Animal KingdomNEET Zoology · Class 11 · NCERT Chapter 4

Introduction

There are roughly 1.5 million named animal species on Earth. Studying each one separately would be impossible, so biologists group them by shared body-plan features. That grouping is what this chapter covers. Expect 2 to 4 NEET questions from Animal Kingdom every year. Most of them are single-fact questions: which phylum has flame cells, which class is jawless, which mammal lays eggs.

The trick is to remember one diagnostic feature and at least one example per phylum. Once you know the diagnostic feature, the rest follows quickly.

Basis of Classification

Animals are classified using five overlapping criteria. These are the axes you will be tested on, so make sure you can apply each one to any animal you are given.

Levels of Organisation

How the body of an animal is built up from cells:

• Cellular level: body is a loose collection of cells with little co-operation. Example: Porifera (sponges).
• Tissue level: cells of similar function are grouped into tissues. Example: Cnidaria, Ctenophora.
• Organ level: tissues are grouped into organs that perform a single function. Example: Platyhelminthes.
• Organ-system level: several organs work together as a system. Example: Aschelminthes onwards (all higher animals).

Body Symmetry

• Asymmetrical: body cannot be divided into equal halves by any plane. Most sponges.
• Radial symmetry: any plane passing through the central axis divides the body into two equal halves. Examples: Cnidaria, Ctenophora, and adult Echinoderms.
• Bilateral symmetry: body can be divided into two identical halves by only one plane (the median plane). Examples: Platyhelminthes onwards, including all chordates.

NEET trap: echinoderm larvae are bilateral, but adult echinoderms are radial. Do not blindly call echinoderms radial.

Germ Layers (Diploblastic vs Triploblastic)

• Diploblastic: only two germ layers, ectoderm (outside) and endoderm (inside), with a non-cellular jelly called mesoglea between them. Examples: Cnidaria, Ctenophora.
• Triploblastic: three germ layers (ectoderm, mesoderm, endoderm). Mesoderm gives rise to muscles, bones, blood and the reproductive system. Examples: Platyhelminthes onwards.

Coelom (Body Cavity)

The coelom is the space between the gut and the body wall. Whether it exists and how it is lined decides the body plan:

• Acoelomate: no body cavity. Mesoderm fills the space solid. Example: Platyhelminthes (tapeworm, planaria, liver fluke).
• Pseudocoelomate: body cavity is present but mesoderm does not line it on both sides. Example: Aschelminthes (Ascaris, Wuchereria).
• Coelomate: true body cavity lined by mesoderm on both sides. Example: Annelida, Arthropoda, Mollusca, Echinodermata, all chordates.

Segmentation and Notochord

• Metameric segmentation: the body is divided into a series of similar segments arranged front to back, both externally and internally. The classic example is the earthworm, where each segment is repeated. Annelida has true segmentation. Many arthropods also show segmentation, but in a modified form.
• Notochord: a flexible rod-like structure running along the back, formed from mesoderm. Animals that have a notochord at any stage of life are called chordates. In vertebrates the notochord is later replaced by a vertebral column.

Non-Chordate Phyla

Animals that lack a notochord at every stage of life are non-chordates. NCERT covers 10 non-chordate phyla. Learn one diagnostic feature and at least one example for each.

Porifera (Sponges)

• Mostly marine, asymmetrical.
• Cellular level of organisation.
• Body has a canal system. Water enters through small pores called ostia, passes through canals into a central cavity called the spongocoel, and goes out through the osculum.
• The canals are lined by special flagellated cells called choanocytes (collar cells) that maintain water flow.
• Skeleton of spicules or spongin fibres.
• Hermaphrodite (sexes not separate). Fertilization is internal.
• Examples: Sycon (Scypha), Spongilla (freshwater sponge), Euspongia (bath sponge).

Cnidaria (Coelenterata)

• Mostly marine, radially symmetrical.
• Tissue level of organisation. Diploblastic.
• Have cnidoblasts or nematocysts (stinging cells) on tentacles, used for anchorage, defence and capturing prey.
• Central gastrovascular cavity with a single opening (mouth) for both ingestion and egestion.
• Two body forms: polyp (sessile, cylindrical, like Hydra) and medusa (free-swimming, umbrella-shaped, like jellyfish). Many species alternate between the two (metagenesis).
• Examples: Hydra, Adamsia (sea anemone), Physalia (Portuguese man-of-war), Pennatula (sea pen), Aurelia (jellyfish), Meandrina (brain coral).

Ctenophora

• All marine, radially symmetrical, diploblastic.
• Tissue level of organisation.
• Locomotion by eight rows of ciliated comb plates (ctene plates). This is the most reliable identifying feature.
• Bioluminescence (give off light) is well marked in ctenophores.
• Sexes are not separate (hermaphrodite). Only sexual reproduction.
• Examples: Pleurobrachia, Ctenoplana.

Platyhelminthes (Flatworms)

• Body is flat dorsoventrally (top to bottom). Bilateral symmetry.
• Triploblastic, acoelomate, organ-level organisation.
• Mostly endoparasites of humans and other animals.
• Excretion by flame cells (protonephridia). This is a NEET-favorite.
• Hermaphrodite. Fertilization internal. Many parasitic forms have hooks and suckers.
• Examples: Taenia (tapeworm), Fasciola (liver fluke), Planaria.

Aschelminthes (Roundworms, also called Nematoda)

• Body cylindrical and tapered at both ends, hence "round" worms.
• Triploblastic, pseudocoelomate, organ-system level of organisation.
• Sexes are separate (dioecious) with marked sexual dimorphism (females are usually larger).
• Many are parasitic and cause disease in humans, animals and plants.
• Examples: Ascaris (intestinal roundworm), Wuchereria (filarial worm), Ancylostoma (hookworm).

Annelida (Segmented Worms)

• Bilateral symmetry, triploblastic, coelomate.
• True metameric segmentation: body divided into similar segments inside and outside.
• Excretion by nephridia. Closed circulatory system.
• Nervous system has a paired ventral nerve cord with ganglia.
• Examples: Nereis (sandworm, marine), Pheretima (earthworm), Hirudinaria (leech).

Arthropoda

• The largest animal phylum. Around two-thirds of all named animal species are arthropods.
• Bilateral symmetry, triploblastic, coelomate, organ-system level.
• Chitinous exoskeleton. Body usually divided into head, thorax and abdomen. Jointed appendages (the name Arthropoda means "jointed feet").
• Open circulatory system. Respiration by gills, book-gills, book-lungs or tracheal system. Excretion by Malpighian tubules.
• Sexes separate. Fertilization mostly internal. Mostly oviparous.
• Common examples: Cockroach (Periplaneta), butterfly, locust, mosquito, scorpion, spider, prawn (Palaemon), crab.
• Economically important arthropods: Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect). NEET asks this trio every other year.
• Vectors: Anopheles, Culex and Aedes mosquitoes carry malaria, filaria and dengue.

Mollusca

• Second largest animal phylum. Body soft, divided into head, muscular foot and visceral hump.
• A soft fold of skin called the mantle covers the visceral hump and usually secretes a calcareous shell. The space between the mantle and the visceral hump is the mantle cavity.
• Feeding organ: radula (a file-like rasping tongue in most molluscs). Respiration by gills (ctenidia) inside the mantle cavity.
• Sexes generally separate. Mostly oviparous.
• Examples: Pila (apple snail), Pinctada (pearl oyster, source of pearls), Sepia (cuttlefish), Loligo (squid), Octopus (lost its shell), Aplysia (sea hare).

Echinodermata

• All marine. Adults radially symmetrical (pentaradial), but larvae are bilateral.
• Triploblastic, coelomate, organ-system level.
• Most unique feature: a water vascular system with tube feet for movement, capture of food and respiration.
• Endoskeleton of calcareous ossicles. Spiny skin (hence the name "spiny skinned").
• Sexes separate. Fertilization usually external. Development indirect.
• Examples: Asterias (starfish), Echinus (sea urchin), Antedon (sea lily), Ophiura (brittle star), Holothuria (sea cucumber).

Hemichordata

• Used to be considered a subphylum of Chordata. NCERT now places it as a separate phylum because it does not really have a notochord (the structure once called the notochord is a buccal diverticulum).
• Marine, worm-like, bilateral, triploblastic, coelomate.
• Body divided into three regions: proboscis, collar and trunk.
• Open circulatory system. Respiration by gills.
• Sexes separate. Fertilization external.
• Examples: Balanoglossus, Saccoglossus.

Phylum Chordata

All chordates share three fundamental features at some stage of life:

1. Notochord (a flexible rod-like body support).
2. Dorsal hollow nerve cord (the spinal cord, lying on the back).
3. Pharyngeal gill slits (slits in the wall of the pharynx).

Phylum Chordata is divided into three subphyla. In vertebrates the notochord is later replaced by a vertebral column.

• Subphylum Urochordata (Tunicata): notochord is present only in the larval tail. Examples: Ascidia, Salpa, Doliolum.
• Subphylum Cephalochordata: notochord runs along the whole length of the body throughout life. Example: Branchiostoma (amphioxus, the lancelet).
• Subphylum Vertebrata: notochord is replaced by a bony or cartilaginous vertebral column in the adult. All vertebrates are chordates, but not all chordates are vertebrates. Vertebrates have a ventral muscular heart, kidneys, and paired appendages.

Subphylum Vertebrata: the seven classes

NCERT places the vertebrates in seven classes. Each one is recognised by a small set of features. Memorise those features and at least two example organisms per class.

Class Cyclostomata

• Most primitive vertebrates. Jawless. Body eel-like, lacks scales and paired fins.
• Mouth is circular and sucking, with rasping teeth.
• Marine in adult life; many migrate to fresh water for spawning, then die after spawning.
• Ectoparasitic on other fish.
• Examples: Petromyzon (lamprey), Myxine (hagfish).

Class Chondrichthyes (Cartilaginous Fishes)

• Marine. Streamlined body. Skeleton entirely of cartilage.
• Mouth ventral. Gills (5 to 7 pairs) are not covered by an operculum.
• Skin has tough placoid scales.
• No air bladder, so they must swim continuously to avoid sinking. Heart 2-chambered.
• Sexes separate. Internal fertilization. Many are viviparous.
• Examples: Scoliodon (shark), Pristis (sawfish), Carcharodon (great white shark), Trygon (sting ray).

Class Osteichthyes (Bony Fishes)

• Marine and freshwater. Skeleton is bony.
• Mouth terminal. Gills are covered by an operculum.
• Skin has cycloid or ctenoid scales.
• Air bladder (swim bladder) gives buoyancy and avoids sinking.
• Heart 2-chambered. External fertilization. Mostly oviparous.
• Examples: Exocoetus (flying fish), Hippocampus (sea horse), Labeo (rohu), Catla, Clarias (magur).

Class Amphibia

• Can live both in water and on land. Skin is moist and without scales.
• Two pairs of limbs. Eyes have eyelids.
• Respiration by gills, lungs and through skin. 3-chambered heart (2 atria + 1 ventricle).
• Sexes separate. External fertilization. Oviparous; development indirect (tadpole larva).
• Examples: Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra (salamander), Ichthyophis (limbless amphibian).

Class Reptilia

• The name means "to creep or crawl".
• Skin is dry and covered by epidermal scales or scutes.
• Heart usually 3-chambered; crocodiles have a 4-chambered heart (NEET trap).
• Cold-blooded (poikilothermic). Internal fertilization. Lay shelled eggs on land (cleidoic eggs).
• They shed scales as skin cast (ecdysis).
• Examples: Chelone (turtle), Testudo (tortoise), Chameleon (tree lizard), Calotes (garden lizard), Crocodilus, Alligator, Naja (cobra), Hemidactylus (wall lizard).

Class Aves (Birds)

• Body covered with feathers. Forelimbs modified into wings. Hind limbs for walking and perching.
• Beak is present, no teeth.
• Bones are hollow with air cavities (pneumatic bones), making flight easier.
• 4-chambered heart. Warm-blooded (homoiothermic). Respiration by lungs assisted by air sacs.
• Sexes separate. Internal fertilization. Oviparous. Direct development.
• Examples: Corvus (crow), Columba (pigeon), Psittacula (parrot), Pavo (peacock), Aptenodytes (penguin, flightless), Struthio (ostrich, flightless), Neophron (vulture).

Class Mammalia

• Most distinguishing features: hair (fur) on body, mammary glands that produce milk for the young, and an external ear (pinna).
• 4-chambered heart. Warm-blooded. Respiration by lungs. Diaphragm separates thoracic and abdominal cavities (unique to mammals).
• Two sets of teeth (heterodont, diphyodont).
• Internal fertilization. Mostly viviparous (give birth to young).
• Three groups based on mode of reproduction:
  • Prototheria (monotremes): egg-laying mammals. Examples: Ornithorhynchus (platypus), Echidna (spiny anteater).
  • Metatheria (marsupials): young are born undeveloped and complete development in a pouch (marsupium). Example: Macropus (kangaroo), Didelphis (opossum).
  • Eutheria (placental mammals): young develop inside the mother through a placenta. Examples: Homo (human), Felis (cat), Canis (dog), Equus (horse), Elephas (elephant), Balaenoptera (blue whale), Pteropus (flying fox/bat), Macaca (monkey).

Worked NEET Problems

Summary Cheat Sheet

• Five axes: levels of organisation, symmetry, germ layers, coelom, segmentation.
• Acoelomate: Platyhelminthes. Pseudocoelomate: Aschelminthes. Coelomate: Annelida onwards.
• Diploblastic: Cnidaria, Ctenophora. Triploblastic: Platyhelminthes onwards.
• Porifera: canal system + choanocytes + spongocoel; Sycon, Spongilla, Euspongia.
• Cnidaria: nematocysts + polyp/medusa + diploblastic; Hydra, Aurelia, Adamsia, Physalia.
• Ctenophora: comb plates + bioluminescence; Pleurobrachia, Ctenoplana.
• Platyhelminthes: flat body + flame cells + acoelomate + mostly parasitic; Taenia, Fasciola, Planaria.
• Aschelminthes: cylindrical + pseudocoelomate + dioecious; Ascaris, Wuchereria, Ancylostoma.
• Annelida: metameric segmentation + nephridia + closed circulation; Nereis, Pheretima, Hirudinaria.
• Arthropoda: largest phylum + chitinous exoskeleton + jointed appendages + Malpighian tubules + open circulation. Economic: Apis, Bombyx, Laccifer.
• Mollusca: soft body + mantle + foot + radula + calcareous shell; Pila, Pinctada (pearl), Sepia, Loligo, Octopus.
• Echinodermata: water vascular system + tube feet + all marine + adults radial, larvae bilateral; Asterias, Echinus, Holothuria.
• Hemichordata: proboscis + collar + trunk; Balanoglossus, Saccoglossus.
• 3 chordate features: notochord, dorsal hollow nerve cord, pharyngeal gill slits.
• 3 chordate subphyla: Urochordata (notochord in larval tail only), Cephalochordata (notochord throughout body and life; amphioxus), Vertebrata.
• Cyclostomata: jawless, parasitic; Petromyzon, Myxine.
• Chondrichthyes: cartilage, ventral mouth, no operculum, placoid scales, no air bladder; Scoliodon, Pristis, Carcharodon, Trygon.
• Osteichthyes: bony, operculum, cycloid/ctenoid scales, air bladder; Labeo, Catla, Clarias, Hippocampus, Exocoetus.
• Amphibia: moist skin (no scales), 3-chambered heart; Rana, Bufo, Hyla, Salamandra, Ichthyophis.
• Reptilia: dry skin with scales, 3-chambered heart (4 in crocodile), cleidoic eggs; Calotes, Naja, Crocodilus, Chelone.
• Aves: feathers + beak + hollow bones + 4-chambered heart + warm-blooded; Corvus, Columba, Pavo, Aptenodytes (penguin), Struthio (ostrich).
• Mammalia: hair + mammary glands + pinna + 4-chambered heart + diaphragm. Prototheria (Ornithorhynchus, Echidna), Metatheria (Macropus), Eutheria (Homo, Canis, Felis, Pteropus).

Next: use the interactive learning widgets to drill phylum identification, compare body plans (symmetry, coelom, germ layers), and the seven vertebrate classes side by side, or work through the 60+ NEET PYQs with full solutions. To time yourself, take the free 10-question mock test.