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Classification of Animals – Complete Notes 

By Home Academy

Animals are multicellular, eukaryotic, heterotrophic organisms that lack cell walls and mostly show active movement. To study their vast diversity systematically, animals are classified based on body structure, symmetry, coelom, segmentation, and presence of notochord.

👉 Animal Kingdom = Kingdom Animalia

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Basis of Classification of Animals 

Basis	Types
Level of organization	Cellular, Tissue, Organ, Organ-system
Body symmetry	Asymmetrical, Radial, Bilateral
Germ layers	Diploblastic, Triploblastic
Body cavity (Coelom)	Acoelomate, Pseudocoelomate, Coelomate
Segmentation	Segmented / Unsegmented
Notochord	Present / Absent

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Major Classification of Animal Kingdom

Animal Kingdom is divided into:

1. Non-Chordates

2. Chordates

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1. Non-Chordates

Non-Chordates are animals that do not possess a notochord, dorsal hollow nerve cord, or post-anal tail at any stage of their life cycle. They constitute the largest and most diverse group of the animal kingdom, ranging from microscopic organisms to large invertebrates like octopus. The study of non-chordates is important for understanding evolution, body organization, symmetry, and adaptation, and questions from this topic are frequently asked in NEET and board examinations.

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Basis of Classification of Non-Chordates

Non-chordates are classified on the basis of:
Level of organization
Body symmetry
Type of body cavity (coelom)
Segmentation
Presence or absence of true tissues

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Major Phyla of Non-Chordates

Phylum Porifera (Sponges)

Porifera includes the simplest multicellular animals, commonly known as sponges. Their bodies are full of pores (ostia) that allow water to circulate through a canal system. They show cellular level of organization and lack true tissues.

Key Features
Body asymmetrical or radially symmetrical
Cellular level of organization
Presence of canal system
Skeleton made of spicules or spongin

Examples
Sycon
Spongilla
Euspongia

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Phylum Coelenterata / Cnidaria

These animals are aquatic, mostly marine, and show tissue level of organization. They possess specialized stinging cells called cnidoblasts used for defense and capturing prey.

Key Features
Radial symmetry
Diploblastic body
Presence of gastrovascular cavity
Cnidoblasts present

Examples
Hydra
Aurelia (jellyfish)
Adamsia (sea anemone)

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Phylum Ctenophora

Ctenophores are marine animals commonly known as comb jellies. They move using eight rows of ciliary plates called comb plates.

Key Features
Radial symmetry
Diploblastic
Presence of comb plates
Bioluminescence common

Examples
Pleurobrachia
Ctenoplana

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Phylum Platyhelminthes (Flatworms)

These are dorsoventrally flattened animals showing organ level of organization. Most members are parasitic.

Key Features
Bilateral symmetry
Triploblastic
Acoelomate body
Incomplete digestive system

Examples
Planaria
Taenia (tapeworm)
Fasciola (liver fluke)

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Phylum Nematoda / Aschelminthes

Nematodes are roundworms with a pseudocoelom. Many species are parasitic and cause diseases in humans.

Key Features
Bilateral symmetry
Triploblastic
Pseudocoelom present
Complete digestive tract

Examples
Ascaris
Wuchereria
Enterobius

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Phylum Annelida

Annelids are segmented worms showing true coelom and metameric segmentation.

Key Features
Bilateral symmetry
Triploblastic
True coelom present
Segmented body

Examples
Earthworm (Pheretima)
Leech (Hirudinaria)
Nereis

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Phylum Arthropoda

This is the largest phylum of the animal kingdom. Arthropods possess jointed appendages and an exoskeleton made of chitin.

Key Features
Bilateral symmetry
Segmented body
Jointed appendages
Open circulatory system

Examples
Cockroach
Housefly
Prawn
Spider

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Phylum Mollusca

Molluscs are soft-bodied animals, often protected by a calcareous shell.

Key Features
Bilateral symmetry
Soft body with head, foot, and visceral mass
Mantle secretes shell
Radula present (except bivalves)

Examples
Pila
Octopus
Unio

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Phylum Echinodermata

Echinoderms are exclusively marine animals with spiny skin and a unique water vascular system.

Key Features
Radial symmetry in adults
Triploblastic
True coelom
Endoskeleton made of calcareous plates

Examples
Starfish (Asterias)
Sea urchin (Echinus)
Sea cucumber (Holothuria)

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Phylum Hemichordata

Hemichordates show some chordate-like characters but lack a true notochord.

Key Features
Bilaterally symmetrical
Triploblastic
Body divided into proboscis, collar, and trunk
Gills present

Examples
Balanoglossus
Saccoglossus

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Importance of Studying Non-Chordates

Helps understand the evolution of higher animals
Important for medical science (parasites)
Useful in ecological balance and environmental studies
Highly significant for competitive exams

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Exam Highlights

Porifera show cellular level of organization
Cnidoblasts are unique to Cnidaria
Flatworms are acoelomate
Arthropoda is the largest phylum
Echinodermata are exclusively marine

Animals without notochord at any stage of life.

Classification of Non-Chordates (With Examples)

Phylum	Key Characteristics	Examples
Porifera	Pore-bearing body, canal system, asymmetrical	Sycon, Spongilla
Coelenterata (Cnidaria)	Radial symmetry, stinging cells (nematocysts)	Hydra, Jellyfish
Platyhelminthes	Flat body, acoelomate, bilateral symmetry	Planaria, Taenia
Nematoda	Roundworms, pseudocoelom, unsegmented	Ascaris, Wuchereria
Annelida	True coelom, segmented body	Earthworm, Leech
Arthropoda	Jointed legs, exoskeleton, largest phylum	Cockroach, Butterfly
Mollusca	Soft body, muscular foot, shell present	Snail, Octopus
Echinodermata	Spiny skin, water vascular system	Starfish, Sea urchin
Hemichordata	Worm-like, gill slits present	Balanoglossus

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Important Exam Highlights – Non-Chordates

🔹 Largest phylum – Arthropoda
🔹 Only phylum with water vascular system – Echinodermata
🔹 First segmented animals – Annelida
🔹 Body cavity absent – Platyhelminthes
🔹 Parasitic worms common – Nematoda

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2. Chordates

Animals having notochord at some stage of life.

Common Features of Chordates

✔ Notochord
✔ Dorsal hollow nerve cord
✔ Pharyngeal gill slits
✔ Post-anal tail

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Classification of Chordates

Group	Description	Examples
Urochordata	Notochord present in larva	Ascidia
Cephalochordata	Notochord throughout life	Amphioxus
Vertebrata	Backbone present	Fish, Birds, Humans

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Classification of Vertebrata 

A. Pisces (Fishes)

Feature	Details
Habitat	Aquatic
Respiration	Gills
Heart	Two-chambered
Examples	Rohu, Shark

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B. Amphibia

Feature	Details
Habitat	Land & water
Skin	Moist
Respiration	Skin & lungs
Examples	Frog, Toad

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C. Reptilia

Feature	Details
Skin	Dry, scaly
Respiration	Lungs
Heart	Three-chambered (except crocodile Crocodile is the only reptile with a four-chambered heart.
Examples	Snake, Lizard

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D. Aves (Birds)

Feature	Details
Body covering	Feathers
Forelimbs	Wings
Heart	Four-chambered
Examples	Pigeon, Eagle

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E. Mammalia

Feature	Details
Body covering	Hair
Mammary glands	Present
Heart	Four-chambered
Examples	Human, Cow

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Key Differences for Exam

Chordates vs Non-Chordates

Feature	Chordates	Non-Chordates
Notochord	Present	Absent
Nerve cord	Dorsal	Ventral
Heart	Ventral	Dorsal

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Most Repeated Exam Points (Must Remember)

⭐ Humans belong to – Mammalia
⭐ Frog belongs to – Amphibia
⭐ Cockroach belongs to – Arthropoda
⭐ Starfish belongs to – Echinodermata
⭐ Earthworm belongs to – Annelida

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MOST REPEATED MCQs – CLASSIFICATION OF ANIMALS (NEET)

1. The largest phylum of Animal Kingdom is
   A. Mollusca
   B. Arthropoda
   C. Annelida
   D. Chordata
   Correct Answer: B

2. Which of the following animals shows radial symmetry?
   A. Earthworm
   B. Cockroach
   C. Starfish
   D. Frog
   Correct Answer: C

3. Water vascular system is a characteristic feature of
   A. Mollusca
   B. Annelida
   C. Echinodermata
   D. Arthropoda
   Correct Answer: C

4. Which phylum is characterized by the presence of jointed appendages?
   A. Mollusca
   B. Arthropoda
   C. Annelida
   D. Echinodermata
   Correct Answer: B

5. Acoelomate animals belong to which phylum?
   A. Annelida
   B. Platyhelminthes
   C. Nematoda
   D. Mollusca
   Correct Answer: B

6. Presence of flame cells for excretion is seen in
   A. Annelida
   B. Nematoda
   C. Platyhelminthes
   D. Arthropoda
   Correct Answer: C

7. Which of the following is diploblastic?
   A. Hydra
   B. Earthworm
   C. Frog
   D. Human
   Correct Answer: A

8. Which phylum includes animals with a pseudocoelom?
   A. Annelida
   B. Platyhelminthes
   C. Nematoda
   D. Mollusca
   Correct Answer: C

9. Exoskeleton made of chitin is found in
   A. Mollusca
   B. Arthropoda
   C. Annelida
   D. Echinodermata
   Correct Answer: B

10. Which of the following is a triploblastic and acoelomate animal?
    A. Hydra
    B. Ascaris
    C. Planaria
    D. Earthworm
    Correct Answer: C

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NEET PREVIOUS YEAR QUESTIONS (PYQ TYPE)

11. Notochord is present throughout life in
    A. Urochordata
    B. Vertebrata
    C. Cephalochordata
    D. Hemichordata
    Correct Answer: C
    (NEET PYQ)

12. Which one of the following is a jawless vertebrate?
    A. Shark
    B. Rohu
    C. Lamprey
    D. Tuna
    Correct Answer: C
    (NEET PYQ)

13. Which of the following animals has a three-chambered heart?
    A. Fish
    B. Frog
    C. Bird
    D. Human
    Correct Answer: B
    (NEET PYQ)

14. Crocodile differs from other reptiles in having
    A. Dry scales
    B. Oviparous nature
    C. Four-chambered heart
    D. Cold blooded body
    Correct Answer: C
    (NEET PYQ – Very Repeated)

15. Which of the following groups is correctly matched?
    A. Pisces – lungs
    B. Amphibia – gills only
    C. Reptilia – moist skin
    D. Aves – feathers
    Correct Answer: D
    (NEET PYQ)

16. Which mammalian feature is not found in other vertebrates?
    A. Hair
    B. Mammary glands
    C. Four-chambered heart
    D. Warm blooded nature
    Correct Answer: B
    (NEET PYQ)

17. Which of the following animals is uricotelic?
    A. Frog
    B. Fish
    C. Bird
    D. Earthworm
    Correct Answer: C
    (NEET PYQ)

18. The larval stage with notochord present only in tail belongs to
    A. Cephalochordata
    B. Urochordata
    C. Vertebrata
    D. Hemichordata
    Correct Answer: B
    (NEET PYQ)

19. Metameric segmentation is best exhibited by
    A. Mollusca
    B. Platyhelminthes
    C. Annelida
    D. Nematoda
    Correct Answer: C
    (NEET PYQ)

20. Which of the following is a cold-blooded vertebrate?
    A. Bird
    B. Mammal
    C. Reptile
    D. Whale
    Correct Answer: C
    (NEET PYQ)

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EXAM TREND ALERT (VERY IMPORTANT)

Arthropoda, Echinodermata, heart chambers, coelom types, and chordate features are high-frequency NEET topics. At least 2–3 questions every year come from these areas.