Classification of animals: phyla and classes

The broad classification of animals

In plain words: once you can answer six questions about an animal (cells, symmetry, germ layers, body cavity, segments, notochord), most phyla fall into place. Each phylum then has a few distinctive features of its own, such as the collar cells of sponges or the stinging cells of cnidarians. The accounts below follow NCERT’s broad summaries; where a summary is not universal, the qualification is stated explicitly.

Phylum Porifera

Sponges are generally marine, mostly asymmetrical, primitive multicellular animals with a cellular level of organisation (page 40).

• Canal system: water enters through minute pores, the ostia, into a central cavity, the spongocoel, and leaves through the osculum. This water transport helps in food gathering, respiratory exchange and removal of waste.
• Digestion: intracellular. Skeleton: spicules or spongin fibres.
• Reproduction: hermaphrodite (eggs and sperms from the same individual); asexual by fragmentation, sexual by gametes; fertilisation internal; development indirect, with a larva morphologically distinct from the adult.
• Examples: Sycon (Scypha), Spongilla (fresh-water sponge), Euspongia (bath sponge).

Choanocytes or collar cells line the spongocoel and the canals.

NCERT Biology (Class 11), Section 4.2.1, page 40

Phylum Coelenterata (Cnidaria)

Coelenterates are aquatic, mostly marine, sessile or free-swimming and radially symmetrical (page 41). The name cnidaria comes from the cnidoblasts or cnidocytes on the tentacles and body, which contain stinging capsules called nematocysts; cnidoblasts are used for anchorage, defence and the capture of prey.

Cnidarians exhibit tissue level of organisation and are diploblastic.

NCERT Biology (Class 11), Section 4.2.2, page 41

• A central gastro-vascular cavity with a single opening, the mouth on the hypostome; digestion extracellular and intracellular.
• Some, such as corals, have a skeleton of calcium carbonate.
• Two body forms: the polyp, sessile and cylindrical (Hydra, Adamsia), and the medusa, umbrella-shaped and free-swimming (Aurelia, jelly fish). Those that exist in both forms show alternation of generations (metagenesis): polyps produce medusae asexually and medusae form polyps sexually, as in Obelia.
• Examples: Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula (sea-pen), Gorgonia (sea-fan), Meandrina (brain coral).

Phylum Ctenophora

Ctenophores, the sea walnuts or comb jellies, are exclusively marine, radially symmetrical and diploblastic, with tissue level of organisation (page 42).

• The body bears eight external rows of ciliated comb plates, which help in locomotion.
• Digestion is both extracellular and intracellular.
• Sexes are not separate; reproduction only by sexual means; fertilisation external; development indirect.
• Examples: Pleurobrachia, Ctenoplana.

Bioluminescence (the property of a living organism to emit light) is well-marked in ctenophores.

NCERT Biology (Class 11), Section 4.2.3, page 42

Phylum Platyhelminthes

Flatworms have a dorso-ventrally flattened body and are mostly endoparasites of animals, including human beings (page 42).

• Bilaterally symmetrical, triploblastic and acoelomate, with an organ level of organisation.
• Parasitic forms have hooks and suckers; some absorb nutrients from the host directly through the body surface.
• Sexes are not separate; fertilisation internal; development through many larval stages.
• Some, such as Planaria, have high regeneration capacity.
• Examples: Taenia (tapeworm), Fasciola (liver fluke).

Specialised cells called flame cells help in osmoregulation and excretion.

NCERT Biology (Class 11), Section 4.2.4, page 42

Phylum Aschelminthes

The body is circular in cross-section, hence the name roundworms (page 43). They may be free-living, aquatic and terrestrial, or parasitic in plants and animals.

• Organ-system level; bilaterally symmetrical, triploblastic and pseudocoelomate.
• Alimentary canal complete, with a well-developed muscular pharynx.
• Sexes are separate (dioecious); females are often longer than males; fertilisation internal; development direct (the young resemble the adult) or indirect.
• Examples: Ascaris (roundworm), Wuchereria (filaria worm), Ancylostoma (hookworm).

An excretory tube removes body wastes from the body cavity through the excretory pore.

NCERT Biology (Class 11), Section 4.2.5, page 43

Phylum Annelida

Annelids may be aquatic (marine and fresh water) or terrestrial; free-living and sometimes parasitic (page 43). They have an organ-system level of organisation and bilateral symmetry, and are triploblastic, metamerically segmented and coelomate. The body surface is marked out into segments or metameres, hence the name (Latin, annulus: little ring).

They possess longitudinal and circular muscles which help in locomotion.

NCERT Biology (Class 11), Section 4.2.6, page 43

• Aquatic annelids such as Nereis have lateral appendages, parapodia, for swimming.
• Closed circulatory system; nephridia for osmoregulation and excretion.
• Nervous system of paired ganglia connected by lateral nerves to a double ventral nerve cord.
• Nereis is dioecious, but earthworms and leeches are monoecious; reproduction is sexual.
• Examples: Nereis, Pheretima (earthworm), Hirudinaria (blood-sucking leech).

Phylum Arthropoda

This is the largest phylum of Animalia and includes the insects: NCERT cites over two-thirds of named species as arthropods (page 44), a textbook estimate rather than a current census.

• Organ-system level; bilaterally symmetrical, triploblastic, segmented and coelomate.
• Body covered by a chitinous exoskeleton; head, thorax and abdomen in the familiar insect body plan (other arthropods combine or modify these regions); jointed appendages (arthros: joint; poda: appendages).
• Respiration by gills, book gills, book lungs or a tracheal system; open circulatory system.
• Sense organs: antennae, compound and simple eyes, statocysts (balancing organs).
• Mostly dioecious; fertilisation usually internal; mostly oviparous; development direct or indirect.
• Examples: economically important insects Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect); vectors Anopheles, Culex and Aedes (mosquitoes); gregarious pest Locusta (locust); living fossil Limulus (king crab).

Excretion takes place through malpighian tubules.

NCERT Biology (Class 11), Section 4.2.7, page 44

This excretory summary is not universal. NCERT Chapter 16 specifies Malpighian tubules in most insects, but antennal or green glands in crustaceans such as prawns.

Phylum Mollusca

The second largest animal phylum: terrestrial or aquatic, with an organ-system level of organisation; bilaterally symmetrical, triploblastic and coelomate (page 44).

• Body usually covered by a calcareous shell (NCERT Table 4.2, page 51: “usually present”; beyond the NCERT text, it is internal in Sepia and Loligo and absent in Octopus); unsegmented, with a distinct head, muscular foot and visceral hump.
• A soft, spongy layer of skin, the mantle, covers the visceral hump; the mantle cavity between them holds feather-like gills with respiratory and excretory functions.
• The anterior head has sensory tentacles.
• Usually dioecious and oviparous, with indirect development (page 45).
• Examples: Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish), Aplysia (sea-hare), Dentalium (tusk shell), Chaetopleura (chiton).

The mouth contains a file-like rasping organ for feeding, called radula.

NCERT Biology (Class 11), Section 4.2.8, page 44

Beyond NCERT’s general radula description, bivalves such as oysters lack a radula; the Australian Antarctic Division’s invertebrate identification guide states this exception.

Phylum Echinodermata

Echinoderms have an endoskeleton of calcareous ossicles, hence the name (spiny bodied). All are marine, with organ-system level of organisation (page 45).

• Adults are radially symmetrical but larvae are bilaterally symmetrical; triploblastic and coelomate.
• NCERT describes a complete gut with mouth below and anus above. Beyond this generalisation, some echinoderms lack an anus: the Australian Museum documents this in brittle stars. “Complete” is therefore not a universal phylum character.
• An excretory system is absent; sexes separate; fertilisation usually external; development indirect, with a free-swimming larva.
• Examples: Asterias (star fish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), Ophiura (brittle star).

The most distinctive feature of echinoderms is the presence of water vascular system which helps in locomotion, capture and transport of food and respiration.

NCERT Biology (Class 11), Section 4.2.9, page 45

Phylum Hemichordata

Hemichordata was earlier considered a subphylum of Chordata but is now a separate phylum under non-chordata (page 45).

Hemichordates have a rudimentary structure in the collar region called stomochord, a structure similar to notochord.

NCERT Biology (Class 11), Section 4.2.10, page 45

• A small group of worm-like marine animals; organ-system level; bilaterally symmetrical, triploblastic and coelomate.
• Cylindrical body of an anterior proboscis, a collar and a long trunk.
• Open circulatory system; respiration through gills; the excretory organ is the proboscis gland.
• Sexes separate; fertilisation external; development indirect.
• Examples: Balanoglossus, Saccoglossus.

Phylum Chordata

Chordates are characterised by a notochord, a dorsal hollow nerve cord, pharyngeal structures and a post-anal tail, present at least during development. NCERT names paired pharyngeal gill slits (pages 45–46); beyond this summary, human embryos form pharyngeal pouches, not functional fish-like gills, as described in NCBI’s embryology account. Chordates are triploblastic coelomates with organ-system organisation. NCERT lists closed circulation; treat this as the school summary, since tunicate circulation varies and does not fit a universal vertebrate-like model.

NCERT Table 4.1 (page 46) compares chordates and non-chordates:

• Notochord: present in chordates; absent in non-chordates.
• Central nervous system: dorsal, hollow and single in chordates; ventral, solid and double in non-chordates.
• Pharynx: perforated by gill slits in chordates; gill slits absent in non-chordates.
• Heart: ventral in chordates; dorsal (if present) in non-chordates.
• Post-anal tail: present in chordates; absent in non-chordates.

The table is a simplified comparison, not five universal diagnostic tests. Non-chordate nervous systems vary: Hydra has a nerve net (NCERT Chapter 18, page 231), for example. Hemichordates also have pharyngeal gill slits, as noted below, and adult chordates need not retain all embryonic features.

Subphyla (page 46). Chordata is divided into Urochordata (Tunicata), Cephalochordata and Vertebrata. Urochordata and Cephalochordata, the protochordates, are exclusively marine.

• Urochordata: the notochord is present only in the larval tail; examples Ascidia, Salpa, Doliolum.
• Cephalochordata: the notochord extends from head to tail and persists throughout life; example Branchiostoma (Amphioxus or lancelet).
• Vertebrata: the notochord is present in the embryo and is replaced by a cartilaginous or bony vertebral column in the adult. Vertebrates also have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation, and paired appendages, fins or limbs.

Thus all vertebrates are chordates but all chordates are not vertebrates.

NCERT Biology (Class 11), Section 4.2.11, page 46

Classes of vertebrates

Vertebrata is divided into two divisions, Agnatha (lacking jaws) and Gnathostomata (bearing jaws); Gnathostomata has two super classes, Pisces (bearing fins) and Tetrapoda (bearing limbs) (page 47).

Class Cyclostomata

NCERT describes all living cyclostomes as ectoparasites on some fishes, with an elongated body bearing 6–15 pairs of gill slits for respiration (page 47).

Their body is devoid of scales and paired fins.

NCERT Biology (Class 11), Section 4.2.11, page 47

• A sucking, circular mouth without jaws; cartilaginous cranium and vertebral column; closed circulation.
• NCERT says they are marine but migrate to fresh water for spawning, die within a few days of spawning, and their larvae return to the ocean after metamorphosis.
• Examples: Petromyzon (lamprey), Myxine (hagfish).

Beyond the NCERT text: these statements fit many lampreys, but not every cyclostome. Hagfishes are marine scavengers that never enter fresh water, and some lampreys are not parasitic. For NEET, answer from NCERT’s description.

Class Chondrichthyes

Marine fishes with a streamlined body and a cartilaginous endoskeleton (page 47).

• Mouth ventral; NCERT says the notochord persists throughout life; gill slits separate and without an operculum.
• Tough skin with minute placoid scales; the teeth are modified placoid scales, directed backwards; very powerful jaws; predaceous.
• Heart two-chambered (one auricle, one ventricle); some have electric organs (Torpedo) or a poison sting (Trygon); cold-blooded (page 48).
• Sexes separate; males have claspers on the pelvic fins; fertilisation internal; many are viviparous.
• Examples: Scoliodon (dog fish), Pristis (saw fish), Carcharodon (great white shark), Trygon (sting ray).

Due to the absence of air bladder, they have to swim constantly to avoid sinking.

NCERT Biology (Class 11), Section 4.2.11, page 47

Beyond the quoted NCERT statement, constant swimming is not required in every cartilaginous fish: some sharks and rays rest on the seabed. NOAA distinguishes such bottom-resting forms from continuously swimming forms; lack of a swim bladder is not proof that the animal never stops swimming.

Class Osteichthyes

Marine and fresh-water fishes with a bony endoskeleton and a streamlined body (page 48).

• Mouth mostly terminal; four pairs of gills covered by an operculum on each side; cycloid or ctenoid scales.
• Heart two-chambered; cold-blooded; sexes separate; fertilisation usually external; mostly oviparous, with direct development.
• Examples: marine Exocoetus (flying fish), Hippocampus (sea horse); fresh-water Labeo (rohu), Catla (katla), Clarias (magur); aquarium Betta (fighting fish), Pterophyllum (angel fish).

Air bladder is present which regulates buoyancy.

NCERT Biology (Class 11), Section 4.2.11, page 48

Beyond the NCERT Biology text: by changing the gas in its air bladder, a fish changes its overall density until the upthrust of the water, which arises because pressure increases with depth, balances its weight, so it can stay at a depth without swimming.

Class Amphibia

Amphibians can live in aquatic as well as terrestrial habitats (Greek amphi: dual; bios: life) (page 48).

• Most have two pairs of limbs; body of head and trunk; a tail in some.
• Eyes with eyelids; a tympanum represents the ear.
• The alimentary canal, urinary and reproductive tracts open into a common cloaca.
• NCERT’s frog-like summary gives respiration by gills, lungs and skin; a three-chambered heart; external fertilisation, oviparity and indirect development. Beyond NCERT, amphibian reproduction varies: a comparative study of salamanders in Scientific Reports documents both internal and external fertilisation.
• Examples: Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra (salamander), Ichthyophis (limbless amphibian).

The amphibian skin is moist (without scales).

NCERT Biology (Class 11), Section 4.2.11, page 48

Class Reptilia

The name refers to their creeping or crawling locomotion (Latin repere, reptum: to creep or crawl). Reptiles are mostly terrestrial, with a dry, cornified skin of epidermal scales or scutes (page 49).

• NCERT states “No external ear openings”, but this is not universal: Fauna of Australia notes that most lizards have them, while snakes do not. Limbs, when present, are generally two pairs.
• Poikilotherms; snakes and lizards shed their scales as a skin cast.
• Sexes separate; fertilisation internal; development direct. NCERT gives oviparity as the summary, but some reptiles bear live young, as the Australian Museum notes.
• Examples: Chelone (turtle), Testudo (tortoise), Chamaeleo (chameleon), Calotes (garden lizard), Crocodylus (crocodile), Alligator, Hemidactylus (wall lizard); venomous snakes (NCERT’s “poisonous”) Naja (cobra), Bungarus (krait), Vipera (viper).

Heart is usually three-chambered, but four-chambered in crocodiles.

NCERT Biology (Class 11), Section 4.2.11, page 49

NCERT prints three of these genus names wrongly or in an outdated form (page 49): the krait is Bungarus, not “Bangarus”; the accepted spelling of the crocodile genus is Crocodylus, not “Crocodilus”; and the chameleon genus is Chamaeleo, which NCERT writes as “Chameleon (Tree lizard)”. Exam questions may still use NCERT’s spellings.

Class Aves

Birds have feathers, a beak, and forelimbs modified into wings; most can fly, except flightless birds such as the ostrich (page 49).

• Hind limbs generally scaly, modified for walking, swimming or clasping branches.
• Dry skin without glands except the oil gland at the base of the tail.
• Endoskeleton fully ossified; long bones hollow with air cavities (pneumatic).
• Heart completely four-chambered; warm-blooded (homoiothermous); respiration by lungs, supplemented by air sacs (pages 49–50).
• Sexes separate; fertilisation internal; oviparous, with direct development.
• Examples: Corvus (crow), Columba (pigeon), Psittacula (parrot), Struthio (ostrich), Pavo (peacock), Aptenodytes (penguin), Neophron (vulture).

The digestive tract of birds has additional chambers, the crop and gizzard.

NCERT Biology (Class 11), Section 4.2.11, page 49

Class Mammalia

Mammals live in habitats from polar ice caps to deserts, mountains, forests, grasslands and dark caves; some fly or live in water (page 50).

The most unique mammalian characteristic is the presence of milk producing glands (mammary glands) by which the young ones are nourished.

NCERT Biology (Class 11), Section 4.2.11, page 50

• Two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying.
• Skin with hair; external ears (pinnae); different types of teeth; four-chambered heart; homoiothermous; lungs (page 51).
• Sexes separate; fertilisation internal; viviparous with few exceptions; development direct.
• Examples: oviparous Ornithorhynchus (platypus); viviparous Macropus (kangaroo), Pteropus (flying fox), Camelus (camel), Macaca (monkey), Rattus (rat), Canis (dog), Felis (cat), Elephas (elephant), Equus (horse), Delphinus (common dolphin), Balaenoptera (blue whale), Panthera tigris (tiger), Panthera leo (lion).

Where NCERT needs care

• Platyhelminthes: NCERT Sections 4.1.1 and 4.2.4 give them an organ level of organisation, but Table 4.2 (page 51) lists “Organ & Organ-system”. Use organ level, as in the text.
• The notochord of cartilaginous fishes: page 46 says the vertebrate notochord is replaced by a vertebral column in the adult, yet page 47 says it persists throughout life in Chondrichthyes. Beyond the NCERT text: the notochord and vertebral structures must be distinguished; persistence and replacement vary across vertebrate groups. For an NCERT-specific comparison, identify which of its two summaries the question is asking about.
• The Summary (page 52): it adds “The body is covered with external skeleton made of chitin” directly after the molluscs, but the chitinous exoskeleton belongs to the arthropods (page 44); the mollusc body is covered by a calcareous shell. It also calls cyclostomes “the most primitive chordates”; the protochordates are chordates too, so use “living jawless vertebrates” rather than treating all modern animals as a ladder of increasing primitiveness.
• Gill slits in hemichordates: NCERT Table 4.1 (page 46) says gill slits are absent in non-chordates, yet hemichordates, placed under non-chordata, respire through gills (page 45). Beyond the NCERT text: these are pharyngeal gill slits, so gill slits alone do not mark a chordate.
• Beyond the NCERT text: Antedon, which NCERT calls a sea lily, is a feather star; sea lilies and feather stars are both crinoids.

NCERT Table 4.2 in brief

NCERT Table 4.2 (page 51) sets the salient features of all the phyla side by side:

• Coelom: absent from Porifera to Platyhelminthes; a pseudocoelom in Aschelminthes; a coelom from Annelida to Chordata.
• Segmentation: only in Annelida, Arthropoda and Chordata.
• Digestive system: absent in Porifera; incomplete in Coelenterata, Ctenophora and Platyhelminthes; complete from Aschelminthes on.
• Circulatory system from Annelida on; respiratory system from Arthropoda on.

Solved examples

Common mistakes to avoid

Quick revision summary

• Porifera: cellular level; canal system (ostia, spongocoel, osculum); choanocytes; intracellular digestion; hermaphrodite.
• Coelenterata: tissue level, diploblastic, radial; cnidoblasts; polyp and medusa; metagenesis (Obelia).
• Ctenophora: eight rows of comb plates; bioluminescence. Platyhelminthes: flat, acoelomate; hooks, suckers, flame cells.
• Aschelminthes: round, pseudocoelomate, dioecious. Annelida: metameric segments, nephridia, parapodia in Nereis.
• Arthropoda: largest phylum; chitinous exoskeleton, jointed appendages, open circulation; Malpighian tubules in most insects, green glands in prawns.
• Mollusca: mantle, usually shell, unsegmented; radula absent in bivalves. Echinodermata: water vascular system; radial adults, bilateral larvae.
• Hemichordata: proboscis, collar, trunk; stomochord. Chordata: notochord, dorsal hollow nerve cord, gill slits, post-anal tail.
• Vertebrates: Cyclostomata (jawless); Chondrichthyes and Osteichthyes (fins); Amphibia, Reptilia, Aves and Mammalia (limbs).

Questions students ask

What is the difference between chordates and non-chordates?

Chordates have a notochord, at least in the embryo or larva, while non-chordates never form one; NCERT Table 4.1 adds four more differences. The table summarises a dorsal hollow nerve cord, pharyngeal gill slits, ventral heart and post-anal tail in chordates, but these are developmental/general patterns rather than an adult checklist. Its non-chordate contrast is also simplified: nerve nets and other arrangements occur, and hemichordates have pharyngeal gill slits. Chordata is a single phylum, from the protochordates to fishes and mammals; the non-chordates are the ten phyla from Porifera to Hemichordata, such as sponges, earthworms, insects and the star fish; hemichordates, which respire through gills, are the exception.

Are non-chordates the same as invertebrates?

Non-chordates and invertebrates are not quite the same: every non-chordate is an invertebrate, an animal without a vertebral column, but some chordates are invertebrates too. The protochordates, Urochordata such as Ascidia, Salpa and Doliolum and Cephalochordata such as Branchiostoma, have a notochord, so they are chordates, yet they never form a vertebral column. ‘Invertebrate’ is an informal grouping, not a taxon NCERT uses: NCERT divides animals into chordates and non-chordates, and only the subphylum Vertebrata forms a vertebral column.

How many classes of vertebrates are there?

NCERT Class 11 divides vertebrates into seven classes: Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia. The ‘five classes’ often quoted, fishes, amphibians, reptiles, birds and mammals, count all fishes as a single class; NCERT instead puts the jawless cyclostomes alone in the division Agnatha and splits the jawed fishes into the cartilaginous Chondrichthyes and the bony Osteichthyes, its super class Pisces. For NEET, use NCERT’s seven.

How do you remember the order of the animal phyla in NCERT?

NCERT describes the eleven animal phyla in this order: Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata and Chordata; a memory aid for it, not from NCERT, is ‘Pretty Cats Can Purr And Act Annoyed; Mice Escape Hungry Cats’. The order broadly follows the bases of classification, from the cellular sponges and diploblastic coelenterates and ctenophores to the acoelomate, pseudocoelomate and coelomate phyla. As some initials repeat, pin a signature feature on each: collar cells, cnidoblasts, comb plates, flame cells, pseudocoelom, metameres, jointed appendages, radula, water vascular system, stomochord, notochord.

Is the animal kingdom chapter in the NEET 2026 syllabus?

The animal kingdom chapter is in the NEET (UG) 2026 syllabus: Biology Unit 1, Diversity in Living World, lists “Salient features and classification of animals-nonchordate up to phyla level and chordate up to classes level (three to five salient features and at least two examples)”. So for each non-chordate phylum and each chordate class, learn three to five salient features and at least two NCERT examples, as this page gives them. The fundamental features used to classify animals, such as symmetry and coelom, are in NCERT Section 4.1, Basis of Classification (pages 37–39).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Match Physalia, Pennatula, Gorgonia and Meandrina with sea-fan, brain coral, Portuguese man-of-war and sea-pen.

   Show answerHide answer

   Physalia, Portuguese man-of-war; Pennatula, sea-pen; Gorgonia, sea-fan; Meandrina, brain coral (NCERT page 41).

2. Q2. Name the three parts of the body of a hemichordate.

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   Proboscis, collar and trunk (NCERT page 45).

3. Q3. In which subphylum is the notochord present only in the larval tail?

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   Urochordata (Tunicata) (NCERT page 46).

4. Q4. Give the excretory organs of annelids, most insects and prawns.

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   Nephridia in annelids; Malpighian tubules in most insects; antennal or green glands in prawns (NCERT Chapter 4, pages 43–44, refined by Chapter 16).