Animal Kingdom – NEET NOTES

1. Levels of Organisation

Animals show different levels of structural organization:

  • Cellular level: Cells are loosely arranged; no true tissues (e.g., Porifera).
  • Tissue level: Cells group to form tissues (e.g., Cnidaria).
  • Organ level: Tissues form organs with specific functions (e.g., Platyhelminthes).
  • Organ system level: Organs form systems for physiological processes (e.g., Annelida onwards).

2. Symmetry

Symmetry is the arrangement of body parts:

  • Asymmetrical: No definite shape or plane of symmetry (e.g., Sponges).
  • Radial symmetry: Body can be divided into equal halves along multiple planes (e.g., Cnidarians, Echinoderms in adults).
  • Bilateral symmetry: Body can be divided into two mirror halves in one plane (e.g., Platyhelminthes to Chordates).

3. Germ Layers (Embryonic Layers)

Animals form different numbers of germ layers during embryogenesis:

  • Diploblastic: Two germ layers – ectoderm and endoderm (e.g., Cnidaria).
  • Triploblastic: Three germ layers – ectoderm, mesoderm, endoderm (e.g., Platyhelminthes to Chordata).

4. Body Cavity (Coelom)

Coelom is the body cavity between the body wall and gut wall:

  • Acoelomates: No coelom (e.g., Platyhelminthes).
  • Pseudocoelomates: False coelom derived from blastocoel (e.g., Aschelminthes).
  • Coelomates: True coelom lined by mesoderm (e.g., Annelida to Chordata).

5. Segmentation

Body division into repeated segments:

  • Absent in lower phyla.
  • Present in Annelids, Arthropods, and Chordates.
  • Segmentation allows greater flexibility and specialization of body parts.

6. Notochord

  • A flexible rod-like structure that supports the body in embryonic stages.
  • Non-chordates: Lack notochord (Porifera to Echinodermata).
  • Chordates: Possess a notochord (at least during embryonic stage).

7. Other Criteria

  • Digestive system: Incomplete (one opening) or complete (two openings).
  • Circulatory system: Open (blood not enclosed in vessels) or closed.
  • Reproduction: Sexual or asexual; internal or external fertilization.
  • Habitat and mode of life: Aquatic, terrestrial, parasitic, etc.

Classification of Animal Kingdom

1. Phylum: Porifera (Sponges)

  1. Aquatic, mostly marine; simplest multicellular animals.
  2. Body is asymmetrical or radially symmetrical.
  3. Cellular level of organization; no tissues or organs.
  4. Body has pores (ostia) for water transport — canal system.
  5. Skeleton made of spicules or spongin fibers.
  6. Digestion is intracellular; respiration and excretion by diffusion.
  7. Examples: Sycon, Spongilla, Euspongia (bath sponge).

2. Phylum: Cnidaria (Coelenterata)

  1. Aquatic, mostly marine; radially symmetrical.
  2. Diploblastic with tissue-level organization.
  3. Body has a central gastrovascular cavity and a single opening (incomplete digestive system).
  4. Stinging cells (cnidocytes) present on tentacles for defense and prey capture.
  5. Two body forms: polyp (sessile) and medusa (free-swimming).
  6. Show alternation of generation (metagenesis).
  7. Examples: Hydra, Jellyfish (Aurelia), Sea anemone (Adamsia), Obelia.

3. Phylum: Ctenophora (Comb jellies)

  1. Marine, radially symmetrical, transparent, and gelatinous animals.
  2. Diploblastic with tissue-level organization.
  3. Eight rows of ciliated comb plates for locomotion.
  4. Show bioluminescence (light production).
  5. Digestion is extracellular and intracellular.
  6. Reproduction is sexual and hermaphroditic; external fertilization.
  7. Examples: Pleurobrachia, Ctenoplana.

4. Phylum: Platyhelminthes (Flatworms)

  1. Mostly parasitic; dorsoventrally flattened, bilaterally symmetrical.
  2. Triploblastic and acoelomate.
  3. Organ-level of organization; no segmentation.
  4. Incomplete digestive system; some lack it.
  5. Flame cells help in excretion and osmoregulation.
  6. Mostly hermaphroditic; show high regeneration ability.
  7. Examples: Planaria, Fasciola (liver fluke), Taenia (tapeworm).

5. Phylum: Aschelminthes (Nematoda or Roundworms)

  1. Cylindrical, unsegmented, bilaterally symmetrical, pseudocoelomates.
  2. Triploblastic with organ-system level of organization.
  3. Complete digestive system (mouth and anus present).
  4. Body covered with tough cuticle.
  5. Sexes separate (dioecious); sexual dimorphism present.
  6. Many are parasitic; others are free-living.
  7. Examples: Ascaris, Wuchereria (filarial worm), Ancylostoma (hookworm).

6. Phylum: Annelida (Segmented worms)

  1. Bilaterally symmetrical, triploblastic, coelomates.
  2. Metameric segmentation (repeated body segments).
  3. Closed circulatory system.
  4. Nephridia for excretion and osmoregulation.
  5. Nervous system is ladder-like.
  6. Locomotion by setae/parapodia.
  7. Examples: Earthworm (Pheretima), Leech (Hirudinaria), Nereis.

7. Phylum: Arthropoda

  1. Largest phylum; jointed legs (arthro = joint, poda = foot).
  2. Segmented body with exoskeleton made of chitin.
  3. Open circulatory system; haemocoel present.
  4. Compound eyes and sensory antennae.
  5. Respiratory organs: gills, tracheae, or book lungs.
  6. Sexes separate; high reproductive capability.
  7. Examples: Insects (Apis), Crab, Prawn, Scorpion, Cockroach.

8. Phylum: Mollusca

  1. Soft-bodied, unsegmented animals with bilateral symmetry.
  2. Body has head, foot, and visceral mass.
  3. Mantle secretes calcium carbonate shell.
  4. Coelomate with open circulatory system (except octopus).
  5. Respiration via gills (ctenidia) or lungs (in terrestrial forms).
  6. Kidney-like organ for excretion (metanephridia).
  7. Examples: Snail (Pila), Octopus, Unio, Sepia.

9. Phylum: Echinodermata

  1. Exclusively marine; spiny-skinned animals.
  2. Adult forms show radial symmetry; larvae are bilaterally symmetrical.
  3. Endoskeleton made of calcium carbonate.
  4. Water vascular system for locomotion, feeding, respiration.
  5. No excretory system; coelomate.
  6. Regeneration ability present.
  7. Examples: Starfish (Asterias), Sea urchin, Sea cucumber, Antedon.

10. Phylum: Chordata

  1. Notochord present at some stage.
  2. Dorsal, hollow nerve cord; pharyngeal slits; post-anal tail.
  3. Bilaterally symmetrical, triploblastic, coelomates.
  4. Closed circulatory system; well-developed systems.
  5. Subdivided into Protochordata and Vertebrata.
  6. Vertebrates have a backbone and cranium.
  7. Examples: Amphioxus, Frog, Fish, Birds, Humans, Reptiles.

Comparative Table of Animal Phyla

CharacteristicPoriferaCnidariaCtenophoraPlatyhelminthesAschelminthesAnnelidaArthropodaMolluscaEchinodermataChordata
Body SymmetryAsymmetrical / RadialRadialRadialBilateralBilateralBilateralBilateralBilateralRadial (adult), Bilateral (larva)Bilateral
Germ LayersDiploblasticDiploblasticDiploblasticTriploblasticTriploblasticTriploblasticTriploblasticTriploblasticTriploblasticTriploblastic
Body OrganisationCellularTissueTissueOrganOrgan-systemOrgan-systemOrgan-systemOrgan-systemOrgan-systemOrgan-system
CoelomAbsentAbsentAbsentAcoelomatePseudocoelomateCoelomateCoelomateCoelomateCoelomateCoelomate
SegmentationAbsentAbsentAbsentAbsentAbsentPresentPresentAbsentAbsentPresent in some
NotochordAbsentAbsentAbsentAbsentAbsentAbsentAbsentAbsentAbsentPresent
Digestive SystemAbsent / IncompleteIncompleteCompleteIncompleteCompleteCompleteCompleteCompleteCompleteComplete
Circulatory SystemAbsentAbsentAbsentAbsentAbsentClosedOpen (mostly)Open (except cephalopods)AbsentClosed
Respiratory SystemDiffusionDiffusionDiffusionDiffusionDiffusionDiffusion / GillsGills / Trachea / Book lungsGills / LungsTube feet / DiffusionGills / Lungs
Excretory StructuresAbsentAbsentAbsentFlame cellsRenette cellsNephridiaMalpighian tubules / Green glandsMetanephridiaAbsentKidneys
Nervous SystemAbsent / SimpleNerve netDiffusedLadder-likeRing + NervesGanglia + Nerve cordBrain + NervesGangliaNerve ringDorsal hollow nerve cord
ReproductionAsexual (budding), SexualSexual, asexualSexualHermaphroditeDioeciousMostly hermaphroditeMostly dioeciousDioecious / HermaphroditeSexualDioecious (mostly)
LocomotionSessile (canal system)TentaclesCiliary combsCilia / muscularMuscle / body wallSetae / parapodiaJointed appendagesFootTube feetFins / Limbs
HabitatAquaticAquaticMarineMostly parasiticParasitic / free-livingAquatic / terrestrialAll habitatsMostly aquaticMarineAquatic / Terrestrial
Special FeaturesCanal system, SpiculesCnidocytesBioluminescenceFlat body, Flame cellsPseudocoelomMetamerismExoskeleton, jointed legsShell, MantleWater vascular systemNotochord, Vertebral column
Examples (NCERT)Spongilla, EuspongiaHydra, AureliaPleurobrachiaTaenia, FasciolaAscaris, WuchereriaPheretima, LeechCockroach, ApisPila, SepiaAsterias, AntedonFrog, Birds, Humans

Subphylum 1: Urochordata (Tunicata)

  1. Habitat: Exclusively marine animals, mostly found attached to submerged surfaces like rocks or ship bottoms.
  2. Notochord: Present only in larval tail; lost in adult stage — hence “Uro” (tail) chordates.
  3. Body Covering: Adult body is enclosed in a tunic made of tunicin (cellulose-like).
  4. Symmetry: Bilateral in larvae, may appear asymmetrical in adults.
  5. Nervous System: Larvae have a dorsal nerve cord; adults have a reduced nerve ganglion.
  6. Circulatory System: Open type; heart shows a unique reversible flow of blood.
  7. Digestive System: Complete with large pharyngeal basket with gill slits for filter feeding.
  8. Excretion: Via nephrocytes; simple excretory organs.
  9. Reproduction: Mostly hermaphroditic with external fertilization. Asexual reproduction (budding) also occurs in some.
  10. Special Feature: Exhibits retrogressive metamorphosis—larva is more advanced than adult.
  11. Examples: Ascidia, Salpa, Herdmania (NCERT).

Subphylum 2: Cephalochordata

  1. Habitat: Marine, burrow in sandy ocean bottoms.
  2. Notochord: Extends throughout life and the whole length of the body, including the head — “Cephalo” means head.
  3. Symmetry: Bilateral symmetry; fish-like body shape.
  4. Level of Organization: Organ-system level; well-developed systems.
  5. Circulatory System: Closed type; no true heart but vessels contract to pump blood.
  6. Nervous System: Dorsal tubular nerve cord with a cerebral vesicle.
  7. Digestive System: Complete; pharynx with numerous gill slits for filter feeding.
  8. Respiration: By pharyngeal gill slits; no lungs or gills.
  9. Excretion: Protonephridia-like structures for excretion.
  10. Reproduction: Sexes are separate (dioecious); external fertilization.
  11. Special Feature: Considered a living link between invertebrates and vertebrates.
  12. Example: Branchiostoma (NCERT), also known as Amphioxus or Lancelet.

Subphylum 3: Vertebrata

  1. Notochord: Present in embryonic stage; replaced by vertebral column (backbone) in adults.
  2. Endoskeleton: Internal skeleton made of cartilage or bone; protects and supports the body.
  3. Nervous System: Highly developed; brain enclosed in a cranium, connected to a dorsal spinal cord.
  4. Circulatory System: Closed type with a chambered heart (2–4 chambers depending on class).
  5. Respiration: Gills (in aquatic forms), lungs (in terrestrial), or both (amphibians).
  6. Excretion: Pair of kidneys that remove nitrogenous waste (urea, uric acid, or ammonia).
  7. Reproduction: Mostly sexual; sexes separate (dioecious); internal or external fertilization.
  8. Body Structure: Triploblastic, coelomate, bilaterally symmetrical, and segmented.
  9. Vertebral Column: Forms the main axial support, replacing notochord function.
  10. Classes: Vertebrata is divided into two superclasses:
    • Agnatha (jawless; e.g., Petromyzon)
    • Gnathostomata (jawed vertebrates: Pisces to Mammalia)
  11. Special Feature: Presence of paired appendages (fins or limbs), high cephalization.
  12. Examples: Frog, Pigeon, Shark, Human, Lizard (depending on class).

Chordata
│
├──► 1. Urochordata (Tunicata)
│ • Notochord in larval tail only
│ • Marine, sessile adults
│ • Retrogressive metamorphosis
│ • Ex: Ascidia, Salpa, Herdmania
│
├──► 2. Cephalochordata
│ • Notochord extends from head to tail
│ • Persistent throughout life
│ • Fish-like, burrowing marine animals
│ • Ex: Branchiostoma (Amphioxus)
│
└──► 3. Vertebrata
• Notochord replaced by vertebral column
• Well-developed brain and organ systems
• Cranium and endoskeleton present
• Divided into:
├──► Agnatha (jawless) – Ex: Petromyzon
└──► Gnathostomata (jawed vertebrates)
├──► Pisces (fishes)
├──► Amphibia
├──► Reptilia
├──► Aves (birds)
└──► Mammalia

Class: Cyclostomata

  1. Habitat: Exclusively marine but migrate to freshwater for spawning.
  2. Body Form: Elongated, cylindrical, eel-like body without paired appendages.
  3. Skeleton: Cartilaginous endoskeleton; notochord is persistent throughout life.
  4. Jaws and Fins: Jawless vertebrates; paired fins absent — hence grouped under Agnatha (jawless).
  5. Mouth: Circular, sucking mouth with keratinized teeth — hence called “Cyclostomes.”
  6. Respiration: 6–15 pairs of gill slits for aquatic respiration.
  7. Skin: Soft, scaleless, slimy; aids in defense and locomotion.
  8. Circulatory System: Closed type with a two-chambered heart.
  9. Nervous System: Brain with dorsal nerve cord protected by cranium and vertebral column.
  10. Excretion: Paired kidneys remove nitrogenous waste (ammonia).
  11. Reproduction: Dioecious; fertilization external; development indirect with larval stage.
  12. Special Feature: Show characteristics of both invertebrates and vertebrates.
  13. Examples (NCERT): Petromyzon (lamprey), Myxine (hagfish).

Superclass: Pisces

(Jawed aquatic vertebrates — Gnathostomata)

Class 1: Chondrichthyes (Cartilaginous Fishes)

  1. Skeleton: Entirely cartilaginous.
  2. Body: Streamlined for swift movement.
  3. Mouth: Ventral; teeth are modified placoid scales.
  4. Gill Slits: 5–7 pairs, uncovered (no operculum).
  5. Scales: Tough placoid scales reduce drag.
  6. Buoyancy: No air bladder; must swim to avoid sinking.
  7. Fertilization: Internal; many are viviparous.
  8. Heart: Two-chambered.
  9. Sexes: Separate (dioecious).
  10. Examples (NCERT): Scoliodon (dogfish), Trygon (stingray), Carcharodon (great white shark).

Class 2: Osteichthyes (Bony Fishes)

  1. Skeleton: Made of bone.
  2. Body: Streamlined; fins used for movement.
  3. Mouth: Usually terminal (at front).
  4. Gill Slits: Covered by operculum.
  5. Scales: Covered with cycloid or ctenoid scales.
  6. Buoyancy: Have air bladder for balance.
  7. Fertilization: Mostly external; many are oviparous.
  8. Heart: Two-chambered.
  9. Sexes: Dioecious.
  10. Examples (NCERT): Rohu, Catla, Clarias (magur), Betta (fighting fish), Hippocampus (seahorse).

Superclass: Tetrapoda

(Four-limbed vertebrates – Gnathostomata)

Class: Amphibia

  1. Habitat: Dual life – aquatic and terrestrial.
  2. Skin: Moist, glandular, without scales; respiration also through skin.
  3. Limbs: Two pairs; used for locomotion.
  4. Respiration: By gills, lungs, and skin.
  5. Heart: Three-chambered (two atria + one ventricle).
  6. Ears: Tympanum present.
  7. Fertilization: External; oviparous; development indirect.
  8. Examples (NCERT): Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra, Ichthyophis (limbless amphibian).

Class: Reptilia

  1. Habitat: Primarily terrestrial.
  2. Skin: Dry, scaly (epidermal scales), impermeable to water.
  3. Limbs: Two pairs; claws present in most.
  4. Respiration: Exclusively through lungs.
  5. Heart: Mostly 3-chambered; crocodiles have 4-chambered heart.
  6. Fertilization: Internal; oviparous with shelled eggs.
  7. Temperature Regulation: Cold-blooded (poikilothermic).
  8. Examples (NCERT): Chelone (turtle), Testudo (tortoise), Chameleon, Calotes (garden lizard), Crocodilus.

Class: Aves (Birds)

  1. Body: Streamlined; modified forelimbs as wings.
  2. Skeleton: Light bones with air cavities (pneumatic).
  3. Skin: Covered with feathers; oil glands at tail base.
  4. Respiration: Lungs with air sacs – efficient system.
  5. Heart: Four-chambered.
  6. Excretion: Uricotelic (excrete uric acid); no urinary bladder (except ostrich).
  7. Fertilization: Internal; oviparous; parental care common.
  8. Temperature Regulation: Warm-blooded (homeothermic).
  9. Examples (NCERT): Corvus (crow), Columba (pigeon), Struthio (ostrich), Psittacula (parrot), Neophron (vulture).

Class: Mammalia

  1. Habitat: Terrestrial, aquatic, aerial, and arboreal.
  2. Skin: Hair-covered; sweat and sebaceous glands present.
  3. Mammary Glands: Present; nourish young with milk.
  4. Limbs: Adapted for walking, swimming, flying, etc.
  5. Teeth: Thecodont, heterodont, and diphyodont.
  6. Heart: Four-chambered.
  7. Respiration: Lungs; diaphragm aids breathing.
  8. Temperature Regulation: Warm-blooded.
  9. Reproduction: Internal fertilization; mostly viviparous (except platypus and echidna).
  10. Excretion: Ureotelic; kidneys excrete urea.
  11. Examples (NCERT): Oryctolagus (rabbit), Macropus (kangaroo), Panthera leo (lion), Homo sapiens.

NEET 2025: Animal Kingdom – Most Important Topics (Brief Points)

1. Basis of Classification

  • Levels: Cellular, tissue, organ, organ-system
  • Symmetry: Radial vs. Bilateral
  • Germ layers: Diploblastic vs. Triploblastic
  • Coelom: Acoelomate / Pseudocoelomate / Coelomate
  • Notochord: Present (Chordates) / Absent (Non-chordates)

2. Unique Features of Phyla (Porifera to Echinodermata)

  • Porifera: Canal system, choanocytes
  • Cnidaria: Cnidoblasts, polyp-medusa
  • Ctenophora: Comb plates, bioluminescence
  • Platyhelminthes: Flame cells, flat body
  • Aschelminthes: Pseudocoelom, unsegmented roundworms
  • Annelida: Metamerism, closed circulation
  • Arthropoda: Jointed appendages, chitinous exoskeleton
  • Mollusca: Mantle, radula, muscular foot
  • Echinodermata: Water vascular system, radial symmetry in adult

3. Subphyla of Chordates

  • Urochordata: Notochord in tail (larva only)
  • Cephalochordata: Notochord throughout life
  • Vertebrata: Notochord replaced by vertebral column

4. Class Cyclostomata

  • Jawless, sucking mouth, no paired fins
  • Example: Petromyzon, Myxine

5. Superclass Pisces (Fishes)

  • Chondrichthyes: Cartilage skeleton, ventral mouth, internal fertilization (Shark, Trygon)
  • Osteichthyes: Bony skeleton, air bladder, external fertilization (Rohu, Catla)

6. Superclass Tetrapoda

  • Amphibia: Moist skin, lungs + skin respiration, 3-chambered heart (Frog)
  • Reptilia: Dry scales, lungs only, mostly 3-chambered heart (Lizard, Crocodile)
  • Aves: Feathers, hollow bones, 4-chambered heart, uricotelic (Pigeon, Ostrich)
  • Mammalia: Hair, mammary glands, diaphragm, viviparous (Human, Rabbit)

7. NCERT Examples & Matching

  • Match phylum/class with examples
  • Focus on Ascidia, Herdmania, Amphioxus, Petromyzon, Frog, Lizard, Pigeon, Rabbit etc.

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