Structural Organisation in Animals – Comprehensive Bullet Notes
Properties of Animals
Multicellular (Metazoans) with division of labour (specialized cells and tissues perform specific functions, increasing efficiency).
Eukaryotic cells (possess a well-defined nucleus and membrane-bound organelles).
Heterotrophic; majority exhibit holozoic nutrition, meaning they ingest complex organic food materials.
Complete food is first ingested (taken into the body) then digested (broken down into simpler forms) inside the body, followed by absorption and assimilation.
Exhibit growth (increase in size/number of cells), reproduction (asexual or sexual), locomotion (change of place), and movement (change in posture or position of body parts).
Organised hierarchy: Cells (basic unit) Tissues (groups of similar cells with a common function) Organs (structures composed of multiple tissues) Organ-systems (groups of organs working together for specific physiological functions).
Levels of Structural Organisation
Unicellular organisms (e.g., Amoeba, Paramecium) perform all vital activities (nutrition, respiration, excretion, reproduction) within a single cell.
Multicellular organisms (e.g., Hydra to humans) show division of labour through specialized tissues, organs, and organ systems, leading to increased complexity and efficiency.
Animal Tissues – Overview
Four basic tissue types based on their primary structure and function:
Epithelial (E) – functions in covering body surfaces, lining cavities and ducts, diffusion, secretion (e.g., glands), and absorption (e.g., intestines).
Connective (C) – provides support, binds and connects other tissues, stores fat, and transports substances (e.g., blood, bone, cartilage).
Muscular (M) – specialized for contraction, facilitating locomotion (movement of the entire body) and various internal movements (e.g., pumping blood, moving food through digestive tract).
Neural (N) – responsible for control and coordination by transmitting electrical signals (nerve impulses) throughout the body.
All organs within the body contain specific proportions of these four tissue types; multiple organs interacting functionally form organ systems (e.g., digestive system, circulatory system).
EPITHELIAL TISSUE
General Features
Always rests on an underlying connective tissue via a non-cellular basement membrane (BM).
The BM is composed of glycoproteins and mucopolysaccharides with protein fibres, secreted jointly by both the epithelial and connective tissue layers, providing structural support and acting as a selective barrier.
Cells are compactly packed with minimal inter-cellular matrix, making the tissue avascular (lacking direct blood supply).
Nutrients and oxygen diffuse from the rich blood supply of the underlying connective tissue.
The free (apical) surface faces external environment or a body fluid (e.g., lumen of an organ), while the basal surface rests on the basement membrane.
Exhibits high regenerative capacity, allowing for rapid repair and replacement of damaged cells, and is well supplied by nerves.
Develops from all three embryonic germ layers:
Skin epithelium (epidermis) originates from the ectoderm.
Gut lining (mucosa) develops from the endoderm.
Endothelium (lining of blood vessels) and mesothelium (lining of serous cavities) are derived from the mesoderm.
Surface modifications enhance specific functions:
Microvilli – minute, finger-like projections found on the apical surface, forming a 'brush-border'; significantly increase surface area for efficient absorption (e.g., small intestine) or secretion.
Cilia – hair-like, motile projections that beat in a coordinated, unidirectional manner to move materials (e.g., mucus, ova) along the epithelial surface.
Classification
1. SIMPLE (single layer of cells) – main roles: secretion, absorption, filtration, diffusion
Simple Squamous (also known as pavement or tessellated epithelium)
Consists of extremely thin, flat, and irregular-shaped cells with a flattened, centrally located nucleus.
Locations: Found where rapid diffusion or filtration is required, such as the air sacs of lungs (alveoli), the lining of Bowman’s capsule in kidneys, and the lining of blood vessels (called endothelium) and body cavities (called mesothelium).
Function: Primarily involved in diffusion (gases, nutrients) and filtration (waste products).
Simple Cuboidal
Composed of cube-like cells with a spherical, centrally located nucleus.
Types:
Plain: Found in thyroid follicles, small ducts of glands (e.g., salivary, pancreatic ducts), and the germinal epithelium of the ovary. Its main functions are secretion and absorption.
Brush-bordered: Possesses microvilli on its apical surface, significantly increasing surface area for absorption. A prime example is the Proximal Convoluted Tubule (PCT) of the nephron in the kidney, crucial for reabsorption of water and solutes.
Ciliated: Features cilia on its surface, which move substances. Present in small bronchioles and larger bronchi of the respiratory tract, where cilia help to move mucus and trapped particles out of the airways.
Simple Columnar (tall, slender cells with an oval nucleus typically located near the base)
Types:
Plain: Lines the stomach, where its primary function is secretion (e.g., gastric juice) and some absorption.
Brush-bordered: Characterized by microvilli and often interspersed with goblet cells (unicellular glands). Abundant in the small intestine, facilitating maximum absorption of digested nutrients and mucus secretion (from goblet cells) for lubrication and protection.
Ciliated: Found in the fallopian tube (oviduct), where ciliary action helps to move the ovum towards the uterus, and in larger bronchioles, aiding mucus clearance.
Glandular epithelium (specialized cuboidal or columnar cells for secretion)
Can be:
Unicellular: Consists of isolated glandular cells, like the goblet cell in the alimentary canal and respiratory tract, which secretes mucus.
Multicellular: Comprises clusters of cells forming glands, such as sweat glands, salivary glands, mammary glands, etc.
Mode of secretion:
Exocrine glands: Secrete products (e.g., milk, saliva, digestive enzymes, sebum, sweat, ear-wax) onto an epithelial surface or into a duct system that leads to a surface. Examples include salivary glands, sweat glands, and sebaceous glands.
Endocrine glands: Ductless glands that secrete hormones directly into the bloodstream to act on distant target cells. Examples include the pituitary gland, thyroid gland, and adrenal gland.
Heterocrine/Mixed glands: Possess both exocrine and endocrine functions. Notable examples are the pancreas (exocrine: digestive enzymes; endocrine: insulin and glucagon) and gonads (testes/ovaries) (exocrine: gametes; endocrine: sex hormones).
2. COMPOUND (multilayered cells) – primary function: protection against chemical/mechanical stress; limited absorption
Stratified/Non-stretchable (multiple layers of cells where only the deepest layer rests on the basement membrane)
Dry (keratinised): The superficial layers of cells are dead and filled with keratin, forming a tough, waterproof, and abrasion-resistant layer. This provides an impervious surface. Found in the epidermis of the skin, protecting against desiccation, pathogens, and mechanical injury.
Moist (non-keratinised): The superficial cells are living and retain their nuclei, keeping the surface moist. Provides protection against abrasion and wear and tear. Lines the buccal cavity, pharynx, oesophagus, vagina, and large ducts of salivary glands, where surfaces are subject to friction but need to remain moist.
Transitional (stretchable)
A specialized stratified epithelium with several layers of cuboidal to pear-shaped cells that can slide over one another, allowing the tissue to stretch significantly without tearing. The outermost cells are often dome-shaped, and the basement membrane is very thin or absent, contributing to its distensibility.
Found exclusively in the urinary tract, including the urinary bladder, ureter, and renal pelvis, where it accommodates changes in volume as urine accumulates.
Cell Junctions (highest density in epithelium)
Tight Junctions (zona occludens): Form a continuous seal between the plasma membranes of adjacent cells, effectively preventing the leakage of substances across the epithelial layer (e.g., forming barriers like the blood-brain barrier and blood-testis barrier).
Adhering Junctions (zona adherens and desmosomes): Provide strong mechanical attachments and cementing between neighbouring cells, anchoring them together. They are common in tissues subjected to mechanical stress, such as the skin epithelium and myocardial cells (e.g., intercalated discs of cardiac muscle).
Gap Junctions: Consist of protein channels (connexons) that create direct cytoplasmic continuity between adjacent cells, allowing rapid transfer of ions, small molecules, and chemical signals. This facilitates instantaneous communication and synchronized activity between cells, particularly important in smooth muscle and cardiac muscle for coordinated contraction.
CONNECTIVE TISSUE (CT)
Mesodermal origin; it is the most abundant and widely distributed tissue type in the body, binding, supporting, and protecting other tissues and organs.
Basic components:
Matrix (extracellular matrix) – typically large in volume and non-living, composed of:
Ground substance: A colorless, transparent, viscous fluid or gel varying from fluid to semi-solid, made of mucopolysaccharides (e.g., hyaluronic acid, chondroitin sulfate) and glycoproteins. It fills the space between cells and fibres, acting as a medium for nutrient and waste exchange.
Fibres: Provide structural support and tensile strength.
Collagen fibres: White, unbranched, inelastic, and very strong; made of collagen protein, providing high tensile strength (resistance to stretching).
Elastic fibres: Yellow, thin, branched, and highly elastic; made of elastin protein, allowing tissues to stretch and recoil to their original shape.
Reticular fibres: A delicate, highly branched network of fine collagen subtype fibres, providing support and flexibility, forming frameworks for organs like lymph nodes, spleen, and bone marrow.
Cells: Diverse cell types embedded within the matrix, each with specialized functions:
Fibroblasts: The most common type; synthesize and secrete the protein subunits of the extracellular fibres and ground substance.
Macrophages (histiocytes): Large, irregular cells, part of the immune system; renowned for their phagocytic activity, engulfing debris, dead cells, and foreign particles.
Mast cells: Large, oval cells filled with granules; play a crucial role in inflammatory and allergic responses by secreting chemical mediators.
Plasma cells: Derived from B lymphocytes; produce and secrete antibodies, playing a vital role in humoral immunity.
Adipocytes (fat cells): Specialized for storing lipids (fat), providing energy storage, insulation, and cushioning.
Neutrophils & Eosinophils: Types of white blood cells (WBCs) that migrate from blood into connective tissue during inflammation or immune responses.
1. LOOSE CT
a. Areolar
Characterized by a semi-fluid, gelatinous matrix with loosely arranged collagen and elastic fibres, interspersed with various cell types. It serves as a universal packing tissue.
Functions: Widely distributed beneath the skin, binding it to underlying muscles. Forms the framework (lamina propria) beneath the epithelium of many organs, and surrounds blood vessels, nerves, and muscles, providing support and allowing for movement.
Cell highlights (examples of cells commonly found in areolar tissue):
Fibroblast – As described above, secretes fibres and ground substance necessary for tissue maintenance and repair.
Macrophage – Actively performs phagocytosis, cleaning up cellular debris and defending against pathogens.
Mast cell – Releases chemical substances during inflammation and allergic reactions:
Histamine: A vasodilator, increasing blood flow and capillary permeability, leading to swelling and redness.
Serotonin: A vasoconstrictor, often regulating blood vessel diameter.
Heparin: An anticoagulant, preventing blood clotting.
Plasma cell – Produces and secretes antibodies as part of the body's immune response.
Neutrophils/Eosinophils – Types of WBCs involved in the immune response, particularly in combating bacterial infections (neutrophils) and parasitic infections/allergic reactions (eosinophils).
b. Adipose
A specialized type of loose connective tissue dominated by adipocytes, which are large cells filled with a single, large fat droplet (composed of triglycerides), pushing the nucleus to the periphery.
Functions: Primarily serves as an energy reserve, stores fat, insulates the body against heat loss, and cushions organs to protect them from mechanical shocks.
Locations: Found beneath the skin (subcutaneous fat), around kidneys, surrounding visceral organs (e.g., heart, intestines), and as blubber in aquatic mammals like whales, providing significant insulation against cold.
2. DENSE CT
Characterized by a matrix with a high proportion of densely packed fibres and fewer cells than loose CT, providing greater strength and less flexibility.
Dense Regular
Collagen fibres are arranged in parallel bundles, providing immense tensile strength in a single direction.
Tendon: Strong, flexible, inelastic cords of dense regular CT that connect skeletal muscles to bones, transmitting muscle contraction force to bone.
Ligament: Bundles of dense regular CT that are more elastic than tendons due to a higher proportion of elastic fibres. They connect bone to bone, stabilizing joints and preventing excessive movement.
Dense Irregular
Collagen fibre bundles are irregularly or randomly arranged, forming a mesh-like network. This arrangement provides strength and resistance to stress from multiple directions.
Locations: Forms the dermis of the skin (the deep, tough layer beneath the epidermis), and joint capsules, periosteum (covering of bone), and perichondrium (covering of cartilage).
3. SPECIALISED SKELETAL CT
Cartilage
A semi-rigid, yet pliable (flexible) connective tissue. Its solid matrix is composed of a complex network of protein fibres embedded in hydrated ground substances containing and . This unique composition allows it to bear mechanical stress without permanent deformation.
Cells: Immature cartilage cells called chondroblasts secrete the matrix and fibres. Once enclosed within small cavities called lacunae, they mature into chondrocytes, which maintain the matrix.
The outer surface of most cartilage is covered by a dense irregular connective tissue layer called the perichondrium, which provides nutrients and regeneration capacity.
Types/Locations: The highly resilient and smooth surface of cartilage reduces friction in joints. Found in the outer ear, tip and septum of the nose, tracheal rings (preventing collapse of airways), larynx (voice box), intervertebral discs (shock absorbers between vertebrae), costal cartilage (connecting ribs to sternum), and forms the embryonic skeleton which is later replaced by bone, also found in the limb buds during development.
Bone
The hardest, non-pliable (rigid) connective tissue, providing the main structural framework of the body.
Composition: Approximately of its dry weight is inorganic material, primarily calcium salts like (calcium carbonate) and (calcium phosphate), which give bone its hardness. The remaining is organic material, mainly ossein collagen fibres, which provide tensile strength and flexibility.
Structural unit (mammals): The osteon or Haversian system. This unit consists of concentric layers (lamellae) of bony matrix arranged around a central Haversian canal (containing blood vessels and nerves). Mature bone cells, osteocytes, are located in lacunae between the lamellae and are connected to each other and to the Haversian canal via minute channels called canaliculi, allowing for nutrient and waste exchange.
Bone is covered externally by a tough fibrous membrane called the periosteum. It contains marrow cavities:
Red marrow: Found in spongy bone, particularly in the ends of long bones and flat bones; responsible for haematopoiesis (formation of blood cells).
Yellow marrow: Primarily found in the shafts of long bones; largely composed of adipose tissue and serves for fat storage.
4. FLUID CT
Blood
A specialized fluid connective tissue composed of a liquid matrix called plasma and formed elements suspended within it.
Plasma contains water, proteins (e.g., albumin, globulins, fibrinogen), salts, hormones, and waste products.
Formed elements include:
RBCs (red blood cells/erythrocytes): Biconcave discs containing hemoglobin for oxygen transport. They are nucleated in frogs but non-nucleated (anucleated) and biconcave in mammals, optimizing oxygen carrying capacity.
WBCs (white blood cells/leukocytes): Involved in the immune response, defending the body against pathogens. They are larger than RBCs and nucleated, categorized into granulocytes and agranulocytes.
Platelets (thrombocytes): Small, anucleated cell fragments crucial for blood clotting and hemostasis.
Lymph
Filtered blood plasma that has leaked out of capillaries and into the interstitial spaces, then collected by lymphatic vessels. It is similar to plasma but contains no RBCs and fewer proteins. It also carries WBCs (lymphocytes).
Flows in a one-way system of lymphatic vessels and passes through lymph nodes, where immune responses are initiated.
Functions: Returns interstitial fluid to the bloodstream, transports dietary fats from the small intestine, and plays a vital role in the immune system.
MUSCULAR TISSUE
Mesodermal origin (except for the iris and ciliary muscles of the eye, which are ectodermal).
Possesses four key properties:
Excitability: Ability to respond to stimuli (e.g., nerve impulses) by generating electrical signals.
Contractility: Ability to shorten forcefully when stimulated, producing tension.
Extensibility: Ability to stretch without being damaged.
Elasticity: Ability to return to its original length and shape after contraction or extension.
Types & features:
Feature | Skeletal | Smooth | Cardiac |
|---|---|---|---|
Shape | Cylindrical, unbranched, elongated | Fusiform/spindle-shaped, tapering ends | Cylindrical, branched, interconnected |
Nuclei | Multinucleate (many), peripherally located | Single, centrally located | Single (sometimes two), centrally located |
Striations | Prominent, visible alternating light and dark bands (A and I bands) | Absent, hence 'smooth' | Faint, less distinct than skeletal |
Control | Voluntary (under conscious control) | Involuntary (autonomic nervous system control) | Involuntary (autonomic nervous system and intrinsic pacemaker cells) |
Contraction | Fast, powerful, but fatigues quickly | Slow, prolonged, sustained, resistant to fatigue | Fastest, rhythmic, non-fatiguing, continuous |
Blood Supply | Rich, highly vascularized | Low, less vascularized | Abundant, highly vascularized |
Special Features | Found attached to bones via tendons; responsible for body movement and posture. Does not have cell junctions directly connecting cells. | Found in walls of internal organs (e.g., stomach, intestines, blood vessels, bladder); responsible for peristalsis, vasoconstriction, etc. Cells are connected by gap junctions. | Found only in the wall of the heart; characterized by intercalated discs, which are specialized cell junctions (containing gap junctions and adhering junctions) that electrically and mechanically connect adjacent cardiac muscle cells, allowing rapid spread of electrical impulses and synchronized contraction. |
NEURAL TISSUE
Ectodermal origin (except microglia, which are mesodermal); primarily composed of two main cell types:
Excitable neurons: The basic functional units responsible for transmitting nerve impulses.
Non-excitable neuroglia (glial cells): Support, nourish, and protect neurons, composing more than of the Central Nervous System (CNS) volume but do not transmit impulses themselves.
Neuron: Structurally and functionally specialized to transmit electrical and chemical signals.
Consists of a cell body (soma/perikaryon), which contains the nucleus and prominent Nissl granules (aggregations of rough endoplasmic reticulum and free ribosomes, involved in protein synthesis for neuronal growth and regeneration).
Dendrites: Short, highly branched processes extending from the cell body; they are the primary receptive (input) regions of the neuron, transmitting graded potentials towards the cell body.
Axon: A single, long, slender process that extends from the cell body; it is the output pathway, generating and transmitting action potentials away from the cell body towards other neurons, muscles, or glands. The axon contains axolemma (axon membrane) and axoplasm (axon cytoplasm).
Myelin sheath: A lipid-rich insulating layer that surrounds many axons, formed by Schwann cells in the PNS (Peripheral Nervous System) and Oligodendrocytes in the CNS. It increases the speed of nerve impulse conduction dramatically through saltatory conduction (where the impulse jumps from one Node of Ranvier to the next).
Nodes of Ranvier: Gaps in the myelin sheath along the axon, where the action potential is regenerated.
Neurons communicate with each other or with effector cells (muscles, glands) at specialized junctions called synapses. Here, electrical signals are converted into chemical signals via the release of neurotransmitters (e.g., acetylcholine, dopamine). Neurotransmitters can be either excitatory (promoting nerve impulse generation in the post-synaptic neuron) or inhibitory (suppressing impulse generation).
FROG (Rana tigrina)
Classification & General Characters
Phylum Chordata Subphylum Vertebrata Class Amphibia Order Anura (tail-less amphibians).
Poikilothermic (cold-blooded): Their body temperature fluctuates with the ambient temperature, as they cannot regulate it internally.
Show aestivation (summer sleep) during hot, dry periods and hibernation (winter sleep) during cold periods to escape extreme temperatures and conserve energy, burrowing into mud or moist soil.
Exhibit camouflage (dorsal olive-green with irregular dark patches to blend with vegetation) and mimicry on land, while the ventral side is uniformly pale yellow for countershading.
Morphology
Body is divisible into a distinct head and trunk; there is no discernible neck (allowing head movement independently of the trunk) or tail (in adults).
Skin: Moist, smooth, and glandular, containing mucus glands (keeping it moist for cutaneous respiration) and poison glands (for defense). It is non-scaly and acts as a significant organ for water absorption (frogs do not drink water but absorb it through the skin).
External nares: Paired openings located above the mouth, serving as the entry point for air into the respiratory system.
Bulged simple eyes: Prominent eyes that provide vision, protected by a transparent nictitating membrane (a third eyelid) that covers and protects the eye in water and keeps it moist on land.
Tympanum: A pair of membranous eardrums located laterally to the eyes, serving as the organ for hearing.
Forelimbs: Shorter, with four digits (fingers); used for support and cushioning during landing.
Hindlimbs: Longer and more muscular, with five digits connected by webs; highly adapted for swimming, leaping, and burrowing.
Sexual dimorphism: Males can be distinguished from females by the presence of vocal sacs (pouches on the throat that inflate to produce croaking sounds) and a copulatory pad (nuptial pad) of thickened skin on the first digit of each forelimb, which aids in gripping the female during amplexus (mating).
Digestive System
The alimentary canal is a complete tube, extending from the mouth to the cloacal aperture.
Route: Mouth buccal cavity pharynx oesophagus (a short tube) stomach (J-shaped, muscular) duodenum (U-shaped, first part of small intestine) ileum (coiled, main part of small intestine) rectum (straight, expanded terminal part) cloaca (a common chamber that receives digestive wastes, urine, and reproductive products) cloacal aperture.
Accessory glands: Aid digestion by secreting digestive juices:
Liver: A large, three-lobed gland located anterior to the stomach; produces bile, which is stored in the gall bladder.
Pancreas: A cream-coloured gland situated between the stomach and duodenum; produces pancreatic juice (containing digestive enzymes).
Both bile and pancreatic juice are released into the duodenum via a common opening, the common bile duct.
Tongue: A large, sticky, bilobed, and highly protrusible (prehensile) organ attached at the front of the mouth. It can be rapidly everted to capture insects.
Gastric digestion: In the stomach, glands secrete (hydrochloric acid) and inactive pepsinogen. activates pepsinogen into pepsin, which initiates the digestion of proteins into proteoses and peptones.
Chyme (partially digested food from the stomach) enters the duodenum.
Bile: Emulsifies fats (breaks large fat globules into smaller ones, increasing surface area for enzyme action).
Pancreatic enzymes: Further break down carbohydrates, proteins, and fats.
Final digestion and maximum absorption of nutrients occur in the ileum, which has a folded inner lining with villi and microvilli to maximize surface area.
Undigested matter passes from the ileum to the rectum, then into the cloaca, and is finally eliminated through the cloacal aperture (egestion).
Respiration
Frogs show different modes of respiration depending on their environment and state:
Cutaneous respiration (skin): The primary mode of respiration in water, and during aestivation and hibernation. The moist, highly vascularized skin allows for efficient diffusion of oxygen and carbon dioxide. This is the most important mode of respiration.
Buccopharyngeal respiration: Occurs on land, involving the moist, highly vascular lining of the buccal cavity and pharynx. Air is pumped in and out through the mouth and nostrils.
Pulmonary respiration (lungs): Occurs on land, via a pair of relatively small, sac-like lungs located in the thorax (chest cavity).
Air path: External nares buccal cavity lungs. Air is forced into the lungs by a process called buccal pumping, involving coordinated movements of the floor of the buccal cavity.
Tadpole larvae: Being aquatic, they respire exclusively through gills (external gills initially, then internal gills). Tadpoles are also herbivorous.
Circulatory System
Closed circulatory system: Blood flows within blood vessels, entirely separate from interstitial fluid. It consists of blood, lymph, and a heart that pumps blood through a network of vessels.
Heart: A three-chambered organ (two atria to receive blood and one ventricle to pump blood) located in the upper part of the body cavity.
Associated structures: Sinus venosus (thin-walled chamber dorsal to the right atrium, receiving deoxygenated blood) and conus arteriosus (a ventral, tubular structure arising from the ventricle, carrying blood to arterial arches). The conus arteriosus has a spiral valve that helps to direct blood flow.
Incomplete double circulation: Deoxygenated blood from the body and oxygenated blood from the lungs/skin enter the single ventricle, leading to some mixing of oxygenated and deoxygenated blood. Thus, the distribution of blood to organs is mixed blood (partially oxygenated).
Arterial arches: The conus arteriosus divides into three pairs of arterial arches that distribute blood to different parts of the body.
Pulmocutaneous arteries: Carry mixed blood to the lungs and skin for oxygenation.
Systemic arches: Supply mixed blood to various systemic organs.
Blood: Contains nucleated RBCs (unlike mammals), WBCs, and platelets suspended in plasma.
Lymph: Forms the tissue fluid, circulates in lymphatic vessels, and is responsible for fluid balance and immunity.
Portal systems: Specialized venous systems where a capillary bed drains into another capillary bed before returning to the heart.
Hepatic portal system: Collects nutrient-rich blood from the gut and delivers it to the liver for processing before it enters general circulation.
Renal portal system: Collects blood from the hind-limbs and posterior body parts and delivers it to the kidneys, allowing for additional filtration.
Excretory System
Consists of a pair of dark red, bean-shaped kidneys located posteriorly in the dorsolateral region of the trunk.
Ureters: Tubes that emerge from the kidneys, carrying urine. They open into the cloaca, which then connects to a thin-walled urinary bladder located ventral to the rectum.
Ureotelic: Adult frogs excrete urea as their primary nitrogenous waste product, which is less toxic than ammonia and requires less water for excretion, an adaptation for semi-aquatic life.
Ammonotelic: Tadpole larvae, being fully aquatic, excrete ammonia, which is highly toxic but readily diffuses into the surrounding water.
Reproductive System
Dioecious: Males and females are separate individuals, exhibiting sexual dimorphism.
Fertilization is external, occurring in water, where eggs and sperm are released simultaneously.
Male
Testes: Paired, yellowish, ovoid structures located near the anterior part of the kidneys. They are attached to the kidneys by a fold of peritoneum called mesorchium.
Vasa efferentia: fine tubules arising from each testis, which enter the kidney.
Bidder’s canal: Within the kidney, vasa efferentia join a common canal called Bidder's canal, which then connects to the ureter.
Ureter: In males, the ureter also functions as a urinogenital duct, carrying both urine from the kidney and sperm from Bidder's canal to the cloaca.
Fat bodies: Finger-like, yellowish structures attached to the testes; they serve as an energy reserve, particularly important for gonad development and reproduction.
Female
Ovaries: Paired, large, irregular structures containing developing ova, located near the kidneys (but with no direct connection to the kidneys, unlike testes).
Oviducts: A pair of long, coiled tubes that emerge from the ovaries and open separately into the cloaca. They are responsible for transporting eggs and secreting a jelly-like covering around them.
Spermatheca: Absent in frogs, meaning there is no specialized structure for sperm storage after mating.
Eggs: Females typically lay eggs at a time into the water, where external fertilization takes place.
Development
Indirect development: Involves a larval stage called a tadpole larva, which is aquatic and very different from the adult frog in morphology and habits.
Metamorphosis: The tadpole undergoes a significant transformation to develop into an adult frog, involving:
Tail regression: The tail is gradually absorbed.
Limb formation: Forelimbs and hindlimbs develop.
Gills disappear: Replaced by lungs for pulmonary respiration.
Diet change: From herbivorous (tadpole) to carnivorous (adult).
Nervous & Endocrine Systems
CNS (Central Nervous System): Comprises the brain and spinal cord.
Brain: Protected by the cranium. Divided into three main parts:
Forebrain: Includes the olfactory lobes (for smell), paired cerebral hemispheres (for voluntary actions, learning), and diencephalon (involved in sensory relay, autonomic functions).
Midbrain: Consists of paired optic lobes (for processing visual information).
Hindbrain: Includes the cerebellum (for muscular coordination and balance) and medulla oblongata (controlling vital functions like respiration and heart rate). The medulla exits the skull through the foramen magnum and continues as the spinal cord.
Spinal cord: Extends posteriorly from the medulla oblongata, protected within the vertebral column.
PNS (Peripheral Nervous System): Consists of nerves extending from the CNS.
pairs of cranial nerves arising from the brain.
Spinal nerves arising from the spinal cord.
Autonomic Nervous System: Subdivided into sympathetic and parasympathetic systems, which control involuntary functions of internal organs.
Sense organs: Specialized structures for perception:
Sensory papillae: Distributed on the skin for touch sensation.
Taste buds: On the tongue for taste.
Nasal epithelium: In the nasal cavity for smell.
Simple eyes: For vision, with eyelids and a nictitating membrane.
Tympanum and internal ear: For hearing and maintaining equilibrium/balance.
Endocrine glands: Ductless glands that secrete hormones regulating various physiological processes:
Pituitary gland, pineal gland, thyroid gland, parathyroid glands, thymus, adrenal glands, pancreatic islets (of Langerhans), and gonads (testes and ovaries).
Importance to Humans
Insect control: Frogs are voracious eaters of insects, including agricultural pests and mosquitoes, thus playing a significant role in pest control.
Ecological food-web link: They form an important link in the food chain, consuming insects and serving as food for larger predators like snakes and birds.
Food source: The muscular hind legs of frogs are consumed as a delicacy in some regions of the world.
Bioindicators: Their permeable skin makes them sensitive to environmental toxins, so their health can indicate pollution levels.
COCKROACH (Periplaneta americana)
Classification & General Traits
Phylum Arthropoda Class Insecta.
Commonly dark brown (some tropical species can be bright green or red-brown); length typically ranges from . They are nocturnal (active at night), omnivorous (feed on a wide range of organic matter), and cursorial (adapted for running).
Body covered by a hard, protective, chitinous exoskeleton that is shed periodically (moulting). The exoskeleton consists of hardened plates called sclerites:
Dorsal plates: tergites (or terga)
Ventral plates: sternites (or sterna)
Lateral plates: pleura (or pleurites)
These sclerites are connected by flexible, thin, and elastic arthrodial membrane, allowing for movement.
Body segments (metameres): The body of an adult cockroach is distinctly segmented, divided into three regions:
Head: Composed of fused segments.
Thorax: Composed of segments (prothorax, mesothorax, metathorax).
Abdomen: Composed of segments.
Morphology
Head: A triangular capsule, highly movable, and articulated by a flexible neck (cervix). It is hypognathous, meaning the mouth parts are directed downwards, suitable for biting and chewing.
Sensory organs on the head:
Long, filiform (thread-like) antennae: A pair of long, multi-jointed antennae arising from the anterior end of the head; highly sensitive for touch and smell, helping in navigation and food location.
Compound eyes: A pair of large, black, kidney-shaped eyes located dorsolaterally on the head. Each eye is composed of approximately individual hexagonal units called ommatidia. These eyes provide mosaic vision (also known as apposition vision), which is characterized by high sensitivity (especially in low light), but low resolution, making it suitable for nocturnal activities but poor for detailed image formation.
Ocelli: Tiny, simple eyes located between the compound eyes, serving as rudimentary light sensors but not forming images.
Mouth parts: Adapted for biting and chewing, arranged around the mouth opening. They include:
Labrum (upper lip)
Paired, strong mandibles (jaws with chitinous teeth for grinding food)
Paired maxillae (accessory jaws with sensory maxillary palps for holding and manipulating food)
Labium (lower lip, formed by fused second maxillae, also with sensory labial palps)
Hypopharynx (or lingua): A flexible, tongue-like central mouthpart, acting as a common salivary duct outlet.
Thorax: Divided into three segments, each bearing a pair of walking legs:
Pro-, meso-, metathorax: Each segment carries one pair of strong, jointed walking legs (coxopodal type), adapted for rapid running.
Pronotum: A large, chitinous shield that covers the entire prothorax and partially extends over the head.
Wings: Two pairs of wings originate from the meso- and metathorax.
Forewings (tegmina/elytra): Arise from the mesothorax; leathery, dark, opaque, and primarily protective, covering the hindwings when at rest. Not directly involved in flight.
Hindwings: Arise from the metathorax; thin, membranous, transparent, and used for flight.
Abdomen: Consists of segments in both sexes.
Terminal appendages (on segment 10):
Anal cerci: A pair of jointed, filamentous sensory structures present in both males and females; highly sensitive to vibrations and air currents (sound receptors).
Anal styles: A pair of short, unsegmented, unjointed, hair-like structures found only in males, used during copulation. Females lack anal styles.
Genital structures (associated with the posterior abdominal segments):
Female: The 7th sternum is boat-shaped and, along with the 8th and 9th sterna, forms a brood or genital pouch. This pouch contains the female gonopore (genital opening), the openings of the spermathecae (sperm storage sacs), and the collateral glands (secrete ootheca material).
Male: The genital pouch (genital chamber) is bounded posteriorly by the 9th and 10th terga and ventrally by the 9th sternum. It contains the male gonopore (genital opening) and phallomeres (also called external genitalia or gonapophyses), which are asymmetrical, chitinous structures involved in sperm transfer.
Digestive System
The alimentary canal is a long tube divided into three regions: foregut, midgut, and hindgut.
Foregut (stomodaeum): Lined with cuticle.
Mouth short pharynx narrow oesophagus large, sac-like crop (used for temporary food storage) gizzard (proventriculus). The gizzard is a muscular, conical structure containing six chitinous teeth for grinding food particles. It also has a valvular fold (stomodeal valve) that prevents regurgitation.
Midgut (mesenteron): Not lined by cuticle. It is the main site of digestion and absorption.
At the junction of the foregut and midgut, a ring of finger-like, blind tubules called hepatic or gastric caecae are present. These caecae secrete digestive enzymes into the midgut.
Hindgut (proctodaeum): Wider than the midgut and lined with cuticle. It reabsorbs water and salts.
The midgut opens into the hindgut, which is differentiated into ileum colon rectum (which stores undigested food as fecal pellets) anus (the terminal opening for defecation).
At the junction of the midgut and hindgut, a ring of thin, yellow, filamentous Malpighian tubules are present. These are the primary excretory organs.
Salivary apparatus: Consists of paired salivary glands and salivary reservoirs located in the thorax. They secrete saliva into a pre-oral cavity, which contains amylase to initiate carbohydrate digestion.
Respiratory System
Cockroaches respire through a highly efficient tracheal system, a network of silvery, branched air tubes that penetrate all tissues of the body, delivering oxygen directly to the cells.
Spiracles: The tracheal system opens to the exterior via pairs of small, slit-like external openings called spiracles (stigmata) located on the lateral sides of the body: pairs on the thorax and pairs on the abdomen. Each spiracle is guarded by sphincters that regulate the opening and closing, controlling air flow and minimizing water loss.
Tracheae: The spiracles lead into large tracheal trunks, which branch into finer and finer tubes called tracheoles.
Tracheoles: The finest terminal branches of the tracheal system, which directly bathe the tissues and individual cells. Gas exchange (oxygen diffusion into cells, carbon dioxide diffusion out) occurs directly across the moist walls of the tracheoles because no respiratory pigments (like hemoglobin) are present in the haemolymph to transport gases.
Circulatory System
Open type: The blood (called haemolymph) does not flow entirely within vessels but bathes the organs directly in fluid-filled body cavities called haemocoel (or pericardial, perivisceral, and perineural sinuses).
Haemolymph: The circulatory fluid, composed of colorless plasma and specialized blood cells called haemocytes (which perform phagocytosis and clotting but do not transport gases).
Heart: An elongated, tubular, 13-chambered muscular tube located along the mid-dorsal line of the thorax and abdomen. Each chamber has a pair of small, valved openings called ostia on its posterior end, allowing haemolymph to enter from the surrounding pericardial sinus.
Alary muscles: A series of fan-shaped muscles that anchor the heart to the dorsal diaphragm and contract rhythmically to help pump haemolymph.
Circulation: Haemolymph enters the heart through the ostia. Rhythmic contractions of the heart (from posterior to anterior) pump haemolymph forward into the anterior aorta, which empties into the head region. From the head, haemolymph flows freely through various sinuses (pericardial sinus surrounding the heart, perivisceral sinus surrounding visceral organs, and perineural sinus surrounding the nerve cord), bathing the organs.
Important note: Haemolymph in cockroaches is not involved in gas transport (oxygen and carbon dioxide); this function is exclusively handled by the tracheal system.
Excretory System
Principal organs: The Malpighian tubules, which are numerous (100-150), slender, yellow, branched tubules attached at the junction of the midgut and hindgut.
Mechanism: They absorb nitrogenous wastes (primarily potassium and sodium urate) from the haemolymph. These urates are converted into uric acid (a solid or semi-solid form) within the tubules, which then empties directly into the hindgut.
Uricotelic: Cockroaches excrete uric acid as their main nitrogenous waste product. This is an adaptation to terrestrial life, as uric acid is insoluble and requires very little water for excretion, conserving body water.
Accessory structures that also aid in excretion:
Nephrocytes: Specialized cells in the haemocoel that store and possibly detoxify waste products by filtering the haemolymph.
Urinose glands: Specialized glands found only in male cockroaches, believed to be involved in excretion.
Fat bodies: Diffuse masses of tissue that store fat, glycogen, and also play a role in metabolism, storage of urates, and detoxification.
Nervous System & Sense Organs
The nervous system consists of a series of primitively arranged paired ganglia joined by longitudinal connectives, forming a ventral nerve cord along the body.
Head: Contains a supra-oesophageal ganglion (often called the 'brain') located above the oesophagus, which primarily controls sensory input from the antennae and eyes. Connected to a sub-oesophageal ganglion located below the oesophagus, which controls the mouthparts and legs.
Thorax: Possesses large ganglia (prothoracic, mesothoracic, metathoracic ganglia), one in each thoracic segment, controlling the movement of legs and wings.
Abdomen: Contains smaller ganglia, one in each of the first abdominal segments, controlling abdominal functions.
Decapitated cockroaches: Can survive for approximately week because a significant portion of their nervous system (major control centres for locomotion and some vital functions) is located in the ventral nerve cord in the thorax and abdomen, not just the head.
Sense organs:
Antennae: Highly sensitive to touch and smell.
Palps: (Maxillary and labial palps) aid in food manipulation and tactile sensation.
Compound eyes: For vision, providing mosaic vision as described earlier.
Anal cerci: Extremely sensitive to vibrations, air currents, and sound waves, acting as mechanoreceptors.
Tactile hairs: Scattered over the body surface, providing general touch sensation.
Reproductive System
Sexual reproduction, with distinct male and female individuals.
Male
Testes: A pair of trilobed (three-lobed), yellowish structures located in the abdominal segments; produce sperm.
Vasa deferentia: A pair of thin ducts originating from the testes, which carry sperm.
Seminal vesicles: The vasa deferentia converge to open into the seminal vesicles, which store sperm in bundles called spermatophores.
Ejaculatory duct: The seminal vesicles open into a common, short ejaculatory duct, which finally opens at the male gonopore (genital opening).
Accessory glands:
Mushroom/Utricular gland: A large, mushroom-shaped gland located in the abdominal segments. It consists of two types of tubules (long and short) that provide nourishment to the sperm and form the inner layers of the spermatophore.
Phallic/Conglobate gland: A long, club-shaped gland that secretes a protective outer covering for the spermatophore.
Spermatophore: During copulation, sperms are glued together and packaged into a complex capsule called a spermatophore, which is then transferred to the female.
External genitalia (phallomeres): Asymmetrical, chitinous structures around the male gonopore, involved in gripping the female and depositing the spermatophore.
Female
Ovaries: A pair of large, irregularly shaped structures located in the abdominal segments. Each ovary is composed of ovarioles.
Ovarioles: A string of bead-like ovarian tubules, each containing a series of developing eggs (ova) in a progressive order of maturity.
Oviducts: The eight ovarioles of each ovary unite to form a single oviduct. The two oviducts then join to form a median, short, and wide vagina (genital chamber), which opens to the outside via the female gonopore.
Spermathecae: A pair of small, pouch-like structures located in the segment, extending from the genital chamber. They are responsible for storing sperm received from the male after copulation. One of the spermathecae is larger and connected to a branched accessory gland.
Accessory collateral glands: Located posteriorly in the abdomen, these glands secrete a hard proteinaceous material that forms the dark reddish-to-black protective capsule around the eggs, called the ootheca.
Ootheca: Each ootheca typically contains eggs arranged in two rows. The female carries the ootheca partly protruding from her genital pouch until a suitable humid place is found to deposit it.
Brood pouch: A spacious chamber formed by the posterior abdominal sterna where the ootheca is formed and retained for a period.
Development
Paurometabolous (incomplete metamorphosis): The development is gradual, without a pupal stage.
Life cycle: Egg (laid in ootheca) nymph adult.
Nymph: The immature stage that hatches from the egg. It resembles the adult but is smaller, paler, lacks wings, and is sexually immature. The nymph grows by undergoing about moults (ecdysis), shedding its exoskeleton to increase in size.
Last nymphal stage: Develops wing pads but fully developed wings are observed only in the adult stage.
Economic Importance
Domestic pest: Cockroaches are significant household and public health pests. They spoil food and other products with their excreta and characteristic foul-smelling secretions (odour).
Mechanical vector of pathogens: They readily pick up various pathogenic microorganisms (bacteria, viruses, fungi) from garbage, sewers, and contaminated surfaces and mechanically transport them to food and food products, contributing to the spread of diseases like cholera, typhoid, dysentery, plague, etc.