Animal Kingdom — NEET Notes

Basis of classification — five fundamental criteria

Animal classification does not rest on appearance. Inspite of obvious differences in structure and form, NCERT lists five fundamental features common to all animals that are used to assign them to phyla: arrangement of cells (level of organisation), body symmetry, nature of germ layers (diploblastic or triploblastic), nature of coelom, and presence or absence of a notochord. Two more — segmentation and pattern of digestive/circulatory systems — are used as supporting characters. Master these five and every phylum slots into a predictable place on the table.

Levels of organisation describe how cells are arranged. In sponges, cells form loose aggregates with some division of labour — cellular level. In coelenterates, cells performing the same function group into tissues — tissue level. In platyhelminthes, tissues group into organs — organ level. From annelids onwards, organs associate into functional systems — organ-system level, the most complex pattern. Organ systems themselves vary in complexity: the digestive system may be incomplete (single opening, like Platyhelminthes) or complete (mouth + anus); the circulatory system may be open (blood pumped out, bathes tissues directly) or closed (vessels of varying diameters — arteries, veins, capillaries).

Symmetry — asymmetrical, radial, bilateral

Sponges are mostly asymmetrical — no plane through the centre divides them into equal halves. When any plane passing through the central axis divides the organism into two identical halves, the body is radially symmetrical; coelenterates, ctenophores and adult echinoderms exhibit this plan. Animals like annelids and arthropods, in which the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry. NCERT explicitly flags one trap: echinodermata exhibit radial or bilateral symmetry depending on the stage — larvae are bilateral, adults are radial. NEET 2023 tested the symmetry axis directly by asking which phylum's adults do not show radial symmetry — Hemichordata, which is bilaterally symmetrical throughout life.

Diploblastic vs triploblastic organisation

The number of embryonic germ layers separates the early-branching phyla from the higher animals. In diploblastic animals, cells are arranged in two layers — an external ectoderm and an internal endoderm — with an undifferentiated layer, the mesoglea, sandwiched between them. Coelenterates are the classic example. In triploblastic animals, the developing embryo has a third germ layer, the mesoderm, lying between ectoderm and endoderm. Platyhelminthes to Chordates are all triploblastic. Ctenophora is also diploblastic with mesoglea, which is one of the reasons it sits alongside Coelenterata rather than higher up the tree.

Coelom — acoelomate, pseudocoelomate, coelomate

The body cavity between the gut wall and the body wall is one of the most NEET-tested distinguishing features. The body cavity that is lined by mesoderm is called a coelom; animals possessing it are coelomates — annelids, molluscs, arthropods, echinoderms, hemichordates and chordates. In some animals, the body cavity is not lined by mesoderm; instead, the mesoderm sits as scattered pouches between ectoderm and endoderm. Such a cavity is a pseudocoelom, and the animals are pseudocoelomates — aschelminthes are the only phylum in this category. Animals in which the body cavity is altogether absent are acoelomates — platyhelminthes. NEET 2020 set this as a direct one-line question: "Bilaterally symmetrical and acoelomate animals are exemplified by — Platyhelminthes."

Segmentation & the notochord

In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs. The earthworm is the textbook example — this pattern is called metameric segmentation, and the phenomenon is metamerism. NCERT places its first appearance at Annelida; Arthropoda and Chordata also show segmentation. NEET 2016 used this directly: parapodia are a feature of polychaete annelids, not arthropods — a common cross-phylum confusion.

The notochord is a mesodermally derived rod-like structure that forms on the dorsal side during embryonic development. Animals that develop a notochord at some stage of life are called chordates; those that never form this structure are non-chordates — Porifera to Echinodermata, and (despite the name) Hemichordata. Hemichordates were earlier placed as a subphylum under Chordata; NCERT now lists them as a separate phylum under non-chordata because their stomochord, a structure in the collar region, is similar to but not a true notochord.

The 11 phyla — one tree, two halves

The broad classification of Kingdom Animalia recognises eleven phyla. The first ten are non-chordates; the eleventh, Chordata, is the only chordate phylum. Their order in NCERT reflects increasing organisational complexity — from the cellular-level sponges to the organ-system, four-chambered-hearted mammals.

Phylum Porifera — sponges

Members of phylum Porifera are commonly known as sponges. They are generally marine and mostly asymmetrical animals — primitive multicellular animals with a cellular level of organisation. Sponges have a characteristic water transport (canal) system: water enters through minute pores called ostia in the body wall into a central cavity, the spongocoel, and exits through the osculum. This pathway helps in food gathering, respiratory exchange and waste removal. The spongocoel and canals are lined by flagellated cells called choanocytes or collar cells. NEET 2017 asked this directly: "In case of poriferans, the spongocoel is lined with flagellated cells called Choanocytes."

Digestion is intracellular. The body is supported by a skeleton made up of spicules or spongin fibres. Sexes are not separate — sponges are hermaphrodite, with eggs and sperms produced by the same individual. They reproduce asexually by fragmentation and sexually by formation of gametes. Fertilisation is internal and development is indirect, having a larval stage morphologically distinct from the adult. NCERT examples: Sycon (Scypha), Spongilla (Fresh water sponge), Euspongia (Bath sponge).

Phylum Coelenterata (Cnidaria)

Coelenterates are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals. The name Cnidaria is derived from the cnidoblasts or cnidocytes (containing the stinging capsules or nematocysts) present on the tentacles and the body. Cnidoblasts serve in anchorage, defence and prey capture. Cnidarians exhibit tissue level of organisation and are diploblastic. They have a central gastro-vascular cavity with a single opening, the mouth on hypostome. Digestion is both extracellular and intracellular. Some cnidarians, like corals, have a skeleton composed of calcium carbonate.

Cnidarians exhibit two basic body forms — polyp and medusa. The polyp is sessile and cylindrical (e.g., Hydra, Adamsia); the medusa is umbrella-shaped and free-swimming (e.g., Aurelia, the jellyfish). Cnidarians that exist in both forms exhibit alternation of generations (metagenesis) — polyps produce medusae asexually and medusae form polyps sexually (e.g., Obelia). NCERT examples include Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).

Phylum Ctenophora — comb jellies

Ctenophores, commonly known as sea walnuts or comb jellies, are exclusively marine, radially symmetrical, diploblastic organisms with tissue level of organisation. The body bears eight external rows of ciliated comb plates, which help in locomotion. Digestion is both extracellular and intracellular. Bioluminescence — the property of a living organism to emit light — is well-marked in ctenophores. Sexes are not separate. Reproduction takes place only by sexual means; fertilisation is external with indirect development. Examples: Pleurobrachia and Ctenoplana.

NEET 2021 baited students with the wrong claim that "comb plates present in ctenophores help in digestion" — they help in locomotion. NEET 2020 paired Ctenoplana with bioluminescence in a matching question. Ctenophores resemble cnidarians in being diploblastic, radial, and aquatic, but lack cnidoblasts and possess comb plates — that one structure tells them apart.

Phylum Platyhelminthes — flatworms

Platyhelminthes have a dorso-ventrally flattened body, hence the common name flatworms. These are mostly endoparasites found in animals including human beings. Flatworms are bilaterally symmetrical, triploblastic and acoelomate animals with organ level of organisation. Hooks and suckers are present in the parasitic forms. Some of them absorb nutrients from the host directly through their body surface.

Specialised cells called flame cells help in osmoregulation and excretion. Sexes are not separate. Fertilisation is internal and development is through many larval stages. Some members like Planaria possess a high regeneration capacity. Examples: Taenia (Tapeworm), Fasciola (Liver fluke). NEET 2023 placed Taenia opposite flame cells in a matching question, and NEET 2019 did the same with Taenia and flame cells — the diagnostic pair to remember.

Phylum Aschelminthes — roundworms

The body of aschelminthes is circular in cross-section, hence the name roundworms. They may be free-living, aquatic and terrestrial, or parasitic in plants and animals. Roundworms have organ-system level of body organisation. They are bilaterally symmetrical, triploblastic and pseudocoelomate animals. The alimentary canal is complete with a well-developed muscular pharynx. An excretory tube removes body wastes from the body cavity through the excretory pore.

Sexes are separate (dioecious) — males and females are distinct, and often females are longer than males. Fertilisation is internal and development may be direct (the young resemble the adult) or indirect. NCERT examples: Ascaris (Roundworm), Wuchereria (Filaria worm), Ancylostoma (Hookworm). NEET 2021 paired Ancylostoma with hookworm in a List-I/List-II match — a direct recall question. NEET 2021 also tested that "Round worms have organ-system level of body organization" as a correct statement.

Phylum Annelida — segmented worms

Annelids may be aquatic (marine and fresh water) or terrestrial; free-living and sometimes parasitic. They exhibit organ-system level of body organisation and bilateral symmetry. They are triploblastic, metamerically segmented and coelomate animals. Their body surface is distinctly marked out into segments or metameres — hence the phylum name Annelida (Latin annulus: little ring). They possess longitudinal and circular muscles which help in locomotion. Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming.

A closed circulatory system is present. Nephridia help in osmoregulation and excretion. The neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord. Nereis, an aquatic form, is dioecious, but earthworms and leeches are monoecious. Reproduction is sexual. NCERT examples: Nereis, Pheretima (Earthworm) and Hirudinaria (Blood-sucking leech). NEET 2023 paired Pheretima with nephridia in a direct match — a five-second answer if you know the phylum table.

Phylum Arthropoda — largest animal phylum

Arthropoda is the largest phylum of Animalia, which includes insects. Over two-thirds of all named species on earth are arthropods. They have organ-system level of organisation. They are bilaterally symmetrical, triploblastic, segmented and coelomate animals. The body of arthropods is covered by a chitinous exoskeleton. The body consists of head, thorax and abdomen. They have jointed appendages (arthros-joint, poda-appendages, hence the phylum name).

Respiratory organs are gills, book gills, book lungs or tracheal system. Circulatory system is of open type. Sensory organs include antennae, compound and simple eyes, and statocysts (balancing organs). Excretion takes place through malpighian tubules. They are mostly dioecious; fertilisation is usually internal; they are mostly oviparous; development may be direct or indirect. NCERT examples — economically important insects: Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect). Vectors: Anopheles, Culex, Aedes (Mosquitoes). Gregarious pest: Locusta (Locust). Living fossil: Limulus (King crab). NEET 2020 paired Locusta with "gregarious, polyphagous pest"; NEET 2021 paired Limulus with "living fossil" — both straight out of this list.

Phylum Mollusca — second-largest phylum

Mollusca is the second-largest animal phylum. Molluscs are terrestrial or aquatic (marine or fresh water) having an organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate animals. The body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot and visceral hump. A soft and spongy layer of skin forms a mantle over the visceral hump. The space between the hump and the mantle is the mantle cavity, in which feather-like gills are present. They have respiratory and excretory functions.

The anterior head region has sensory tentacles. The mouth contains a file-like rasping organ for feeding, called radula. Molluscs are usually dioecious and oviparous, with indirect development. NCERT examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Sea-hare), Dentalium (Tusk shell), Chaetopleura (Chiton). NEET 2021 paired Pinctada with pearl oyster; NEET 2019 paired Pila with radula — both classic mollusc identifiers.

Phylum Echinodermata — spiny-skinned

Echinoderms have an endoskeleton of calcareous ossicles — hence the name Echinodermata (spiny bodied). All are marine with organ-system level of organisation. The adult echinoderms are radially symmetrical but the larvae are bilaterally symmetrical. They are triploblastic and coelomate animals. The digestive system is complete, with the mouth on the lower (ventral) side and the anus on the upper (dorsal) side.

The most distinctive feature of echinoderms is the presence of a water vascular system, which helps in locomotion, capture and transport of food, and respiration. An excretory system is absent. Sexes are separate; reproduction is sexual; fertilisation is usually external; development is indirect with a free-swimming larva. NCERT examples: Asterias (Star fish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber) and Ophiura (Brittle star). NCERT exercise 8 has been recycled by NEET essentially verbatim: "Water vascular system is the characteristic of which group — Echinodermata."

Phylum Hemichordata — proboscis, collar, trunk

Hemichordata was earlier considered as a subphylum under phylum Chordata, but is now placed as a separate phylum under non-chordata. Hemichordates have a rudimentary structure in the collar region called the stomochord — a structure similar to but not identical with a notochord. This phylum consists of a small group of worm-like marine animals with organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate animals.

The body is cylindrical and composed of an anterior proboscis, a collar and a long trunk. Circulatory system is of open type. Respiration takes place through gills. Excretory organ is the proboscis gland. Sexes are separate; fertilisation is external; development is indirect. Examples: Balanoglossus and Saccoglossus. NEET 2017 asked which feature hemichordates share with chordates — the answer is pharyngeal gill slits. NEET 2023 used Hemichordata as the odd-one-out in a "radial symmetry NOT found in" question — because hemichordates alone among the listed phyla are bilateral.

Phylum Chordata & the vertebrate classes

Animals belonging to phylum Chordata are fundamentally characterised by four features: the presence of a notochord, a dorsal hollow nerve cord, paired pharyngeal gill slits, and a post-anal tail. They are bilaterally symmetrical, triploblastic, coelomate with organ-system level of organisation. They possess a closed circulatory system and a ventral heart.

Phylum Chordata is divided into three subphyla — Urochordata (or Tunicata), Cephalochordata and Vertebrata. Subphyla Urochordata and Cephalochordata are referred to as protochordates and are exclusively marine. In Urochordata, the notochord is present only in the larval tail; in Cephalochordata, it extends from head to tail and is persistent throughout life. Examples: Urochordata — Ascidia, Salpa, Doliolum; Cephalochordata — Branchiostoma (Amphioxus or Lancelet).

The members of subphylum Vertebrata possess a notochord during the embryonic period; in the adult it is replaced by a cartilaginous or bony vertebral column. Thus all vertebrates are chordates but all chordates are not vertebrates — NEET 2022 set this exact line as an assertion-reason question with the correct reason being "Notochord is replaced by vertebral column in the adult vertebrates." Besides basic chordate characters, vertebrates have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation, and paired appendages (fins or limbs).

Subphylum Vertebrata is split into the jawless Agnatha (one class, Cyclostomata) and the jawed Gnathostomata (super-classes Pisces and Tetrapoda). The five classical NEET classes follow below.

Class Cyclostomata

All living members of class Cyclostomata are ectoparasites on some fishes. They have an elongated body bearing 6–15 pairs of gill slits for respiration. Cyclostomes have a sucking and circular mouth without jaws. Their body is devoid of scales and paired fins. Cranium and vertebral column are cartilaginous. Circulation is closed. Cyclostomes are marine but migrate to fresh water for spawning; after spawning they die within a few days, and their larvae, after metamorphosis, return to the ocean. Examples: Petromyzon (Lamprey) and Myxine (Hagfish). NEET 2020 paired "6–15 pairs of gill slits" with cyclostomes — a direct lift from NCERT.

Class Chondrichthyes — cartilaginous fishes

Chondrichthyes are marine animals with streamlined body and cartilaginous endoskeleton. Mouth is located ventrally. Notochord is persistent throughout life. Gill slits are separate and without operculum (gill cover). The skin is tough, containing minute placoid scales. Teeth are modified placoid scales, backwardly directed. Their jaws are very powerful; these animals are predaceous. Due to the absence of air bladder, they have to swim constantly to avoid sinking.

Heart is two-chambered (one auricle, one ventricle). Some have electric organs (e.g., Torpedo) and some possess a poison sting (e.g., Trygon). They are cold-blooded (poikilothermous). Sexes are separate; in males pelvic fins bear claspers. They have internal fertilisation and many of them are viviparous. Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray). NEET 2016 set "cartilaginous endoskeleton" as a feature that always holds for Chondrichthyes; NEET 2020 paired Trygon with poison sting.

Class Osteichthyes — bony fishes

Osteichthyes includes both marine and fresh-water fishes with a bony endoskeleton. Body is streamlined. Mouth is mostly terminal. They have four pairs of gills covered by an operculum on each side. Skin is covered with cycloid/ctenoid scales. Air bladder is present, which regulates buoyancy. Heart is two-chambered. They are cold-blooded. Sexes are separate; fertilisation is usually external; they are mostly oviparous and development is direct.

NCERT examples — Marine: Exocoetus (Flying fish), Hippocampus (Sea horse); Freshwater: Labeo (Rohu), Catla (Katla), Clarias (Magur); Aquarium: Betta (Fighting fish), Pterophyllum (Angel fish). NEET 2020 matched Osteichthyes with "air bladder" — its diagnostic feature.

Class Amphibia

As the name indicates (Gr. amphi: dual, bios: life), amphibians can live in aquatic as well as terrestrial habitats. Most have two pairs of limbs. Body is divisible into head and trunk; a tail may be present in some. The amphibian skin is moist (without scales). The eyes have eyelids. A tympanum represents the ear. Alimentary canal, urinary and reproductive tracts open into a common chamber called the cloaca, which opens to the exterior. Respiration is by gills, lungs and through skin. The heart is three-chambered (two auricles and one ventricle). These are cold-blooded animals; sexes are separate; fertilisation is external; they are oviparous and development is indirect. NCERT examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless amphibia).

Class Reptilia

The class name refers to their creeping or crawling mode of locomotion (Latin repere or reptum, to creep or crawl). They are mostly terrestrial and their body is covered by dry and cornified skin, epidermal scales or scutes. They do not have external ear openings; the tympanum represents the ear. Limbs, when present, are two pairs. Heart is usually three-chambered, but four-chambered in crocodiles. Reptiles are poikilotherms. Snakes and lizards shed their scales as skin cast. Sexes are separate; fertilisation is internal; they are oviparous and development is direct. NCERT examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), Alligator, Hemidactylus (Wall lizard), poisonous snakes — Naja (Cobra), Bangarus (Krait), Vipera (Viper). NEET 2018 used Chelone as the non-homeotherm (reptile) in a four-option set.

Class Aves — birds

The characteristic features of Aves (birds) are the presence of feathers and the ability to fly (except flightless birds like the Ostrich). They possess a beak. The forelimbs are modified into wings. Hind limbs generally have scales and are modified for walking, swimming or clasping the tree branches. Skin is dry without glands except the oil gland at the base of the tail. Endoskeleton is fully ossified (bony), and the long bones are hollow with air cavities (pneumatic). The digestive tract of birds has additional chambers, the crop and gizzard. Heart is completely four-chambered. They are warm-blooded (homoiothermous). Respiration is by lungs; air sacs connected to lungs supplement respiration. Sexes are separate; fertilisation is internal; they are oviparous and development is direct.

NCERT examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture). NEET 2018 and 2022 both asked about the crop-and-gizzard combination — the diagnostic Aves identifier. NEET 2021 used pneumatic bones to single out Neophron against three non-bird genera.

Class Mammalia

Mammals are found in a variety of habitats — polar ice caps, deserts, mountains, forests, grasslands and dark caves. Some have adapted to fly or to live in water. The most unique mammalian characteristic is the presence of milk-producing glands (mammary glands) by which the young ones are nourished. They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying. The skin of mammals is unique in possessing hair. External ears or pinnae are present. Different types of teeth are present in the jaw. Heart is four-chambered. They are homoiothermous. Respiration is by lungs. Sexes are separate; fertilisation is internal. They are viviparous with few exceptions, and development is direct.

NCERT 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). NEET 2023 asked the unique mammalian characteristics: hairs, pinna and mammary glands — option 3, against three distractors.