NEET BiologyNCERT Class 11Chapter 4

Animal Kingdom: NEET notes

More than a million animal species have been described, so the chapter first sets up the few body features used to sort them: level of organisation, symmetry, germ layers, body cavity, segmentation and notochord. It then walks through the eleven phyla from Porifera to Chordata, and inside Chordata through the classes from Cyclostomata to Mammalia, giving each group's defining features and standard examples.

What NEET asks

NEET leans heavily on this chapter. It asks which phylum shows a given feature (choanocytes, cnidoblasts, comb plates, flame cells, nephridia, malpighian tubules, radula, water vascular system, proboscis gland), matches genera with their common names and groups, and tests Table 4.1 (chordates versus non-chordates) and Table 4.2 (the phylum summary). Marks are lost on the exceptions: echinoderm larvae are bilateral, crocodiles have a four-chambered heart, Nereis is dioecious while earthworms and leeches are monoecious, the platypus lays eggs, and Hemichordata is now a non-chordate phylum.

1. Levels of organisation

NCERT §4.1, §4.1.1

Animals are grouped by features they share in spite of differences in form: how cells are arranged, body symmetry, the kind of coelom, and the pattern of digestive, circulatory and reproductive systems.
Every animal is multicellular, but cells are organised at different levels. Sponges have the cellular level: cells form loose aggregates with some division of labour among them.
Coelenterates have the tissue level: cells doing the same job are grouped into tissues.
Platyhelminthes show the organ level, where tissues are grouped into organs, each specialised for one function.
Annelids, arthropods, molluscs, echinoderms and chordates show the organ-system level: organs work together as systems, each serving one physiological function.
An incomplete digestive system has a single opening that acts as both mouth and anus, as in Platyhelminthes. A complete one has two openings, a mouth and an anus.
In an open circulatory system the heart pumps blood out so that it bathes cells and tissues directly. In a closed system blood stays inside vessels of different diameters: arteries, veins and capillaries.

2. Symmetry and germ layers

NCERT §4.1.2, §4.1.3

Sponges are mostly asymmetrical: no plane through the centre cuts them into two equal halves.
Radial symmetry: any plane passing through the central axis gives two identical halves. Coelenterates, ctenophores and adult echinoderms are built this way.
Bilateral symmetry: only one plane divides the body into identical left and right halves, as in annelids and arthropods.
Diploblastic animals develop from two embryonic layers, an outer ectoderm and an inner endoderm, with an undifferentiated mesoglea between them. Coelenterates are the example.
Triploblastic animals have a third germ layer, the mesoderm, between ectoderm and endoderm. Every phylum from Platyhelminthes to Chordata is triploblastic.
Ctenophores, like cnidarians, are diploblastic; the diploblastic phyla are the two with radial symmetry and tissue-level organisation.

3. Coelom, segmentation and notochord

NCERT §4.1.4–§4.1.6, Figure 4.4

Classification leans heavily on the body cavity: is there a space between the gut and the body wall, and what lines it?
A coelom is a body cavity lined by mesoderm. Coelomates: annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
In a pseudocoelom the cavity is not lined by mesoderm; the mesoderm lies as scattered pouches between ectoderm and endoderm. Aschelminthes are pseudocoelomates.
Acoelomates have no body cavity at all; Platyhelminthes are the example.
Metameric segmentation (metamerism): the body is divided externally and internally into segments, with at least some organs repeated in series, as in the earthworm.
The notochord is a rod-like structure of mesodermal origin formed on the dorsal side of the embryo. Animals that form it are chordates; those that do not (Porifera to Echinodermata) are non-chordates.
Figure 4.4 sorts the phyla step by step: cellular level (Porifera) versus tissue or higher; radial versus bilateral symmetry; then no coelom, pseudocoelom or true coelom; then segmented or not; then notochord or not.

4. Sponges, cnidarians and comb jellies

NCERT §4.2.1–§4.2.3

Porifera (sponges) are mostly marine, mostly asymmetrical, primitive multicellular animals at the cellular level. Water enters through tiny pores called ostia, reaches the central spongocoel and leaves through the osculum; this canal system serves food gathering, gas exchange and waste removal.
The spongocoel and the canals are lined by choanocytes (collar cells), and food is digested inside cells. Spicules or spongin fibres form the skeleton. Sponges are hermaphrodite, reproduce asexually by fragmentation and sexually by gametes, and have internal fertilisation with a distinct larval stage. Examples: Sycon (also called Scypha), Spongilla (a freshwater sponge), Euspongia (the bath sponge).
Coelenterata (Cnidaria) are aquatic, mostly marine, sessile or free-swimming and radially symmetrical. They are named after cnidoblasts (cnidocytes), cells on the tentacles and body that hold stinging nematocysts used for anchorage, defence and catching prey.
Cnidarians are diploblastic with tissue-level organisation. They have a gastro-vascular cavity with a single opening, the mouth, on the hypostome. Digestion is both extracellular and intracellular. Corals have a calcium carbonate skeleton.
Two body forms occur: the polyp, sessile and cylindrical (Hydra, Adamsia), and the medusa, umbrella-shaped and free-swimming (Aurelia). Where both occur there is metagenesis: polyps give rise to medusae asexually and medusae give rise to polyps sexually, as in Obelia.
Cnidarian examples: the sea anemone Adamsia, the Portuguese man-of-war Physalia, the brain coral Meandrina, the sea-pen Pennatula and the sea-fan Gorgonia.
Ctenophora (sea walnuts, comb jellies) are exclusively marine, radially symmetrical, diploblastic and at the tissue level. Eight external rows of ciliated comb plates drive locomotion. Digestion is extracellular and intracellular, and bioluminescence is well marked.
Ctenophores are hermaphrodite and reproduce only sexually, with external fertilisation and indirect development. Examples: Pleurobrachia, Ctenoplana.

5. Flatworms, roundworms and annelids

NCERT §4.2.4–§4.2.6

Platyhelminthes (flatworms) have a body flattened from top to bottom (dorso-ventrally). Most are endoparasites of animals, humans included. They are bilateral, triploblastic, acoelomate and at the organ level.
Parasitic flatworms have hooks and suckers, and some take in food from the host straight through the body surface. Flame cells handle osmoregulation and excretion. They are hermaphrodite with internal fertilisation and many larval stages. Planaria regenerates very well. Examples: Taenia (tapeworm), Fasciola (liver fluke).
Aschelminthes (roundworms) are circular in cross-section. They may be free-living in water or soil, or parasites of plants and animals. They are bilateral, triploblastic, pseudocoelomate and at the organ-system level.
The roundworm alimentary canal is complete, and its pharynx is muscular and well developed. Wastes leave the body cavity through an excretory tube that opens at the excretory pore. Sexes are separate (dioecious) and females are often longer than males. Fertilisation is internal; development may be direct or indirect. Examples: the roundworm Ascaris, the hookworm Ancylostoma and the filaria worm Wuchereria.
Annelida live in water (marine and fresh) or on land, are free-living and sometimes parasitic. They are bilateral, triploblastic, coelomate, organ-system level and metamerically segmented; the name comes from the Latin annulus, a little ring.
Longitudinal and circular muscles move annelids. Nereis, an aquatic form, swims with lateral appendages called parapodia. Circulation is closed. Nephridia handle osmoregulation and excretion. Paired ganglia joined by lateral nerves to a double ventral nerve cord form the nervous system.
Earthworms and leeches are monoecious (both sexes in one animal), whereas Nereis is dioecious; all reproduce sexually. Examples: Pheretima (earthworm), Hirudinaria (a blood-sucking leech) and Nereis.

6. Arthropods and molluscs

NCERT §4.2.7, §4.2.8

Arthropoda, the phylum that holds the insects, is the largest in the animal kingdom: more than two-thirds of the species named on earth belong to it.
Arthropods are bilateral, triploblastic, segmented, coelomate and at the organ-system level. A chitinous exoskeleton covers a body of head, thorax and abdomen, with jointed appendages (arthros, joint; poda, appendages).
They breathe with gills, book gills, book lungs or a tracheal system. Circulation is open. Sense organs include antennae, compound and simple eyes, and statocysts for balance. Malpighian tubules carry out excretion.
Most are dioecious and oviparous; fertilisation is usually internal and development direct or indirect.
Arthropod examples by role. Useful insects: the honey bee Apis, the silkworm Bombyx and the lac insect Laccifer. Disease vectors: the mosquitoes Anopheles, Culex and Aedes. A gregarious pest: the locust Locusta. A living fossil: the king crab Limulus.
Mollusca is the second largest animal phylum. Molluscs live on land or in water, marine or fresh, and are bilateral, triploblastic, coelomate and at the organ-system level.
The unsegmented body has a distinct head, a muscular foot and a visceral hump, and is covered by a calcareous shell. A soft, spongy skin fold, the mantle, covers the hump; the mantle cavity between them holds feather-like gills that serve both respiration and excretion.
Sensory tentacles sit on the head, and a file-like rasping organ, the radula, in the mouth is used for feeding. Most molluscs have separate sexes, lay eggs and develop through a larva. Examples: the apple snail Pila, the pearl oyster Pinctada, the cuttlefish Sepia, the squid Loligo, Octopus (devil fish), the sea-hare Aplysia, the tusk shell Dentalium and the chiton Chaetopleura.

7. Echinoderms and hemichordates

NCERT §4.2.9, §4.2.10

Echinodermata (spiny-bodied) are named for their endoskeleton of calcareous ossicles. All are marine, organ-system level, triploblastic and coelomate.
Adult echinoderms are radially symmetrical but their larvae are bilaterally symmetrical.
The gut is complete, with the mouth on the lower (ventral) side and the anus on the upper (dorsal) side.
Their most distinctive feature is the water vascular system, used for locomotion, capturing and transporting food, and respiration. There is no excretory system.
Sexes are separate and reproduction is sexual. Fertilisation is usually external, and development is indirect with a free-swimming larva. Examples: the star fish Asterias, the brittle star Ophiura, the sea urchin Echinus, the sea cucumber Cucumaria and the sea lily Antedon.
Hemichordata was once a subphylum of Chordata but is now a separate phylum among the non-chordates. In the collar region it has a rudimentary stomochord, a structure similar to a notochord.
Hemichordates are few in number: marine, worm-shaped animals that are bilateral, triploblastic, coelomate and at the organ-system level. The cylindrical body has three parts: proboscis, collar and a long trunk.
Circulation is open, respiration is by gills and the proboscis gland is the excretory organ. Sexes are separate, fertilisation external and development indirect. Examples: Balanoglossus, Saccoglossus.

8. Chordates and non-chordates

NCERT §4.2.11, Table 4.1

Chordates are defined by three features: a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. They are also bilateral, triploblastic, coelomate, organ-system level, with a post-anal tail and closed circulation.
Table 4.1 in brief. Chordates: notochord present; central nervous system dorsal, hollow and single; pharynx pierced by gill slits; heart ventral; post-anal tail present. Non-chordates: no notochord; nerve cord ventral, solid and double; no gill slits; heart dorsal where there is one; no post-anal tail.
Chordata has three subphyla: Urochordata (Tunicata), Cephalochordata and Vertebrata. The first two are called protochordates and are all marine.
In Urochordata the notochord is found only in the larval tail. In Cephalochordata it runs from head to tail and lasts through life. Examples: Ascidia, Salpa, Doliolum (urochordates); Branchiostoma, also called Amphioxus or lancelet (cephalochordate).
Vertebrates have a notochord only in the embryo; in the adult it is replaced by a vertebral column of cartilage or bone. So every vertebrate is a chordate, but not every chordate is a vertebrate.
On top of the chordate features, vertebrates have a muscular ventral heart with two, three or four chambers, kidneys that excrete and osmoregulate, and paired appendages in the form of fins or limbs.
Vertebrata splits into Agnatha (no jaws: class Cyclostomata) and Gnathostomata (jaws). Gnathostomata has two super classes: Pisces with fins (Chondrichthyes, Osteichthyes) and Tetrapoda with limbs (Amphibia, Reptilia, Aves, Mammalia).

9. Jawless and jawed fishes

NCERT §4.2.11.1–§4.2.11.3

All living cyclostomes are ectoparasites on some fishes. The elongated body has 6 to 15 pairs of gill slits for breathing and a circular sucking mouth with no jaws. There are no scales and no paired fins; cranium and vertebral column are cartilaginous, and circulation is closed.
Cyclostomes are marine but migrate to fresh water to spawn, and die within a few days after spawning. Their larvae metamorphose and return to the sea. Examples: Petromyzon (lamprey), Myxine (hagfish).
Chondrichthyes are marine with a streamlined body and a cartilaginous endoskeleton. The mouth is ventral, the notochord lasts throughout life, and the gill slits are separate with no operculum.
Their tough skin carries minute placoid scales, and their teeth are modified placoid scales pointing backwards. The jaws are very powerful and they are predators. With no air bladder they must keep swimming to avoid sinking.
The cartilaginous fish heart has two chambers, one auricle and one ventricle. Some have electric organs (Torpedo) and some a poison sting (Trygon). They are cold-blooded (poikilothermous). Sexes are separate, males have claspers on the pelvic fins, fertilisation is internal and many give birth to live young. Examples: the dog fish Scoliodon, the great white shark Carcharodon, the saw fish Pristis and the sting ray Trygon.
Osteichthyes live in the sea and in fresh water and have a bony endoskeleton, a streamlined body and a mostly terminal mouth. Four pairs of gills are covered by an operculum on each side, and the skin has cycloid or ctenoid scales.
Bony fishes have an air bladder that regulates buoyancy, a two-chambered heart, and are cold-blooded. Sexes are separate, fertilisation is usually external, most are oviparous and development is direct.
Bony fish examples. Marine: the sea horse Hippocampus and the flying fish Exocoetus. Freshwater: rohu (Labeo), katla (Catla) and magur (Clarias). Aquarium: the fighting fish Betta and the angel fish Pterophyllum.

10. Amphibians and reptiles

NCERT §4.2.11.4, §4.2.11.5

Amphibians (Greek amphi, dual; bios, life) can live both in water and on land. Most have two pairs of limbs, the body is divided into head and trunk, and some have a tail.
Amphibian skin is moist and has no scales. The eyes have eyelids and a tympanum serves as the ear. The gut, the urinary tract and the reproductive tract all end in one shared chamber, the cloaca, which opens outside.
Amphibians breathe by gills, lungs and the skin. The heart has three chambers, two auricles and one ventricle, and they are cold-blooded. Sexes are separate; eggs are laid and fertilised outside the body, and a larval stage (indirect development) follows.
Amphibian examples: the frog Rana, the toad Bufo, the tree frog Hyla, the salamander Salamandra and the limbless amphibian Ichthyophis.
Reptilia takes its name from creeping or crawling (Latin repere or reptum). Reptiles are mostly terrestrial with dry, cornified skin bearing epidermal scales or scutes. There is no external ear opening; a tympanum serves as the ear. Limbs, where present, are two pairs.
Most reptiles have a three-chambered heart; crocodiles are the exception with four chambers. All reptiles are poikilotherms. Snakes and lizards cast off their scales as a skin cast.
Reptiles have separate sexes, internal fertilisation, lay eggs and develop directly. Examples: the turtle Chelone, the tortoise Testudo, the garden lizard Calotes, the wall lizard Hemidactylus, the tree lizard Chameleon, Crocodilus and Alligator; and the poisonous snakes cobra (Naja), krait (Bangarus) and viper (Vipera).

11. Birds and mammals

NCERT §4.2.11.6, §4.2.11.7, Table 4.2

Birds (Aves) are marked by feathers, and most can fly; flightless ones such as the ostrich are the exception. They have a beak, and the forelimbs are modified into wings.
The hind limbs usually bear scales and are adapted for walking, swimming or clasping branches. Bird skin is dry, and its only gland is the oil gland at the tail base.
The bird endoskeleton is fully ossified, and the long bones are pneumatic: hollow, with air cavities inside. The gut has two extra chambers, the crop and the gizzard.
Birds have a completely four-chambered heart and are warm-blooded (homoiothermous), keeping a constant body temperature. They breathe with lungs, helped by air sacs connected to the lungs. Sexes are separate, fertilisation internal, eggs are laid and development is direct. Examples: the crow Corvus, the pigeon Columba, the parrot Psittacula, the peacock Pavo, the vulture Neophron, the penguin Aptenodytes and the flightless ostrich Struthio.
Mammals live in every kind of habitat, from polar ice caps and deserts to mountains, forests, grasslands and dark caves, and some fly or live in water. Their most distinctive feature is mammary glands, which make milk to feed the young.
Mammals have two pairs of limbs, which may be built for walking, running, climbing, burrowing, swimming or flying. The skin has hair, external ears (pinnae) are present, and the jaw carries different types of teeth.
Mammals have a four-chambered heart, are homoiothermous and breathe by lungs. Sexes are separate and fertilisation is internal. Nearly all give birth to live young, and development is direct.
Mammal examples. Egg-laying: the platypus Ornithorhynchus. Viviparous: the kangaroo Macropus, the flying fox Pteropus, the blue whale Balaenoptera, the common dolphin Delphinus, the camel Camelus, the monkey Macaca, the rat Rattus, the dog Canis, the cat Felis, the elephant Elephas, the horse Equus, the tiger Panthera tigris and the lion Panthera leo.
Table 4.2 ties the phyla together: segmentation is present only in Annelida, Arthropoda and Chordata; the gut is absent in Porifera and incomplete in Coelenterata, Ctenophora and Platyhelminthes; circulatory and respiratory systems appear in the table from Annelida and Arthropoda onwards respectively.

Must-know facts

Cellular level: Porifera. Tissue level: Coelenterata and Ctenophora. Organ level: Platyhelminthes. Organ-system level: Aschelminthes onwards.
Incomplete gut (one opening): Platyhelminthes. Open circulation: arthropods and hemichordates; closed: annelids and chordates.
Radial symmetry: coelenterates, ctenophores, adult echinoderms. Echinoderm larvae are bilateral. Sponges are mostly asymmetrical.
Diploblastic with mesoglea: Coelenterata (and Ctenophora). Triploblastic: Platyhelminthes to Chordata.
Acoelomate: Platyhelminthes. Pseudocoelomate: Aschelminthes. Coelomate: annelids onwards, including hemichordates and chordates.
Metamerism is first seen in Annelida (earthworm).
Water path in sponges: ostia → spongocoel → osculum; choanocytes line the canals; digestion intracellular.
Cnidoblasts with nematocysts: Coelenterata. Metagenesis: Obelia. Coral skeleton: calcium carbonate.
Ctenophores: eight rows of comb plates, bioluminescence, only sexual reproduction.
Flame cells: Platyhelminthes. Nephridia: Annelida. Malpighian tubules: Arthropoda. Proboscis gland: Hemichordata. No excretory system: Echinodermata.
Parapodia: Nereis (dioecious). Earthworm and leech: monoecious.
Arthropoda: largest phylum, over two-thirds of named species; Limulus is a living fossil.
Mollusca: second largest phylum; radula for feeding; gills in the mantle cavity.
Water vascular system: Echinodermata. Stomochord: Hemichordata (now a non-chordate phylum).
Notochord only in the larval tail: Urochordata. Head to tail and lifelong: Cephalochordata.
Cyclostomes: 6-15 pairs of gill slits, jawless sucking mouth, ectoparasites on fishes.
Chondrichthyes: placoid scales, no air bladder, no operculum, claspers in males. Osteichthyes: four pairs of gills under an operculum, air bladder.
Heart: fishes 2 chambers; amphibians and most reptiles 3; crocodiles, birds and mammals 4.
Poikilotherms: fishes, amphibians, reptiles. Homoiotherms: birds and mammals.
Birds: pneumatic bones, crop and gizzard, air sacs, only an oil gland at the tail base. Platypus: an egg-laying mammal.

Common traps

Calling echinoderms bilaterally symmetrical, or radially symmetrical at every stage.

The adult is radial, the larva is bilateral. The chapter flags Echinodermata as radial or bilateral depending on the stage.

Placing Hemichordata inside Chordata because it has a notochord.

It has only a stomochord, similar to a notochord, in the collar. It is now a separate non-chordate phylum.

Saying all reptiles have a three-chambered heart.

Usually three chambers, but crocodiles have four.

Saying all annelids are hermaphrodite.

Earthworms and leeches are monoecious, but Nereis is dioecious.

Writing that aschelminthes are acoelomate or coelomate.

They are pseudocoelomate: the cavity is not lined by mesoderm. Flatworms are the acoelomates.

Assuming all mammals give birth to live young.

Viviparous with few exceptions; Ornithorhynchus (platypus) lays eggs.

Mixing up the protochordates: putting the lifelong notochord in Urochordata.

Urochordata: notochord only in the larval tail. Cephalochordata (Branchiostoma): head to tail, throughout life.

Giving cartilaginous fishes an air bladder or an operculum.

Chondrichthyes lack both, so they swim constantly to avoid sinking. Bony fishes have both.

Thinking sponges have separate sexes or external fertilisation.

Sponges are hermaphrodite with internal fertilisation and indirect development.

Key terms

Choanocyte: A collar cell lining the spongocoel and canals of a sponge.
Spongocoel: The central cavity of a sponge; water leaves it through the osculum.
Cnidoblast: A cnidarian cell carrying a stinging capsule, the nematocyst.
Metagenesis: Alternation of polyp and medusa forms in a cnidarian such as Obelia.
Mesoglea: The undifferentiated layer between ectoderm and endoderm in diploblastic animals.
Coelom: A body cavity lined by mesoderm, between body wall and gut wall.
Pseudocoelom: A body cavity not lined by mesoderm; the mesoderm lies as scattered pouches.
Metamerism: Division of the body into serially repeated segments, outside and inside.
Notochord: A dorsal, mesoderm-derived rod formed in the embryo of chordates.
Flame cells: Excretory and osmoregulatory cells of flatworms.
Nephridia: Excretory and osmoregulatory organs of annelids.
Parapodia: Lateral swimming appendages of aquatic annelids such as Nereis.
Malpighian tubules: Excretory organs of arthropods.
Radula: The file-like rasping feeding organ in a mollusc's mouth.
Water vascular system: The echinoderm system used for locomotion, food capture and transport, and respiration.
Stomochord: A rudimentary notochord-like structure in the collar of hemichordates.
Placoid scales: Minute scales in the skin of cartilaginous fishes; their teeth are modified placoid scales.
Cloaca: A common chamber into which the gut, urinary and reproductive tracts open.
Pneumatic bones: Hollow long bones of birds with air cavities.
Homoiothermous: Able to keep a constant body temperature; birds and mammals.

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