Classification of Living Organisms Chart Tnpsc

Classification of Living Organisms

The Classification of Living Organisms serves as the roadmap to deciphering this intricate biological mosaic, guiding us through the diversity of life forms that inhabit Earth.

Welcome to our blog, where we embark on a fascinating expedition into the heart of taxonomy, the science of classification. Whether you’re a curious mind seeking to unravel the mysteries of biodiversity or a budding biologist eager to deepen your understanding, this exploration promises to be a riveting journey.

As we navigate through the realms of kingdoms, phyla, classes, orders, families, genera, and species, we’ll uncover the principles that underpin the classification of living organisms. From the ancient wisdom of Aristotle to the modern advancements in molecular biology, join us in tracing the evolution of our understanding of life’s myriad forms.

The Classification of Living Organisms isn’t just a subject for textbooks; it’s a dynamic field that reflects the ever-evolving relationships among living beings. Whether you’re captivated by the minuscule microorganisms or awestruck by the diversity of flora and fauna, this blog promises to be your companion in exploring the wonders of life’s classification.

Classification of Living Organisms Chart Tnpsc
Classification of Living Organisms Chart

Three domains of classification

All the organisms are classified into 3 major domains.


Archaea, formerly known as archaebacteria, is one of the three domains of life, the other two being Bacteria and Eukaryota. Archaea are single-celled microorganisms that share some characteristics with bacteria but are also similar to eukaryotes in certain molecular aspects. They are known for their ability to thrive in extreme environments, such as hot springs, acidic lakes, deep-sea hydrothermal vents, and salt flats.

They are thermophilic or heat-loving bacteria. These bacteria live in high-temperature vents.



Eubacteria is a single-celled organism that is without a well-developed nucleus. Eubacteria, also known simply as bacteria, is one of the two major domains of prokaryotic microorganisms, the other being Archaea. Unlike Archaea, Eubacteria includes the more familiar and commonly studied bacteria. These microorganisms are widespread and can be found in a variety of environments, ranging from soil and water to the human body.


Eukarya means ‘True Nucleus’. Eukarya are organisms that have a well-formed nucleus in their cell/cells. Eukarya is one of the three domains of life, the other two being Bacteria and Archaea. Eukarya encompasses all eukaryotic organisms, which are characterized by having cells with a true nucleus and membrane-bound organelles. This domain is diverse and includes a wide range of organisms, from microscopic protists to complex multicellular organisms like plants, animals, and fungi.


Five Kingdoms of Life

In the beginning, living beings were classified only into two categories, that is plants and animals. But in 1969 Whittaker suggested that the bacteria should not be in the plant kingdom and protozoa should be in the Animal kingdom.

Thereby Whittaker suggested five-kingdom classifications. The Five kingdoms of living beings are:

  1. Monera
  2. Protista
  3. Fungi
  4. Plantae
  5. Animalia


It is single-celled. It does not have a well-formed nucleus (Prokaryotes).

The term “Monera” was historically used as a taxonomic kingdom that included unicellular prokaryotic organisms, such as bacteria. However, the classification of Monera is now considered outdated and has been replaced by more modern and accurate taxonomic schemes.

The classification of living organisms has undergone significant revisions based on advances in molecular biology and genetics. The three-domain system, proposed by Carl Woese and his colleagues, has become widely accepted. According to this system, living organisms are classified into three domains: Bacteria, Archaea, and Eukarya.

The domain Bacteria includes unicellular prokaryotic organisms, commonly referred to as bacteria. These organisms lack a true nucleus and membrane-bound organelles. They are found in various environments and play essential roles in ecological processes, including nutrient cycling.

The domain Archaea also consists of unicellular prokaryotic organisms, but they exhibit distinct molecular and biochemical features that set them apart from bacteria. Archaea are often found in extreme environments, such as hot springs, acidic lakes, and deep-sea hydrothermal vents.

The domain Eukarya encompasses all eukaryotic organisms, including protists, fungi, plants, and animals. Eukaryotic cells have a true nucleus and membrane-bound organelles, distinguishing them from prokaryotic cells.

In summary, the term “Monera” is no longer used in contemporary taxonomy. Instead, organisms once classified within the kingdom Monera are now appropriately placed in the domains Bacteria and Archaea, depending on their molecular and biochemical characteristics. Eukaryotic organisms belong to the domain Eukarya.



It is single-celled with a well-formed nucleus (Eukaryotes).

The term “Protoctista” was historically used as a taxonomic kingdom to group together various eukaryotic organisms that did not fit neatly into the plant, animal, or fungi kingdoms. However, similar to the classification of Monera, the kingdom Protoctista is considered outdated in modern taxonomy.


As our understanding of the diversity and evolutionary relationships among eukaryotic organisms has advanced, the classification system has been revised. The concept of the kingdom Protoctista has been replaced with more accurate and specific classifications for various groups of organisms. Many of the organisms that were once placed in Protoctista are now distributed across different eukaryotic kingdoms.

The most widely accepted modern classification recognizes the following major eukaryotic kingdoms:

  1. Protista: This term is sometimes used as an informal category to refer to eukaryotic organisms that do not fit into the plant, animal, or fungi kingdoms. However, “Protista” itself is not a formal taxonomic rank.
  2. Plantae: Includes multicellular, photosynthetic organisms such as plants and green algae.
  3. Animalia: Comprises multicellular, heterotrophic organisms, including animals.

It is eukaryotes, multicelled, and is heterotrophs.

These five kingdoms are further divided into several divisions i.e. As bacteria, fungi, and Plantae or Phyla (Protoctista and Animalia). A family is made of several genera (Genus). Each genus has many species.

Every species is separated from its related species under the same genus by reproductive barriers. That is a member of one species cannot interbreed with another species to produce fertile offspring.

For example: Lion and Tiger cannot interbreed, Donkey and Zebra cannot interbreed, Humans and Chimpanzee cannot, etc.

  1. Fungi: Encompasses non-motile, heterotrophic organisms, such as mushrooms, molds, and yeasts.
  2. Chromista: Includes various algae with chloroplasts containing chlorophylls a and c, as well as some other protists.
  3. Excavata: A diverse group of protists, including some that have flagella and are involved in various ecological roles.
  4. Amoebozoa: A group of amoeboid protists, including amoebas and slime molds.

It’s important to note that the classification of eukaryotic organisms continues to evolve as our understanding of their genetics, molecular biology, and evolutionary relationships improves. Taxonomists use molecular data, including DNA sequencing, to refine classifications and better reflect the evolutionary history of organisms. The use of terms like “Protista” and “Protoctista” has diminished in favor of more specific and phylogenetically informed classifications.

Classification of Animals

  • Non-Chordata
    • Phylum Porifera
    • Phylum Cnidaria
      • Phylum Aschelminthes
      • Annelida
      • Phylum Arthropoda
      • Echinodermata
      • Phylum Mollusca
    • Phylum Platyhelminthes
  • Chordata
    • Phylum Chordata
      • Sub-Phylum Vertebrata
        • Class Chondrichthyes
        • Class Osteichthyes
        • Class Amphibia
        • Class Reptilia
        • Class Aves
        • Class Mammalia


Non-Chordata” is a term used to collectively refer to all animals that do not belong to the phylum Chordata. Chordata is a diverse phylum that includes animals with a notochord, a flexible rod-like structure providing support, at some stage of their development. Chordates encompass familiar animals such as vertebrates (fish, amphibians, reptiles, birds, and mammals) as well as some invertebrates like tunicates and lancelets.

Non-Chordata, therefore, encompasses a vast array of invertebrate animals that lack a notochord.


Phylum Porifera

Phylum Porifera comprises the sponges, which are simple, aquatic, and primarily marine animals. Sponges are considered one of the earliest forms of multicellular life and represent the most primitive animals within the animal kingdom. Despite their simplicity, they play important roles in marine ecosystems and have certain unique characteristics:

  1. Body Structure: Sponges have a simple body organization without true tissues or organs. They consist of a loose aggregation of cells separated by a gelatinous matrix called mesohyl. The body has numerous pores, canals, and chambers.
  2. Cell Types: Sponges have specialized cell types, including choanocytes (collar cells) that line the internal chambers and flagella for creating water currents, amoeboid cells for nutrient uptake, and spicule-producing cells for skeletal support.
  3. Water Circulation: Sponges are filter feeders. They rely on the flow of water through their bodies to obtain nutrients and oxygen. Water is drawn in through tiny pores called ostia, moves through canals and chambers, and exits through larger openings called oscula.
  4. Skeletal Structures: Many sponges have a skeleton made of spicules, which are small, needle-like structures composed of calcium carbonate, silica, or a flexible protein called spongin. The arrangement of spicules helps in classifying different sponge species.
  5. Reproduction: Sponges can reproduce both sexually and asexually. Asexual reproduction often involves the formation of gemmules—resistant structures that can survive harsh conditions. Sexual reproduction can involve the production of eggs and sperm, with fertilization taking place in the water.
  6. Habitats: Sponges are predominantly marine, but some species can be found in freshwater environments. They are often attached to substrates like rocks, shells, or the ocean floor.
  7. Ecological Roles: Sponges play important roles in marine ecosystems. They contribute to nutrient cycling by filtering and processing large volumes of water, and they provide habitat for various organisms. Some species also contain chemical compounds with potential pharmaceutical applications.
  8. Regeneration: Sponges are capable of remarkable regeneration. If a sponge is fragmented, the individual fragments can reorganize and regenerate into new, functional sponges.

Despite their apparent simplicity, sponges exhibit a range of adaptations to their environments and have unique biological features. They represent an important group in the study of early animal evolution and continue to be of interest to biologists and ecologists.

Phylum Porifera
Phylum Porifera

Phylum Cnidaria

Phylum Cnidaria includes a diverse group of aquatic animals, many of which are commonly known as cnidarians. This phylum is characterized by the presence of specialized stinging cells called cnidocytes, which contain harpoon-like structures called nematocysts. Cnidarians exhibit radial symmetry and have a simple body plan, typically organized into two main body forms: the polyp and the medusa.

Here are some key characteristics and examples of organisms within Phylum Cnidaria:

  1. Radial Symmetry: Cnidarians exhibit radial symmetry, meaning their body parts are arranged around a central axis. This symmetry is well-suited for their sessile (attached) or free-floating lifestyles.
  2. Cnidocytes: Specialized cells called cnidocytes are used for defense and prey capture. These cells contain nematocysts, which are microscopic capsules that can discharge a thread-like structure containing toxins. Cnidarians use nematocysts to paralyze or capture prey.
  3. Two Body Forms:
    • Polyp: The polyp form is generally cylindrical and attached to a substrate (e.g., coral reefs, rocks). The mouth is facing upward, surrounded by tentacles equipped with cnidocytes. Examples include sea anemones and coral polyps.
    • Medusa: The medusa form is typically free-swimming and bell-shaped. The mouth is located on the underside, surrounded by tentacles. Examples include jellyfish.
  4. Gastrovascular Cavity: Cnidarians have a central digestive compartment known as a gastrovascular cavity. This cavity serves for both digestion and nutrient distribution.
  5. Nervous System: Cnidarians have a simple nerve net, which allows them to coordinate basic movements and responses. However, they lack a centralized brain.
  6. Life Cycle: Many cnidarians exhibit both asexual and sexual reproduction. Asexual reproduction often involves budding, while sexual reproduction involves the release of gametes into the water.
  7. Examples: Common examples of cnidarians include:
    • Hydra: A small, freshwater polyp.
    • Jellyfish: Medusae of various species, such as moon jellyfish and box jellyfish.
    • Corals: Colonial polyps that secrete calcium carbonate skeletons and form coral reefs.
    • Sea Anemones: Sessile polyps often found in marine environments.
  8. Ecological Importance: Cnidarians play important roles in marine ecosystems. Coral reefs, built by colonial coral polyps, are particularly valuable ecosystems that provide habitat for numerous marine species.

It’s worth noting that while cnidarians share certain characteristics, there is considerable diversity within the phylum, and not all species fit neatly into the polyp or medusa categories. The study of cnidarians provides insights into the evolution of multicellular animals and the development of specialized structures for prey capture and defense.

Phylum Cnidaria
Phylum Cnidaria

Phylum Aschelminthes

The term “Aschelminthes” is an older and now obsolete taxonomic grouping that was used to refer to a diverse assemblage of pseudocoelomate, unsegmented worms. This grouping included several phyla of roundworms, nematodes, and related organisms. However, the classification has undergone significant changes with advancements in molecular biology and phylogenetics.

Phylum Aschelminthes
Phylum Aschelminthes

The phylum Aschelminthes included the following major groups:

  1. Nematoda (Roundworms): Nematodes are unsegmented, pseudocoelomate worms with a cylindrical body. They are abundant in various environments, including soil, water, and the tissues of plants and animals. Some nematodes are parasitic, causing diseases in plants, animals, and humans.
  2. Nematomorpha (Horsehair Worms): These worms are long, slender, and often found in water. The larvae are parasitic in arthropods, while the adults are free-living.
  3. Kinorhyncha: Kinorhynchs are small, marine pseudocoelomates that inhabit sediments. They have a segmented body divided into head, neck, and trunk regions.
  4. Loricifera: Loriciferans are tiny marine animals with a protective covering called a lorica. They were discovered in the 1980s, and their biology is not fully understood.

As of the current understanding of animal taxonomy, the phylum Aschelminthes is no longer considered valid, and its members have been reclassified into various phyla within the Ecdysozoa clade. The Ecdysozoa clade includes animals that undergo ecdysis, or molting, during their life cycle. Nematodes, for example, are now placed within the phylum Nematoda, and other groups have been recognized as separate phyla or integrated into larger taxonomic groups.

It’s important to note that taxonomic classifications are dynamic and subject to change as new molecular and genetic data become available, providing more accurate insights into the evolutionary relationships among organisms. The use of modern molecular techniques has significantly reshaped our understanding of the relationships among different animal groups.


Phylum Annelida includes a diverse group of segmented worms, commonly known as annelids. These organisms exhibit a segmented body plan, which is characterized by the presence of repeated units called metameres or segments. Annelids are found in various aquatic and terrestrial environments, and they play important roles in nutrient cycling and soil health. The phylum Annelida is part of the larger group Lophotrochozoa.


Key characteristics of annelids include:

  1. Segmentation: The body of annelids is divided into a series of repeated, distinct segments. Each segment typically contains a specific set of organs and structures.
  2. Coelom: Annelids have a true coelom, which is a fluid-filled body cavity surrounded by mesoderm. The coelom provides space for internal organs and allows for more complex body movements.
  3. Metamerism: The segments of annelids are arranged in a linear series, and this metamerism is often evident externally. Segmentation allows for flexibility and coordinated movement.
  4. Setae: Many annelids have small, bristle-like structures called setae on their body segments. Setae are used for locomotion and anchor the worm in the substrate.
  5. Digestive System: Annelids typically have a complete digestive system with a mouth, pharynx, esophagus, crop (storage organ), gizzard (grinding organ), and intestine.
  6. Circulatory System: Annelids often have a closed circulatory system with blood vessels that help distribute nutrients throughout the body.
  7. Respiration: Annelids use various mechanisms for respiration, including the diffusion of gases across their moist body surfaces. Some species also have specialized respiratory structures, such as gills.
  8. Reproduction: Annelids can reproduce both sexually and asexually. Sexual reproduction involves the release of eggs and sperm, while asexual reproduction may occur through fragmentation or budding.

Common examples of annelids include:

  • Earthworms (Class Oligochaeta): Terrestrial worms that burrow in soil and play a crucial role in soil aeration and nutrient cycling.
  • Polychaetes (Class Polychaeta): Mostly marine worms with well-developed parapodia (appendages) on each segment, aiding in swimming and crawling.
  • Leeches (Class Hirudinea): Aquatic or terrestrial worms known for their suckers and often associated with freshwater habitats. Some leeches are blood-feeders, while others are carnivorous.

Annelids are important ecologically and contribute significantly to soil health and marine ecosystems. Their segmented body plan and diverse adaptations make them a fascinating group of organisms for study in biology.

Phylum Arthropoda

Phylum Arthropoda is an incredibly diverse and successful group of invertebrate animals, characterized by their jointed appendages and exoskeleton made of chitin. Arthropods are found in nearly all environments on Earth and comprise a vast number of species, making them the largest phylum in the animal kingdom. They exhibit a remarkable range of adaptations, morphologies, and ecological roles.

Phylum Arthropoda
Phylum Arthropoda

Key characteristics of arthropods include:

  1. Exoskeleton: Arthropods have a hard exoskeleton made of chitin, a tough and flexible material. This exoskeleton provides support, protection, and a surface for muscle attachment. It is periodically molted (shed) to allow for growth.
  2. Segmented Body: The bodies of arthropods are organized into distinct segments, typically grouped into three main regions: head, thorax, and abdomen. Each segment often bears a pair of jointed appendages.
  3. Jointed Appendages: Arthropods have jointed limbs that are adapted for various functions, such as walking, swimming, grasping, and feeding. The presence of jointed appendages is a defining characteristic of the phylum.
  4. Open Circulatory System: Arthropods have an open circulatory system in which blood, or hemolymph, is pumped into body cavities (hemocoel) and comes into direct contact with tissues.
  5. Nervous System: Arthropods have a well-developed nervous system with a brain and a ventral nerve cord. Their sensory organs, including compound eyes and antennae, are often highly developed.
  6. Respiratory Systems: Arthropods use a variety of respiratory structures, including gills, book lungs, and tracheae, depending on the species. The respiratory system is adapted to meet the needs of different environments.
  7. Metamorphosis: Many arthropods undergo metamorphosis during their life cycle, transitioning through distinct developmental stages (egg, larva, pupa, and adult). The type of metamorphosis varies among different groups.

Major subphyla and classes within Arthropoda include:

  • Subphylum Chelicerata: Includes spiders, scorpions, ticks, and horseshoe crabs.
  • Subphylum Myriapoda: Includes millipedes and centipedes.
  • Subphylum Hexapoda (Class Insecta): Includes insects, the most numerous and diverse group of arthropods.
  • Subphylum Crustacea: Includes crabs, lobsters, shrimp, barnacles, and isopods.

Arthropods have successfully colonized a wide range of habitats, including terrestrial, freshwater, and marine environments. Their ecological roles are diverse, encompassing pollination, decomposition, predation, and serving as a crucial component of various food webs. The success of arthropods is attributed to their versatile body plan, adaptability, and ability to exploit diverse ecological niches.


Phylum Echinodermata includes a group of marine animals known for their distinctive radial symmetry, spiny skin, and unique water vascular system. Echinoderms exhibit a variety of body forms, and many are recognizable for their pentaradial symmetry, which typically radiates from a central disk. This phylum encompasses a diverse range of species, including familiar organisms like starfish, sea urchins, and sea cucumbers.


Key characteristics of echinoderms include:

  1. Pentaradial Symmetry: Most adult echinoderms display a form of fivefold radial symmetry, often with arms or rays radiating from a central disk. However, this symmetry is often modified or secondarily bilateral in some species.
  2. Endoskeleton: Echinoderms possess an internal skeleton composed of calcareous plates or ossicles. These plates are often covered by a layer of spines, which provides protection and support.
  3. Water Vascular System: Echinoderms have a unique hydraulic system called the water vascular system. It consists of a network of fluid-filled canals and tube feet. The water vascular system is involved in locomotion, feeding, and gas exchange.
  4. Tube Feet: Tube feet are small, fluid-filled projections extending from the body surface. They operate through the water vascular system and are used for various functions, including locomotion, capturing prey, and attaching to surfaces.
  5. Digestive System: Echinoderms have a complete digestive system with a mouth, stomach, and intestine. Some species have specialized structures for extracellular digestion.
  6. Respiration: Gas exchange occurs through simple diffusion across the body surface or through specialized structures in the tube feet.
  7. Nervous System: Echinoderms have a decentralized nervous system with a nerve ring around the mouth and radial nerves extending into the arms. They lack a centralized brain.
  8. Regeneration: Many echinoderms have impressive regenerative abilities. They can regenerate lost arms or even entire body parts under certain conditions.

Major classes within Phylum Echinodermata include:

  • Class Asteroidea: Includes starfish or sea stars.
  • Class Ophiuroidea: Includes brittle stars, characterized by long, slender arms.
  • Class Echinoidea: Includes sea urchins and sand dollars, typically with a spherical or flattened body and movable spines.
  • Class Holothuroidea: Includes sea cucumbers, elongated and cucumber-shaped animals.
  • Class Crinoidea: Includes sea lilies and feather stars, which are characterized by a cup-shaped body and feathery arms.

Echinoderms are primarily marine and are found in a variety of habitats, from shallow coastal waters to the deep sea. They are ecologically important as grazers, filter feeders, and predators. Additionally, echinoderms contribute to marine biodiversity and play roles in nutrient cycling. Their unique features and evolutionary adaptations make them fascinating subjects for scientific study.

Phylum Mollusca

Phylum Mollusca is a diverse and large group of invertebrate animals that includes well-known and ecologically important organisms. Mollusks are characterized by a soft body, often protected by a hard shell, and a muscular foot for locomotion. This phylum is one of the most diverse in the animal kingdom, with a wide range of body forms, sizes, and ecological roles.

Phylum Mollusca
Phylum Mollusca

Key characteristics of mollusks include:

  1. Soft Body: Mollusks have a soft, unsegmented body that is typically divided into three main parts: the head, visceral mass (containing organs), and foot.
  2. Mantle: A fold of tissue called the mantle covers the visceral mass and often secretes a protective shell. In some mollusks, the mantle cavity houses gills or a lung for respiration.
  3. Radula: Many mollusks have a radula, a rasping, tongue-like organ covered with tiny, chitinous teeth. The radula is used for feeding and varies in structure depending on the species’ diet.
  4. Shell: Mollusks may have an external shell made of calcium carbonate, an internal shell, or no shell at all. The diversity of shell types is a key feature within the phylum.
  5. Foot: Mollusks typically have a muscular foot used for various forms of locomotion, such as crawling, burrowing, or swimming.
  6. Circulatory System: Mollusks have an open circulatory system in which blood, or hemolymph, is pumped by a heart into spaces called sinuses.
  7. Nervous System: Mollusks have a simple nervous system, often with a pair of ganglia (nerve clusters) and nerve cords.
  8. Reproduction: Mollusks exhibit a range of reproductive strategies, including sexual reproduction with separate sexes or hermaphroditism. Fertilization may be internal or external, depending on the species.

Major classes within Phylum Mollusca include:

  • Class Gastropoda: Includes snails and slugs, characterized by a single, coiled shell or no shell at all.
  • Class Bivalvia: Includes clams, mussels, and oysters, which have two hinged shells (valves) that enclose the soft body.
  • Class Cephalopoda: Includes squids, octopuses, and nautiluses, which are characterized by a reduced or absent external shell, a highly developed head, and tentacles.
  • Class Polyplacophora: Includes chitons, which have a series of eight overlapping plates on their dorsal surface.
  • Class Scaphopoda: Includes tusk shells, which have a tubular shell open at both ends.

Mollusks inhabit a variety of environments, from oceans and freshwater to terrestrial habitats. They play essential roles in ecosystems as filter feeders, grazers, scavengers, and predators. Mollusks are also economically important, providing food for humans and contributing to the cultural and economic aspects of various societies.

Phylum Platyhelminthes

Phylum Platyhelminthes, commonly known as flatworms, comprises a diverse group of bilaterally symmetrical, unsegmented, and soft-bodied invertebrates. These animals are characterized by their flattened, ribbon-like body shape. Platyhelminthes are found in various aquatic and moist terrestrial environments, ranging from freshwater to marine habitats.

Phylum Platyhelminthes
Phylum Platyhelminthes

Key characteristics of flatworms include:

  1. Flattened Body: Platyhelminthes have a dorsoventrally flattened body with a distinct front (anterior) end and back (posterior) end. This body plan allows for efficient gas exchange and nutrient absorption across the body surface.
  2. Bilateral Symmetry: Flatworms exhibit bilateral symmetry, meaning that their bodies can be divided into two mirror-image halves along a central plane.
  3. Cephalization: Some flatworms show cephalization, where a concentration of nervous and sensory structures is found in the anterior (head) region. This is a characteristic associated with animals that have a distinct front end with sensory organs.
  4. Gastrovascular Cavity: Platyhelminthes have a simple gastrovascular cavity with a single opening serving as both the mouth and anus. This cavity allows for digestion and nutrient absorption.
  5. No Coelom: Flatworms lack a true body cavity (coelom). Instead, their bodies are filled with parenchyma, a loose network of cells that provides support.
  6. Excretion: Flatworms excrete metabolic wastes through simple diffusion across their body surface.
  7. Reproduction: Platyhelminthes exhibit a variety of reproductive strategies, including sexual and asexual reproduction. Some species are hermaphroditic, possessing both male and female reproductive organs.

Major classes within Phylum Platyhelminthes include:

  • Class Turbellaria: Free-living flatworms, many of which are found in freshwater environments. Planarians are a well-known example.
  • Class Trematoda: Parasitic flukes that often infect vertebrates, including humans. Examples include liver flukes and blood flukes.
  • Class Cestoda: Parasitic tapeworms that live in the digestive tracts of vertebrates. They are characterized by a long, segmented body.

Flatworms play important ecological roles, both as free-living organisms in aquatic ecosystems and as parasites that can affect the health of various animals, including humans. Some flatworms have regenerative abilities and can regenerate into complete individuals from fragments of their bodies. Additionally, the parasitic flatworms have complex life cycles often involving multiple host species.

Phylum Chordata

Phylum Chordata is a diverse group of animals that share certain key characteristics at some stage in their life cycle. Chordates are bilaterally symmetrical, coelomate animals with a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. These characteristics set them apart from other animal phyla and provide a basis for understanding their evolutionary relationships.

Class Chondrichthyes

Class Chondrichthyes is a group of cartilaginous fishes that includes sharks, rays, and skates. Chondrichthyes are characterized by having skeletons made of cartilage rather than bone. They are a diverse group of marine animals that have adapted to a wide range of aquatic environments.

Class Chondrichthyes
Class Chondrichthyes

Key characteristics of Class Chondrichthyes include:

  1. Cartilaginous Skeleton: Unlike bony fishes, chondrichthyans have a skeleton made of cartilage, which is lighter and more flexible than bone.
  2. Jaws and Teeth: Chondrichthyans are known for their powerful jaws and sharp teeth. Most species have multiple rows of teeth, and these teeth are often replaced continuously throughout the animal’s life.
  3. Paired Fins: Chondrichthyans have paired pectoral and pelvic fins that help with stabilization and maneuvering in the water.
  4. Placoid Scales: The skin of chondrichthyans is covered in small, tooth-like scales called placoid scales. These scales reduce drag and contribute to the streamlined body shape.
  5. Gill Slits: Chondrichthyans typically have five to seven pairs of gill slits on the sides of their heads, which are used for extracting oxygen from the water.
  6. Internal Fertilization: Many chondrichthyans have internal fertilization, where the male transfers sperm to the female’s reproductive system using specialized structures called claspers.
  7. Heterocercal Tail: The tails of chondrichthyans are often heterocercal, meaning that the upper lobe is larger than the lower lobe. This tail shape provides lift and helps maintain buoyancy.
  8. Ampullae of Lorenzini: Specialized sensory organs called ampullae of Lorenzini enable chondrichthyans to detect weak electric fields produced by other animals, aiding in prey detection.

Two main groups within Class Chondrichthyes are:

  • Subclass Elasmobranchii: Includes sharks, rays, and skates. Sharks are characterized by their streamlined bodies, multiple rows of teeth, and a predatory lifestyle. Rays and skates have flattened bodies and often spend much of their time on the ocean floor.
  • Subclass Holocephali: Includes chimaeras, also known as ghost sharks or ratfish. Chimaeras have characteristics of both sharks and bony fishes and are found in deep-sea environments.

Chondrichthyans play important roles in marine ecosystems as both predators and prey. They are key components of ocean food webs and contribute to the regulation of fish populations. Additionally, some species of chondrichthyans, such as sharks, are important to fisheries for their fins, meat, and other products. Conservation efforts are underway to address concerns about overfishing and the impact of human activities on chondrichthyan populations.

Class Osteichthyes

Class Osteichthyes is a group of bony fishes, which are characterized by having skeletons made of bone rather than cartilage. This class is the largest and most diverse group of vertebrates, comprising the majority of fish species. Bony fishes are found in a wide range of aquatic environments, including freshwater and marine habitats.

Class Osteichthyes
Class Osteichthyes

Key characteristics of Class Osteichthyes include:

  1. Bony Skeleton: Unlike cartilaginous fishes (Class Chondrichthyes), bony fishes have skeletons made of true bone. This provides support and rigidity to the body.
  2. Swim Bladder: Many bony fishes have a swim bladder, an internal gas-filled sac that helps control buoyancy. This adaptation allows them to control their position in the water column.
  3. Operculum: Bony fishes have a bony plate called the operculum that covers and protects the gills. The operculum is part of the bony gill cover, allowing for efficient extraction of oxygen from water.
  4. Paired Fins: Bony fishes have paired pectoral and pelvic fins, which aid in stabilization, maneuvering, and maintaining balance.
  5. Scales: Most bony fishes have overlapping scales that cover their bodies, providing protection and reducing friction as they move through the water.
  6. Gill Covers: Bony fishes typically have a bony operculum covering their gills, unlike the gill slits seen in cartilaginous fishes.
  7. External Fertilization: Many bony fishes practice external fertilization, where eggs are fertilized by sperm outside the body.
  8. Lateral Line System: Bony fishes possess a lateral line system, a series of sensory organs along their bodies that detect changes in water pressure. This system helps them sense movement and vibrations in the water.

Class Osteichthyes is further divided into two subclasses:

  • Subclass Actinopterygii: Ray-finned fishes, which make up the vast majority of bony fishes. Their fins are supported by long, bony rays.
  • Subclass Sarcopterygii: Includes lobe-finned fishes, which have fleshy, lobed pectoral and pelvic fins. The lobe-finned fishes include the coelacanths and lungfishes. Tetrapods (four-limbed vertebrates) evolved from a group of lobe-finned fishes.

Bony fishes are ecologically diverse, filling various niches in aquatic ecosystems. They range from small, brightly colored reef fish to large, migratory species like tuna and salmon. Bony fishes are economically important as a food source and are also popular in aquaculture and the aquarium trade. The evolutionary success of bony fishes is evident in their adaptability to diverse environments and their widespread distribution in both freshwater and marine habitats.

Class Amphibia

Class Amphibia includes a diverse group of vertebrates commonly known as amphibians. Amphibians are characterized by a dual life cycle that typically involves an aquatic larval stage with gills and a terrestrial adult stage with lungs. This class encompasses a variety of organisms, including frogs, toads, salamanders, newts, and caecilians.

Class Amphibia
Class Amphibia

Key characteristics of Class Amphibia include:

  1. Moist, Permeable Skin: Amphibians have thin, moist skin that is permeable to gases, allowing for cutaneous respiration. Cutaneous respiration is especially important for oxygen uptake, particularly in species that live in aquatic or humid environments.
  2. Metamorphosis: Many amphibians undergo metamorphosis, a process of transformation from aquatic larvae with gills to terrestrial adults with lungs. During metamorphosis, there are often significant changes in body form, including the development of limbs and the resorption of the tail.
  3. Bimodal Respiration: Amphibians are capable of both cutaneous and pulmonary respiration. While lungs are present in most adult amphibians, they often retain the ability to respire through their skin.
  4. Ectothermic: Amphibians are ectothermic or cold-blooded, meaning their body temperature is dependent on the external environment. They often bask in the sun to regulate their body temperature.
  5. Paired Limbs: Most amphibians have four limbs, with webbed or non-webbed digits. Limbs are used for locomotion on land and in water.
  6. Three-Chambered Heart: Amphibians typically have a three-chambered heart with two atria and one ventricle. This arrangement partially separates oxygenated and deoxygenated blood.
  7. Aquatic Reproduction: Amphibians usually lay eggs in water, and their young undergo development in an aquatic environment. Eggs lack a shell and are vulnerable to desiccation.
  8. Parental Care: Some amphibian species exhibit parental care, with adults guarding eggs or even carrying tadpoles on their backs.

Class Amphibia is divided into three orders:

  1. Order Anura: Includes frogs and toads, which are characterized by a tailless body during the adult stage. Frogs typically have a more slender body and long hind limbs adapted for jumping, while toads have a shorter, stockier build.
  2. Order Caudata: Includes salamanders and newts, which have a long tail during the adult stage. Salamanders are known for their slender bodies and smooth, moist skin, while newts often have brightly colored markings.
  3. Order Gymnophiona: Includes caecilians, which are legless, burrowing amphibians with a snake-like appearance. Caecilians are found in tropical regions and are often fossorial, living in soil or leaf litter.

Amphibians play important roles in ecosystems as both predators and prey. They are also sensitive to environmental changes, making them valuable indicators of environmental health. However, many amphibian populations worldwide face threats, including habitat loss, pollution, and the spread of infectious diseases, contributing to declines in amphibian diversity. Conservation efforts are underway to address these challenges and protect amphibian species.

Class Reptilia

Class Reptilia is a diverse group of vertebrates that includes turtles, snakes, lizards, crocodiles, and tuataras. Reptiles are characterized by several key features that distinguish them from other vertebrates. Here are some key characteristics of Class Reptilia:

  1. Scaly Skin: Reptiles have dry, scaly skin that provides protection against water loss and external threats. The scales are made of keratin, a tough protein.
  2. Ectothermy: Reptiles are ectothermic, meaning they regulate their body temperature through external sources. They rely on the environment, such as basking in the sun, to maintain their body temperature.
  3. Lungs: Reptiles have well-developed lungs for efficient respiration. Unlike amphibians, they do not rely on cutaneous respiration (breathing through the skin).
  4. Amniotic Eggs: Reptiles lay amniotic eggs, which have a protective shell and membranes. These eggs can be laid on land, allowing reptiles to reproduce independently of aquatic environments.
  5. Internal Fertilization: Reproduction involves internal fertilization, where sperm is transferred directly to the female’s reproductive tract. Many reptiles have specialized copulatory organs to facilitate this process.
  6. Well-Developed Limbs: Most reptiles have limbs adapted for various forms of locomotion, such as walking, running, climbing, or swimming. Some reptiles, like snakes, have limbless bodies.
  7. Diverse Skull Morphology: The skulls of reptiles can vary widely depending on their feeding habits. Snakes, for example, have highly flexible skulls to accommodate swallowing large prey.
  8. Three-Chambered Heart: Most reptiles have a three-chambered heart with two atria and one partially divided ventricle. Crocodilians have a four-chambered heart, similar to birds and mammals.

Class Reptilia is further divided into several orders, each containing different groups of reptiles:

  1. Order Testudines (Turtles and Tortoises): Turtles are characterized by a bony or cartilaginous shell that encases their body.
  2. Order Squamata (Lizards and Snakes): This diverse order includes lizards, snakes, and amphisbaenians (worm lizards).
  3. Order Crocodylia (Crocodiles, Alligators, Caimans, and Gharials): This order includes semi-aquatic reptiles with powerful jaws and a fourth tooth in the lower jaw fitting into a pit in the upper jaw.
  4. Order Sphenodontia (Tuatara): Tuatara are reptiles found in New Zealand and represent a primitive group with unique characteristics.

Reptiles play crucial roles in ecosystems as predators, prey, and contributors to biodiversity. They have adapted to various environments, from deserts to rainforests, and their diverse characteristics make them fascinating subjects of study in biology. Conservation efforts are often needed to protect reptile species facing threats such as habitat loss, pollution, and overexploitation.

Class Reptilia
Class Reptilia

Class Aves

Class Aves includes birds, a diverse group of warm-blooded, feathered, and typically flying animals. Birds are found in a wide range of habitats, from polar regions to tropical forests, and they exhibit various adaptations for flight, foraging, and reproduction. The class Aves is characterized by several distinctive features that set birds apart from other animals.

Class Aves
Class Aves

Key characteristics of Class Aves include:

  1. Feathers: Birds have feathers, which serve multiple functions, including insulation, aerodynamics for flight, and displays for communication. Feathers are unique to birds and are composed of keratin.
  2. Endothermy (Warm-Blooded): Birds are warm-blooded animals, meaning they can regulate their body temperature internally. This adaptation allows for sustained activity and flight.
  3. Beak: Birds have a beak or bill instead of teeth. The shape and size of the beak are often adapted to the bird’s diet and feeding habits.
  4. Hollow Bones: Bird bones are lightweight and often hollow, which contributes to their ability to fly. The respiratory system of birds is highly efficient, providing a constant supply of oxygen during flight.
  5. Highly Developed Vision: Birds have well-developed eyesight, adapted to their specific ecological needs. Many birds have a keen sense of color vision and depth perception.
  6. Forelimbs Modified as Wings: The forelimbs of birds are modified into wings, allowing for powered flight. Some flightless birds have wings adapted for other functions, such as swimming (e.g., penguins) or balance (e.g., ostriches).
  7. Highly Efficient Respiratory System: Birds have a unique respiratory system with air sacs that provide a continuous flow of air through the lungs, ensuring a constant supply of oxygen during both inhalation and exhalation.
  8. Oviparous Reproduction: Birds lay eggs, and most species are oviparous. The eggs are enclosed in a protective shell, and parental care varies among species.

Class Aves is further divided into numerous orders and families, encompassing a vast diversity of bird species. Some well-known orders of birds include:

  1. Order Passeriformes: Passerines or perching birds, including sparrows, finches, and songbirds.
  2. Order Falconiformes: Birds of prey, including eagles, hawks, and falcons.
  3. Order Columbiformes: Pigeons and doves.
  4. Order Psittaciformes: Parrots and cockatoos.
  5. Order Struthioniformes: Ostriches and emus (flightless birds).

Birds play crucial roles in ecosystems as pollinators, seed dispersers, and predators of insects and other animals. They are also important culturally, economically, and aesthetically, with many species being admired for their songs, plumage, and behaviors. Conservation efforts are underway to address threats such as habitat loss, climate change, and pollution that impact bird populations globally.

Class Mammalia

Mammalia is a class of vertebrate animals characterized by certain common features. Here are some key characteristics of mammals:

  1. Mammary Glands: All mammals have mammary glands that produce milk to nourish their young.
  2. Hair or Fur: Mammals typically have some form of hair or fur on their bodies, although it may be reduced or absent in certain species.
  3. Warm-Blooded: Mammals are warm-blooded, meaning they can regulate their internal body temperature independent of the external environment.
  4. Live Birth or Milk Production: Most mammals give birth to live young, although there are exceptions such as monotremes (platypus and echidna) that lay eggs. Mammals also provide maternal care, and the young are often fed with milk.
  5. Vertebrates: Mammals have a backbone or spine composed of vertebrae.
  6. Placenta (in most cases): Most mammals have a placenta, a temporary organ that provides nutrients and exchanges wastes between the mother and the developing young during pregnancy.
  7. Specialized Teeth: Mammals often have specialized teeth adapted to their diet. For example, carnivores have sharp, pointed teeth for tearing meat, while herbivores have flat teeth for grinding plant material.
  8. Advanced Brain: Mammals generally have well-developed brains compared to other vertebrates.

Mammals are a diverse group, ranging from tiny shrews to massive whales, and they inhabit various environments worldwide, including land, air, and water. The class Mammalia is further divided into subclasses, orders, families, genera, and species, reflecting the hierarchical classification system in biology. Examples of mammalian orders include Primates (humans, monkeys, and apes), Carnivora (dogs, cats, and bears), and Rodentia (rats, mice, and squirrels).

Class Mammalia
Class Mammalia


In conclusion, delving into the vast and intricate world of the classification of living organisms unveils the remarkable diversity and interconnectedness of life on Earth. From the smallest microorganisms to the towering giants of the natural world, the systematic categorization provides us with a framework to understand the complexities of biological diversity. The journey through the kingdoms and phyla, exploring the relationships between species, emphasizes the unity of life and the shared evolutionary history that unites all living beings.

As we navigate the branches of the tree of life, it becomes clear that the classification of living organisms is not just a scientific endeavor but a key to conservation, understanding ecosystems, and even discovering potential applications in various fields. The ongoing advancements in technology and genetic research continue to refine our understanding of taxonomy, challenging traditional classifications and opening new frontiers in the study of life.

In this exploration, we’ve witnessed the importance of adaptability and resilience in the face of changing environments, as well as the delicate balance that sustains the intricate web of life. The study of classification not only aids scientists but also fosters a deeper appreciation for the richness of biodiversity and the interconnectedness of all living things.

In our quest to understand and classify living organisms, we are not only deciphering the past and present but also laying the groundwork for a sustainable future. As stewards of this planet, armed with knowledge and appreciation for the diversity of life, we can strive to protect and preserve the delicate balance that allows every living organism to thrive. So, let us continue to explore, learn, and marvel at the wonders of the living world, recognizing that each classification holds a unique piece of the puzzle in the grand tapestry of life.


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* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.
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