Structural Organisation in Animals: Comprehensive Study Notes

Levels of Biological Organisation and Animal Tissues

• Organisms in the animal kingdom range from unicellular to complex multicellular forms.
• In unicellular organisms, a single cell performs all essential functions, including digestion, respiration, and reproduction.
• Multicellular animals exhibit a higher level of organisation where basic functions are distributed across different groups of cells:
  • Hydra is a simple multicellular organism composed of thousands of cells of different types.
  • The human body consists of billions of cells that work together.
• A tissue is defined as a group of similar cells together with intercellular substances that perform a specific function.
• All complex animals are composed of only four basic types of tissues: Epithelial, Connective, Muscular, and Neural.
• Organs like the stomach, lung, heart, and kidney are formed when these tissues are organised in specific patterns and proportions.
• An organ system (e.g., digestive or respiratory) is formed when two or more organs interact physically or chemically to perform a common function.
• The division of labour among cells, tissues, organs, and organ systems is essential for the survival of the body as a whole.

Epithelial Tissue

• Epithelial tissue, commonly known as epithelium ($Plural: epithelia$), provides a covering or lining for body parts and has a free surface facing either body fluids or the external environment.
• Cells are compactly packed with very little intercellular matrix.
• Simple Epithelium consists of a single layer of cells and functions as a lining for tubes, ducts, and body cavities. It is divided into three types based on structural modifications:
  1. Squamous Epithelium: Made of a single thin layer of flattened cells with irregular boundaries. Found in the walls of blood vessels and air sacs of lungs. Functions as a diffusion boundary.
  2. Cuboidal Epithelium: Composed of a single layer of cube-like cells. Found in gland ducts and the tubular parts of nephrons in kidneys. Functions include secretion and absorption. The proximal convoluted tubule ($PCT$) of the nephron features microvilli.
  3. Columnar Epithelium: Composed of a single layer of tall, slender cells with nuclei located at the base. It may have microvilli on the free surface. Found in the lining of the stomach and intestine for secretion and absorption.
• Ciliated Epithelium occurs when columnar or cuboidal cells bear cilia on their free surface. This moves particles or mucus in a specific direction. It is found in the inner surface of hollow organs like bronchioles and fallopian tubes.
• Glandular Epithelium consists of specialised cuboidal or columnar cells for secretion:
  • Unicellular Glands: Isolated glandular cells like goblet cells in the alimentary canal.
  • Multicellular Glands: Clusters of cells such as salivary glands.
  • Exocrine Glands: Secrete products ($mucus, saliva, earwax, oil, milk, digestive enzymes$) through ducts or tubes.
  • Endocrine Glands: Ductless glands that secrete hormones directly into the fluid bathing the gland.
• Compound Epithelium is multi-layered and has a limited role in secretion and absorption. Its primary function is protection against mechanical and chemical stresses. It covers the dry surface of the skin, the moist surface of the buccal cavity, pharynx, and internal linings of salivary and pancreatic ducts.
• Cell Junctions provide structural and functional links between cells:
  • Tight Junctions: Stop substances from leaking across a tissue.
  • Adhering Junctions: Cement neighboring cells together.
  • Gap Junctions: Facilitate communication by connecting the cytoplasm of adjoining cells for the rapid transfer of ions, small molecules, and sometimes large molecules.

Connective Tissue

• Connective tissues are the most abundant and widely distributed tissues in complex animals. They link and support other tissues and organs.
• Except for blood, all connective tissue cells secrete structural protein fibres called collagen or elastin, providing strength, elasticity, and flexibility.
• The cells also secrete modified polysaccharides that accumulate as the matrix or ground substance.
• Connective tissue is classified into three types:
  1. Loose Connective Tissue: Cells and fibres are loosely arranged in a semi-fluid ground substance.
     • Areolar Tissue: Located beneath the skin, serving as a support framework for epithelium. Contains fibroblasts ($fibre-producing cells$), macrophages, and mast cells.
     • Adipose Tissue: Located mainly beneath the skin, specialised for storing fats. Excess nutrients are converted to fats and stored here.
  2. Dense Connective Tissue: Fibres and fibroblasts are compactly packed.
     • Dense Regular: Collagen fibres are in rows between parallel bundles. Examples include tendons ($attach skeletal muscles to bones$) and ligaments ($attach one bone to another$).
     • Dense Irregular: Fibroblasts and fibres ($mostly collagen$) are oriented differently. Found in the skin.
  3. Specialised Connective Tissue:
     • Cartilage: Solid, pliable intercellular material that resists compression. Chondrocytes ($cells$) are enclosed in small cavities within the matrix. Present in vertebrate embryos, tip of nose, outer ear, vertebral column joints, and limbs.
     • Bone: Hard, non-pliable ground substance rich in calcium salts and collagen fibres. Provides the structural frame, supports, and protects soft tissues. Osteocytes ($bone cells$) reside in spaces called lacunae. Long bones bear weight and contain bone marrow, the site of blood cell production.
     • Blood: Fluid connective tissue containing plasma, red blood cells ($RBC$), white blood cells ($WBC$), and platelets. Functions as the main circulating transport fluid.

Muscle Tissue

• Muscle tissue is composed of many long, cylindrical fibres arranged in parallel arrays, made of fine fibrils called myofibrils.
• Muscle fibres contract ($shorten$) and relax ($lengthen$) in response to stimulation to move the body and maintain position.
• Three types of muscle tissue:
  1. Skeletal Muscle: Striated (striped) and bundled in parallel; closely attached to skeletal bones (e.g., biceps). Encapsulated by a tough connective tissue sheath.
  2. Smooth Muscle: Fusiform (tapered at both ends), non-striated, and involuntary. Bundled in connective tissue sheaths within walls of internal organs like blood vessels, stomach, and intestine.
  3. Cardiac Muscle: Contractile tissue exclusive to the heart. Plasma membranes are fused by cell junctions. Intercalated discs ($communication junctions$) allow cells to contract as a single unit.

Neural Tissue

• Neural tissue exerts the greatest control over the body's responsiveness to changing conditions.
• Neurons are the excitable units of the neural system.
• Neuroglia protect and support neurons and constitute more than one-half the volume of neural tissue.
• Stimulation of a neuron generates an electrical disturbance traveling along the plasma membrane to the neuron's endings ($output zone$), triggering stimulation or inhibition of adjacent cells.

Earthworm (Pheretima and Lumbricus)

• Reddish-brown terrestrial invertebrates inhabiting the moist upper soil layer. They live in burrows and leave faecal deposits known as worm castings.
• Morphology:
  • Long cylindrical body divided into $100-120$ similar segments ($metameres$).
  • Dorsal surface has a dark median mid-dorsal line ($dorsal blood vessel$).
  • Anterior end has the mouth and prostomium ($sensory lobe covering the mouth$).
  • First body segment is the peristomium ($buccal segment$).
  • Clitellum: A dark band of glandular tissue in segments $14-16$. Regions are defined as preclitellar, clitellar, and postclitellar.
  • Spermathecal apertures: Four pairs on the ventro-lateral sides of intersegmental grooves ($5^{th}-9^{th}$ segments).
  • Female genital pore: Mid-ventral line of the $14^{th}$ segment.
  • Male genital pores: Ventro-lateral sides of the $18^{th}$ segment.
  • S-shaped setae: Embedded in epidermal pits for locomotion, present in all segments except the first, last, and clitellum.
• Anatomy:
  • Body Wall: Cuticle, epidermis ($single layer of columnar cells$), circular and longitudinal muscle layers, and coelomic epithelium.
  • Digestive System: Alimentary canal runs from first to last segment. Includes buccal cavity ($1-3$), pharynx, oesophagus ($5-7$), muscular gizzard ($8-9$ for grinding soil), stomach ($9-14$), and intestine ($15^{th}$ to last). Calciferous glands in the stomach neutralise humic acid. Typhlosole is an internal median fold beginning after segment $26$ ($except last 23-25 segments$) to increase absorption area.
  • Circulatory System: Closed type. Blood is confined to vessels and heart. Blood glands ($4^{th}-6^{th}$ segments) produce phagocytic blood cells and haemoglobin dissolved in plasma. Heart contractions ensure one-way flow.
  • Respiration: Occurs via diffusion through the moist body surface; no specialised organs.
  • Excretion: Segmentally arranged coiled tubules called nephridia. Types include septal ($15^{th}$ to last, open into intestine), integumentary ($3^{rd}$ to last, open to body surface), and pharyngeal ($paired tufts in 4^{th}, 5^{th}, 6^{th}$ segments).
  • Nervous System: Segmental ganglia on a ventral paired nerve cord. Anteriorly, nerves encircle the pharynx and join cerebral ganglia to form a nerve ring.
  • Sensory System: Lacks eyes but has light and touch receptors and chemoreceptors ($taste receptors$).
  • Reproduction: Hermaphrodite ($bisexual$). Testes ($10^{th}, 11^{th}$ segments); vasa deferentia lead to $18^{th}$ segment; accessory glands in $17^{th}, 19^{th}$ segments. Spermathecae ($6^{th}-9^{th}$ segments) store sperm. Ovaries ($inter-segmental septum of 12^{th}-13^{th}$ segments). Exchange of spermatophores occurs during mating. Cocoons produced by clitellum contain eggs and sperm. Development is direct ($no larva$).
  • Economic Importance: 'Friends of farmers' due to soil aeration and vermicomposting.

Cockroach (Periplaneta americana)

• Cockroaches are nocturnal omnivores in Phylum Arthropoda, Class Insecta. Size ranges from $1/4$ to $3$ inches ($0.6-7.6\,cm$).
• Morphology:
  • Body divided into head, thorax, and abdomen. Covered by a chitinous exoskeleton with hardened plates called sclerites ($tergites dorsally, sternites ventrally$) joined by arthrodial membranes.
  • Head: Triangular, formed by fusion of six segments. Bears compound eyes and sensory antennae. Mouthparts are biting/chewing type: labrum ($upper lip$), mandibles, maxillae, labium ($lower lip$), and hypopharynx ($tongue$).
  • Thorax: Prothorax, mesothorax, and metathorax. Three pairs of legs. Wings: Forewings ($tegmina$) from mesothorax are leathery and protective; hind wings from metathorax are used for flight.
  • Abdomen: $10$ segments. Females have a boat-shaped $7^{th}$ sternum forming a genital pouch. Males have anal styles ($absent in females$). Both sexes have anal cerci on the $10^{th}$ segment.
• Anatomy:
  • Digestive System: Foregut ($mouth, pharynx, oesophagus, crop for storage, gizzard with chitinous teeth$), Midgut ($mesenteron$), and Hindgut ($ileum, colon, rectum$). Hepatic caeca ($6-8$ blind tubules$) at foregut-midgut junction secrete juice. Malpighian tubules ($100-150) at midgut-hindgut junction for excretion.\n * Circulatory System: Open type; visceral organs in haemocoel are bathed in haemolymph (colourless plasma and haemocytes). Heart is a mid-dorsal tube with funnel-shaped chambers and ostia.\n * Respiratory System: Network of trachea opening via 10 pairs of spiracles. Gas exchange via tracheoles by diffusion.\n * Excretion: Malpighian tubules convert nitrogenous waste to uric acid (uricotelic). Fat body, nephrocytes, and urecose glands also assist.\n * Nervous System: Ventral nerve cord with 3$thoracic and$6$abdominal ganglia. Supra-oesophageal ganglion serves as the brain. Mosaic vision via approximately$2000$hexagonal ommatidia ($nocturnal vision).\n * Reproduction: Dioecious. Males: Testes (4^{th}-6^{th}$segments), mushroom gland ($6^{th}-7^{th}$segments), and phallomeres. Sperms are in spermatophores. Females: Ovaries ($2^{nd}-6^{th}$segments),$8$ovarioles per ovary, spermatheca in$6^{th}$segment. Fertilized eggs in oothecae ($capsules$). Development is paurometabolous ($nymphal stages$); nymphs moult$13 times.\n\n# Frog (Rana tigrina)\n\n* Amphibians and poikilotherms (cold-blooded$). Capable of camouflage ($mimicry$) and undergo aestivation ($summer sleep$) and hibernation ($winter sleep).\n* Morphology:\n * Smooth, moist skin (mucus-covered). Dorsal side olive green with spots; ventral side pale yellow.\n * Body consists of head and trunk (neck and tail absent). Bulged eyes with nictitating membranes. Tympanum receives sound.\n * Hind limbs have 5$webbed digits; forelimbs have$4 digits. Males have vocal sacs and copulatory pads on the first forelimb digit.\n* Anatomy:\n * Digestive System: Short alimentary canal (carnivorous$). Food captured by bilobed tongue. Stomach secretes$HCl$and gastric juices. Partially digested food is chyme. Duodenum receives bile ($liver/gall bladder) and pancreatic juice. Absorption occurs via villi and microvilli in the intestine; waste exits via cloaca.\n * Respiration: Cutaneous respiration (skin$) in water. Pulmonary respiration ($lungs) on land. Buccal cavity also acts as respiratory organ. During sleep, only skin is used.\n * Circulatory System: Closed type. Three-chambered heart (2$atria,$1$ventricle$). Sinus venosus and conus arteriosus are present. Features hepatic portal and renal portal systems. RBCs are nucleated and contain haemoglobin. Lymph lacks RBCs and some proteins.
  • Excretory System: Pair of kidneys ($nephrons$), ureters, cloaca, and urinary bladder. Ureotelic ($excretes urea$). In males, ureters act as urinogenital ducts.
  • Control and Coordination: Endocrine glands ($pituitary, thyroid, adrenal, etc.$) and neural system ($Central: brain/spinal cord; Peripheral: cranial/spinal nerves; Autonomic: sympathetic/parasympathetic$). $10$ pairs of cranial nerves. Brain is in a cranium; includes Fore-brain ($olfactory lobes, cerebrum, diencephalon$), Mid-brain ($optic lobes$), and Hind-brain ($cerebellum, medulla oblongata$).
  • Sense Organs: Sensory papillae ($touch$), taste buds, nasal epithelium, simple eyes ($one unit$), and tympanum ($hearing/balancing$).
  • Reproduction: Males have yellowish ovoid testes adhered to kidneys by mesorchium. Vasa efferentia ($10-12$) open into Bidder's canal. Females have ovaries near kidneys; oviducts open into cloaca. A female lays $2500-3000$ ova. External fertilisation in water. Development via tadpole larva ($metamorphosis$).

Questions & Discussion

• Common name of Periplaneta americana: Cockroach.
• Number of spermathecae in earthworm: Four pairs ($found in segments 6-9$).
• Position of ovaries in cockroach: $2^{nd}-6^{th}$ abdominal segments.
• Number of segments in cockroach abdomen: $10$ segments.
• Location of Malpighian tubules: At the junction of the midgut ($mesenteron$) and hindgut.
• Function of nephridia: Regulation of the volume and composition of body fluids and excretion of wastes.
• Types of nephridia in earthworm: Septal, integumentary, and pharyngeal.
• Difference between Prostomium and Peristomium: Prostomium is a sensory lobe covering the mouth used for burrowing; Peristomium is the first body segment containing the mouth.
• Cellular components of blood: Plasma, red blood cells ($erythrocytes$), white blood cells ($leucocytes$), and platelets.
• Chondrocytes: Cartilage cells found in small cavities of the matrix.
• Axons: Long processes of neurons found in neural tissue.
• Ciliated epithelium location: Inner surface of bronchioles and fallopian tubes.
• Ureters in frog function: In males, they act as urinogenital ducts to transport both urine and sperm; in females, they only transport urine to the cloaca.
• Impact of cockroaches: They are pests that contaminate food and act as vectors for bacterial diseases.