Understanding Biology: The Study of Life

Biology is a natural science focused on living organisms and how they function—from the tiniest molecules within cells to large, complex systems. It helps us understand life, how different species operate, adapt, and evolve, and how they interact with one another and their surroundings.

Biology covers a wide range of fields, such as:

• Botany – the study of plants
• Zoology – the study of animals
• Genetics, Evolution, Physiology,
• Microbiology, and Molecular Biology

This subject explores every physical and chemical process related to life. Progress in biological sciences has played a major role in improving health, medicine, agriculture, and overall human wellbeing.

Essential Features of Living Beings

All living organisms, no matter how big or small, share a set of fundamental traits:

1. Structured Organization

Every organism is systematically built from one or more cells—the fundamental units of life.

• Single-celled organisms (like bacteria and yeast) carry out all life processes within one cell.
• Multicellular organisms (like animals and plants) have various specialized cells that perform distinct tasks to support the whole organism.

2. Reacting to Environment

Living beings are sensitive to external signals or changes.

• Plants lean toward light or respond to touch.
• Bacteria can move toward or away from light (phototaxis) or chemicals (chemotaxis).

3. Reproduction

All organisms have the ability to produce offspring to continue their species.

• Sexual reproduction: Offspring inherit combined traits from both parents.
• Asexual reproduction: One parent replicates itself to produce a genetically identical clone.

Many organisms develop special reproductive structures to assist this process.

4. Adaptation to Surroundings

All living things must adjust to their environment. These adaptations develop over time through the process of natural selection, which is a driving force behind evolution.

5. Growth and Development

Organisms follow genetic instructions that guide their growth. These instructions help in forming new cells and shaping the development of the entire organism.

6. Regulation of Internal Functions

To survive, organisms must regulate internal systems, including how they move nutrients, respond to changes, and maintain balance within their bodies.

7. Maintaining Balance (Homeostasis)

Homeostasis refers to the ability to keep internal conditions stable, even when the environment outside changes. For instance, humans maintain a steady body temperature.

8. Energy Usage

All living beings require energy to function.

• Plants capture sunlight and convert it into chemical energy via photosynthesis.
• Animals consume food to obtain energy stored in organic molecules.

Levels of Biological Organization

Life is organized in a step-by-step hierarchy, from the smallest building blocks to entire organisms:

1. Cells and Their Components

Cells are the foundation of all life forms. Inside each cell are structures called organelles, each performing specific roles to keep the cell running.

• Prokaryotic cells: Do not have a nucleus or membrane-bound organelles. Example: bacteria.
• Eukaryotic cells: Contain a well-defined nucleus and other membrane-bound structures. Example: animal and plant cells.

Typical components of a cell include:

• Cell membrane – outer layer controlling movement in and out
• Nucleus – contains genetic material
• Cytoplasm – jelly-like substance where activities occur

2. Tissues

Cells with similar roles group together to form tissues.

Examples:

• Muscle tissue – for movement
• Connective tissue – for support
• Nervous tissue – for transmitting signals

3. Organs

Tissues combine to create organs, which perform specific biological tasks.

Examples: Heart, Brain, Stomach

4. Organ Systems

Organs that work together form an organ system.

Examples:

• Digestive system – breaks down food
• Circulatory system – moves blood
• Nervous system – processes information

5. Complete Organism

When all organ systems work in harmony, they form a living organism—a fully functioning individual like a human, plant, or animal.

 

Molecular Biology: The Science Behind Life’s Building Blocks

Molecular biology is a branch of science that focuses on understanding the biological processes at the molecular level. It explores how different molecules like DNA, RNA, and proteins interact within cells and how these interactions are controlled and regulated to support life.

Proteins: The Functional Units of Life

Proteins are fundamental to the structure and operation of all living organisms. They carry out a wide range of functions within cells, including:

• Providing support and shape to cells and tissues
• Facilitating biochemical reactions (as enzymes)
• Regulating cellular processes

Proteins are made from smaller units called amino acids, which link together in long chains and fold into specific shapes to perform their roles effectively.

DNA: The Carrier of Genetic Information

Deoxyribonucleic acid (DNA) holds the genetic blueprint for most life forms on Earth, except for certain RNA viruses. It is responsible for passing genetic traits from one generation to the next.

Key features of DNA:

• Found mainly in the cell nucleus
• Consists of two strands coiled into a right-handed double helix
• Made up of units called nucleotides, each consisting of:
  • A five-carbon sugar (deoxyribose)
  • A phosphate group
  • A nitrogen-containing base

There are four types of nitrogen bases in DNA:

• Adenine (A)
• Thymine (T)
• Cytosine (C)
• Guanine (G)

The bases pair specifically (A with T, and C with G) to maintain the DNA structure and store genetic information.

RNA: The Messenger and Translator of Genetic Code

Ribonucleic acid (RNA) is present in every living cell and plays a vital role in using the instructions coded in DNA to make proteins. Unlike DNA, RNA is typically:

• Single-stranded
• Made up of nucleotides, but with uracil (U) replacing thymine (T)

RNA is essential in decoding the genetic message and turning it into functional proteins.

Types of RNA and Their Roles

Based on function, RNA is classified into three main types, each playing a unique role in protein synthesis:

1. Messenger RNA (mRNA)
   • Carries the genetic instructions from DNA to the ribosome, where proteins are made.
   • Acts like a blueprint for assembling amino acids in the correct sequence.
2. Transfer RNA (tRNA)
   • Brings amino acids to the ribosome during protein production.
   • Matches each amino acid to the corresponding codon in the mRNA.
3. Ribosomal RNA (rRNA)
   • A key component of ribosomes, the cellular machines that build proteins.
   • Helps link amino acids together into a growing protein chain.

Central Dogma: The Blueprint of Life

The central dogma of molecular biology explains how genetic information flows within a biological system. It follows a one-way path:

DNA → mRNA → Protein

This means that genes (segments of DNA) provide the instructions to form mRNA, which in turn tells the cell how to assemble proteins.

Key Processes in the Central Dogma

• Replication
  Before a cell divides, it must duplicate its DNA so that each new cell receives an identical copy. This process is known as DNA replication.
• Transcription
  In transcription, a segment of DNA is used as a template to create messenger RNA (mRNA). This happens inside the nucleus.
• Translation
  The mRNA then travels to ribosomes, where it is decoded to build a chain of amino acids, forming proteins. This step is called translation.

Where Do These Happen?

• Replication & Transcription: Inside the nucleus
• Translation: On ribosomes (either floating freely in the cell or attached to the Endoplasmic Reticulum)

Human Diseases: When the Body’s Balance Breaks

A disease is any condition that disrupts the normal functioning of the body. Diseases are mainly classified into two types:

1. Communicable Diseases

These diseases are contagious—they can spread from one person to another through:

• Contaminated surfaces
• Bodily fluids or blood
• Insect bites
• Airborne transmission

They are usually caused by pathogens like bacteria, viruses, protozoa, or fungi.

Examples by Cause:

• Bacterial Infections:
  • Leprosy – Mycobacterium leprae
  • Tuberculosis – Mycobacterium tuberculosis
  • Typhoid – Salmonella typhi
• Viral Infections:
  • Ebola – Ebola virus
  • Genital Herpes – Herpes simplex virus
  • Influenza – Influenza virus
  • Measles – Rubeola virus
  • Nipah virus infection
  • Monkeypox – Monkeypox virus
  • COVID-19 – SARS-CoV-2 virus
• Protozoan Infections:
  • Malaria – Plasmodium species
  • Kala-azar – Leishmania donovani
  • Amoebiasis – Entamoeba histolytica
• Fungal Infections:
  • Candidiasis – Candida auris
  • Aspergillosis – Aspergillus mold
  • Athlete’s Foot – Dermatophyte fungi
  • Blastomycosis – Blastomyces dermatitidis

2. Non-Communicable Diseases (NCDs)

These are long-term illnesses that do not spread from person to person. They develop due to a combination of:

• Genetic makeup
• Environmental influences
• Lifestyle factors (like diet, stress, physical inactivity)

Major Types of NCDs:

• Cardiovascular diseases – e.g., heart attacks, strokes
• Cancers – various types of malignancies
• Chronic respiratory diseases – e.g., asthma, COPD
• Diabetes – problems with blood sugar regulation

Fact: Non-communicable diseases are responsible for 41 million deaths annually, making up 74% of global deaths.

Understanding Immunity: The Body’s Natural Defense System

Immunity refers to the body’s built-in ability to defend itself against harmful invaders like bacteria, viruses, and toxins. This protective function is carried out by the immune system, a highly sophisticated network made up of lymphoid organs (like the bone marrow, thymus, and spleen) and specialized immune cells.

Types of Immunity

The immune system uses two main types of defense mechanisms: Innate Immunity and Adaptive Immunity.

1. Innate Immunity (Natural Immunity)

• This is the body’s first line of defense and is present from birth.
• It offers a fast and generalized response to any foreign substances.
• Innate immunity includes physical barriers (like skin), chemical barriers (like stomach acid), and cellular defenses (like phagocytes).
• It acts immediately after exposure to a pathogen, without needing prior contact.

2. Adaptive Immunity (Acquired Immunity)

• This form of immunity develops after the body encounters specific antigens (foreign substances).
• It involves a targeted and long-lasting response tailored to a specific pathogen.
• Adaptive immunity depends on immune memory, allowing the body to respond more efficiently to repeated exposures.

Adaptive immunity is further divided into:

a. Active Immunity

• Developed when the body is exposed to a pathogen naturally or through vaccination.
• The immune system produces its own antibodies and memory cells.
• This type of immunity is long-lasting and often permanent.

b. Passive Immunity

• Achieved by directly receiving antibodies from an external source (e.g., through mother’s milk or antibody injections).
• It provides immediate but short-term protection, as the body doesn’t produce its own immune memory.

Summary Table

Type of Immunity	Key Features	Duration	Example
Innate Immunity	Present from birth, non-specific, fast-acting	Short-term	Skin barrier, white blood cells
Active Immunity	Self-generated after exposure or vaccination	Long-term	Immunity after recovering from chickenpox
Passive Immunity	Received from another source	Short-term	Antibodies from mother to infant

By understanding how the immune system works, we can better appreciate the importance of vaccinations, hygiene, and overall health in building a strong defense against disease.

Vaccines: Training the Immune System to Fight Disease

Vaccines are preventive tools—administered via injections, oral drops, pills, or nasal sprays—that teach the immune system how to recognize and combat harmful pathogens such as bacteria and viruses. By mimicking disease-causing organisms, vaccines stimulate the body's defense mechanisms without causing illness, helping build immunity safely and effectively.

Types of Vaccines and How They Work

Different types of vaccines are designed based on how they interact with the immune system and the nature of the pathogen they target.

1. Inactivated Vaccines

• Contain killed (inactive) forms of the virus or bacteria.
• These vaccines are safe for people with weakened immune systems but may require multiple doses to build strong immunity.

Examples: Polio vaccine, Hepatitis A vaccine, Rabies vaccine.

2. Live-Attenuated Vaccines

• Use a weakened form of the live virus or bacteria that causes the disease.
• They generate a strong and long-lasting immune response but are typically not recommended for people with compromised immunity.

Examples: MMR vaccine (Measles, Mumps, Rubella), Rotavirus vaccine.

3. mRNA Vaccines

• Contain genetic instructions (mRNA) that prompt the body to produce a protein resembling part of the virus, triggering an immune response.
• Do not contain live virus, so there’s no risk of causing the disease.

Examples: Pfizer-BioNTech COVID-19 vaccine, Moderna COVID-19 vaccine.

4. Subunit, Recombinant, Polysaccharide, and Conjugate Vaccines

• Use specific pieces of the pathogen—like proteins or sugars—to stimulate immunity.
• These vaccines are suitable for people with weakened immune systems and often have fewer side effects.

Example: Hepatitis B vaccine.

5. Toxoid Vaccines

• Target the toxins released by bacteria rather than the bacteria themselves.
• Help the body develop immunity to the harmful effects of the toxin, not the pathogen directly.

Examples: Diphtheria vaccine, Tetanus vaccine.

6. Viral Vector Vaccines

• Use a modified virus (vector) to deliver genetic material into the body, prompting an immune response.
• The vector virus is harmless and does not cause illness.

Example: Oxford-AstraZeneca COVID-19 vaccine.

Biological Classification: Organizing Life on Earth

Biological classification is the scientific method of grouping living organisms into structured categories based on their shared characteristics and evolutionary relationships. This system allows scientists to understand and study the immense diversity of life in a simplified and systematic way.

The ultimate goal of classification is to organize organisms in a logical, hierarchical manner, making it easier to identify, name, and study them.

Five Kingdom Classification System

In 1969, R.H. Whittaker introduced the widely accepted Five Kingdom Classification, dividing all life forms into:

• Monera
• Protista
• Fungi
• Plantae
• Animalia

This classification was a breakthrough in taxonomy, accommodating a wider range of organisms including unicellular and multicellular life forms.

What is Taxonomy?

Taxonomy is the branch of biology concerned with the description, identification, naming (nomenclature), and classification of organisms. It provides a universal language for naming organisms and establishes the framework for biological classification.

Taxonomic Hierarchy: Levels of Biological Classification

The process of arranging organisms into nested groups based on shared characteristics is known as the taxonomic hierarchy. This hierarchy follows a descending order from the most general to the most specific category:

1. Domain

• The broadest level of classification.
• Three domains: Archaea, Bacteria, and Eukarya.

2. Kingdom

• Each domain is divided into kingdoms.
• For instance, the Eukarya domain includes:
  • Animalia
  • Plantae
  • Fungi
  • Protista
  • Monera

3. Phylum

• Groups organisms within a kingdom based on similar body plans or major features.
• Example: Chordata (organisms with a notochord) is a phylum under Animalia.

4. Class

• Phyla are subdivided into classes, refining the classification further.
• Example: Mammalia (mammals) is a class under Chordata.

5. Order

• Each class is divided into orders, grouping families with similar characteristics.
• Example: Carnivora includes carnivorous mammals like cats, dogs, and bears.

6. Family

• Orders are split into families.
• Example: Canidae is the dog family under Carnivora.

7. Genus

• A genus comprises species that are structurally similar or closely related.
• Example: Vulpes is a genus under Canidae.

8. Species

• The most specific level of classification.
• A species consists of individuals that can interbreed and produce fertile offspring.
• Example: Vulpes vulpes is the scientific name for the Red Fox.

Quick Recap Table: Taxonomic Hierarchy

Level	Example (Red Fox)
Domain	Eukarya
Kingdom	Animalia
Phylum	Chordata
Class	Mammalia
Order	Carnivora
Family	Canidae
Genus	Vulpes
Species	Vulpes vulpes

Plant Kingdom: Detailed Overview

Introduction

The Plant Kingdom comprises multicellular, eukaryotic organisms that are autotrophic and rich in chlorophyll. They are essential components of terrestrial ecosystems and include diverse forms like trees, herbs, grasses, ferns, and mosses. With over 300,000 known species, plants are found in almost every land environment.

Ecological Importance

Plants act as primary producers, harnessing solar energy through photosynthesis to produce chemical energy. This supports nearly all life on Earth. Key ecological roles of plants include:

• Oxygen production
• Soil stabilization
• Nutrient cycling
• Climate regulation
• Providing habitats for numerous animal species

Evolution of Plant Classification

Historical Classification

Early systems categorized plants based on outward appearance (trees, shrubs, herbs). As knowledge progressed, internal structures and reproductive traits were used for classification.

Modern Plant Classification Systems

1. Linnaean System
   • Hierarchical classification: Kingdom → Division → Class → Order → Family → Genus → Species
   • Uses binomial nomenclature (Genus + Species)
2. Phylogenetic System
   • Classifies plants based on evolutionary relationships and DNA evidence
   • Groups like mosses, liverworts, ferns, gymnosperms, and angiosperms reflect common ancestry

Economic Importance

Plants are crucial for:

• Food (crops)
• Medicine
• Timber
• Fibres
• Ornamental and landscaping uses

Major Divisions of the Plant Kingdom

1. Cryptogams vs. Phanerogams

Feature	Cryptogams	Phanerogams
Seeds	Absent	Present
Reproduction	By spores	By seeds
Reproductive Organs	Hidden	Clearly visible
Evolution	Primitive	Advanced

Cryptogams (Seedless Plants)

1. Thallophyta (Algae)

• Autotrophic, aquatic, and simple thalloid plants
• Classified into:

Class	Characteristics	Examples
Chlorophyceae (Green algae)	Chlorophyll a & b, starch as food reserve, cellulose wall	Chlamydomonas, Volvox, Ulothrix, Spirogyra
Phaeophyceae (Brown algae)	Chlorophyll a, c, fucoxanthin; laminarin/mannitol as food reserve; cellulose + algin walls	Ectocarpus, Laminaria, Fucus
Rhodophyceae (Red algae)	Chlorophyll a, d, phycoerythrin; Floridean starch; cellulose + pectin walls	Gracilaria, Gelidium, Porphyra

Reproduction in Algae:

• Vegetative: Fragmentation
• Asexual: Zoospores, aplanospores
• Sexual: Isogamy, anisogamy, oogamy

Significance:

• Food: Porphyra, Laminaria
• Industrial uses: Agar, alginates, carrageenan
• Biofertilizers: Anabaena, Nostoc

2. Bryophytes

• Found in moist, shady environments
• Dominant phase: Gametophyte
• No true vascular tissues
• Divided into:

Class	Key Features	Example
Liverworts	Dorsiventral thallus, rhizoids on underside	Marchantia
Mosses	Erect plant body, leaf and stem-like structures	Funaria, Sphagnum

Reproduction:

• Vegetative: Fragmentation, gemmae
• Asexual: Spores in sporangium
• Sexual: Antheridia and archegonia form gametes

Significance:

• Soil conservation
• Antibiotic production
• Packaging material

3. Pteridophytes

• First vascular land plants
• Plant body: Sporophyte with true roots, stems, leaves
• Vascular tissues present
• Reproduce by spores
• Divided into:

Class	Features	Examples
Psilopsida	Simple, dichotomous stems; no roots	Psilotum
Lycopsida	Club mosses; small leaves; strobili present	Selaginella
Sphenopsida	Jointed stems; whorled leaves	Equisetum
Pteropsida	Large megaphyll leaves; sori on leaves	Pteris, Adiantum

Reproduction:

• Vegetative: Fragmentation, budding
• Asexual: Spore formation
• Sexual: Gametophyte forms male and female gametes → zygote → sporophyte

Uses:

• Medicinal, ornamental, soil binders, biofertilizers

Phanerogams (Seed-Bearing Plants)

1. Gymnosperms

• Naked seeds (not enclosed in fruit)
• Heterosporous and wind-pollinated
• Vascular tissues present
• Subdivided into:

Class	Features	Examples
Cycadales	Palm-like; coralloid roots with cyanobacteria	Cycas
Coniferales	Needle-leaves; resin ducts; cones	Pinus, Cedrus
Ginkgoales	Only living species: Ginkgo biloba	Ginkgo
Gnetales	Xerophytes; scale leaves; no xylem vessels	Ephedra, Gnetum

Reproduction:

• Microspores → Male gametophytes
• Megaspores → Female gametophytes
• Wind pollination → Fertilization → Embryo and seeds

Significance:

• Timber, resin, turpentine oil, ornamentals

2. Angiosperms

• Flowering plants
• Seeds enclosed in fruits
• Divided into:

Class	Features	Examples
Dicotyledons	Two cotyledons, reticulate venation, taproot	Rose, Potato, Pea
Monocotyledons	One cotyledon, parallel venation, fibrous roots	Lily, Wheat, Rice

Reproduction:

• Vegetative: Grafting, cuttings
• Asexual: Apomixis (seeds without fertilization)
• Sexual:
  • Pollen grains (male gametophyte) develop from microspores
  • Embryo sac (female gametophyte) from megaspores
  • Pollination → Fertilization → Zygote → Embryo → Seed/Fruit formation

Significance:

• Food, medicine, fibre, fuel, timber, dyes, gums, etc.

Plant Structure & Function

Roots

• Anchor the plant and absorb nutrients
• Types: Taproot, fibrous
• Specialized roots store food or aid propagation

Stems

• Support plant body, transport nutrients
• Types: Herbaceous (soft), Woody
• Modified stems: Tubers, rhizomes, bulbs, corms

Leaves

• Main site of photosynthesis
• Structure: Epidermis, stomata, vascular bundles
• Variations in shape, margin, arrangement

Reproductive Organs

• Spores/seeds/cones in non-flowering plants
• Flowers in angiosperms contain sex organs
• Fertilized flowers develop into fruits and seeds

Vascular System

• Xylem: Transports water/minerals from roots
• Phloem: Distributes sugars from photosynthesis
• Provides structural support and nutrient flow

Meristems

• Growth zones with undifferentiated cells
• Apical meristems → length growth
• Lateral meristems → thickness growth

Photosynthesis

• Occurs in chloroplasts of leaf mesophyll cells
• Converts sunlight into glucose, releasing oxygen

🔍 What is Animal Kingdom?

The Animal Kingdom (Kingdom Animalia) includes all multicellular, eukaryotic, heterotrophic organisms. These are living creatures that eat, move, and respond to their environment — like us!

📚 How Animals Are Classified

Animals are grouped based on:

• 🔬 Body structure – Cells, tissues, organs
• 🌀 Symmetry – Asymmetrical, radial, bilateral
• 🧫 Germ layers – 2 or 3 layers during development
• 🕳️ Body cavity (Coelom) – Present or absent
• 🧩 Segmentation – Repeating body parts
• 🧠 Notochord – Present in advanced animals

🔬 Levels of Body Organization

Level	Key Feature	Examples
Cellular	Cells loosely arranged	Sponges
Tissue	Groups of cells form tissues	Cnidarians
Organ	Organs for specific functions	Flatworms
Organ System	Complex systems like humans	From Annelids up

🌀 Body Symmetry: How Are Animals Shaped?

• ❌ Asymmetrical – No symmetry (e.g., Sponges)
• 🌐 Radial – Equal halves in all directions (e.g., Jellyfish)
• ➗ Bilateral – Divided into mirror halves (e.g., Humans, Earthworms)

🧫 Germ Layers (Development Layers)

• 🧬 Diploblastic – 2 layers: Ectoderm + Endoderm (e.g., Cnidaria)
• 🧬 Triploblastic – 3 layers: Ectoderm + Mesoderm + Endoderm (most animals)

🕳️ Coelom: Do They Have a Body Cavity?

• ❌ Acoelomate – No cavity (e.g., Flatworms)
• 💡 Pseudocoelomate – Partially lined cavity (e.g., Roundworms)
• ✅ Coelomate – Fully developed cavity (e.g., Humans, Insects)

🧩 Segmentation

• Animals like Earthworms show metameric segmentation – their body is divided into repeated units.

🧠 Notochord: Backbone Basics

• Animals with a notochord (primitive spine) are Chordates.
• Those without it are Non-Chordates.

🐚 Classification: From Simple to Complex

Let’s dive into the main phyla!

🧽 Porifera – The Sponges

• Aquatic, porous body, no true tissues
• Water canal system for food & oxygen
• Examples: Sycon, Spongilla, Euspongia

🌊 Cnidaria – Jellyfish Family

• Stinging cells (cnidoblasts), radial symmetry
• Tissue level, diploblastic
• Examples: Hydra, Jellyfish (Aurelia), Sea Anemone

💡 Ctenophora – Comb Jellies

• Marine, bioluminescent, use 8 comb plates to swim
• Examples: Pleurobrachia, Ctenoplana

🪱 Platyhelminthes – Flatworms

• Bilateral, acoelomate, parasitic
• Flame cells for excretion
• Examples: Tapeworm, Liver fluke

🔄 Aschelminthes – Roundworms

• Pseudocoelomate, unsegmented
• Separate sexes, parasites
• Examples: Ascaris, Filarial worm (Wuchereria)

🪶 Annelida – Segmented Worms

• True coelom, metamerism
• Closed circulation, nephridia present
• Examples: Earthworm, Leech, Nereis

🦗 Arthropoda – The Largest Phylum!

• Jointed legs, chitinous exoskeleton
• Open circulation, Malpighian tubules
• Examples: Butterfly, Spider, Crab, Honeybee

🐌 Mollusca – Soft Bodies with Shells

• Second largest phylum
• Muscular foot, mantle, radula
• Examples: Snail, Octopus, Pearl Oyster

⭐ Echinodermata – Spiny Skinned Animals

• Marine, radial symmetry (adult), tube feet
• Water vascular system
• Examples: Starfish, Sea urchin

🔬 Hemichordata – Half Chordates

• Marine, with proboscis-collar-trunk body
• Stomochord (similar to notochord)
• Examples: Balanoglossus

🧬 Chordata – Animals with a Backbone

✅ Features:

• Notochord (at least during development)
• Dorsal nerve cord
• Gill slits
• Post-anal tail
• Closed blood system

🔎 Sub-phyla of Chordata:

🐚 1. Urochordata – Tunicates

E.g., Ascidia

🔺 2. Cephalochordata – Amphioxus

E.g., Branchiostoma

🧍‍♂️ 3. Vertebrata – With backbone (most advanced)

🐟 Vertebrate Classes

⚙️ Cyclostomata – Jawless Fish

• Round, sucker-like mouth
• Examples: Lamprey, Hagfish

🦈 Chondrichthyes – Cartilaginous Fish

• Marine, ventral mouth, no swim bladder
• Examples: Shark, Stingray

🐠 Osteichthyes – Bony Fish

• Operculum covers gills
• Swim bladder present
• Examples: Rohu, Tuna, Angel fish

🐸 Amphibia – Dual Life

• Live in water and land
• Moist skin, cold-blooded
• Examples: Frog, Toad

🐍 Reptilia – Crawling Creatures

• Dry skin, shelled eggs
• Crocodile has 4-chambered heart!
• Examples: Snake, Lizard, Crocodile

🐦 Aves – Birds

• Feathers, flight, warm-blooded
• Hollow bones, beak (no teeth)
• Examples: Parrot, Ostrich, Eagle

🐻 Mammalia – Hairy and Smart

• Mammary glands, fur/hair, external ears
• Mostly give birth to young
• Examples: Humans, Bats, Whales, Kangaroo

✅ Quick Comparison Table

Class	Heart	Body Cover	Blooded	Example
Fish	2	Scales	Cold	Rohu, Shark
Amphibia	3	Moist skin	Cold	Frog, Salamander
Reptilia	3-4	Scales	Cold	Snake, Turtle
Aves	4	Feathers	Warm	Crow, Ostrich
Mammalia	4	Hair	Warm	Human, Bat