Structural Organisation in Animals

Animal Tissues

• Unicellular vs. Multicellular
  • Unicellular organisms: A single cell performs all vital functions such as digestion, respiration, excretion, and reproduction.
  • Multicellular animals: Show division of labour.
    • Different groups of cells are specialised to perform specific functions in a coordinated manner.
• Example: Hydra
  • A simple multicellular organism composed of different types of cells,
  • Each cell present in large numbers and specialised for particular functions.
• Human Body
  • Made up of billions of cells
  • Cells function together in an organised way.
• Tissue
  • A group of similar cells, along with their intercellular substances, having a common origin and performing a specific function.
  • In complex animals, tissues are organised into organs.
  • And organs work together to form organ systems such as the digestive system, respiratory system, and circulatory system.
  • This hierarchical organisation ensures efficient functioning and survival of the organism.

Animal Tissues

• All complex animals possess four basic types of tissues:
  1. Epithelial Tissue
  2. Connective Tissue
  3. Muscular Tissue
  4. Neural Tissue

These tissues are arranged in specific proportions to form organs and organ systems.

1. Epithelial Tissue

Definition and General Features:

• Epithelial tissue, also called epithelium, forms the covering or lining of body surfaces, cavities, ducts, and organs.
• It has a free surface that faces either the external environment or a body fluid.
• The cells are tightly packed with very little intercellular material, forming a continuous sheet.

Types of Epithelial Tissues:

1. Simple Epithelium
   • Consists of a single layer of cells
   • Forms the lining for body cavities, ducts, and tubes.
   • Mainly functions in absorption, secretion, and diffusion.
   • Types of Simple Epithelium:
     • Squamous Epithelium:
       • It is made of a single thin layer of flattened cells with irregular boundaries.
       • It occurs in the walls of blood vessels and air sacs of the lungs.
       • Its thin structure allows diffusion of gases and other substances.
     • Cuboidal Epithelium:
       • Consists of cube-shaped cells arranged in a single layer.
       • Commonly found in ducts of glands and tubular parts of nephrons in the kidneys.
       • Main functions are secretion and absorption.
       • In some regions, such as the proximal convoluted tubule of the nephron, cells possess microvilli to increase the surface area for absorption.
     • Columnar Epithelium:
       • Composed of tall, slender cells with nuclei located near the base.
       • It lines the stomach, intestine, gall bladder, and bile duct.
       • Its primary functions are absorption and secretion.
       • In the intestine, the presence of microvilli forms a brush border that enhances absorption.
     • Ciliated Epithelium:
       • When cuboidal or columnar epithelial cells bear cilia on their free surface, the tissue is called ciliated epithelium.
       • It is found in bronchioles and fallopian tubes, where the cilia help move mucus, eggs, or fluids in a specific direction.
2. Glandular Epithelium:
   • Glandular epithelium is specialised for secretion.
   • It develops from simple epithelial cells.
   • Types:
     • Unicellular Glands: Consist of single secretory cells, such as goblet cells found in the alimentary canal (Stomach).
     • Multicellular Glands: Made of clusters of secretory cells (e.g., salivary gland).
   • Glands Based on the mode of secretion::
     • Exocrine Glands: Secrete substances like enzymes, mucus, or sweat through ducts onto an epithelial surface.
     • Endocrine Glands: Release hormones directly into the blood or tissue fluid and do not have ducts.

Some glands, like the pancreas and gonads, function as both exocrine and endocrine glands and are called heterocrine glands.

• 3. Compound Epithelium:
  • Consists of multiple layers of cells.
  • Plays a protective role rather than absorption or secretion.
  • Covers the skin, buccal cavity, pharynx, and the ducts of salivary and pancreatic glands, protecting underlying tissues from mechanical and chemical stress.

Cell Junctions in Epithelial Tissues:

Epithelial cells are held together by specialised junctions that maintain tissue integrity and coordination.

1. Tight Junctions: Prevent leakage of substances across tissues.
2. Adhering Junctions: Cement/Hold neighbouring cells firmly together, providing mechanical strength.
3. Gap Junctions: Facilitate communication between adjacent cells by connecting their cytoplasm for rapid transfer of ions and molecules.

2. Connective Tissue

Definition and General Features:

• Most abundant and widely distributed tissues in the bodies of complex animals.
• Their primary function is to link, support, bind, and protect other tissues and organs.
• These tissues range from soft forms like areolar and adipose tissue to specialised types such as cartilage, bone, and blood.

• Most connective tissues, except blood, contain cells that secrete structural protein fibres called collagen and elastin.
  • These fibres provide strength, elasticity, and flexibility.
• The cells also secrete modified polysaccharides that accumulate between cells and fibres to form the matrix or ground substance, which binds the tissue components together.

Types of Connective Tissue:

1. Loose Connective Tissue:
   • Cells and fibres loosely arranged in a semi-fluid ground substance.
   • It acts mainly as a binding and packing tissue.
   • Examples:
     • Areolar Tissue:
       • Found beneath the skin and between organs.
       • It contains fibroblasts (which produce fibres), macrophages (defence cells), and mast cells.
       • Its main function is to support and anchor epithelial tissues and to act as a packing material.
     • Adipose Tissue:
       • Adipose tissue is located mainly beneath the skin and around organs.
       • The cells store excess nutrients in the form of fat.
       • This tissue provides insulation, stores energy, and cushions organs.
2. Dense Connective Tissue:
   • In these tissue, fibres and fibroblasts are closely packed, providing great strength.
   • Types:
     1. Dense Regular Connective Tissue:
        • In this tissue, collagen fibres are arranged in parallel bundles.
        • This arrangement provides high tensile strength in one direction.
        • Examples: Tendons (attach skeletal muscles to bones), ligaments (attach one bone to another bone).
     2. Dense Irregular Connective Tissue:
        • Here, collagen fibres and fibroblasts are arranged in an irregular pattern.
        • This arrangement provides strength in multiple directions.
        • Example: Dermis of the skin.
     3. White Fibrous Connective Tissue:
        • It mainly consists of thick bundles of collagen fibres with fibroblasts arranged in rows.
        • It is very strong but has limited flexibility.
        • It forms flat sheets found in structures such as the dermis of skin, connective tissue sheaths of muscles and nerves, periosteum, perichondrium, and coverings of organs.
     4. Yellow Elastic Connective Tissue:
        • This tissue contains a network of elastic fibres, giving it high elasticity along with strength.
        • It forms elastic ligaments and sheets present in walls of blood vessels, lungs, bronchi, true vocal cords, trachea, and larynx.
3. Specialized Connective Tissue:
   • Includes cartilage, bones, and blood.
   • Cartilage:
     • Cartilage is a soft skeletal connective tissue.
     • Its cells, called chondrocytes, are enclosed in small spaces within a solid, pliable matrix.
     • The matrix resists compression and provides flexibility.
     • Example: Found at the tip of the nose, outer ear, joints, vertebral column, and respiratory passages.
       • In embryos, most cartilage is later replaced by bone.
     • Types of cartilage include:
       1. Hyaline cartilage: Found at joints of long bones, trachea, ribs, nasal septum, and larynx.
       2. Fibrocartilage: Strongest cartilage; found in intervertebral discs and pubic symphysis.
       3. Elastic cartilage: Found in pinna of ear, epiglottis, and Eustachian tube.
   • Bones:
     • Bone is a hard, non-pliable connective tissue.
     • Bone cells, called osteocytes, are present in spaces known as lacunae.
     • The matrix is rich in calcium salts and collagen fibres, giving bone strength and rigidity.
     • Bones provide structural support, protect vital organs, and serve as attachment sites for muscles, enabling movement.
     • Limb bones bear body weight.
     • Red bone marrow is responsible for the formation of blood cells.
   • Blood:
     • Blood is a fluid connective tissue.
     • It consists of plasma and formed elements—red blood cells, white blood cells, and platelets.
     • Blood circulates throughout the body and transports gases, nutrients, hormones, waste materials, and other substances.

Key Cells of Connective Tissue:

• Fibroblasts: Produce collagen and elastin fibres.
• Chondrocytes: Cells of cartilage.
• Osteocytes: Cells of bone.
• RBCs, WBCs, Platelets: Cellular components of blood.

Summary

• Connective tissues support, bind, and protect body organs, store fats, provide structural strength, and transport substances.
• Their diverse structure allows them to perform specialised functions essential for the survival and proper functioning of the body.

3. Muscle Tissue

General Features:

• Muscle tissue is specialised for contraction and movement.
• It is made up of long, cylindrical cells called muscle fibres, which are arranged in parallel bundles.
• Each muscle fibre contains fine contractile fibrils known as myofibrils, present in the cytoplasm called sarcoplasm.

• Muscle fibres have the ability to contract (shorten) and relax (lengthen) in response to stimulation.
• This coordinated contraction and relaxation produces movement of the body and helps maintain posture and position of various body parts.

Types of Muscle Tissue:

Based on structure, location, and function, muscle tissue is of three types:

1. Skeletal muscle
2. Smooth muscle
3. Cardiac muscle

1. Skeletal Muscle:
   • Skeletal muscle fibres are long, cylindrical, unbranched, and striated (striped).
   • These fibres are arranged in parallel bundles.
   • Several bundles of muscle fibres are enclosed within a sheath of tough connective tissue.
   • Skeletal muscles are attached to bones and are found in limbs, body walls, tongue, and the beginning of the oesophagus.
     • Example: biceps muscle.
   • These muscles are voluntary in nature, meaning their contraction is under conscious control.
   • Skeletal muscles help in body movement, posture maintenance, and locomotion.
   • They fatigue relatively quickly.
2. Smooth Muscle:
   • These are spindle-shaped (fusiform), tapering at both ends, and lack striations.
   • The fibres are held together by cell junctions and are bundled within a connective tissue sheath.
   • Found in the walls of internal organs such as blood vessels, stomach, intestines, urinary bladder, and uterus.
   • These muscles are involuntary, meaning their activity is not under conscious control.
   • They contract slowly and continuously and do not fatigue easily.
   • Smooth muscles help in processes like peristalsis, regulation of blood flow, and movement of contents within organs.
3. Cardiac Muscle:
   • Cardiac muscle is found only in the heart.
   • The muscle fibres are cylindrical, branched, and striated.
   • Adjacent cardiac muscle cells are connected by special cell junctions called intercalated discs, which fuse the plasma membranes of neighbouring cells.
     • Intercalated discs allow rapid transmission of impulses, so when one cell contracts, the neighbouring cells also contract.
     • This enables the heart to function as a coordinated unit.
   • Cardiac muscles are involuntary and never get fatigued.
   • They are also present in the walls of large veins such as pulmonary veins and superior vena cava where these veins enter the heart.

Key Differences in Function

• Skeletal muscles are voluntary and responsible for body movements and posture.
• Smooth muscles are involuntary and control movements within internal organs.
• Cardiac muscles are involuntary and specialised for continuous rhythmic contraction to pump blood.

Summary:

• Muscle tissue plays a vital role in movement, posture maintenance, circulation, and functioning of internal organs.
• The structural differences among skeletal, smooth, and cardiac muscles enable them to perform specialised functions essential for survival.

4. Neural Tissue

General Features:

• Neural tissue exerts the greatest control over the body’s responsiveness to changing internal and external conditions.
• It is specialised for receiving stimuli, conducting impulses, and coordinating body activities.
• Neural tissue is composed of two main cell types: neurons and neuroglial cells.

Neurons:

• Neurons are the structural and functional units of the neural system.
• They are excitable cells capable of generating electrical disturbances (nerve impulses) when stimulated.
• Each neuron consists of:
  1. Cell body (cyton or soma)
  2. Dendrites – short, branched processes that receive impulses
  3. Axon – a long process that transmits impulses away from the cell body
• The axon usually ends in terminal arborisations, which transmit signals to other neurons, muscles, or glands.
• When a neuron is stimulated, an electrical impulse travels rapidly along its plasma membrane.
• On reaching the terminal ends, the impulse may stimulate or inhibit adjacent neurons or effector cells.

An axon or dendrite covered with a sheath is called a nerve fibre.

Types of Nerve Fibres

Based on the presence or absence of myelin sheath, nerve fibres are of two types:

• 1. Medullated (Myelinated) Nerve Fibres
  • These fibres are covered with a myelin sheath.
  • The myelin sheath is interrupted at regular intervals by gaps called nodes of Ranvier, which help in faster conduction of nerve impulses.
• 2. Non-medullated (Non-myelinated) Nerve Fibres
  • These fibres lack a myelin sheath and are commonly found in autonomic nerves.
  • Impulse conduction in these fibres is comparatively slower.

Neuroglial Cells:

• Neuroglial cells support, protect, and nourish neurons.
• They constitute more than 50% of the volume of neural tissue.
• Although neuroglial cells do not conduct nerve impulses, they play an essential role in maintaining the structural and functional integrity of neurons.

Importance of Neural Tissue

Neural tissue enables communication within the body by transmitting electrical signals. It helps in coordination, regulation, perception of stimuli, and maintenance of homeostasis.

Organ and Organ System

Organ and Organ System

Formation of Organs and Organ Systems

• Basic tissues organise to form organs, and organs organise to form organ systems.
• Organ systems work together to coordinate the activities of millions of cells, ensuring efficient functioning of the organism.

Example:

• The heart is an organ composed of all four basic animal tissues:
  1. Epithelial tissue – lining of chambers
  2. Connective tissue – support and blood
  3. Muscular tissue – cardiac muscle for pumping
  4. Neural tissue – regulation of heartbeat

Significance of Organisation

• This organisation of tissues into organs and organ systems allows:
  • Better coordination of functions
  • Division of labour among cells
  • Increased efficiency and survival

The increasing complexity of tissues, organs, and organ systems reflects an evolutionary trend toward higher organisation.

Morphology and Anatomy

• Morphology refers to the study of externally visible features of organisms.
• Anatomy refers to the study of the internal structure of organisms.

Earthworm

Habitat and Habit

• Habitat: Earthworms live in moist soil and burrows.
  • They commonly inhabit the upper layers of soil rich in organic matter.
• Common Indian Species/Genus: genera Pheretima and Lumbricus.

Morphology

• Body: Long, cylindrical and divided into about 100–120 similar segments called metameres.
• Dorsal Surface: Marked by a dark median mid-dorsal line due to the dorsal blood vessel.
• Ventral Surface: bears genital openings.
• Anterior End: consists of a mouth and a lobe-like prostomium.
  • The prostomium is sensory in function, helps in covering the mouth and acts as a wedge to open soil cracks.
• First Body Segment: is called peristomium and it contains the mouth.
• Clitellum: A thick dark glandular band formed by segments 14 to 16 in mature earthworms.
  • Body Regions: Based on Clitellum, the body is divided into three regions:
  • 1. Pre-clitellar region
  • 2. Clitellar region
  • 3. Post-clitellar region
• Setae: S-shaped bristles like structures in most segments aiding in locomotion.
  • Setae present in each segment except the first, last and clitellar segments
• External Openings include:
  • Four pairs of spermathecal apertures on segments 5-9
  • A single female genital pore on the mid-ventral side of the 14th segment
  • A pair of male genital pores on the ventro-lateral side of the 18th segment
  • Numerous nephridiopores on the body surface.

Anatomy of Earthworm

Body Wall and Coelom

• The body wall consists of:
  • Thin non-cellular cuticle
  • Epidermis of columnar epithelial cells with gland cells
  • Circular muscle layer
  • Longitudinal muscle layer
  • Inner coelomic epithelium
• The coelom is a fluid-filled cavity between the body wall and alimentary canal.
• The coelomic fluid acts as a hydroskeleton and helps in locomotion.
• It contains various corpuscles including phagocytes and chloragogen cells.

Digestive System

• Alimentary Canal: A straight tube extending from the mouth to the anus.
• Parts of the alimentary canal:
  • Mouth: Opens into the buccal cavity (segments 1-3).
  • Pharynx: Muscular part, up to 4th segment.
  • Oesophagus: Narrow tube (segments 5-7).
  • Gizzard: Thick-walled muscular organ, grinds soil and food (segments 8-9).
  • Stomach: segments 9–14; contains calciferous glands that neutralise humic acid.
  • Intestine: Starts from 15th segment, ends at anus.
    • Typhlosole: The intestine shows a dorsal internal fold called typhlosole from the 26th segment onwards, which increases the absorptive surface area.

The alimentary canal ends at the anus.

Blood Vascular System

• Closed Circulatory System: Blood flows through blood vessels and capillaries.
• There are longitudinal, lateral and intestinal blood vessels.
  • Smaller Vessels and Capillaries: Supply blood to gut, nerve cord, and body wall.
  • Blood Glands: Located in segments 4, 5, and 6; produce blood cells and hemoglobin.
  • Blood cells are phagocytic in nature.

Respiratory System

• No Specialized Organs: Exchange of gases occurs through the moist body surface by diffusion.

Excretory System

• Nephridia: Slender & coiled tubules for excretion.
• Types of nephridia:
  1. Septal Nephridia: present from segment 15 to the last; open into the intestine (enteronephric)
  2. Pharyngeal nephridia: present in segments 4–6; open into alimentary canal
  3. Integumentary nephridia: present from segment 3 to the last; open outside through nephridiopores (exonephric)

Chloragogen cells also help in excretion.

Nervous System

• Ganglia: The nervous system consists of paired ventral nerve cord with segmentally arranged ganglia.
  • Nerve Ring: In the anterior region, the nerve cord forms a nerve ring around the pharynx by joining cerebral ganglia dorsally.
• Earthworms possess:
  • Light-sensitive cells
  • Touch-sensitive receptors
  • Chemoreceptors (taste receptors)

Reproductive System

• Hermaphrodite (monoecious): both male and female reproductive organs in the same individual.
• Male reproductive system:
  • Testes: Two pairs in segments 10 and 11.
  • Seminal vesicles, vasa deferentia, prostate glands and accessory glands
  • Male genital pores open in the 18th segment
• Female reproductive system:
  • Ovaries: One pair of ovaries attached to septum between 12th and 13th segments
  • Short oviducts opening by a single female genital pore in the 14th segment
• Spermathecae: Four pairs of spermathecae in segments 6–9 store sperm received during copulation.
• Mating: During mating, two worms exchange sperms.
  • Fertilisation and development occur inside cocoons deposited in soil.
  • Development is direct, with no larval stage.

Importance to Humans

• Earthworms are called “friends of farmers” because:
  • Soil Fertility: They make soil porous and aerated
  • Vermicomposting: Improve soil fertility through vermicomposting
  • Fishing Bait: Enhance root penetration and water absorption

Cockroach

General Information

• Appearance: Brown or black, sometimes yellow, red, or green in tropical regions.
• Size: Ranges from 0.6 cm to 7.6 cm.
• Habitat & Diet: Nocturnal, omnivorous animals commonly found in damp places worldwide, common in human homes.
• Features: Due to their habit of contaminating food, some species act as pests and disease carriers (vectors).

Classification

Phylum: Arthropoda
Class: Insecta
Scientific name: Periplaneta americana

Morphology

The body of cockroach is elongated, bilaterally symmetrical and dorsoventrally flattened. It measures about 34–53 mm in length.

The body is divided into three regions:
• Head
• Thorax
• Abdomen

• The entire body is covered by a hard, brown, chitinous exoskeleton.
• The hardened plates of exoskeleton are called sclerites.
• Dorsal sclerites are called tergites and ventral sclerites are called sternites.
• These are connected by thin, flexible arthrodial membranes.

Head

• Shape: Triangular and formed by the fusion of six segments.
• Mobility: Head highly mobile due to the presence of a flexible neck.
• Head bears:
  • Eyes: Compound eyes (1 pair).
  • Antennae: Long and thread-like with sensory receptors.
• Mouthparts: Biting and chewing type and include:
  • Labrum (Upper lip).
  • Mandibles (1 pair): For Biting and chewing.
  • Maxillae: 1 Pair.
  • Labium (Lower lip).
  • Hypopharynx (tongue-like structure)

Thorax

• The thorax consists of three segments:
  • Prothorax
  • Mesothorax
  • Metathorax
• Neck: Short extension of prothorax.
• Legs: Each thoracic segment bears one pair of walking legs.
  • Each leg consists of several segments called podomeres.
• Wings:
  • Forewings (Tegmina): Arise from mesothorax, dark; they are dark, opaque and leathery
  • Hind Wings: Arise from metathorax; they are transparent, membranous and used for flight

Abdomen

• The abdomen consists of 10 segments in both males and females.
• In Females:
  • 7th Sternum: Boat-shaped.
  • Brood/Genital Pouch: 7th sternum along with the 8th and 9th sterna forms a brood or genital pouch. This pouch contains:
    1. Female genital pore
    2. Spermathecal pores
    3. Collateral glands
• In Males:
  • The genital pouch lies at the hind end of abdomen and contains:
    1. Anus (dorsal)
    2. Male genital pore (ventral)
    3. Gonapophysis
  • Anal Styles: Males possess a pair of short, thread-like anal styles, which are absent in females.
• Anal Cerci: A jointed filamentous structures on the 10th segment in both sexes.

Anatomy of Cockroach

Digestive System

• Alimentary Canal: Divided into three regions – foregut, midgut, and hindgut.
  • Foregut:
    • Mouth opens into the pharynx.
    • Oesophagus leads to a sac-like crop used for food storage.
    • Gizzard (proventriculus) follows the crop, contains six chitinous teeth and muscular walls for grinding food.
  • Midgut:
    • It is short and narrow.
    • At the junction of foregut and midgut, 6–8 blind tubules called hepatic or gastric caecae are present.
      • These secrete digestive enzymes.
  • Hindgut:
    • At the junction of midgut and hindgut, about 100–150 Malpighian tubules are present.
    • Hindgut is divided into ileum, colon and rectum.
    • Rectum contains six rectal glands and opens outside through the anus.

Foregut and hindgut are ectodermal and lined by cuticle, whereas midgut is endodermal and lacks cuticle.

Blood Vascular System

• Open Circulatory System: Blood (haemolymph) flows freely in the body cavity called haemocoel and directly bathes the organs.
• Haemolymph consists of colourless plasma and haemocytes and lacks respiratory pigments.
• Heart:
  • The heart is an elongated muscular tube with funnel-shaped chambers having ostia on either side.
  • Blood enters the heart through ostia and is pumped anteriorly.
  • The heart is neurogenic.

Respiratory System

• Tracheal System: A network of tubes.
  • Spiracles: 10 pairs of openings located laterally (sides) on the body.
    • Spiracles are guarded by bristles to prevent entry of dust.
  • Tracheoles: Tracheae branch into tracheoles where gaseous exchange occurs by diffusion.

Excretory System

• Malpighian Tubules: Remove nitrogenous wastes from haemolymph and convert them into uric acid.
  • Hence, cockroach is uricotelic.
• Other excretory structures include: Fat body, nephrocytes, and urecose glands also help in excretion.

Nervous System

• The nervous system consists of:
  • Central nervous system
  • Peripheral nervous system
  • Sympathetic nervous system
• Ganglia: There are 9 segmentally arranged ganglia.
  • Thorax: 3 ganglia.
  • Abdomen: 6 ganglia.
• The brain is represented by a bilobed supra-oesophageal ganglion which supplies nerves to antennae and compound eyes.
• Sense organs include:
  • Antennae
  • Compound eyes
  • Maxillary and labial palps
  • Anal cerci
• Compound Eyes: consists of about 2000 ommatidia and produces mosaic vision, suitable for nocturnal life.

Reproductive System

• Dioecious: Separate male and female cockroaches.
  • Male Reproductive System:
    • Testes (1pair): In 4th-6th abdominal segments.
    • Vas deferens: Leads to ejaculatory duct.
    • Spermatophores: Seminal vesicles storing sperms as spermatophores
    • Mushroom-shaped Gland: An accessory gland in 6th-7th segments.
    • Phallomeres: External genitalia called phallomeres
  • Female Reproductive System:
    • Ovaries (1pair): In 2nd-6th abdominal segments.
    • Ovarioles (several): Tubules containing ova.
    • Oviducts: unite into a single median oviduct
    • Spermatheca (1pair): Stores sperm.
    • Collateral glands: secrete material forming ootheca

Reproduction and Development

• Spermatophores: Sperms are transferred in the form of spermatophores.
• Ootheca: Fertilised eggs are enclosed in a reddish-brown capsule called ootheca.
  • Each ootheca contains 14–16 eggs. A female produces about 9–10 oothecae.
• Paurometabolous Development: meaning dev. occurs through nymph stages.
  • Nymph resembles adult but lacks wings and reproductive organs.
  • After several (13) moults, the adult is formed.

Importance/Role

• Pests: They spoil food and spread diseases by contaminating food with their excreta.
• Wild Species: Many wild species, however, have no economic importance.

Frogs

General Information

• Habitat: Live on land and in freshwater (amphibians).
• Class: Class Amphibia, Phylum Chordata.
• Common Indian frog Species: Rana tigrina.
• Body Temperature: Cold-blooded (poikilotherms), body temperature varies with the temperature of the environment.
• Camouflage: A protective coloration ability; Change color to blend in with surroundings (mimicry).
• Aestivation and Hibernation: Hide in burrows during extreme heat (summer sleep) and cold (winter sleep).

Morphology

• Skin:
  • Smooth, slippery and always moist due to mucus.
    • Dorsal side: Olive green with dark irregular spots
    • Ventral side: Uniformly pale yellow
  • Frogs do not drink water; instead, water is absorbed directly through the skin.
• Body Parts:
  • Divided into head and trunk.
  • Neck and Tail: Absent in adult frogs.
  • A membranous tympanum (ear drum) is present on both sides of the head and receives sound vibrations.
  • Nostrils (1pair external): Present above the mouth.
  • Eyes: Bulged, with a nictitating membrane for vision & protection in water.
  • Tympanum (Ear drum): Membranous, present on both sides of the head and receives sound vibrations.
• Limbs:
  • Forelimbs: Shorter, end in four digits
  • Hind Limbs: Longer, muscular, end in five digits
    • The digits of hindlimbs are webbed, helping in swimming, leaping, walking and burrowing.
• Sexual Dimorphism:
  • Male frogs:
    • Possess vocal sacs for sound production
    • Have a copulatory pad (for mating purpose) on the first digit of forelimbs
    • Are usually smaller and darker
  • Females: Lack vocal sacs and copulatory pads

Anatomy

Body Cavity

• Contains various organ systems like digestive, circulatory, respiratory, nervous, excretory, and reproductive systems.

Digestive System

• Parts: Alimentary canal and digestive glands.
• Alimentary Canal: Short due to carnivorous diet.
  • Pathway: Mouth → Buccal cavity → Pharynx → Oesophagus → Stomach → Small intestine → Rectum → Cloaca
• The stomach is divided into:
  1. Cardiac stomach
  2. Pyloric stomach
• The small intestine consists of:
  1. Duodenum
  2. Ileum
• The inner wall of intestine has villi and microvilli for absorption.
• Digestive Glands:
  • Liver: Secretes bile, stored in the gall bladder.
  • Pancreas: Produces pancreatic enzymes.
  • Gastric and intestinal glands
• Digestion Process:
  • Food is captured by a bilobed tongue.
  • HCl and gastric juices digest food in stomach.
  • Bile emulsifies fats and pancreatic juice digests carbohydrates and proteins.
  • Final digestion and absorption occur in intestine..

Respiratory System

• Adult frog respires by three methods:
• 1. Cutaneous respiration
  • Occurs through moist, vascular skin.
  • It is the only method in water and during aestivation and hibernation.
• 2. Buccopharyngeal respiration
  • Occurs on land or during partial immersion in water.
• 3. Pulmonary respiration
  • Occurs through lungs when frog is outside water.
  • Lungs are pink, sac-like structures.

Circulatory System

• Type: Well developed and closed.
• Heart:
  • Muscular
  • Three chambers: two atria and one ventricle
  • Covered by pericardium
• A sinus venosus opens into the right atrium and the ventricle opens into conus arteriosus.
• Blood:
  • Plasma
  • RBCs (nucleated, with haemoglobin)
  • WBCs
  • Platelets
• Lymphatic System: includes lymph, lymph vessels and lymph nodes.
• Special Portal Systems:
  1. Hepatic Portal System: Between intestine and liver.
  2. Renal Portal System: Between kidneys and lower body parts.

Excretory System

• Components:
  • A pair of kidneys
  • Ureters
  • Urinary bladder
  • Cloaca
• Kidneys: Bean-shaped and contain nephrons.
• Ureters:
  • In males, ureters act as urinogenital ducts.
  • In females, ureters and oviducts open separately into cloaca.
• Bladder: Thin-walled, ventral to the rectum.
• Excretion: Frog excretes urea and is therefore ureotelic.

Nervous System

• Nervous system includes:
  1. Central nervous system: Brain and spinal cord.
  2. Peripheral nervous system: Cranial and spinal nerves.
  3. Autonomic nervous system: Sympathetic and parasympathetic.
• Brain: Brain is enclosed in a bony brain box and divided into:
  1. Forebrain
  2. Midbrain
  3. Hindbrain
• Sense Organs: Sense organs include:
  • Touch – sensory papillae
  • Taste – taste buds
  • Smell – nasal epithelium
  • Vision – eyes
  • Hearing – tympanum and internal ear

Chemical coordination is controlled by endocrine glands.

Reproductive System

Frogs are dioecious, having separate sexes.

• Male Reproductive System:
  • Testes (1pair): Yellowish, near/attached to kidneys.
  • Vasa Efferentia: 10-12.
  • Urinogenital Duct: Opens into the cloaca.
• Female Reproductive System:
  • Ovaries (1pair): Near kidneys.
  • Oviducts (1pair): Open into the cloaca.
  • Eggs: Females lay about 2500–3000 ova at a time
• Fertilization: external and occurs in water.
• Development: Involves a tadpole stage undergoing metamorphosis to become an adult.

Importance/Role

• Ecological Role: Eat insects, protect crops, and maintain ecological balance.
• Food: In some countries, frog legs are used as food.

Chapter Summary:

Cells, Tissues, Organs, and Organ Systems

• Living organisms show organisation at different levels.
• Cells are the basic structural and functional units of life.
• Groups of similar cells performing one or more specific functions form tissues.
• Different tissues combine to form organs, and organs work together as organ systems to ensure survival of the organism.

Tissues in Animals

• Epithelial tissues are sheet-like tissues that cover the body surface and line internal organs and cavities.
• They mainly function in protection, absorption, secretion and exchange.

• Connective tissues support, bind, protect and strengthen different parts of the body.
• Soft connective tissues consist of cells and protein fibres embedded in a ground substance.
• Specialised connective tissues include cartilage, bone, blood and adipose tissue, which provide support, transport materials and store energy.

• Muscle tissue helps in movement and posture.
• Skeletal muscles are attached to bones and are voluntary.
• Smooth muscles are present in internal organs and work involuntarily.
• Cardiac muscle is found only in the heart and works continuously without fatigue.

• Nervous tissue controls and coordinates body activities.
• Neurons are the basic structural and functional units of nervous tissue and help in transmission of impulses.

Earthworm (Pheretima posthuma)

• The body of earthworm is long, cylindrical and covered by a thin cuticle.
• It is divided into many similar segments, except segments 14, 15 and 16 which form the clitellum.
• S-shaped setae present in most segments help in locomotion.

• Four pairs of spermathecal openings are present between segments 5 and 9.
• The female genital pore is present on the 14th segment and a pair of male genital pores is present on the 18th segment.

• The alimentary canal is a straight tube consisting of mouth, buccal cavity, pharynx, gizzard, stomach, intestine and anus.

• Earthworm has a closed blood vascular system with blood vessels and valves.
• The nervous system consists of a ventral nerve cord with segmentally arranged ganglia.

• Earthworm is hermaphrodite, possessing both male and female reproductive organs.
• Fertilisation and development take place inside the cocoon.

Cockroach (Periplaneta americana)

• The body of cockroach is covered by a hard chitinous exoskeleton and is divided into head, thorax and abdomen.

• The thorax consists of three segments, each bearing a pair of walking legs.
• Two pairs of wings arise from the second and third thoracic segments.
• The abdomen consists of ten segments.

• The alimentary canal includes mouth, pharynx, oesophagus, crop, gizzard, midgut, hindgut and anus.
• Hepatic caecae are present at the junction of foregut and midgut.
• Malpighian tubules are present at the junction of midgut and hindgut and function in excretion.
• Salivary glands are present near the crop.

• Cockroach has an open blood vascular system.
• Respiration occurs through a tracheal system that opens outside by spiracles.

• The nervous system consists of a series of ganglia connected by a ventral nerve cord.

• Cockroaches are dioecious.
• In males, testes are present in the 4th to 6th abdominal segments.
• In females, ovaries are present in the 2nd to 6th abdominal segments.
• Fertilisation is internal.
• Females produce about 9–10 oothecae.

Frog (Rana tigrina)

• The frog is a common amphibian found in India.
• Its body is covered by moist skin with mucous glands.
• The body is divided into head and trunk.

• Respiration occurs through skin in water and through lungs on land.
• The muscular, bilobed tongue helps in capturing prey.

• The alimentary canal includes oesophagus, stomach, intestine, rectum and cloaca.
• Digestive glands include liver and pancreas.

• The circulatory system is closed and shows single circulation.
• Red blood cells are nucleated.

• The nervous system includes central, peripheral and autonomic components.

• In males, a pair of testes is present, while females possess a pair of ovaries.
• Fertilisation is external.
• Females lay about 2500–3000 eggs.
• Development includes a tadpole larval stage which undergoes metamorphosis to form an adult frog.