Animal Kingdom NEET Biology Notes 2026 | Complete NCERT-Based Revision Notes

Animal Kingdom NEET Biology Notes are essential for mastering one of the most important chapters in NEET Biology. This chapter covers animal classification, levels of organisation, symmetry, coelom, segmentation, non-chordates, chordates, and vertebrate classes. Since multiple questions are asked every year from Animal Kingdom, understanding characteristic features, examples, and important exceptions can significantly improve accuracy and scoring potential in the NEET examination.
 

Animal Kingdom is one of the most frequently tested chapters in NEET Biology. On average, 2-4 questions come directly from this chapter every year. Questions are mostly based on characteristic features of each phylum/class, examples of organisms, classification basis, and diagram-based identification. Memorising examples along with their phylum is essential.

1. BASIS OF CLASSIFICATION

Animals are classified based on the following fundamental features. These criteria are extremely important because almost every MCQ in this chapter tests whether you can match an organism to its correct classification basis.

Levels of Organisation

• Cellular level organisation: Cells are arranged as loose aggregates (e.g., sponges/Porifera)
• Tissue-level organisation: Cells performing the same function are arranged into tissues (e.g., Cnidarians like Hydra)
• Organ-level organisation: Tissues group to form organs (e.g., Platyhelminthes/flatworms)
• Organ system level organization: Organs associate to form functional systems; can be of two types:
  • Incomplete digestive system - single opening (mouth), e.g., Platyhelminthes
  • Complete digestive system - two openings (mouth + anus), e.g., Annelida, Arthropoda, etc.

Symmetry

• Asymmetrical: No body symmetry - e.g., most sponges
• Radial symmetry: Body parts arranged around a central axis (any plane through the axis divides into similar halves) - e.g., Coelenterates, Ctenophores, Echinoderms (adult)
• Bilateral symmetry: Body can be divided into identical left and right halves by a single plane through the axis - e.g., Annelids, Arthropods, Chordates

Diploblastic and Triploblastic Organisation

• Diploblastic: Two embryonic layers - ectoderm and endoderm, with a non-cellular mesoglea in between. E.g., Cnidaria (Coelenterates)
• Triploblastic: Three embryonic layers — ectoderm, mesoderm, and endoderm. E.g., all animals from Platyhelminthes onwards

Coelom (Body Cavity) - VERY IMPORTANT FOR NEET

The space between the body wall and the gut wall, lined by mesoderm, is called coelom.

Segmentation (Metamerism)

External and internal division of the body into segments with serial repetition of organs is called metameric segmentation. Seen in Annelida onwards (Annelida, Arthropoda, Chordata).

Notochord - KEY DIFFERENTIATOR

A mesodermally derived rod-like structure formed on the dorsal side during embryonic development.

• Chordates: Notochord present (at some stage of life)
• Non-chordates: Notochord absent

2. NON-CHORDATES (Invertebrates) - PHYLUM-WISE NOTES

Phylum Porifera (Sponges)

• Cellular level of organisation, mostly asymmetrical
• Possess a unique water canal system (canal system) for circulation of water to bring food and oxygen, and remove waste — Types: Ascon, Sycon, Leucon
• The body has numerous pores called ostia (water enters) and a large opening called osculum (water exits)
• Outer layer: pinacocytes; Inner layer lined by flagellated cells called choanocytes (collar cells)
• Skeleton made of spicules or spongin fibres
• Mostly marine and sessile, all are hermaphrodites
• Reproduction: asexual (budding, gemmules) and sexual; development includes a larval stage
• Examples: Sycon (Scypha), Spongilla (freshwater sponge), Euspongia (bath sponge)

Phylum Cnidaria (Coelenterata)

• Aquatic, mostly marine, sessile or free-swimming
• Diploblastic, radially symmetrical
• Cnidoblasts/cnidocytes (stinging cells) present on tentacles — used for defence and capture of prey
• Two basic body forms: Polyp (sessile, e.g., Hydra) and Medusa (umbrella-shaped, free-swimming, e.g., jellyfish)
• Many cnidarians show polymorphism — both polyp and medusa forms exist (e.g., Obelia)
• Digestion: extracellular and intracellular
• Some Cnidarians, like corals, have a skeleton of calcium carbonate
• Examples: Hydra, Physalia (Portuguese man-of-war), Adamsia, Pennatula, Meandrina (brain coral), Gorgonia

Phylum Ctenophora (Sea walnuts/Comb jellies)

• Exclusively marine, free-swimming
• Diploblastic, biradial symmetry
• The body bears eight external rows of cilia called comb plates, used for locomotion
• Digestion: extracellular and intracellular
• Bioluminescence present (property of emitting light)
• Sexes are not separate (Hermaphrodite/Monoecious). Reproduction is only sexual, with external fertilization and indirect development.
• Reproduction only sexual; external fertilisation, indirect development
• Examples: Ctenoplana, Pleurobrachia

Phylum Platyhelminthes (Flatworms)

• Dorsoventrally flattened body (flat from top to bottom)
• Triploblastic, acoelomate, bilaterally symmetrical
• Mostly endoparasites in animals (some free-living)
• Have specific organs for attachment to host called hooks/suckers in parasitic forms
• Body surface covered with tegument (in parasitic forms) instead of cilia
• Hermaphrodites (sexes are not separate).
• Examples: Fasciola hepatica (liver fluke), Taenia solium (tapeworm), Planaria (free-living - capable of regeneration)

Phylum Aschelminthes (Nematoda - Roundworms)

• Body circular in cross-section, hence the name roundworms
• Triploblastic, pseudocoelomate, bilaterally symmetrical
• Mostly endoparasites causing diseases in humans, plants, and animals
• Alimentary canal complete with a well-developed muscular pharynx
• Excretory substances are removed through the excretory pore
• Sexes are separate (dioecious) - males are generally smaller than females
• Fertilisation internal; development direct or indirect
• Examples: Ascaris (roundworm), Wuchereria (filaria worm, causes elephantiasis), Ancylostoma (hookworm)

Phylum Annelida

• Body metamerically segmented (true segmentation), hence the name (Latin annulus = little ring)
• Triploblastic, coelomate (true coelom present), bilaterally symmetrical
• Found in marine, freshwater, and terrestrial habitats
• Closed circulatory system present
• Excretion through nephridia
• Nervous system: paired ganglia connected by lateral nerves to a central nerve ring
• Examples: Nereis (marine), Pheretima (earthworm), Hirudinaria (blood-sucking leech)

Phylum Arthropoda - LARGEST PHYLUM (in number of species)

• The most species-rich phylum in the Animal Kingdom (over two-thirds of all named species)
• Triploblastic, coelomate (body cavity mainly haemocoel), bilaterally symmetrical.
• Jointed legs (Greek: arthros = joint, podos = foot)
• Body covered by a chitinous exoskeleton
• Body divided into head, thorax, and abdomen
• Respiration: through gills, tracheae, or book lungs
• Open circulatory system; sensory organs include antennae, eyes (compound and simple), statocysts
• Excretion through malpighian tubules
• Examples: Locusta (locust), Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect), Limulus (king crab — "living fossil"), Palaemon (prawn)

Phylum Mollusca - SECOND LARGEST PHYLUM

• The second-largest animal phylum
• Triploblastic, coelomate, bilaterally symmetrical
• Body covered by a calcareous shell (in most), divisible into head, muscular foot, and visceral hump
• Mantle present, covering soft body; space between mantle and body = mantle cavity
• A rasping/file-like organ for feeding called radula (absent in bivalves)
• Excretion through the kidneys
• Examples: Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish — most advanced mollusc), Aplysia (sea hare), Dentalium (tusk shell), Chaetopleura (chiton)

Phylum Echinodermata

• Exclusively marine animals
• Body unsegmented, triploblastic, coelomate
• Radially symmetrical as adults but bilaterally symmetrical as larvae
• Calcium carbonate spicules/ossicles form an endoskeleton, giving spiny skin appearance (hence the name)
• Unique water vascular system for locomotion, capture and transport of food
• Sexes are separate (dioecious); reproduction is sexual, fertilization is external, and development is indirect with a free-swimming larva.
• Examples: Asterias (starfish/sea star), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), Ophiura (brittle star)

Phylum Hemichordata

• Small group of worm-like marine animals, formerly considered a sub-phylum under Chordata, now a separate phylum
• Triploblastic, coelomate, bilaterally symmetrical
• Body divided into proboscis, collar, and trunk
• Circulatory system open type
• Excretion by a single proboscis gland
• Sexes separate; fertilisation external; development indirect
• Examples: Balanoglossus, Saccoglossus

3. PHYLUM CHORDATA

General Characteristics of Chordates (Three Key Features)