Chapter 1: Diversity of Living Organisms

(i) The Living World

Need for Classification

• Organization: To systematically study the vast number of organisms.
• Identification: To accurately identify unknown organisms.
• Relationships: To understand evolutionary relationships among organisms.
• Communication: To establish a universal system for naming and describing organisms, avoiding ambiguity.
• Conservation: To assess biodiversity and prioritize conservation efforts.

Taxonomy & Systematics

• Taxonomy: The branch of biology dealing with the classification, nomenclature, and identification of organisms.
• Systematics: The study of the evolutionary relationships among organisms, including their classification, nomenclature, and identification. It is broader than taxonomy.
  • Numerical Taxonomy (Phenetics): Based on all observable characteristics. Uses computers to analyze similarities and differences, assigning equal weight to each character.
  • Cytotaxonomy: Based on cytological information like chromosome number, structure, and behavior.
  • Chemotaxonomy: Uses chemical constituents of plants (e.g., amino acids, proteins, DNA sequences) to resolve taxonomic problems.

Concept of Species and Taxonomical Hierarchy

• Species: A group of individuals that can interbreed naturally to produce fertile offspring. It is the basic unit of classification.
• Taxonomical Hierarchy: The arrangement of various taxa (groups) in a hierarchical order, from lower to higher categories.
  • Kingdom: The highest category (e.g., Animalia, Plantae).
  • Phylum/Division: (Phylum for animals, Division for plants) A group of related classes.
  • Class: A group of related orders.
  • Order: A group of related families.
  • Family: A group of related genera.
  • Genus: A group of related species.
  • Species: The lowest obligate category.

Binomial Nomenclature

• Definition: A system of naming organisms where each organism is given a two-part name: the first part is the genus name, and the second part is the species epithet.
• Rules:
  1. Biological names are generally in Latin and written in italics.
  2. The first word represents the genus, starting with a capital letter.
  3. The second word is the species epithet, starting with a small letter.
  4. When handwritten, both names are separately underlined; when typed, they are italicized.
  5. The name of the author appears after the species epithet in an abbreviated form (e.g., Mangifera indica Linn.).
• Advantages:
  • Universality: Provides a unique and universally accepted name for each organism, avoiding confusion caused by local names.
  • Clarity: Indicates the genus to which an organism belongs, showing its relationship with other species in the same genus.
  • Stability: Names are stable and do not change frequently.

Three Systems of Classification

• Artificial System: Based on one or a few superficial morphological characters (e.g., habitat, color, number of stamens).
  • Example: Linnaeus's system based on sexual characters of plants.
  • Limitations: Does not reflect natural relationships or evolutionary history.
• Natural System: Based on natural affinities among organisms, considering both external and internal features (e.g., anatomy, embryology, phytochemistry).
  • Example: Bentham and Hooker's system for flowering plants.
  • Advantages: Provides a more comprehensive and natural grouping.
• Phylogenetic System: Based on evolutionary relationships among organisms. It assumes that organisms belonging to the same taxa have a common ancestor.
  • Example: Engler and Prantl's system.
  • Advantages: Reflects true evolutionary history and relationships.

(ii) Biological Classification

Three Domains of Life

• Archaea: Prokaryotes, often extremophiles (e.g., methanogens, halophiles, thermoacidophiles). Distinct cell wall and membrane composition.
• Bacteria (Eubacteria): True bacteria, prokaryotes with peptidoglycan cell walls. Includes most common bacteria.
• Eukarya: All eukaryotes, including protists, fungi, plants, and animals. Possess a true nucleus and membrane-bound organelles.

Five Kingdoms Classification (R.H. Whittaker, 1969)

Based on cell structure, thallus organization, mode of nutrition, reproduction, and phylogenetic relationships.

• Monera:
  • Features: Prokaryotic, unicellular, cell wall present (non-cellulosic, peptidoglycan), autotrophic (chemosynthetic/photosynthetic) or heterotrophic.
  • Examples: Bacteria, Cyanobacteria (blue-green algae), Mycoplasma.
• Protista:
  • Features: Eukaryotic, mostly unicellular, aquatic, autotrophic or heterotrophic.
  • Examples: Amoeba, Paramecium, Euglena, Diatoms.
• Fungi:
  • Features: Eukaryotic, multicellular (except yeast), heterotrophic (saprophytic/parasitic), cell wall of chitin, reproduce by spores.
  • Examples: Mushrooms, molds, yeasts.
• Plantae:
  • Features: Eukaryotic, multicellular, autotrophic (photosynthetic), cell wall of cellulose.
  • Examples: Algae, Bryophytes, Pteridophytes, Gymnosperms, Angiosperms.
• Animalia:
  • Features: Eukaryotic, multicellular, heterotrophic (holozoic), no cell wall.
  • Examples: All animals from sponges to mammals.

Kingdom Monera

• Structure of Bacteria: Simple, prokaryotic cell structure. Lacks membrane-bound organelles. Possesses a cell wall, cell membrane, cytoplasm, ribosomes, and a nucleoid region (containing genetic material). Some have flagella for motility.
• Classification of Bacteria:
  • Shape:
    • Coccus: Spherical (e.g., Streptococcus)
    • Bacillus: Rod-shaped (e.g., Lactobacillus)
    • Vibrio: Comma-shaped (e.g., Vibrio cholerae)
    • Spirillum: Spiral-shaped (e.g., Spirillum minus)
  • Nutrition:
    • Autotrophic: Photosynthetic (e.g., Cyanobacteria) or Chemosynthetic (e.g., Nitrifying bacteria).
    • Heterotrophic: Saprophytic (decomposers) or Parasitic (cause diseases).
  • Respiration:
    • Aerobic: Require oxygen (e.g., Bacillus subtilis).
    • Anaerobic: Do not require oxygen (e.g., Clostridium).
    • Facultative Anaerobic: Can survive with or without oxygen (e.g., E. coli).
• Gram +ve/-ve Bacteria: Based on Gram staining technique.
  • Gram-positive: Retain crystal violet stain and appear purple. Have a thick peptidoglycan cell wall.
  • Gram-negative: Do not retain crystal violet and appear pink/red after counterstaining. Have a thin peptidoglycan layer and an outer membrane.
• Reproduction in Bacteria:
  • Fission (Binary Fission): Asexual reproduction where a single bacterium divides into two identical daughter cells. Most common method.
  • Conjugation: Transfer of genetic material (plasmid) from one bacterium to another through a pilus.
  • Transduction: Transfer of genetic material from one bacterium to another via a bacteriophage (virus).
  • Transformation: Uptake of naked DNA from the environment by a bacterium.
• Archaebacteria: Ancient bacteria, adapted to extreme conditions.
  • Methanogens: Produce methane, found in marshy areas and guts of ruminants (e.g., Methanobacterium).
  • Halophiles: Live in extreme salty conditions (e.g., Halobacterium).
  • Thermoacidophiles: Live in hot, acidic environments (e.g., hot springs).
• Mycoplasma:
  • Features: Smallest living cells, lack a cell wall, can survive without oxygen, pathogenic to plants and animals.
• Economic Importance of Bacteria:
  • Sewage Treatment: Decompose organic matter in sewage.
  • Antibiotics: Produce antibiotics (e.g., Streptomyces produces streptomycin).
  • Curd/Cheese Production: Lactobacillus converts milk to curd.
  • Nitrogen Fixation: Rhizobium in leguminous plants, Azotobacter in soil.
  • Decomposition: Recycle nutrients in ecosystems.

Kingdom Protista

• Features: Eukaryotic, mostly unicellular, primarily aquatic, diverse modes of nutrition (photosynthetic, heterotrophic, mixotrophic).
• Examples:
  • Chrysophytes: Diatoms (chief producers in oceans, form diatomaceous earth) and Desmids (golden algae).
  • Dinoflagellates: Marine, photosynthetic, often cause red tides (e.g., Gonyaulax).
  • Euglenoids: Freshwater, lack cell wall, have pellicle, photosynthetic in light, heterotrophic in dark (e.g., Euglena).
  • Slime Moulds: Saprophytic protists, form plasmodium under favorable conditions.
  • Protozoans: Heterotrophic, predators or parasites.
    • Rhizopods (Amoeboid): Move and capture food using pseudopodia (e.g., Amoeba, Entamoeba).
    • Flagellates: Have flagella for movement (e.g., Trypanosoma - causes sleeping sickness).
    • Ciliates: Have cilia for movement and feeding (e.g., Paramecium).
    • Sporozoans: Form infectious spore-like stage in life cycle, all are parasites (e.g., Plasmodium - causes malaria).

Kingdom Fungi

• Characteristics: Eukaryotic, heterotrophic (saprophytic or parasitic), body consists of hyphae (filaments) forming a mycelium, cell wall made of chitin and polysaccharides, store food as glycogen.
• Reproduction:
  • Asexual: Fragmentation, budding, fission, spores (conidia, sporangiospores, zoospores).
  • Sexual: Involves plasmogamy (fusion of protoplasts), karyogamy (fusion of nuclei), and meiosis.
• Types (Classes):
  • Zygomycetes (Phycomycetes): Aseptate, coenocytic hyphae. Asexual spores are sporangiospores. Sexual reproduction by fusion of two gametes to form zygospore (e.g., Rhizopus - bread mould).
  • Ascomycetes (Sac Fungi): Septate hyphae. Asexual spores are conidia. Sexual spores are ascospores produced in sac-like asci (e.g., Aspergillus, Penicillium, yeast - Saccharomyces).
  • Basidiomycetes (Club Fungi): Septate hyphae. Asexual spores generally absent. Sexual spores are basidiospores produced on basidia (e.g., Mushrooms, bracket fungi, rusts, smuts).
  • Deuteromycetes (Imperfect Fungi): Only asexual or vegetative phases are known. Once sexual forms are discovered, they are moved to Ascomycetes or Basidiomycetes. Reproduce by conidia (e.g., Alternaria, Colletotrichum, Trichoderma).
• Fungal Terms:
  • Isogamy: Fusion of morphologically similar gametes.
  • Anisogamy: Fusion of morphologically dissimilar gametes.
  • Oogamy: Fusion of a large, non-motile female gamete with a smaller, motile male gamete.
  • Plasmogamy: Fusion of protoplasts (cytoplasm) of two compatible hyphae.
  • Karyogamy: Fusion of two nuclei.
  • Dikaryophase: A stage in the sexual reproduction of some fungi (Ascomycetes and Basidiomycetes) where two haploid nuclei (n+n) exist within the same cell before fusing.
• Roles of Fungi:
  • Medicine: Production of antibiotics (e.g., Penicillin from Penicillium notatum).
  • Baking/Brewing: Yeast (Saccharomyces cerevisiae) used in bread making and alcohol fermentation.
  • Decomposition: Important decomposers, recycling nutrients in ecosystems.
  • Food: Mushrooms are consumed as food.
• Lichens: Symbiotic association between fungi (mycobiont) and algae or cyanobacteria (phycobiont). Fungi provide shelter, water, and minerals; algae/cyanobacteria perform photosynthesis. Pioneer species on bare rocks.
• Mycorrhiza: Symbiotic association between fungi and roots of higher plants. Fungi help in absorption of water and minerals; plant provides food to fungi.
  • Ectomycorrhiza: Fungi form a mantle on the root surface and penetrate intercellular spaces.
  • Endomycorrhiza: Fungi penetrate cortical cells of the root.

Viruses & Related Entities

• Viruses: Non-cellular infectious agents, obligate intracellular parasites. Possess genetic material (DNA or RNA) surrounded by a protein coat (capsid).
  • TMV (Tobacco Mosaic Virus) Structure: Rod-shaped, single-stranded RNA as genetic material, helical capsid.
  • Bacteriophage Structure: Tadpole-shaped, double-stranded DNA as genetic material, head-tail structure. Infects bacteria.
• Scientists:
  • D.J. Ivanowsky (1892): Recognized certain microbes as causal organisms of mosaic disease of tobacco, smaller than bacteria.
  • M.W. Beijerinck (1898): Demonstrated that the extract of infected tobacco plants could cause infection in healthy plants and called the fluid "Contagium vivum fluidum" (infectious living fluid).
  • W.M. Stanley (1935): Showed that viruses could be crystallized and crystals consist largely of proteins.
• Viroids: Smaller than viruses, lack protein coat, consist only of a free RNA molecule of low molecular weight. Cause diseases in plants (e.g., Potato Spindle Tuber disease).
• Prions: Abnormally folded proteins, cause neurological diseases. Lack nucleic acid.
  • Examples: BSE (Bovine Spongiform Encephalopathy) or Mad Cow Disease in cattle, CJD (Creutzfeldt-Jakob Disease) in humans.

(iii) Plant Kingdom

Algae

• Features: Simple, thalloid (undifferentiated body), autotrophic, mostly aquatic.
• Classes:
  • Chlorophyceae (Green Algae):
    • Features: Grass green due to chlorophyll a and b, stored food is starch, cell wall of cellulose, usually have pyrenoids (storage bodies), mostly freshwater.
    • Examples: Chlamydomonas, Volvox, Spirogyra, Chara.
  • Phaeophyceae (Brown Algae):
    • Features: Brown color due to fucoxanthin, stored food is laminarin or mannitol, cell wall of cellulose and algin, mostly marine.
    • Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.
  • Rhodophyceae (Red Algae):
    • Features: Red color due to phycoerythrin, stored food is floridean starch (similar to amylopectin and glycogen), cell wall of cellulose, pectin, and polysulphate esters, mostly marine, found in warmer areas.
    • Examples: Polysiphonia, Porphyra, Gracilaria, Gelidium.
• Uses of Algae:
  1. Food: Porphyra, Laminaria, Sargassum are used as food.
  2. Agar: From Gelidium and Gracilaria, used in culture media and ice-creams.
  3. Algin: From brown algae, used as a hydrocolloid.
  4. Carrageen: From red algae, used as an emulsifier.
  5. Single Cell Protein (SCP): Chlorella is rich in protein.
  6. Biofuel: Some algae can be used for biofuel production.

Bryophyta (Amphibians of the Plant Kingdom)

• Features: Non-vascular, terrestrial but dependent on water for sexual reproduction, thallus-like body (more differentiated than algae), lack true roots, stems, and leaves. Dominant gametophytic phase.
• Liverworts: Thalloid, dorsiventral, closely appressed to the substrate. Asexual reproduction by fragmentation or gemmae. (e.g., Marchantia).
• Mosses: Have upright, slender axes bearing spirally arranged leaves. More elaborate than liverworts. Have rhizoids for anchorage. (e.g., Funaria, Sphagnum).
• Life Cycle of Funaria (Moss):
  • Dominant gametophyte (haploid) produces gametes.
  • Antheridia (male sex organ) produce antherozoids (motile male gametes).
  • Archegonia (female sex organ) produce a single egg.
  • Water is essential for transfer of antherozoids to the egg.
  • Zygote (diploid) develops into a sporophyte.
  • Sporophyte (diploid) is parasitic on gametophyte, consists of foot, seta, and capsule.
  • Capsule produces spores (haploid) by meiosis.
  • Spores germinate to form protonema (filamentous stage), which develops into a new gametophyte.
• Uses of Bryophytes:
  1. Peat: Sphagnum (moss) provides peat, used as fuel and packing material due to its water-holding capacity.
  2. Ecological Succession: Pioneer organisms on bare rocks/soil, help in soil formation and prevent soil erosion.

Pteridophyta (First Terrestrial Plants with Vascular Tissues)

• Features: Vascular plants (possess xylem and phloem), sporophyte is the dominant phase, differentiated into true roots, stems, and leaves. Found in cool, damp, shady places.
• Classes:
  • Psilopsida: Primitive, rootless (e.g., Psilotum).
  • Lycopsida: Club mosses (e.g., Selaginella, Lycopodium).
  • Sphenopsida: Horsetails (e.g., Equisetum).
  • Pteropsida: Ferns (e.g., Dryopteris, Adiantum).
• Life Cycle of Fern:
  • Dominant sporophyte (diploid) bears sporangia on leaves (sporophylls).
  • Spores (haploid) are produced by meiosis in sporangia.
  • Spores germinate to form a small, inconspicuous, heart-shaped gametophyte called prothallus (haploid).
  • Prothallus bears antheridia and archegonia.
  • Water is required for fertilization.
  • Zygote (diploid) develops into a new sporophyte.
• Homospory/Heterospory:
  • Homosporous: Produce only one type of spore (most pteridophytes, e.g., Ferns). Spores germinate to form bisexual gametophytes.
  • Heterosporous: Produce two types of spores: microspores (germinate to form male gametophytes) and megaspores (germinate to form female gametophytes) (e.g., Selaginella, Salvinia). This is a precursor to seed habit.

Gymnosperms (Naked Seeds)

• Features: Plants in which ovules are not enclosed by an ovary wall and remain exposed both before and after fertilization. Seeds are naked. Possess tap roots, stems (branched or unbranched), and leaves (simple or compound). Vascular tissues are well-developed.
• Life Cycle of Pinus:
  • Sporophyte (diploid) is the dominant plant body.
  • Bears male and female cones (strobili) on the same plant (monoecious).
  • Male cones produce microspores (pollen grains) by meiosis.
  • Female cones produce megaspores by meiosis, one of which develops into the female gametophyte (embryo sac) containing archegonia.
  • Pollen grains are carried by wind to the ovule.
  • Fertilization occurs, and the zygote develops into an embryo.
  • Ovules develop into naked seeds.
• Economic Importance of Gymnosperms:
  1. Timber: Wood from conifers (pine, deodar) is used for construction and furniture.
  2. Resins and Turpentine: Obtained from pines.
  3. Food: Seeds of Pinus gerardiana (chilgoza) are edible.
  4. Medicinal: Ephedrine (from Ephedra) used to treat respiratory ailments.
  5. Ornamental: Many species are grown for their aesthetic value.

(iv) Animal Kingdom

Basis of Classification

• Levels of Organization:
  • Cellular Level: Cells are arranged as loose cell aggregates (e.g., Porifera).
  • Tissue Level: Cells performing the same function are arranged into tissues (e.g., Cnidaria, Ctenophora).
  • Organ Level: Tissues are grouped to form organs (e.g., Platyhelminthes).
  • Organ System Level: Organs associate to form organ systems (e.g., Annelida, Arthropoda, Mollusca, Echinodermata, Chordata).
• Body Plan:
  • Cell Aggregate Plan: No true tissues, cells loosely arranged (e.g., Sponges).
  • Blind Sac Plan: Single opening for both ingestion and egestion (e.g., Cnidaria, Platyhelminthes).
  • Tube-within-a-Tube Plan: Complete digestive system with two openings (mouth and anus) (e.g., Annelida to Chordata).
• Symmetry:
  • Asymmetrical: No symmetry (e.g., Sponges).
  • Radial Symmetry: Body can be divided into two identical halves by any plane passing through the central axis (e.g., Cnidaria, Ctenophora, adult Echinodermata).
  • Bilateral Symmetry: Body can be divided into identical left and right halves in only one plane (e.g., Platyhelminthes to Chordata).
• Germ Layers:
  • Diploblastic: Two embryonic germ layers: ectoderm and endoderm (e.g., Cnidaria, Ctenophora).
  • Triploblastic: Three embryonic germ layers: ectoderm, mesoderm, and endoderm (e.g., Platyhelminthes to Chordata).
• Coelom Types (Body Cavity):
  • Acoelomates: No body cavity between body wall and gut (e.g., Platyhelminthes).
  • Pseudocoelomates: Body cavity is not lined by mesoderm; mesoderm is present as scattered pouches (e.g., Aschelminthes/Nematoda).
  • Coelomates (Eucoelomates): True coelom, body cavity lined by mesoderm (e.g., Annelida to Chordata).
• Segmentation (Metamerism): Body is externally and internally divided into segments, with serial repetition of some organs (e.g., Annelida, Arthropoda, Chordata).

Non-Chordates (Phyla from Porifera to Hemichordata)

1. Phylum Porifera (Sponges):
   • Features: Cellular level of organization, asymmetrical, diploblastic, possess water canal system (spongocoel, ostia, osculum, choanocytes), hermaphrodite.
   • Examples: Sycon, Spongilla (freshwater sponge).
2. Phylum Cnidaria (Coelenterata):
   • Features: Tissue level of organization, radial symmetry, diploblastic, possess cnidoblasts/cnidocytes (stinging cells), gastro-vascular cavity, exhibit polymorphism (polyp and medusa forms).
   • Examples: Hydra, Aurelia (jellyfish), Adamsia (sea anemone).
3. Phylum Ctenophora (Comb Jellies/Sea Walnuts):
   • Features: Tissue level of organization, radial symmetry, diploblastic, possess 8 rows of ciliated comb plates for locomotion, show bioluminescence.
   • Examples: Pleurobrachia, Ctenoplana.
4. Phylum Platyhelminthes (Flatworms):
   • Features: Organ level of organization, bilateral symmetry, triploblastic, acoelomate, dorso-ventrally flattened body, hooks and suckers in parasitic forms, flame cells for osmoregulation and excretion.
   • Examples: Taenia (tapeworm), Fasciola (liver fluke), Planaria (free-living).
5. Phylum Aschelminthes (Nematoda/Roundworms):
   • Features: Organ-system level of organization, bilateral symmetry, triploblastic, pseudocoelomate, cylindrical body, complete digestive system, sexes are separate (dioecious).
   • Examples: Ascaris (roundworm), Wuchereria (filarial worm), Ancylostoma (hookworm).
6. Phylum Annelida (Segmented Worms):
   • Features: Organ-system level of organization, bilateral symmetry, triploblastic, true coelomate, metamerically segmented body, longitudinal and circular muscles for locomotion, closed circulatory system, nephridia for osmoregulation and excretion.
   • Examples: Nereis, Pheretima (earthworm), Hirudinaria (blood-sucking leech).
7. Phylum Arthropoda (Joint-legged Animals):
   • Features: Organ-system level of organization, bilateral symmetry, triploblastic, coelomate, segmented body with jointed appendages, chitinous exoskeleton, open circulatory system, respiratory organs (gills, book gills, book lungs, tracheal system).
   • Examples: Insects (cockroach, mosquito), Crustaceans (prawn, crab), Arachnids (spider, scorpion), Myriapods (centipede, millipede).
8. Phylum Mollusca (Soft-bodied Animals):
   • Features: Organ-system level of organization, bilateral symmetry, triploblastic, coelomate, soft unsegmented body, usually covered by a calcareous shell, distinct head, muscular foot, and visceral hump, feather-like gills (ctenidia) for respiration and excretion.
   • Examples: Pila (apple snail), Octopus (devilfish), Unio (freshwater mussel), Sepia (cuttlefish).
9. Phylum Echinodermata (Spiny-skinned Animals):
   • Features: Organ-system level of organization, radial symmetry in adults (bilateral in larva), triploblastic, coelomate, spiny skin, presence of water vascular system (ambulacral system) for locomotion, capture of food, and respiration, absence of excretory system.
   • Examples: Asterias (starfish), Echinus (sea urchin), Antedon (feather star), Cucumaria (sea cucumber).
10. Phylum Hemichordata (Half Chordates):
    • Features: Organ-system level of organization, bilateral symmetry, triploblastic, coelomate, worm-like marine animals, body divided into proboscis, collar, and trunk, presence of stomochord (rudimentary notochord-like structure in collar region), open circulatory system, respiration by gills.
    • Examples: Balanoglossus, Saccoglossus.

Chordates

• Defining Features:

  1. Presence of a notochord (a flexible rod-like structure) at some stage of life.
  2. Presence of a dorsal, hollow nerve cord.
  3. Presence of paired pharyngeal gill slits.
  4. Presence of a post-anal tail.

• Subphyla of Chordata:

  • Urochordata (Tunicata): Notochord present only in larval tail, body covered by a tunic/test.
    • Examples: Ascidia, Salpa.
  • Cephalochordata: Notochord extends from head to tail region and persists throughout life.
    • Examples: Branchiostoma (Amphioxus or Lancelet).

• Vertebrata: Possess a vertebral column (bony or cartilaginous) that replaces the notochord in adults. Have a ventral muscular heart, kidneys for excretion and osmoregulation, and paired appendages.

  • Class Cyclostomata (Circular Mouths):
    • Features: Jawless vertebrates, elongated eel-like body, circular and sucking mouth, scales absent, paired fins absent, cartilaginous cranium and vertebral column, marine but migrate to freshwater for spawning.
    • Examples: Petromyzon (Lamprey), Myxine (Hagfish).
  • Class Chondrichthyes (Cartilaginous Fishes):
    • Features: Marine, streamlined body, cartilaginous endoskeleton, mouth ventral, gill slits separate and without operculum, skin with placoid scales, powerful jaws, no air bladder (so swim constantly to avoid sinking), poikilothermous (cold-blooded).
    • Examples: Scoliodon (Dogfish), Pristis (Sawfish), Carcharodon (Great white shark).
  • Class Osteichthyes (Bony Fishes):
    • Features: Both marine and freshwater, bony endoskeleton, mouth terminal, four pairs of gills covered by operculum, skin with cycloid/ctenoid scales, air bladder present (regulates buoyancy), poikilothermous.
    • Examples: Marine: Hippocampus (Seahorse), Exocoetus (Flying fish). Freshwater: Labeo (Rohu), Catla (Katla).
  • Class Amphibia (Dual Life):
    • Features: Can live in aquatic and terrestrial habitats, body divisible into head and trunk (tail may be present in some), skin moist without scales, two pairs of limbs, respiration by gills, lungs, and skin, three-chambered heart, poikilothermous.
    • Examples: Rana (Frog), Bufo (Toad), Salamandra (Salamander).
  • Class Reptilia (Creeping/Crawling):
    • Features: Mostly terrestrial, body covered by dry and cornified skin, epidermal scales or scutes, limbs are two pairs (if present), respiration by lungs, three-chambered heart (except crocodiles which have four), poikilothermous, lay shelled eggs (cleidoic eggs).
    • Examples: Chelone (Turtle), Naja (Cobra), Crocodilus (Crocodile), Chamaeleon (Chameleon).
  • Class Aves (Birds):
    • Features: Presence of feathers, most can fly (except flightless birds like ostrich), forelimbs modified into wings, hindlimbs for walking/swimming/clasping, beak present (no teeth), pneumatic bones (hollow bones), four-chambered heart, warm-blooded (homoiothermous), lay eggs.
    • Examples: Corvus (Crow), Columba (Pigeon), Struthio (Ostrich).
  • Class Mammalia (Presence of Mammary Glands):
    • Features: Presence of mammary glands (milk-producing glands), hair on body, two pairs of limbs, external ears (pinnae) present, different types of teeth (heterodont dentition), four-chambered heart, warm-blooded, viviparous (give birth to young ones, except monotremes).
    • Examples: Homo sapiens (Human), Macaca (Monkey), Canis (Dog), Elephas (Elephant).