Structural Organisation in Animals

STRUCTURAL ORGANISATION IN ANIMALS

Hierarchical Organisation of Multicellular Animals

In multicellular organisms, cells are organised in a hierarchical manner to ensure efficient functioning and division of labour:

**Cells → Tissues → Organs → Organ Systems → Organism**

**Cells** are the basic unit of life; unicellular organisms perform all functions in a single cell

**Tissues** are groups of similar cells along with intercellular substances that perform specific functions

**Organs** are structures made of two or more tissue types arranged in specific proportions and patterns

**Organ Systems** are formed when two or more organs work together through physical or chemical interactions to perform a common function

Example: The heart consists of epithelial, connective, muscular, and neural tissues all functioning together

This hierarchical organisation exhibits **division of labour**, where different cell groups specialise in specific functions, contributing to the survival of the entire organism.

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ORGAN AND ORGAN SYSTEM

**Definition**: An **organ** is a functional unit composed of different types of tissues arranged in specific proportions and patterns, performing one or more functions.

**Definition**: An **organ system** is a group of organs that physically and/or chemically interact to perform a common function.

Tissue Organisation in Organs

Each organ in the body contains one or more of the four basic tissue types: epithelial, connective, muscular, and neural

Example: The human heart contains all four tissue types

Epithelial tissue forms the inner lining (endocardium)

Connective tissue forms the cardiac skeleton

Muscular tissue forms the myocardium for contraction

Neural tissue provides nerve supply for coordination

Evolutionary Trends in Organ Systems

Complexity in organ and organ systems displays a **discernable evolutionary trend**, indicating increased specialisation and efficiency in higher organisms compared to lower organisms. This concept is studied in detail in Class XII.

Morphology and Anatomy Distinction

**Morphology** refers to the study of external form or externally visible features of organisms

In animals, morphology specifically refers to the external appearance of organs and body parts

**Anatomy** refers to the study of internal structure and organisation of organs in animals

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FROGS: GENERAL CHARACTERISTICS

Taxonomic Classification

**Phylum**: Chordata

**Class**: Amphibia

**Common Indian Species**: *Rana tigrina* (Indian bullfrog)

Frogs can live both on land and in freshwater environments

Key Characteristics

**Temperature Regulation**: Frogs are **cold-blooded** or **poikilothermic** animals, meaning their body temperature varies with the environmental temperature. They lack constant body temperature regulation mechanisms.

**Protective Colouration and Mimicry**: Frogs can change colour to match their surroundings, hiding from predators. This protective colouration is called **mimicry**.

**Seasonal Dormancy**:

**Aestivation** (summer sleep): Frogs take shelter in deep burrows during peak summer to protect from extreme heat

**Hibernation** (winter sleep): Frogs remain dormant during winter months in protected shelters

**Water Absorption**: Frogs do not drink water; they absorb it directly through their skin.

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MORPHOLOGY OF FROG (*Rana tigrina*)

External Features and Structural Organisation

**Body Division**: The frog body is divisible into **head and trunk**. A neck and tail are absent, distinguishing amphibians from other vertebrates.

Head Region

**Nostrils**: A pair of nostrils is present above the mouth, used for air intake and olfaction

**Eyes**: Eyes are bulged (protruding) and positioned dorsolaterally; protected by a **nictitating membrane** (third eyelid) that shields them while in water

**Tympanum**: A membranous structure on either side of the head behind the eyes; serves as the external ear receiving sound signals; lies flush with the skin without an external ear

Skin Characteristics

**Texture**: Smooth and slippery due to presence of mucus glands

**Colour Pattern**:

Dorsal (upper) surface: Olive green with dark irregular spots

Ventral (lower) surface: Uniformly pale yellow

**Moisture**: Always maintained in moist condition

**Function**: Vascularised skin aids in respiration both in water and on land

Limbs

**Forelimbs**:

End in **four digits** (fingers)

Smaller and less muscular than hind limbs

Used in walking and burrowing

Males have a **copulatory pad** (nuptial pad) on the first digit used during mating

**Hind Limbs**:

End in **five digits** (toes)

Larger and more muscular than forelimbs

**Webbed digits** (toes connected by skin membrane) facilitate swimming

Adapted for powerful jumping and swimming

Sexual Dimorphism

Males and females can be distinguished by:

**Males**: Possess **vocal sacs** (sound-producing structures) and **copulatory pads** on first foreleg digit

**Females**: Lack vocal sacs and copulatory pads

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ANATOMY OF FROG: INTERNAL ORGAN SYSTEMS

DIGESTIVE SYSTEM

**Components**: Alimentary canal and digestive glands

**Pathway of Food** (from mouth to cloacal aperture):

1. **Mouth** → **Buccal cavity** (food capture occurs here)

2. **Pharynx** → **Oesophagus** (short tube)

3. **Stomach** (site of protein digestion with HCl and gastric juices)

4. **Small Intestine** (duodenum as first part)

5. **Intestine** (main absorption site)

6. **Rectum** (storage of undigested waste)

7. **Cloaca** (common chamber)

8. **Cloacal aperture** (external opening)

**Digestive Organs and Functions**:

**Tongue**: Bilobed at the tip; muscular structure used for capturing prey by flipping out

**Stomach**: Secretes HCl and gastric juices for protein digestion; converts food into **chyme** (partially digested food)

**Liver**: Secretes **bile** that emulsifies (breaks down) fats; bile is stored in the **gall bladder**

**Pancreas**: Secretes **pancreatic juice** containing digestive enzymes for carbohydrate and protein digestion

**Small Intestine (Duodenum)**: Receives bile from gall bladder and pancreatic juices through a common bile duct; completes digestion

**Intestine**: Contains numerous **villi** (finger-like folds) and **microvilli** (microscopic projections) for absorption of digested nutrients into blood

**Reason for Short Intestine**: Frogs are **carnivores** (meat-eaters); carnivorous animals have shorter intestines compared to herbivores because meat is easier to digest and requires less surface area for absorption.

RESPIRATORY SYSTEM

Frogs respire by **multiple methods** depending on their habitat:

**In Water** (Cutaneous Respiration):

**Skin acts as the primary respiratory organ**

Dissolved oxygen in water diffuses through moist skin into blood capillaries

Carbon dioxide diffuses out through skin

**On Land** (Dual Method):

**Buccal cavity respiration**: Gas exchange occurs through thin buccal cavity lining

**Cutaneous respiration**: Continues through skin even on land

**Pulmonary respiration**: Air enters through nostrils → buccal cavity → **lungs**

**Lungs**: Pair of elongated, pink, sac-like structures in upper thorax region; thin-walled for gas exchange

**During Aestivation and Hibernation**:

Gas exchange occurs exclusively through skin

Lungs are not functional during dormancy

CIRCULATORY SYSTEM

**Type**: **Closed circulatory system** (blood flows through vessels, not in direct contact with tissues)

**Components**:

**Heart**:

Muscular structure in upper body cavity

**Three-chambered heart**: 2 atria (auricles) + 1 ventricle

**Pericardium**: Protective membrane surrounding heart

**Sinus venosus**: Triangular structure joining right atrium; receives blood from major veins (vena cava)

**Conus arteriosus**: Sac-like structure on ventral side of ventricle; conducts blood to arteries

**Blood Vessels**:

**Arteries**: Carry oxygenated blood from heart to all body parts

**Veins**: Collect deoxygenated blood from body parts and return to heart

**Special Venous Systems**:

**Hepatic portal system**: Connection between liver and intestine; ensures filtered blood passes through liver before systemic circulation

**Renal portal system**: Connection between kidneys and lower body parts; filtration of wastes before systemic circulation

**Blood Composition**:

**Plasma**: Liquid matrix carrying nutrients, gases, and water

**Red Blood Cells (RBCs/Erythrocytes)**:

**Nucleated** (contain nucleus, unlike mammalian RBCs)

Contain **haemoglobin** (red respiratory pigment for oxygen transport)

**White Blood Cells (WBCs/Leucocytes)**: Perform immune functions

**Platelets**: Blood clotting function

**Lymphatic System**:

**Components**: Lymph, lymph channels, and lymph nodes

**Lymph**: Similar to blood but lacks RBCs and few proteins

**Functions**: Transport of nutrients and filtration

**Blood Circulation Function**: Nutrients, oxygen, and water are transported to respective tissues; waste products are transported to excretory organs. Circulation is achieved by the pumping action of the muscular heart.

EXCRETORY SYSTEM

**Components**: Pair of kidneys, ureters, urinary bladder, and cloaca

**Kidneys**:

Compact, dark red, bean-like structures

Situated posteriorly in body cavity on both sides of vertebral column

**Structural and Functional Units**: **Uriniferous tubules** or **nephrons** (tiny filtering units)

**Function**: Filtration of nitrogenous waste from blood

**Ureters**:

**In males**: Act as **urinogenital ducts** (conduct both urine and sperm)

**In females**: Ureters and oviduct open separately into cloaca

Emerge from kidneys and open into cloaca

**Urinary Bladder**:

Thin-walled structure ventral (below) to rectum

Stores urine before excretion

Opens into cloaca

**Cloaca**:

Small median chamber

Common opening for faecal matter, urine, and reproductive products

Opens to exterior through **cloacal aperture**

**Nitrogenous Waste Excretion**: Frogs excrete **urea** and are therefore classified as **ureotelic animals**. Excretory wastes are carried by blood to kidneys, filtered, and excreted.

NERVOUS SYSTEM

**Organisation**: Three-part system for control and coordination

**1. Central Nervous System (CNS)**:

**Brain**: Enclosed in **brain box** (cranium); divided into three regions:

**Forebrain**: Contains olfactory lobes, paired cerebral hemispheres, and unpaired diencephalon; controls smell and sensory processing

**Midbrain**: Characterised by paired optic lobes; controls vision and reflex responses

**Hindbrain**: Consists of cerebellum (balance and coordination) and medulla oblongata (vital centres); medulla passes through foramen magnum

**Spinal Cord**: Continuation of medulla oblongata; enclosed in vertebral column; conducts signals to and from brain

**2. Peripheral Nervous System (PNS)**:

**Cranial Nerves**: Ten pairs arising from brain

**Spinal Nerves**: Arise from spinal cord

**3. Autonomic Nervous System**:

**Sympathetic**: Prepares body for emergency

**Parasympathetic**: Promotes resting functions

ENDOCRINE SYSTEM

**Prominent Endocrine Glands**:

**Pituitary gland**: Master gland controlling other endocrine glands

**Thyroid**: Metabolism regulation

**Parathyroid**: Calcium regulation

**Thymus**: Immune function

**Pineal body**: Circadian rhythm regulation

**Pancreatic islets**: Blood glucose regulation

**Adrenals**: Stress response

**Gonads**: Reproductive hormone production

**Function**: Hormones secreted by these glands achieve chemical coordination of various organs.

SENSORY ORGANS

**Types of Sense Organs**:

1. **Touch**: Sensory papillae (cellular aggregations) on skin

2. **Taste**: Taste buds on tongue

3. **Smell**: Nasal epithelium in nasal cavity

4. **Vision**: **Eyes** (well-organised structures)

Pair of spherical structures in orbit within skull

**Simple eyes**: Possess only one optical unit (unlike compound eyes in insects)

Protected by nictitating membrane

5. **Hearing and Equilibrium**: **Tympanum and internal ear** (well-organised structures)

External tympanum (eardrum) visible on lateral surface

Internal ear for balance and hearing

No external ear present

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REPRODUCTIVE SYSTEM

MALE REPRODUCTIVE SYSTEM

**Components** (Figure 7.3):

**Testes**: Pair of yellowish, ovoid structures adhered to upper part of kidneys by **mesorchium** (double fold of peritoneum)

**Vasa efferentia**: 10-12 tubules arising from testes

**Bidder's canal**: Receives vasa efferentia; enters kidney

**Urinogenital duct**: Conducts sperm from kidney to cloaca

**Cloaca**: Common chamber for urine and sperm discharge

**Cloacal aperture**: External opening

**Associated Structures**:

**Fat bodies**: Energy storage structures attached to testes

**Adrenal glands**: Endocrine structures on kidneys

**Urinary bladder**: Stores urine

FEMALE REPRODUCTIVE SYSTEM

**Components** (Figure 7.4):

**Ovaries**: Pair of structures situated near kidneys; **no functional connection with kidneys** (unlike males)

**Oviducts**: Pair of long, coiled tubules arising from ovaries; conduct ova (eggs) to cloaca

**Ova**: Eggs produced by ovaries

**Cloaca**: Common chamber for egg discharge

**Cloacal aperture**: External opening

**Reproductive Capacity**:

A mature female can lay **2500 to 3000 ova at a time**

Eggs are large, unpigmented (in freshwater frogs)

FERTILISATION AND DEVELOPMENT

**Type**: **External fertilisation** (occurs in water)

**Process**:

1. Male and female frogs engage in **amplexus** (mating embrace)

2. Female releases eggs into water; male releases sperm simultaneously

3. Fertilisation occurs in water (external fertilisation)

4. Fertilised eggs develop into larvae

**Larval Development**:

**Tadpole**: Larval stage of frog

Aquatic, fishlike appearance

Possesses tail, gills, and no limbs initially

Feeds on algae and aquatic plants

**Metamorphosis**:

**Definition**: Transformation of tadpole into adult frog

**Process**:

Gradual development of four limbs

Tail gradually resorbed

Gills replaced by lungs

Dietary shift from herbivorous to carnivorous

Adult morphology achieved

**Larval stage in India**: Typically 2-3 months before metamorphosis completes

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ECOLOGICAL IMPORTANCE OF FROGS

**Benefits for Mankind**:

**Pest Control**: Consume large quantities of insects, protecting crops from pests

**Food Source**: Muscular legs used as food in several countries

**Ecological Balance**:

Important links in **food chains and food webs** within ecosystems

Serve as indicator organisms for environmental health

Control insect populations naturally

**Conservation Importance**: Declining frog populations indicate environmental pollution and ecological imbalance.

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KEY DEFINITIONS FOR BOARD EXAMS

**Tissue**: Group of similar cells with intercellular substances performing specific functions

**Organ**: Structure made of different tissue types in specific proportions performing one or more functions

**Organ System**: Group of organs interacting to perform a common function

**Poikilotherm/Cold-blooded**: Animal with body temperature varying with environment

**Aestivation**: Dormancy during summer

**Hibernation**: Dormancy during winter

**Ureotelic**: Organism excreting urea as main nitrogenous waste

**External Fertilisation**: Fusion of gametes occurring outside the body in aquatic environment

**Metamorphosis**: Transformation of larva into adult form

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SUMMARY POINTS FOR QUICK REVISION

Frogs belong to class **Amphibia**, phylum **Chordata**

Body divisible into **head and trunk only**

**Poikilothermic** animals (cold-blooded)

Respiratory organs: **Skin, buccal cavity, and lungs**

Heart: **Three-chambered** (2 atria + 1 ventricle)

Excretion: **Urea** through kidneys

Kidneys composed of **nephrons** (functional units)

**Closed circulatory system** with special portal systems

**Nucleated RBCs** containing haemoglobin

Brain regions: **Forebrain, midbrain, hindbrain**

**10 pairs of cranial nerves**

Reproduction: **External fertilisation** in water

Females lay **2500-3000 eggs** at once

Development involves **tadpole stage with metamorphosis**

Ecological role: **Insect control and food web participation**

This comprehensive outline covers every concept, definition, and process required for CBSE Class 11 board examination preparation on the Structural Organisation in Animals chapter.