nerve ring links with cerebral ganglia.
  • Sensory: photoreceptors/touch cells anteriorly; chemoreceptors.
  • Reproduction
    • Hermaphrodite, protandrous; cross-fertilisation via spermatophores.
    • Testes (10,11); ovaries (12/13 septum); accessory glands (17,19).
    • Cocoons secreted by clitellum; development direct (no larva).
  • Ecological/Practical relevance
    • "Farmer’s friend": burrowing aerates soil; casting ⇒\Rightarrow fertility (vermicomposting).
    • Fishing bait.

Cockroach (Periplaneta americana)

Morphology
  • Class Insecta, Phylum Arthropoda; typical length 34–53 mm34\text{–}53\,\text{mm}.
  • Body regions: head – thorax – abdomen; exoskeleton of chitinous sclerites (tergites + sternites) linked by arthrodial membrane.
  • Head: triangular, 6 fused segments; flexible neck.
    • Compound eyes (≈20002000 ommatidia each) →\rightarrow mosaic vision (high sensitivity, low resolution, nocturnal).
    • Antennae: long, sensory.
    • Mouthparts (biting/chewing) = labrum, mandibles, maxillae, labium + hypopharynx (tongue).
  • Thorax: pro-, meso-, metathorax; each bears walking leg.
    • Tegmina (leathery fore-wings) from mesothorax; membranous hind-wings from metathorax.
  • Abdomen: 10 segments.
    • Female: 7th sternum boat-shaped →\rightarrow genital pouch (female gonopore, spermathecal pores).
    • Male: genital pouch bounded by 9th–10th terga/sternum; anal styles present.
    • Anal cerci: jointed filaments on 10th segment (sensory).
Anatomy & Physiology
  • Digestive system
    • Foregut: mouth → pharynx → oesophagus → crop (storage) → gizzard (6 chitinous teeth plates).
    • Hepatic/Gastric caeca (6-8) at fore-/midgut junction (digestive enzymes).
    • Midgut → hindgut (ileum, colon, rectum).
    • Malpighian tubules (100-150, thin yellow) at mid-/hindgut junction → excretion (uricotelic).
  • Circulation: open type; haemocoel bathed by haemolymph (plasma + haemocytes).
    • Heart: elongated dorsal tube with 1313 chambers & alary muscles; ostia allow inflow.
  • Respiration: tracheal network opening via 1010 pairs of spiracles; sphincters regulate; gas exchange by diffusion.
  • Excretion: Malpighian tubules + fat body, nephrocytes, uricose glands →\rightarrow uric acid.
  • Nervous system: segmental ganglia + ventral connectives; head lobe (supra-oesophageal) controls antennae/eyes. Decapitated cockroach survives ~1 week (body ganglia autonomous).
  • Sense organs: antennae, palps, cerci, compound eyes, ocelli.
  • Reproduction
    • Dioecious.
    • Male: testes (4-6 segments) → vasa deferentia → seminal vesicle → ejaculatory duct → gonopore; mushroom gland (accessory); chitinous phallomeres. Sperms →\rightarrow spermatophores.
    • Female: ovaries (2-6 segments) each with 8 ovarioles; oviducts fuse into vagina; spermatheca (6th seg.) stores sperm; collateral glands secrete oothecal wall.
    • Ootheca: ~8 mm8\,\text{mm} capsule with 14–1614\text{–}16 eggs; female lays 9–109\text{–}10 oothecae.
    • Development paurometabolous (nymph →\rightarrow ≈13\approx13 moults →\rightarrow adult).
  • Economic/Health aspect: household pest; contaminates food; vector for bacteria/allergens.

Frog (Rana tigrina)

Morphology
  • Class Amphibia; poikilothermic; shows camouflage (dorsal olive-green + spots; ventral pale yellow).
  • Aestivation (summer burrow) & hibernation (winter).
  • Skin: thin, moist, mucus-coated; vascular → cutaneous respiration & water absorption (no drinking).
  • Body: head + trunk (no neck/tail).
    • Nostrils above mouth; nictitating membrane over bulged eyes; tympanum lateral.
    • Limbs: forelimbs (4 digits), hindlimbs (5 webbed digits) → swimming/leaping.
  • Sexual dimorphism: males with vocal sacs + copulatory pad on first finger.
Anatomy & Physiology
  • Digestive system
    • Short (carnivorous).
    • Mouth → buccal cavity (bilobed sticky tongue) → pharynx → oesophagus → stomach (HCl + gastric juice) → intestine (duodenum + ileum) → rectum → cloaca.
    • Liver (bile, stored in gall bladder); pancreas (enzymes).
    • Villi/microvilli increase absorption; undigested waste expelled via cloaca.
  • Respiration
    1. Cutaneous (water & land, esp. dormancy).
    2. Buccopharyngeal (moist buccal cavity).
    3. Pulmonary: lungs (pink sacs) used primarily on land.
  • Circulation
    • Closed; heart with 33 chambers (2 atria + 1 ventricle) + sinus venosus + conus arteriosus.
    • RBC nucleated; lymphatic system present (lymph, nodes, channels).
    • Portal systems: hepatic (gut →\rightarrow liver) & renal (hind-body →\rightarrow kidney).
  • Excretory system
    • Pair of kidneys; ureters (males = urinogenital ducts) →\rightarrow cloaca; urinary bladder ventral to rectum.
    • Ureotelic (excrete urea).
  • Neuro-endocrine coordination
    • CNS: brain (fore, mid, hind) in cranium; 10 cranial nerves; spinal cord.
    • Peripheral (cranial + spinal) & autonomic (sympathetic/parasympathetic).
    • Endocrine glands: pituitary, thyroid, parathyroid, thymus, pineal, pancreas (islets), adrenals, gonads.
  • Sense organs
    • Organs of touch, taste, smell, simple eyes, internal ears (hearing & balance).
  • Reproduction & Development
    • Male: yellow ovoid testes attached to kidneys via mesorchium; vasa efferentia → Bidder’s canal → urinogenital duct → cloaca.
    • Female: paired ovaries; oviducts open into cloaca separately.
    • Female lays 2500–30002500\text{–}3000 eggs; external fertilisation in water.
    • Tadpole larva undergoes metamorphosis → adult.
  • Ecological/Economic role
    • Insect control (crop protection).
    • Integral food-web link (prey & predator).
    • In some regions, frog legs consumed by humans (ethical concern on over-harvesting).