Structural Organisation in Animals – Class 11 NEET Notes

1.🧫Introduction to Animal Tissues

In animals, a tissue is a group of similar cells that perform a specific function. Based on structure and function, animal tissues are divided into four major types:
👉 Epithelial tissue,
👉 Connective tissue,
👉 Muscular tissue, and
👉 Nervous tissue.

Let’s understand each in detail:

1.1 Epithelial Tissue

Epithelial tissue is a type of animal tissue that acts as a covering or lining for various parts of the body. It forms the outer layer of the skin, lines the inside of organs, body cavities, and blood vessels, and also forms many glands. This tissue is made up of tightly packed cells with little or no space between them. Because of this arrangement, epithelial tissue acts as a barrier to protect the body from injury, germs, and water loss.

Epithelial cells are arranged in continuous sheets that can be single-layered or multi-layered. A thin, non-living layer called the basement membrane supports the epithelial tissue and connects it to the underlying connective tissue. The cells in this tissue have the ability to regenerate quickly, which helps in healing wounds. Based on the number of cell layers and cell shape, epithelial tissues are classified into different types.

Now, let’s understand the various types of epithelial tissues.

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A. Simple Epithelium

Simple epithelium is made up of a single layer of cells. It is generally found in places where absorption, filtration, or secretion is required. Since it is thin, it does not provide much protection, but it allows easy exchange of materials like gases, nutrients, and wastes. Simple epithelium can be of three main types based on the shape of the cells:

1. Simple Squamous Epithelium

This type is made up of flat, thin, and scale-like cells. These cells fit together like floor tiles and form a smooth, thin layer. Because the layer is thin, it allows materials to pass through easily by diffusion or filtration.

Location and Function:

• Found in the lining of blood vessels (endothelium), air sacs of lungs (alveoli), lining of the heart, and the walls of capillaries.
• Helps in processes like gas exchange, filtration, and diffusion of substances.

2. Simple Cuboidal Epithelium

This tissue consists of cube-shaped cells with a centrally located nucleus. It is thicker than squamous epithelium and often involved in secretion and absorption.

Location and Function:

• Found in kidney tubules, ducts of small glands, thyroid gland, and surface of ovaries.
• Performs functions like absorption, secretion, and protection.

3. Simple Columnar Epithelium

This type has tall and pillar-like cells. The nucleus is usually located at the base of the cell. These cells can sometimes have microvilli (tiny hair-like projections) on their surface to increase surface area for absorption.

Location and Function:

• Found lining the stomach, intestines, and gall bladder.
• Helps in absorption of nutrients and secretion of digestive enzymes and mucus.

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B. Ciliated Epithelium

Ciliated epithelium is a special type of columnar or cuboidal epithelium that has hair-like structures called cilia on the free surface of the cells. These cilia constantly beat in a wave-like motion to move substances over the epithelial surface.

Types:

• Ciliated Columnar Epithelium
• Ciliated Cuboidal Epithelium (less common)

Location and Function:

• Found in the inner lining of the respiratory tract (trachea, bronchi), where it moves mucus and trapped dust particles out of the lungs.
• Also present in the lining of fallopian tubes, where it helps move the egg toward the uterus.

Ciliated epithelium plays an important role in cleaning and movement within tubular organs.

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C. Compound Epithelium

Compound epithelium is made up of more than one layer of cells. Its main role is to provide protection against mechanical or chemical stress. Since it is thicker, it is not involved in absorption or secretion.

There are several types of compound epithelium based on the structure and arrangement of cells:

1. Stratified Squamous Epithelium

This is the most common type of compound epithelium. It has multiple layers of cells, with the outermost being flat and scale-like. There are two types:

• Keratinized: Contains keratin (a protein that makes it waterproof and tough). Found in skin.
• Non-keratinized: Lacks keratin. Found in the lining of mouth, esophagus, and vagina.

Function: Protection from wear and tear, dehydration, and infection.

2. Stratified Cuboidal Epithelium

It consists of several layers of cuboidal cells. It is not very common.

Location: Found in ducts of sweat glands, salivary glands, and mammary glands.

3. Stratified Columnar Epithelium

This has multiple layers with columnar cells on the top layer. It is also rare.

Location: Found in parts of the pharynx, male urethra, and lining of some glandular ducts.

4. Transitional Epithelium

This special type of compound epithelium can stretch and change shape. It has several layers of cells that can slide over each other, making it flexible.

Location and Function:

• Found in the lining of the urinary bladder, ureters, and part of the urethra.
• Helps in stretching and preventing leakage of urine.

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D. Glandular Epithelium

Glandular epithelium is formed by cells that are specialized to produce and release various secretions like enzymes, hormones, sweat, saliva, and mucus. These cells can be present within an epithelial layer or may fold inward to form glands.

Types of Glands Based on Number of Cells:

1. Unicellular Glands: Made up of single secretory cells.
   • Example: Goblet cells (secrete mucus in respiratory and intestinal tract).
2. Multicellular Glands: Made up of many secretory cells grouped together.
   • Examples: Salivary glands, sweat glands, pancreas.

Types Based on Mode of Secretion:

• Merocrine glands: Release secretion without damaging the cell. Example: Salivary gland.
• Apocrine glands: Part of the cell is lost with the secretion. Example: Mammary gland.
• Holocrine glands: Entire cell disintegrates to release secretion. Example: Sebaceous (oil) gland.

Types Based on Where They Secrete:

• Exocrine glands: Secrete products into ducts that open on a surface. Example: Sweat glands.
• Endocrine glands: Secrete hormones directly into the blood. Example: Thyroid gland.
• Mixed glands: Have both exocrine and endocrine parts. Example: Pancreas.

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Summary:

Epithelial tissue forms the basic covering and lining in the body. It is found on skin, lining of internal organs, blood vessels, and glands. Based on structure and function, it is classified as simple, compound, ciliated, and glandular epithelium. Each type has a specific location and role in the body.

Simple epithelium helps in absorption, filtration, and secretion. Compound epithelium offers protection. Ciliated epithelium helps in the movement of substances. Glandular epithelium produces and releases various secretions essential for body functions.

1.2 Connective Tissue

Connective tissue is one of the most important types of animal tissues. Its main function is to connect, support, and bind various body structures. Unlike epithelial tissue, the cells in connective tissue are loosely spaced and embedded in an intercellular matrix. This matrix may be liquid, jelly-like, or solid, depending on the type of connective tissue. The matrix is what gives connective tissue its properties and functions.

Connective tissue is found all over the body, including beneath the skin, around blood vessels and organs, between muscles, and in bones and cartilage. It also helps in the transportation of materials and the defense mechanism of the body. There are various types of connective tissues, each adapted for specific functions.

Let’s understand each type in detail:

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1. Areolar Tissue

Areolar tissue is the most widely distributed type of connective tissue. It is a loose connective tissue that is found beneath the skin, between muscles, around blood vessels and nerves, and in the internal organs. It fills the space between organs and tissues and acts like a packing material.

Structure: Areolar tissue has different types of cells and fibers. The major cells include:

• Fibroblasts (which produce fibers)
• Mast cells (involved in defense)
• Macrophages (engulf bacteria and dead cells)
• Plasma cells (produce antibodies)

Fibers found in the matrix include:

• Collagen fibers (for strength)
• Elastic fibers (for flexibility)

Functions:

• Provides support and flexibility to the body parts
• Helps in repair of tissues after injury
• Plays a role in defense against germs
• Connects skin to underlying muscles

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2. Adipose Tissue

Adipose tissue is a specialized form of connective tissue that stores fat. It is mostly found beneath the skin, around kidneys, heart, eyeballs, and in the abdominal region. The fat stored in this tissue serves as a reserve source of energy.

Structure: Adipose tissue is made up of specialized cells called adipocytes that store fat in large vacuoles. The cytoplasm and nucleus are pushed to the side due to fat accumulation.

Functions:

• Acts as an energy reservoir
• Provides insulation to maintain body temperature
• Acts as a cushion and protects internal organs from shock
• Helps in shaping the body

There are two types of adipose tissue:

1. White Adipose Tissue – Stores energy and insulates the body
2. Brown Adipose Tissue – Produces heat in newborns and hibernating animals

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3. Dense Connective Tissue

Dense connective tissue contains a large number of collagen fibers, which provide strength and flexibility. Based on the arrangement of fibers, it is divided into two types:

a) Dense Regular Connective Tissue

In this type, the collagen fibers are arranged in parallel bundles. This regular arrangement allows the tissue to resist tension and provide strength in one direction.

Location and Function:

• Found in tendons (connect muscles to bones) and ligaments (connect bones to bones)
• Provides strong attachment and support

b) Dense Irregular Connective Tissue

Here, collagen fibers are arranged irregularly. This arrangement allows the tissue to resist tension from different directions.

Location and Function:

• Found in the dermis (inner layer) of the skin, joint capsules, and coverings of various organs
• Provides strength and flexibility in multiple directions

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4. Cartilage

Cartilage is a firm and flexible connective tissue that provides support and shape to various body parts. It is softer than bone and does not contain blood vessels or nerves. The cells of cartilage are called chondrocytes, which lie in small spaces called lacunae within the matrix.

Structure:

• Matrix is firm and contains proteins and sugars
• Chondrocytes are present in lacunae
• Surrounded by a dense layer called perichondrium

Types of Cartilage:

1. Hyaline Cartilage: Most common; found in nose, trachea, larynx, and ends of long bones.
2. Elastic Cartilage: Contains more elastic fibers; found in external ear and epiglottis.
3. Fibrous Cartilage: Contains more collagen fibers; found in intervertebral discs and pubic symphysis.

Functions:

• Provides flexibility and support
• Forms the embryonic skeleton
• Reduces friction between bones in joints

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5. Bone

Bone is a hard and rigid connective tissue that forms the skeleton of the body. It provides support, protection, and helps in movement. Bone also stores minerals like calcium and phosphorus.

Structure:

• The matrix is solid and made up of calcium phosphate and collagen fibers
• Bone cells are called osteocytes, present in spaces called lacunae
• Lacunae are arranged in concentric circles around a central canal (Haversian canal)
• Blood vessels and nerves pass through the central canal

Functions:

• Provides shape and support to the body
• Protects internal organs like brain and heart
• Helps in movement by attachment of muscles
• Produces blood cells in bone marrow (hematopoiesis)
• Stores minerals

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6. Blood

Blood is a fluid connective tissue. It flows throughout the body and transports materials. The fluid matrix of blood is called plasma, in which different types of cells are suspended.

Components:

• Plasma: Yellowish fluid containing water, proteins, hormones, and nutrients
• Red Blood Cells (RBCs): Carry oxygen using hemoglobin
• White Blood Cells (WBCs): Fight infections and provide immunity
• Platelets: Help in blood clotting

Functions:

• Transport of oxygen, carbon dioxide, nutrients, hormones, and waste
• Regulation of body temperature and pH
• Defense against infections
• Blood clotting during injuries

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7. Lymph

Lymph is another fluid connective tissue. It is a colorless fluid that is formed from the leakage of plasma from blood capillaries. It flows in lymphatic vessels and eventually returns to the blood.

Structure:

• Contains lymphocytes (a type of white blood cell)
• Does not have RBCs
• Similar to plasma but with fewer proteins

Functions:

• Helps in the transport of fats from intestines
• Removes waste and drains excess fluid from tissues
• Plays a major role in immunity and defense

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Summary:

Connective tissue is the most widespread and varied type of animal tissue. It includes loose connective tissues like areolar and adipose, dense connective tissues like tendons and ligaments, supportive tissues like bone and cartilage, and fluid connective tissues like blood and lymph. Each type is specialized for specific functions such as binding, support, protection, transportation, and immunity.

1.3 Muscular Tissue

Muscular tissue is a specialized type of body tissue responsible for producing movement in the body. It is made up of elongated cells called muscle fibers, which are capable of contracting and relaxing. This tissue helps in performing a wide range of activities like movement of body parts, pumping of the heart, movement of food through the digestive system, and even facial expressions. Muscular tissues are characterized by their excitability (ability to respond to stimuli), contractility (ability to shorten), extensibility (ability to stretch), and elasticity (ability to return to original shape). Based on structure, control, and location, muscular tissue is classified into three main types: skeletal muscle, smooth muscle, and cardiac muscle.

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1. Skeletal Muscle (Striated Voluntary Muscle)

Skeletal muscles are the muscles attached to the bones of the skeleton, which help in voluntary movements of the body. These muscles are called striated muscles because under a microscope, they show alternating light and dark bands or striations. They are also known as voluntary muscles because they are under conscious control, meaning we can move them by our own will.

The structure of skeletal muscle fibers is very unique. They are long, cylindrical, unbranched, and multinucleated (contain many nuclei). The nuclei are present at the periphery (edges) of the cell. These muscle fibers can be several centimeters long. The striations seen in skeletal muscles are due to the regular arrangement of actin and myosin protein filaments within the fibers. These proteins are responsible for contraction.

Each skeletal muscle fiber is covered by a membrane called sarcolemma, and the cytoplasm inside is known as sarcoplasm, which contains numerous myofibrils (contractile threads). The skeletal muscle fibers contract rapidly but tire easily, which means they cannot work continuously for a long time without rest. This is because skeletal muscles consume a lot of energy and produce lactic acid as a by-product during activity, which causes fatigue.

Skeletal muscles perform several vital functions in the body such as locomotion (walking, running), posture maintenance, facial expressions, speech, and reflex actions. Since they are under the control of the somatic nervous system, their activity can be consciously regulated.

Key Features of Skeletal Muscles:

• Long, cylindrical, unbranched fibers
• Multinucleated with peripheral nuclei
• Striated appearance
• Voluntary control
• Rapid contraction, quick fatigue
• Attached to bones
• Controlled by the somatic nervous system

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2. Smooth Muscle (Unstriated Involuntary Muscle)

Smooth muscles are found in the walls of internal organs such as the stomach, intestines, urinary bladder, uterus, blood vessels, and respiratory passages. Unlike skeletal muscles, smooth muscles do not show any striations under the microscope; hence, they are called non-striated muscles. They are also known as involuntary muscles because they are not under conscious control.

Smooth muscle fibers are spindle-shaped (wider in the middle and tapering at both ends), uninucleated (having a single nucleus), and unbranched. These muscle fibers are shorter than skeletal muscle fibers. The nucleus is centrally located. Since they lack striations, they appear smooth and uniform under the microscope.

Smooth muscles contract and relax slowly but steadily. Unlike skeletal muscles, they do not get tired easily and can sustain contractions for a long time. These muscles play an important role in the involuntary movements of internal organs such as peristalsis (movement of food through the digestive tract), regulation of blood flow, emptying of the bladder, childbirth (uterine contractions), and control of airway diameter in the respiratory tract.

Smooth muscles are regulated by the autonomic nervous system, meaning their activities are automatically controlled by the body without any conscious effort.

Key Features of Smooth Muscles:

• Spindle-shaped, unbranched fibers
• Single, central nucleus
• No striations (non-striated)
• Involuntary control
• Slow, sustained contractions
• Found in internal organs
• Controlled by autonomic nervous system

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3. Cardiac Muscle (Striated Involuntary Muscle)

Cardiac muscles are found only in the walls of the heart. These muscles are responsible for the rhythmic contraction and relaxation of the heart, which pumps blood throughout the body. Cardiac muscles are unique because they have properties of both skeletal and smooth muscles.

Structurally, cardiac muscle fibers are short, cylindrical, and branched. They are joined end to end to form a network. At the junctions between two cells, there are specialized structures called intercalated discs. These discs help in rapid transmission of impulses from one cell to another and ensure coordinated contraction of the heart muscle.

Cardiac muscle fibers are striated like skeletal muscles but are involuntary like smooth muscles. Each fiber contains one or two central nuclei. The contraction of cardiac muscles is rhythmic, strong, and involuntary, which means the heart beats continuously without conscious control.

These muscles are highly resistant to fatigue because they have numerous mitochondria, which help produce large amounts of energy continuously. Cardiac muscles receive oxygen-rich blood directly through coronary arteries, ensuring a constant supply of oxygen and nutrients.

The heartbeat is regulated by a specialized pacemaker system within the heart, which generates electrical impulses automatically, and it is influenced by the autonomic nervous system and hormones.

Key Features of Cardiac Muscles:

• Cylindrical, branched fibers
• One or two central nuclei
• Striated appearance
• Involuntary control
• Strong, rhythmic, and continuous contractions
• Present only in the heart
• Contain intercalated discs
• Controlled by autonomic nervous system and pacemaker cells

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Comparison of Skeletal, Smooth, and Cardiac Muscles

Feature	Skeletal Muscle	Smooth Muscle	Cardiac Muscle
Location	Attached to bones	Walls of internal organs	Heart walls
Shape	Long, cylindrical	Spindle-shaped	Short, cylindrical, branched
Striations	Present	Absent	Present
Nucleus	Multinucleated (peripheral)	Uninucleated (central)	One or two (central)
Control	Voluntary	Involuntary	Involuntary
Speed of Contraction	Fast	Slow	Moderate
Fatigue	Fatigues quickly	Fatigue-resistant	Very fatigue-resistant
Presence of Intercalated Discs	Absent	Absent	Present
Nervous System Control	Somatic nervous system	Autonomic nervous system	Autonomic nervous system + pacemaker

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Functions of Muscular Tissue in the Body

1. Movement: Skeletal muscles help in locomotion and movement of body parts.
2. Internal Organ Functioning: Smooth muscles assist in movement of food, blood, and other materials within the body.
3. Heartbeat: Cardiac muscle keeps the heart beating continuously.
4. Posture Maintenance: Skeletal muscles keep the body in stable positions like standing or sitting.
5. Heat Production: Muscles generate heat during activity and help maintain body temperature.
6. Facial Expression and Communication: Muscles of the face help express emotions and speech.
7. Peristalsis: Smooth muscles in the digestive tract move food through coordinated contractions.
8. Blood Pressure Regulation: Smooth muscles in blood vessels control vessel diameter, influencing blood pressure.

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Conclusion

Muscular tissue is an essential part of the human body, playing a key role in both voluntary and involuntary movements. The three types – skeletal, smooth, and cardiac muscles – each have unique structures and specialized functions. While skeletal muscles help in conscious movement and posture, smooth muscles control involuntary activities inside organs, and cardiac muscles maintain the heartbeat.

1.4 Nervous Tissue

The nervous tissue is one of the most important and specialized tissues in the human body. It is the main tissue of the nervous system, which includes the brain, spinal cord, and nerves. The primary function of nervous tissue is to receive stimuli, process information, and send responses to different parts of the body. This is how the body maintains coordination, communication, and control among its various systems.

The nervous system works like a high-speed information network. It helps us react to changes in the environment (like sound, light, heat, smell, touch) and allows internal communication between organs. The nervous tissue is made up of two main types of cells:
👉 Neurons (the functional units of the nervous system)
👉 Neuroglial cells (the supporting and protecting cells)

Let’s understand each of these in detail.

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1. Neurons – The Structural and Functional Units of the Nervous System

Neurons are the basic units of the nervous system. These are highly specialized cells that are capable of receiving, processing, and transmitting electrical impulses. They help in the transmission of signals from one part of the body to another very rapidly. This allows quick responses to any stimulus.

Each neuron has three major parts:
👉 Cell Body (Soma or Cyton)
👉 Dendrites
👉 Axon

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a) Cell Body (Soma or Cyton)

The cell body is the main part of the neuron that contains the nucleus and other cell organelles like mitochondria, ribosomes, Golgi apparatus, and endoplasmic reticulum. It is responsible for metabolic activities of the neuron. The cytoplasm of the cell body is called neuroplasm, and it contains small granules called Nissl bodies, which are unique to neurons. These Nissl bodies are responsible for protein synthesis and are made of rough endoplasmic reticulum and ribosomes.

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b) Dendrites

Dendrites are short, branched projections arising from the cell body. Their main function is to receive signals (electrical or chemical) from other neurons or sensory receptors and carry them towards the cell body. A neuron can have many dendrites, increasing its ability to receive multiple signals.

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c) Axon

The axon is a long, single projection that extends from the cell body. Its function is to carry impulses away from the cell body to other neurons, muscles, or glands. The axon may be very long, sometimes more than a meter. At the end of the axon, it branches into smaller structures called axon terminals or nerve endings, which form connections called synapses with other neurons or effector cells (like muscle or gland cells).

Many axons are covered by a myelin sheath, which is a fatty insulating layer produced by specialized cells like Schwann cells (in the peripheral nervous system) or oligodendrocytes (in the central nervous system). The myelin sheath increases the speed of nerve impulse conduction and also protects the axon.

The gaps between the myelin sheath are called Nodes of Ranvier. These nodes help in saltatory conduction, where the nerve impulse jumps from one node to the next, allowing faster transmission.

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Types of Neurons (Based on Function)

1. Sensory Neurons (Afferent Neurons):
   These neurons carry signals from sensory organs (like skin, eyes, ears) to the brain or spinal cord. They help us feel touch, temperature, pain, sound, etc.
2. Motor Neurons (Efferent Neurons):
   These neurons carry messages from the brain or spinal cord to muscles or glands, which act in response (like contracting a muscle or secreting a hormone).
3. Interneurons (Association Neurons):
   These are present within the brain and spinal cord, and they connect sensory and motor neurons. They help in processing and integration of information.

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Features of Neurons

• Highly specialized and excitable cells that transmit electrical impulses.
• Can respond rapidly to a stimulus.
• Do not divide or regenerate in most parts of the brain.
• Possess Nissl granules, unique to neurons.
• Can have very long extensions for distant communication.
• Form networks with thousands of other neurons via synapses.

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2. Neuroglial Cells – The Supporting Cells

While neurons are responsible for signal transmission, they cannot function properly without the help of neuroglial cells (also called glial cells or glia). These are non-neuronal cells that provide support, protection, nourishment, and maintenance for neurons. In fact, glial cells are more numerous than neurons in the nervous system.

Neuroglial cells are like the caretakers of the neurons. They do not transmit nerve impulses but are essential for the health and function of neurons.

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Functions of Neuroglial Cells

1. Support and Structure:
   Glial cells help to hold neurons in place and maintain the shape of the nervous tissue.
2. Protection:
   They protect neurons from physical damage and form barriers like the blood-brain barrier, which controls what substances can enter the brain from the blood.
3. Nourishment:
   Glial cells supply nutrients and oxygen to neurons.
4. Insulation:
   Some glial cells produce the myelin sheath, which insulates axons and increases the speed of impulse conduction.
5. Clean-up:
   They remove dead cells, waste materials, and excess neurotransmitters, helping to maintain a clean environment for neurons.
6. Immune Defense:
   Some glial cells like microglia act like immune cells of the brain and fight infections or injuries.

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Types of Neuroglial Cells

There are several types of neuroglial cells, each with specific functions:

1. Astrocytes

• Star-shaped glial cells present in the brain and spinal cord.
• Maintain the blood-brain barrier.
• Provide nutrients to neurons and help in repair after injury.

2. Oligodendrocytes

• Found in the central nervous system (CNS).
• Produce myelin sheath for axons in the CNS.

3. Schwann Cells

• Found in the peripheral nervous system (PNS).
• Produce myelin sheath for axons in the PNS.
• Help in regeneration of damaged axons.

4. Microglia

• Act as immune cells of the nervous system.
• Engulf bacteria, viruses, and damaged cells (phagocytosis).

5. Ependymal Cells

• Line the cavities of the brain and spinal cord.
• Help in the production and circulation of cerebrospinal fluid (CSF).

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Comparison between Neurons and Neuroglial Cells

Feature	Neurons	Neuroglial Cells
Function	Transmit electrical impulses	Support and protect neurons
Number	Less in number	More in number
Ability to divide	Do not divide (mostly)	Can divide and multiply
Involvement in impulse	Directly involved in transmission	Not involved in impulse transmission
Special features	Axons, dendrites, Nissl bodies	Varies (depends on cell type)
Myelin formation	No	Some glial cells form myelin

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Importance of Nervous Tissue in the Body

The nervous tissue plays a central role in maintaining homeostasis, communication, and control of body functions. It helps us:

• Detect environmental changes (stimuli)
• Respond quickly to dangers or injuries
• Coordinate voluntary and involuntary actions
• Think, learn, remember, and feel emotions
• Control functions of internal organs (heartbeat, digestion, respiration)
• Form complex behaviors and cognitive abilities

Without the nervous tissue, there would be no way for different organs and tissues to communicate or work together effectively.

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Conclusion

Nervous tissue is one of the most vital and specialized tissues in the body. It forms the backbone of the nervous system by integrating and coordinating all the body’s functions. Neurons are the chief functional units that receive and send information through electrical impulses, while neuroglial cells support and protect neurons in many essential ways. Though neurons are few in number, their precise and fast action is what makes complex functions like thinking, memory, movement, and reflexes possible. At the same time, without the support system provided by neuroglia, neurons cannot survive or work effectively.

2. Organ and Organ System

Living organisms, especially animals, show different levels of body organisation depending on their complexity. From simple cells to complex systems, every level plays an important role in maintaining the structure and function of the organism. In multicellular animals, the body is organized in a hierarchical manner starting from the cellular level to the organ-system level. This gradual increase in complexity is called the levels of structural organisation.

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🧱 Levels of Structural Organisation in Animals

There are four major levels of structural organisation seen in animals:

1. Cellular Level

This is the most basic level, where the animal’s body is made of individual cells, but these cells do not form tissues. Each cell functions independently. Example: Sponges (Porifera)

2. Tissue Level

Here, cells of similar structure and function come together to form tissues, but true organs are still absent. Example: Cnidarians (like Hydra, Jellyfish)

3. Organ Level

In this level, different tissues are organized to form a specific organ that performs a specific function. But organ systems may not be fully developed. Example: Platyhelminthes (Flatworms)

4. Organ-System Level

This is the highest level of organisation, seen in more advanced animals. Here, organs work together to form organ systems, each with a specific role, like digestion, respiration, circulation, etc. This level provides better efficiency and coordination. Example: Earthworm, Cockroach, Frog, Humans

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🧬 Organ-System Level of Organisation (in Earthworm, Cockroach, and Frog)

Animals like earthworms, cockroaches, and frogs show a well-developed organ-system level of organisation. These animals are more advanced and possess specialised organs that work in coordination as a part of organ systems such as the digestive, respiratory, excretory, circulatory, reproductive, and nervous systems.

Let’s study these animals one by one in detail:

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🪱 Earthworm – Organ-System Level

Earthworm is a terrestrial, segmented, invertebrate belonging to the phylum Annelida. It shows a well-developed organ-system level of body organisation.

🔸 Body Organisation

• The body is long, cylindrical, and divided into many metameric segments.
• It is bilaterally symmetrical and triploblastic (having three germ layers).
• The coelom is true (coelomate) and filled with coelomic fluid.

🔸 Digestive System

• The earthworm has a complete alimentary canal that runs from mouth to anus.
• It includes mouth, buccal cavity, pharynx, oesophagus, crop (storage), gizzard (grinding), intestine (digestion & absorption), and anus.
• The intestinal inner lining has a typhlosole, a fold that increases surface area for absorption.

🔸 Circulatory System

• Earthworm has a closed circulatory system (blood flows within vessels).
• Blood is red due to the presence of haemoglobin in plasma (not in RBCs).
• It contains dorsal, ventral, and lateral blood vessels.
• Blood is pumped by contractile hearts (lateral hearts).

🔸 Excretory System

• Excretion occurs through nephridia, which remove nitrogenous wastes.
• Different types: Septal, Integumentary, and Pharyngeal nephridia.

🔸 Nervous System

• The nervous system consists of a nerve ring around the pharynx and paired ventral nerve cords with segmental ganglia.

🔸 Reproductive System

• Earthworm is hermaphroditic (male and female reproductive organs in same individual).
• Cross-fertilisation occurs during mating.
• Fertilisation is external, and a cocoon is formed around fertilized eggs.

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🪳 Cockroach – Organ-System Level

Cockroach is a terrestrial, nocturnal insect, belonging to the phylum Arthropoda. It shows a complex and distinct organ-system level of organisation.

🔸 Body Organisation

• The body is divided into three parts: Head, thorax, and abdomen.
• It has three pairs of legs, two pairs of wings, and antennae.
• The body is covered with a chitinous exoskeleton.

🔸 Digestive System

• The digestive system includes mouthparts (for chewing), pharynx, oesophagus, crop, gizzard (grinds food), midgut (digestion), and hindgut (absorption).
• Hepatic caecae and Malpighian tubules are attached to aid in digestion and excretion.

🔸 Circulatory System

• Cockroach has an open circulatory system, meaning blood flows freely in body cavities.
• Blood (called haemolymph) is colourless and does not carry oxygen.
• Heart is tubular with 13 chambers, located dorsally.

🔸 Respiratory System

• Cockroach breathes through spiracles (small openings on body surface).
• Air enters through spiracles and moves into tracheae (thin tubes) and reaches tissues directly.
• No respiratory pigment like haemoglobin is present.

🔸 Excretory System

• Malpighian tubules remove waste by absorbing nitrogenous compounds from haemolymph.
• Excretion is mainly in the form of uric acid (uricotelic).

🔸 Nervous System

• Consists of paired ganglia, ventral nerve cord, and sensory organs like compound eyes and antennae.
• Brain is located in the head and connected to the sub-oesophageal ganglion.

🔸 Reproductive System

• Cockroach shows sexual dimorphism – males and females can be identified separately.
• Fertilisation is internal.
• Females lay eggs in a capsule called ootheca.

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🐸 Frog – Organ-System Level

Frog is a cold-blooded vertebrate amphibian, belonging to phylum Chordata and class Amphibia. It shows a highly developed organ-system level of organisation.

🔸 Body Organisation

• The body is bilaterally symmetrical, triploblastic, and coelomate.
• Divided into head and trunk.
• Skin is moist, slimy, and without scales, which helps in respiration.

🔸 Digestive System

• The digestive system is well-developed and includes buccal cavity, pharynx, oesophagus, stomach, small and large intestines, and cloaca.
• Liver, pancreas, and gall bladder help in digestion.
• Digestion is extracellular and occurs in the alimentary canal.

🔸 Circulatory System

• Frog has a closed circulatory system.
• Three-chambered heart (2 auricles + 1 ventricle).
• Blood contains RBCs with nuclei.
• Both oxygenated and deoxygenated blood can mix in the ventricle.

🔸 Respiratory System

• Frogs respire through:
  • Lungs (on land)
  • Moist skin (cutaneous respiration in water)
  • Buccal cavity (buccopharyngeal respiration)
• No diaphragm present.

🔸 Excretory System

• Main excretory organs are kidneys, which remove nitrogenous waste as urea (ureotelic).
• Waste is excreted through cloaca.

🔸 Nervous System

• Nervous system is divided into central (brain and spinal cord), peripheral, and autonomic nervous systems.
• Brain has parts: Forebrain (olfactory & cerebrum), midbrain (optic lobes), and hindbrain (cerebellum & medulla).

🔸 Reproductive System

• Frogs are unisexual and undergo external fertilisation.
• Breeding occurs in water.
• Eggs are laid in clusters, and development is indirect via tadpole larva.

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🧾 Conclusion

The organ-system level of organisation represents the most complex and efficient level of body design. Animals like earthworm, cockroach, and frog have well-developed organ systems that carry out various life functions in a coordinated and organized manner.

• Earthworm, being a segmented invertebrate, has a closed circulatory system and shows hermaphroditism.
• Cockroach, an arthropod, displays open circulation and complex body segmentation with specialized appendages.
• Frog, a vertebrate, exhibits highly evolved systems like closed circulation, three-chambered heart, and cutaneous-lung respiration.

3. Morphology, Anatomy & Functions of Animals

In biology, the study of animals includes understanding both their morphology (external features) and anatomy (internal structure). These two fields, along with the functional understanding of their organs and systems, help in explaining how animals survive, reproduce, and perform different activities. For NEET, we focus on a few selected animals with an emphasis on their body plan, organ systems, and how these structures help in physiological functions.

For NEET, three animals are mainly emphasized:
🪱 Earthworm – A representative of phylum Annelida
🪳 Cockroach – A representative of phylum Arthropoda
🐸 Frog – A representative of phylum Chordata (Amphibia)

3.1 🪱 Earthworm (Pheretima posthuma)

arthworm is a soil-dwelling, segmented invertebrate that plays an important role in agriculture and soil health. It is one of the most commonly studied organisms in biology and is a perfect example of the organ-system level of organization in animals. The scientific name of the Indian earthworm is Pheretima posthuma.

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🔹 A. Habitat and Habit

Earthworms are commonly found in moist soil, especially in gardens, agricultural lands, and areas rich in organic matter. They are burrowing animals and live in shallow tunnels in the upper layers of moist soil. During the day, they stay underground to avoid sunlight and dryness.

Earthworms are nocturnal, meaning they are more active at night. They are herbivorous and feed on decaying organic matter (plant remains, dead leaves). While burrowing, they swallow soil and extract nutrients from it, making the soil rich and fertile. This is why they are often called “friends of farmers” or “natural ploughmen”.

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🔹 B. Morphology of Earthworm

Earthworm’s body shows clear segmentation and bilateral symmetry. It is triploblastic, coelomate, and has no skeleton, but the body wall is muscular and helps in movement.

🔸 1. Body Segmentation

• The body is long, narrow, cylindrical, and divided into about 100–120 segments, also called metameres or somites.
• Each segment is similar in appearance externally, giving the body a metameric segmentation look.
• The first segment contains the mouth, while the last segment has the anus.

🔸 2. Clitellum

• Around the 14th to 16th segments, there is a thick, dark band called the clitellum.
• It is a glandular swelling that plays a crucial role in reproduction.
• The presence of clitellum helps to distinguish an adult earthworm from a juvenile.

🔸 3. Setae

• Except the first, last, and clitellum segments, each segment has small, hair-like structures called setae.
• Setae are embedded in the body wall and can be extended or withdrawn.
• They help in anchorage and locomotion by gripping the soil.

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🔹 C. Digestive System

The earthworm has a complete digestive system, starting from the mouth and ending at the anus. It is a straight tube running through the length of the body.

🔸 Parts of the Digestive Tract:

1. Mouth: Located in the first segment, opens into the buccal cavity.
2. Buccal cavity: Present in segments 1–3, helps in swallowing soil.
3. Pharynx: Segments 4–5, muscular and helps in sucking food.
4. Oesophagus: Segments 5–7, a narrow tube.
5. Crop: Segment 8–9, acts as a storage chamber.
6. Gizzard: Segment 9–11, thick-walled and muscular, helps in grinding soil and food.
7. Intestine: Starts from segment 15 and continues till the last segment. Most digestion and absorption occur here.
8. Typhlosole: A fold of the intestinal wall that increases the surface area for absorption.
9. Anus: Located at the posterior end, expels undigested soil and waste.

The digestive system of the earthworm is adapted to extract nutrients from organic matter in the soil.

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🔹 D. Circulatory System

Earthworm has a closed circulatory system, meaning that the blood flows inside blood vessels.

🔸 Components:

• Dorsal blood vessel: Runs along the back side and carries blood forward.
• Ventral blood vessel: Carries blood backward.
• Lateral hearts (or pseudo-hearts): Present in segments 7 to 11. They are muscular loops that help in pumping blood.

The blood contains haemoglobin dissolved in plasma, which gives it a red colour. However, unlike humans, earthworm blood does not have RBCs.

The closed system helps in efficient circulation of nutrients, gases, and waste materials.

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🔹 E. Excretory System

Earthworm removes its metabolic waste through organs called nephridia. These are long, coiled tubules that help in excreting nitrogenous waste (mostly ammonia and urea).

🔸 Types of Nephridia:

1. Septal Nephridia – Found on the septa between segments.
2. Integumentary Nephridia – Scattered on the body wall, especially in the middle region.
3. Pharyngeal Nephridia – Located in the 4th, 5th, and 6th segments.

These nephridia collect waste from the body fluids and discharge it outside through small pores.

Earthworm is ammonotelic (excretes ammonia) and ureotelic (excretes urea) depending on water availability.

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🔹 F. Nervous System

Earthworm has a well-developed nervous system for a simple invertebrate.

🔸 Structure:

• The nerve ring acts like a primitive brain and is located around the pharynx.
• It consists of:
  • Cerebral ganglia (brain)
  • Circumpharyngeal connectives
  • Subpharyngeal ganglia
• A ventral nerve cord runs along the body length, and has segmental ganglia in each segment.

Earthworms are sensitive to touch, light, chemicals, and temperature. Even though they do not have eyes or ears, they respond to stimuli through photoreceptors and sensory cells in the skin.

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🔹 G. Reproductive System

Earthworms are hermaphrodites, meaning they have both male and female reproductive organs in the same body. However, they cannot fertilize themselves – they undergo cross-fertilization.

🔸 Male Reproductive Organs:

• Testes: Found in segments 10 and 11.
• Vasa deferentia: Ducts that carry sperm to the spermathecal pores.

🔸 Female Reproductive Organs:

• Ovaries: Present in the 13th segment.
• Oviducts: Open to the exterior through female genital pores on segment 14.

During mating, two earthworms align ventrally and exchange sperms. The clitellum secretes a mucous cocoon, which receives eggs and sperms. Fertilisation takes place inside the cocoon, which is then deposited in the soil. A young worm hatches from it after a few weeks.

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🔹 H. Diagram-based Identification (Key Features)

For NEET and practical exams, students should be able to identify and label the external and internal parts of the earthworm based on diagrams.

🔸 External Morphological Features to Identify:

• Segmented body
• Clitellum (14–16 segments)
• Mouth at anterior end
• Anus at posterior end
• Setae (may need magnification)

🔸 Internal Features to Identify in Dissection/Diagram:

• Digestive system: Crop, gizzard, intestine with typhlosole
• Hearts and blood vessels
• Nephridia
• Nerve cord
• Reproductive organs (testes, ovaries, genital pores)

Labelled diagrams usually show lateral and ventral views with body systems.

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✅ Summary Table (Quick Revision)

Feature	Description
Habitat	Moist soil, burrowing, nocturnal
Body structure	Segmented, cylindrical, no limbs
Clitellum	Segments 14–16, used in reproduction
Setae	Bristle-like structures for movement
Digestive system	Mouth → Gizzard → Intestine → Anus
Circulatory system	Closed type, with blood vessels & hearts
Respiratory system	Cutaneous (through moist skin)
Excretory system	Nephridia (Septal, Integumentary, Pharyngeal)
Nervous system	Brain (nerve ring) + Ventral nerve cord
Reproductive system	Hermaphrodite, cross-fertilization, cocoons
Diagram identification	Segmental body, clitellum, mouth, anus, setae

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🧠 Conclusion

The earthworm is a simple yet highly organised animal that shows organ-system level of organization. Its body is adapted to a burrowing lifestyle, with moist skin for respiration, a closed circulatory system, and an effective digestive tract to extract nutrients from soil. Its reproductive system ensures survival through cross-fertilization and cocoon formation.

3.2 🪳Cockroach (Periplaneta americana)

Cockroach is a cold-blooded, terrestrial, omnivorous insect belonging to the phylum Arthropoda, the largest phylum in the animal kingdom. The most commonly studied species for biology and NEET is Periplaneta americana, the American cockroach. It is an important example of an organ-system level of organisation in invertebrates.

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🔹 1. External Features of Cockroach

The cockroach has a hard, chitinous exoskeleton and shows bilateral symmetry. Its body is dorsoventrally flattened, reddish-brown, and usually measures 3–5 cm in length. The outer body is made of rigid plates called sclerites joined by thin flexible membranes.

🔸 Body Division:

The body of a cockroach is divided into three regions:

👉 A. Head

• The head is triangular, and it’s connected to the thorax by a short flexible neck.
• It bears:
  • A pair of long antennae (sensory function)
  • Two large compound eyes
  • Mouthparts adapted for chewing: labrum (upper lip), mandibles (grinding), maxillae (handling food), labium (lower lip), and hypopharynx (tongue-like structure).

👉 B. Thorax

• The thorax has three segments:
  1. Prothorax
  2. Mesothorax
  3. Metathorax
• Each segment bears a pair of walking legs.
• Two pairs of wings are present:
  • Forewings (tegmina) are thick, leathery, and arise from the mesothorax.
  • Hindwings are thin, membranous, and arise from the metathorax, used for flying.

👉 C. Abdomen

• The abdomen has 10 segments.
• In males, the 9th segment has anal styles (absent in females).
• Both sexes have a pair of jointed anal cerci at the posterior end for sensing vibrations.

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🔹 2. Digestive System of Cockroach

The cockroach has a well-developed alimentary canal running from mouth to anus. It is divided into:

1. Foregut (Stomodeum)
2. Midgut (Mesenteron)
3. Hindgut (Proctodeum)

🔸 Foregut

• Includes mouth, pharynx, oesophagus, crop, and gizzard.
• Crop: Thin-walled sac that stores food.
• Gizzard: Thick muscular organ with chitinous plates, used to grind food.

🔸 Midgut

• Primary site of digestion and absorption.
• Contains hepatic caeca (finger-like glands) that secrete digestive enzymes into the midgut.

🔸 Hindgut

• Includes ileum, colon, and rectum.
• Absorbs water and forms solid waste.
• Opens into the anus for egestion.

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🔹 3. Circulatory System of Cockroach

Cockroach has an open circulatory system, which means blood does not flow in blood vessels, but freely in the body cavity (haemocoel).

🔸 Main Features:

• The heart is elongated and tubular, consisting of 13 chambers, located on the dorsal side.
• Blood (called haemolymph) is colourless and lacks respiratory pigments like haemoglobin.
• Haemolymph helps in transport of nutrients, hormones, and waste, but not oxygen.

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🔹 4. Respiratory System (Tracheal System)

Cockroach breathes through a system of air tubes called tracheae. The tracheae open to the outside through small holes called spiracles.

🔸 Key Features:

• There are 10 pairs of spiracles: 2 thoracic + 8 abdominal.
• Spiracles allow air to enter the body.
• Inside, air travels through the tracheae → tracheoles → directly to tissues.
• Oxygen diffuses directly to cells; no blood is involved in oxygen transport.
• Waste gas (CO₂) is released through the same path.

This type of system ensures efficient gaseous exchange, but only suitable for small animals.

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🔹 5. Nervous System (Ganglia and Nerves)

Cockroach has a highly developed nervous system among invertebrates. It is ventrally placed, unlike humans where it is dorsal.

🔸 Nervous System Consists of:

• Cerebral ganglion (brain): Controls basic body functions.
• Sub-oesophageal ganglion: Controls mouthparts and head movement.
• Ventral nerve cord: Double and solid, running along the belly side.
• Segmental ganglia: Present along the ventral nerve cord for local control.

🔸 Sensory Organs:

• Antennae: Sensitive to touch and smell.
• Compound eyes: Located on head, provide mosaic vision.
• Cerci: Posterior appendages that detect air movement/vibrations.

The ganglia control reflexes and help cockroach react quickly to danger, making it a fast-moving insect.

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🔹 6. Reproductive System

Cockroach is dioecious, meaning male and female sexes are separate. Both sexes have distinct reproductive systems.

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🔸 Male Reproductive System:

• Contains a pair of testes (located in abdominal segments 4–6).
• Vasa deferentia: Carry sperms from testes to the seminal vesicles.
• Sperms are stored and packed into capsules called spermatophores.
• During mating, spermatophores are transferred to the female.

🔸 Female Reproductive System:

• Contains a pair of ovaries in abdominal segments 2–6.
• Oviducts from both ovaries join to form a common oviduct (vagina).
• Spermatheca: Stores sperms received during mating.
• After fertilization, the female produces an ootheca (a capsule-like structure).
• Each ootheca contains 16–18 fertilized eggs.
• Female deposits oothecae in warm, dark places.

Development is direct, meaning young ones (nymphs) look like adults but lack wings and mature genitalia.

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🔹 7. Diagram-Based Identification – Key Labeled Parts

For NEET and practical diagrams, students must be able to identify and label external and internal parts of cockroach accurately.

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✅ External Features (Dorsal View)

Label:

• Head
• Compound eyes
• Antennae
• Pronotum (protects the head)
• Thorax (with forewings and hindwings)
• Abdomen
• Cerci
• Anal styles (only in males)

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✅ Internal Anatomy (Dissection View)

Label:

• Crop
• Gizzard
• Hepatic caeca
• Midgut
• Malpighian tubules
• Heart with chambers
• Ventral nerve cord
• Ganglia
• Testes/Ovaries
• Spermatheca (in females)

Clear understanding of these labeled parts helps in solving diagram-based NEET questions confidently.

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🔹 8. Summary Table – Quick NEET Revision

Feature	Description
Body Division	Head, thorax, abdomen
Legs	3 pairs (1 on each thoracic segment)
Wings	2 pairs: forewings (tegmina), hindwings
Eyes	Compound, for mosaic vision
Antennae	Sensory function (touch, smell)
Circulatory System	Open type, with 13-chambered dorsal heart
Respiratory System	Tracheal system with spiracles (10 pairs)
Excretory System	Malpighian tubules (uricotelic)
Nervous System	Brain + Ventral nerve cord + segmental ganglia
Reproductive System	Dioecious; spermatophores (males), oothecae (females)
Development	Direct (no larval stage)

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✅ Conclusion

The cockroach is a perfect example of a highly adapted invertebrate. It shows:

• Segmented, chitinous body
• Advanced systems for digestion, respiration, reproduction
• Efficient nervous and sensory systems
• Direct development and high reproductive potential

3.3 🐸Frog (Rana tigrina / Rana hexadactyla) – Morphology, Anatomy, Physiology

Frogs are cold-blooded (poikilothermic), amphibious vertebrates. They belong to phylum Chordata, class Amphibia. The most commonly studied species in India is Rana tigrina, also called Rana hexadactyla (Indian bullfrog). It serves as an excellent example of organ-system level organization in vertebrates and is important for NEET-level biology.

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🔹 1. Habitat and Adaptations

Frogs live both on land and in water, showing amphibious nature. They are commonly found near ponds, lakes, paddy fields, and moist grassy areas. Their skin must remain moist for respiration, so they avoid dry habitats.

🔸 Key Adaptations:

• Moist skin helps in cutaneous (skin) respiration.
• Webbed hind limbs assist in swimming.
• Nictitating membrane (third eyelid) protects eyes under water.
• Long hind legs with strong muscles allow leaping and swimming.
• Tympanum (ear drum) detects sound vibrations on land.
• They have both lungs and skin for breathing, suitable for life in aquatic and terrestrial environments.
• Camouflaging skin colour protects them from predators.

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🔹 2. Morphology of Frog

The frog’s body is soft, smooth, and streamlined to reduce resistance during swimming. The body is divided into head and trunk (no neck or tail in adults).

🔸 Skin:

• Smooth, slimy, moist, and without scales.
• Contains mucous glands to keep it moist.
• Colour changes slightly to blend with surroundings (camouflage).
• Rich in blood capillaries, enabling skin respiration.

🔸 Head:

• Triangular in shape, bears eyes, nostrils, mouth, and tympanum.
• Mouth is large, with upper jaw fixed and lower jaw movable.
• Tongue is long, bifid (forked), sticky, and attached in front—used to catch insects.

🔸 Eyes:

• Two large, bulging eyes with eyelids and a nictitating membrane (protective transparent layer).
• Help in vision and protection under water.

🔸 Ears:

• Tympanum (external ear drum) is visible on each side.
• Internal ears are present without external structures like pinna.
• Sensitive to air and water vibrations.

🔸 Limbs:

• Frogs have two forelimbs and two hindlimbs.
• Forelimbs are short and used to support the body.
• Hindlimbs are long, muscular, and adapted for jumping and swimming.
• Each hindlimb has five webbed digits, helpful in swimming.

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🔹 3. Digestive System of Frog

The digestive system of the frog is complete and well-organized, adapted for a carnivorous diet (insects, worms, etc.).

🔸 Major Parts:

• Mouth: Opens into the buccal cavity containing tongue and teeth.
• Buccal cavity: Has maxillary teeth on upper jaw and vomerine teeth on roof for gripping prey.
• Pharynx: Connects buccal cavity to oesophagus.
• Oesophagus: A short muscular tube leading to stomach.
• Stomach: J-shaped organ; secretes gastric juices including enzymes and HCl.
• Small intestine: Site of digestion and absorption; has duodenum and ileum.
• Liver: Largest gland, secretes bile, stored in gall bladder.
• Pancreas: Secretes digestive enzymes.
• Large intestine: Absorbs water and stores waste.
• Cloaca: Common chamber for digestive, urinary, and reproductive openings.

🔸 Digestion:

• Frogs digest proteins, fats, and carbohydrates using secreted enzymes.
• Undigested waste is egested through cloacal aperture.

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🔹 4. Circulatory System of Frog

The frog has a closed circulatory system and a three-chambered heart, which pumps blood throughout the body.

🔸 Heart:

• Three chambers – 2 atria (right and left) and 1 ventricle.
• Right atrium receives deoxygenated blood from body.
• Left atrium receives oxygenated blood from lungs and skin.
• Both types of blood mix in the ventricle.
• Conus arteriosus ensures partial separation of oxygenated and deoxygenated blood during pumping.

🔸 Blood:

• Contains RBCs (nucleated), WBCs, and plasma.
• Red in colour due to haemoglobin.
• Blood transports oxygen, nutrients, hormones, and waste materials.

🔸 Types of Circulation:

• Pulmonary circulation: Between heart and lungs.
• Systemic circulation: Between heart and body.
• Cutaneous circulation: Between skin and heart for gas exchange.

This system is known as incomplete double circulation.

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🔹 5. Respiratory System of Frog

Frog uses three types of respiration, depending on its environment.

🔸 A. Cutaneous Respiration:

• Through moist skin.
• Main mode when frog is underwater or inactive.
• Exchange of O₂ and CO₂ occurs directly through skin capillaries.

🔸 B. Pulmonary Respiration:

• Through lungs, when frog is on land.
• Lungs are sac-like, spongy, and richly supplied with blood vessels.
• Frog gulps air into lungs using buccal pumping.

🔸 C. Buccopharyngeal Respiration:

• Occurs through moist lining of buccal cavity and pharynx.
• Happens when frog is resting with mouth closed but nostrils open.

These multiple modes help frogs survive in both water and land environments.

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🔹 6. Nervous System of Frog

Frog has a well-developed central and peripheral nervous system. It controls all activities like movement, feeding, responses, and reproduction.

🔸 Divisions:

1. Central Nervous System (CNS): Brain + Spinal cord
2. Peripheral Nervous System: Cranial and spinal nerves
3. Autonomic Nervous System: Controls involuntary actions

🔸 Brain:

• Protected by skull and covered by meninges.
• Divided into:
  • Forebrain (olfactory lobes, cerebrum): smell and voluntary control
  • Midbrain (optic lobes): vision
  • Hindbrain (cerebellum, medulla): balance and vital functions like heartbeat, breathing

🔸 Sense Organs:

• Eyes: Vision
• Tympanum: Hearing
• Nostrils: Smell
• Skin: Touch and temperature
• Tongue: Taste

The nervous system and sense organs together help frog respond to its surroundings.

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🔹 7. Reproductive System of Frog

Frogs show sexual reproduction, and sexes are separate (dioecious). Fertilization is external, and development is indirect through larval stage (tadpole).

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🔸 A. Male Reproductive System:

• Contains two testes attached to kidneys.
• Vasa efferentia carry sperms to ureters, which open into cloaca.
• During mating, sperms are released into water.

🔸 B. Female Reproductive System:

• Contains two large ovaries.
• Oviducts open into cloaca through separate openings.
• Eggs are released into water during breeding season.

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🔸 Breeding and Fertilization:

• Occurs during monsoon in shallow water bodies.
• Male mounts over female (amplexus) and releases sperm over the eggs as they are laid.
• Fertilization is external and aquatic.
• Eggs develop into tadpoles, which show fish-like features and later undergo metamorphosis to become adult frogs.

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🔹 8. Diagram and Labeled Structures

Frogs are commonly asked in diagram-based questions in NEET. Important labeled diagrams include:

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✅ External View:

• Head
• Eyes
• Nostrils
• Tympanum
• Forelimbs
• Hindlimbs
• Webbed toes
• Cloacal opening

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✅ Internal View (Dissection):

• Mouth cavity
• Oesophagus
• Stomach
• Liver with gall bladder
• Pancreas
• Small and large intestine
• Cloaca
• Lungs
• Heart (3 chambers)
• Testes/Ovaries
• Brain (if cranial cavity is opened)
• Spinal cord

Being able to label and identify these parts correctly will help in both theory and practical exams.

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🔹 Summary Table (Quick Revision)

Feature	Description
Habitat	Freshwater bodies, amphibious lifestyle
Skin	Moist, no scales, used for cutaneous respiration
Eyes	Large, bulging with nictitating membrane
Tympanum	Acts as ear drum
Limbs	Hindlimbs powerful with webbed digits for jumping & swimming
Digestive System	Complete; includes stomach, liver, pancreas, cloaca
Circulatory System	Closed; 3-chambered heart, incomplete double circulation
Respiratory System	Cutaneous, pulmonary, buccopharyngeal
Nervous System	Brain, spinal cord, cranial/spinal nerves
Reproductive System	Dioecious; external fertilization in water
Development	Indirect, via tadpole stage (metamorphosis)

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✅ Conclusion

The frog is a perfect example of an amphibian with adaptations for both aquatic and terrestrial life. Its anatomy and physiology are well-suited for a dual lifestyle:

• It breathes through lungs and skin,
• Moves efficiently using muscular limbs,
• Shows external fertilization and unique larval development,
• Possesses a closed circulatory system, complex brain, and active nervous system.