Animal Kingdom - #1 best NCERT Notes 🙂 – Easy to Understand Guaranteed #1

BASIS OF CLASSIFICATION

When you look around, you will see many different kinds of animals. With over a million species described, classification helps us organize and understand them. Despite their differences, animals share common features used for classification.

Animals are multicellular, heterotrophic organisms lacking cell walls and chlorophyll.

“This chapter focuses on the fundamental principles of animal classification, detailing the various levels of organization, body symmetries, and major animal phyla, including detailed classifications within the vertebrates.”

11 major phyla of the animal kingdom

Phylum	Common Example
Porifera	Sponge
Cnidaria (Coelenterata)	Jellyfish
Ctenophora	Comb jelly
Platyhelminthes	Tapeworm
Aschelminthes	Roundworm
Annelida	Earthworm
Arthropoda	Insect (e.g., beetle)
Mollusca	Snail
Echinodermata	Starfish
Hemichordata	Acorn worm
Chordata	Human

11 major phyla of the animal kingdom

1.1.1 Levels of Organisation

Cellular Level: Simple organization with loose cell aggregates, e.g. Sponges.
Tissue Level: Cells performing the same function form tissues, e.g. Coelenterates & Ctenophora.
Organ Level: Tissues form organs, seen in Platyhelminthes.
Organ System Level: Organs form organ systems for performing specific and complex body fuction, seen in Annelids, Arthropods, Molluscs, Echinoderms, and Chordates.
- Examples:
  - Incomplete digestive system (one opening) in Platyhelminthes.
  - Complete digestive system (mouth and anus) in more complex animals.
  - Open circulatory system (blood bathes tissues directly).
  - Closed circulatory system (blood circulates in vessels).

1.1.2 Symmetry

Body symmetry in classification refers to the arrangement of an organism’s body parts around a central axis or plane.

Asymmetrical: Body can not be divided in 2 or more equal halves. (e.g., sponges).
Radial Symmetry: Any plane through central axis divides into identical halves (e.g., coelenterates, ctenophores, echinoderms).
Bilateral Symmetry: Only one plane divides the body into equal left and right halves (e.g., annelids, arthropods).

1.1.3 Germinal Layers Organisation

Germinal layers are the primary layers of cells (ectoderm, mesoderm, endoderm) that form during embryonic development and give rise to all tissues and organs.

Diploblastic: Two embryonic layers (ectoderm and endoderm) with mesoglea (undifferentiated jelly-like layer) in between (e.g., coelenterates).
Triploblastic: Three embryonic layers (ectoderm, mesoderm, endoderm) (e.g., platyhelminthes to chordates).

1.1.4 Coelom (body cavity)

A coelom is a fluid-filled body cavity completely lined by mesoderm, found between the digestive tract (gut) and the body wall.
It provides space for the development and suspension of internal organs, allowing for greater complexity in body structure and function.
Based on the nature of their coelom, animals are of three types:

Coelomates: Body cavity lined by mesoderm, thus True coelom (e.g., annelids, molluscs, arthropods).
Pseudocoelomates: Body cavity not fully lined by mesoderm, thus False coelom (e.g., aschelminthes).
- the mesodermal tissue does not form a continuous lining but rather exists in separate patches or clusters around the cavity.
Acoelomates: No body cavity (e.g., platyhelminthes).
- bodies filled with solid mesodermal tissue i.e. Parenchyma matrix.

1.1.5 Segmentation (Metamerism)

Metamerism: Body divided into segments (metamers) with serial repetition of organs (e.g., earthworm).
- Segmentation in animals can be external, internal, or both, depending on the species and their evolutionary adaptations.
- External segmentation is visible as distinct segments or somites on the outer body surface, facilitating flexibility, movement, and protection.
- Internal segmentation involves repeated structures within each body segment, such as nerve ganglia, muscles, and organs, enhancing coordination and efficiency.
- Some animals, like chitons, exhibit primarily external segmentation for locomotion and protection.
- while earthworms and centipedes display both external and internal segmentation, integrating movement and complex organ systems.
- In humans, metamerism is internally represented by the segmented arrangement of the vertebral column, nerve distribution along spinal segments, and the grouping of muscles into functional units.

1.1.6 Notochord

Notochord is a mesodermally derived, flexible, rod-like structure on the dorsal side that provides skeletal support in all chordate embryos and some adult chordates.

Chordates: Animals with Notochord during embryonic development (e.g., vertebrates).
Non-chordates: Animals without a notochord (e.g., porifera to echinoderms).

CLASSIFICATION OF ANIMALS

1. Phylum – Porifera

Common Name: Sponges
Habitat and Habit (movement): Aquatic, mostly marine; sedentary; solitary or colonial.
Symmetry: Asymmetrical, few radial.
Organization: Cellular level.
Skeleton: Made of spicules or spongin fibers.
Circulatory system: Absent.
Digestive system: Absent, Intracellular digestion.
Reproduction:
- Hermaphrodites (monoecious);
- Asexual reproduction by fragmentation;
- Sexual reproduction by gametes.
Fertilization & Development: Internal fertilization, indirect development with a larval stage
Unique features:
- Water Canal System: Water enters through ostia (pores), goes to spongocoel (body cavity), exits through osculum (pore).
- Functions of Water Canal System: Food gathering, respiratory exchange, waste removal
- Choanocytes (collar cells): Line the spongocoel and canals
Examples: Sycon (Scypha), Spongilla (Freshwater sponge), Euspongia (Bath sponge)

2. Phylum – Coelenterata (Cnidaria)

Habitat and Habit (movement): Aquatic, mostly marine; sessile or free-swimming.
Symmetry: Radial
Organization: Tissue level
Germ layers: Diploblastic
Coelom: Acoelomate
Digestion: Incomplete, with both extracellular and intracellular digestion.
Respiratory system: Absent
Circulatory system: Absent
Skeleton: Some (like corals) have calcium carbonate (CaCO₃) skeleton
Body Forms:
- Polyp: Sessile and cylindrical (e.g., Hydra, Adamsia)
- Medusa: Umbrella-shaped and free-swimming (e.g., Aurelia)
Reproduction:
- Mostly separate sexes (Dioecious).
- Alternation of generation (Metagenesis) – polyps asexually produce medusae, medusae sexually produce polyps
Fertilization & Development:
- External fertilization.
- Development is indirect.
Unique features:
- Cnidocytes (Cnidoblasts): Stinging cells on Tentacles, for anchorage, defense, prey capture.
- The gastro-vascular cavity (coelenteron): Single opening, located on the mouth (hypostome), serves both for ingestion and egestion.
Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan), Meandrina (Brain coral)

3. Phylum – Ctenophora

Common Name: Sea walnuts or comb jellies
Habit and Habitat: Marine, solitary, and pelagic.
Symmetry: Radial (often described as biradial due to the presence of paired structures).
Organization: Tissue level
Germ layers: Diploblastic
Coelom: Acoelomate
Respiratory system & Circulatory system: Absent
Digestive system: Incomplete, with both intracellular & extracellular digestion.
Reproduction: Only sexual, Hermaphrodite, external fertilization, indirect development
Unique features:
- Body Features: 8 rows of ciliated comb plates for locomotion.
- Bioluminescence: Can emit light.
Examples: Pleurobrachia, Ctenoplana

4. Phylum – Platyhelminthes

Common Name: Flatworms
Body Features: Dorso-ventrally flattened, thus called flatworms (including tapeworms which are also dorso-ventrally flattened).
Habit and Habitat: Mainly aquatic; mostly endoparasites in animals, including humans.
Symmetry: Bilateral
Organization: Organ-system level.
Germ layers: Triploblastic
Body Cavity: Acoelomate
Digestive system: Incomplete, has a single opening that serves both as the mouth and the anus.
Respiratory system & Circulatory system: Absent
Reproduction & Development:
- asexual (fragmentation/regeneration) and Sexual.
- Hermaphroditic, with internal fertilization.
- Indirect development (multiple larval stages).
Special Features:
- Hooks and suckers in parasitic forms, aids in nutrient absorption through body surface
- High regeneration capacity in some (e.g., Planaria)
- Excretion: By flame cells.
Examples: Taenia (Tapeworm), Fasciola (Liver fluke)

5. Phylum – Aschelminthes (Nematoda)

Common Name: Roundworms
Body: Circular in cross-section, thus called roundworms.
Habit and Habitat: Free-living (aquatic and terrestrial) or parasitic
Symmetry: Bilateral
Organization: Organ-system level
Germ layers: Triploblastic
Body Cavity: Pseudocoelomate
Digestive System: Complete with muscular pharynx
Respiratory system & Circulatory system: Absent
Reproduction & Development:
- Dioecious (separate sexes), internal fertilization,
- Development can be direct or indirect.
Special Features:
- Males and females are distinct, with females being longer than males.
- Excretion: Excretory tube and pore
Examples: Ascaris (Roundworm), Wuchereria (Filaria worm), Ancylostoma (Hookworm)

6. Phylum – Annelida

Habitat: Aquatic (marine and freshwater) or terrestrial; free-living or parasitic
Symmetry: Bilateral
Organization: Organ-system level
Layers: Triploblastic
Segmentation: Metamerically segmented, body shows external rings.
Body Cavity: Coelomate
Digestive System: Complete
Respiratory system: Cutaneous (skin) respiration. Some have branchial (gill) respiration.
Circulatory system: Closed, blood circulates within vessels and does not fill body cavities.
Reproduction & Development:
- Sexual reproduction.
- Dioecious in some (e.g., Nereis), monoecious in others (e.g., earthworms, leeches)
- Development can be direct or indirect.
Special Features:
- Movement: Longitudinal and circular muscles for locomotion,
- Locomotory Organs: Setae (in earthworms) or parapodia in aquatic forms (e.g., Nereis).
- Excretion: By Nephridia
- Nervous System: Paired ganglia and double ventral nerve cord
Examples: Nereis, Pheretima (Earthworm), Hirudinaria (Blood-sucking leech)

7. Phylum – Arthropoda

Largest Phylum: Includes insects, over two-thirds of all known species.
Habitat: Cosmopolitan (found in all environments).
Symmetry: Bilateral
Organization: Organ-system level
Germ layers: Triploblastic
Segmentation: Metamerically segmented.
Body Cavity: Coelomate
Digestive System: Complete
Respiratory system: Gills, book gills, book lungs, or tracheal system.
Circulatory system: Open, blood (hemolymph) is not confined to vessels and flows freely through body cavities.
Reproduction & Development:
- Sexual
- Mostly dioecious and usually internal fertilization.
- Mostly oviparous.
- Development can be direct or indirect.
Special Features:
- Body Structure:
  - Chitinous exoskeleton (cuticle)
  - Body parts: head, thorax, abdomen (sometimes cephalothorax and abdomen).
  - Jointed appendages
- Sensory Organs: Antennae, compound and simple eyes, statocysts (balancing organs)
Excretion: Malpighian tubules
Examples:
- Economically important: Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect)
- Vectors: Anopheles, Culex, Aedes (Mosquitoes)
- Gregarious pest: Locusta (Locust)
- Living fossil: Limulus (King crab)

8. Phylum – Mollusca

Second Largest Phylum
Habitat: Terrestrial, aquatic (marine and freshwater).
Symmetry: Bilateral
Organization: Organ-system level
Germ layers: Triploblastic
Segmentation: Unsegmented body
Body Cavity: Coelomate
Digestive System: Complete
Respiratory system: Gills in aqueous forms and mantle cavity/lung in terrestrial forms.
Circulatory system: Open
Reproduction & Development:
- Sexual
- Mostly dioecious (but some are monoecious) and usually internal fertilization.
- Mostly oviparous.
- Development is direct or indirect.
Special Features:
- Calcareous shell
- Distinct head, muscular foot, visceral hump
- Mantle layer over visceral hump
- Mantle cavity with feather-like gills for respiration & excretion.
- Feeding: Radula (file-like rasping organ)
Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Sea-hare), Dentalium (Tusk shell), Chaetopleura (Chiton)

9. Phylum – Echinodermata

Habitat: Exclusively marine.
Symmetry: Adults: radial symmetry, larvae: bilateral symmetry
Organization: Organ-system level
Germ layers: Triploblastic
Segmentation: Unsegmented body
Body Cavity: Coelomate
Digestive System: Complete, with mouth on ventral side and anus on dorsal side
Respiratory system: Skin gills and water vascular system.
Circulatory system: Open
Reproduction & Development:
- Sexual
- Mostly dioecious, with external fertilization.
- Mostly oviparous.
- Development is indirect, with ciliated free-swimming larva.
Special Features:
- Endoskeleton of calcareous ossicles (spiny-bodied).
- Head absent.
- Water vascular system for respiration, locomotion, and capture and transport of food.
- Excretory system absent.
Examples: Asterias (Star fish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber), Ophiura (Brittle star)

10. Phylum – Hemichordata

Previously Considered a sub-phylum of Chordata.
Habitat: Exclusively marine.
Symmetry: Bilateral.
Organization: Organ-system level
Germ layers: Triploblastic
Segmentation: Unsegmented body
Body Cavity: Coelomate
Digestive System: Complete
Respiratory System: Gills or pharyngeal slits.
Circulatory system: Open
Reproduction & Development:
- Sexual
- Mostly dioecious, with external fertilization.
- Mostly oviparous.
- Development is indirect, often involving a larval stage.
Special Features:
- Worm-like marine animals
- Cylindrical body divided into three regions: anterior proboscis, collar, and long trunk.
- Collar bears a structure called the stomochord (a rudimentary structure somewhat similar to the notochord found in chordates).
- Excretion: Proboscis gland
Examples: Balanoglossus (acorn worm / tongue worm), Saccoglossus

11. Phylum – Chordata

Key Features:
- Notochord: A flexible, rod-shaped structure that provides support.
- Dorsal hollow nerve cord: Located above the notochord.
- Paired pharyngeal gill slits: Openings in the pharynx that are present at some stage of development.
- Post-anal tail: Extending beyond the anus at some stage of development.
- Bilateral symmetry, triploblastic, coelomate
- Organ-system level of organisation
- Closed circulatory system: Blood is contained within vessels.

Differences between Chordata and Non-Chordata

Feature	Chordata	Non-Chordata
Notochord	Present in the embryonic stage (may be retained in some adult forms)	Absent
Central Nervous System	Dorsal, hollow, and single	Ventral, solid, and double
Pharyngeal Gill Slits	Present at some stage of development	Absent
Heart Position	Ventral (on the front or belly side)	Dorsal (on the back side, if present)
Post-anal Tail	Present in some form at some stage of development	Absent
Examples	Vertebrates (mammals, birds, reptiles, amphibians, fish), Tunicates, Lancelets	Invertebrates (arthropods, mollusks, annelids, echinoderms, etc.)

Differences between Chordata and Non-Chordata

Phylum Chordata is further classified into 3 subphyla:

Subphyla:
1. Urochordata (Tunicata):
  - Notochord: Present in the larval tail.
  - Body covering: Covered by a tunic made up of tunicin.
  - Habitat: Exclusively marine.
  - Reproduction: Hermaphroditic (having both male and female reproductive organs).
  - Examples: Ascidia, Salpa, Doliolum.
2. Cephalochordata:
  - Notochord: Extends from head to tail, present throughout life.
  - Body structure: Fish-like body.
  - Habitat: Exclusively marine.
  - Reproduction: Sexes are separate (dioecious).
  - Examples: Branchiostoma (commonly known as Amphioxus or Lancelet).
3. Vertebrata (Craniata):
  - Notochord: Present during the embryonic period, replaced by a vertebral column in adults.
  - Additional features: Ventral muscular heart, kidneys for excretion and osmoregulation, paired appendages (fins or limbs).
  - Called “Craniata” because these animals possess a cranium (skull) that encases and protects the brain.
  - Examples: This subphylum includes all vertebrates like fishes, amphibians, reptiles, birds, and mammals.

Classes within Vertebrata

1. Class – Cyclostomata

Features:
- Ectoparasites on fish
- Elongated body, 6-15 pairs of gill slits
- Sucking, circular mouth without jaws
- No scales and no paired fins.
- Cartilaginous cranium and vertebral column
- Marine, but spawn in fresh water
- Die after spawning; larvae (ammocoetes) return to the ocean.
Examples: Petromyzon (Lamprey), Myxine (Hagfish)

2. Class – Chondrichthyes

Features:
- Marine, streamlined body
- Cartilaginous endoskeleton
- Ventral mouth, persistent notochord.
- Separate gill slits, no operculum (gill cover).
- Tough skin with placoid scales
- Powerful jaws, predaceous
- No air bladder, must swim to avoid sinking
- Two-chambered heart
- Electric organs in some (e.g., Torpedo), poison sting in some (e.g., Trygon).
- Cold-blooded (ectothermic), separate sexes.
- In males, pelvic fins bear claspers for mating (internal fertilization).
- Internal fertilization; many are viviparous (give birth to live young).
Examples: Scoliodon (Dogfish), Pristis (Sawfish), Carcharodon (Great white shark), Trygon (Stingray)

3. Class – Osteichthyes

Features:
- Marine and freshwater fishes
- Bony endoskeleton, streamlined body
- Terminal mouth
- Four pairs of gills covered by an operculum.
- Skin with cycloid/ctenoid scales
- Air bladder for buoyancy
- Two-chambered heart
- Cold-blooded (ectothermic), separate sexes.
- External fertilisation, mostly oviparous
- Direct development
Examples:
- Marine: Exocoetus (Flying fish), Hippocampus (Sea horse)
- Freshwater: Labeo (Rohu), Catla (Katla), Clarias (Magur)
- Aquarium: Betta (Fighting fish), Pterophyllum (Angel fish)

4. Class – Amphibia

Features:
- Live in water and on land (terrestrial) habitats and need water for breeding, hence called amphibians.
- Two pairs of limbs
- Body divided into head and trunk; tail may be present
- Moist skin without scales
- Eyes with eyelids, tympanum represents the ear.
- Common chamber for digestive, urinary, and reproductive tracts (i.e. cloaca)
- Respiration: gills (in larva), lungs, and skin
- 3-chambered heart (two auricles, one ventricle)
- Cold-blooded (Poikilotherms), separate sexes
- External fertilisation, oviparous, indirect development
Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless amphibian)

5. Class – Reptilia

Features:
- Mostly terrestrial, with creeping or crawling mode of movement.
- Dry, cornified skin with epidermal scales or scutes.
- Shed scales as skin cast
- No external ear openings, tympanum represents the ear.
- Two pairs of limbs (when present)
- Heart usually 3-chambered (4-chambered in crocodiles)
- Cold-blooded (ectothermic).
- Separate sexes, internal fertilisation
- Oviparous, direct development
Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator), Hemidactylus (Wall lizard), Naja (Cobra), Bangarus (Krait), Vipera (Viper)

6. Class – Aves

Features:
- Feathers, ability to fly (except flightless birds like ostrich)
- Beak present
- Forelimbs modified into wings; hind limbs have scales and are adapted for walking, swimming, or perching.
- Dry skin without glands (except uropygial oil gland at the base of the tail)
- Bony endoskeleton, with hollow bones containing air cavities (pneumatic bones).
- Digestive tract with crop and gizzard
- 4-chambered heart
- Warm-blooded (homeothermic).
- Respiration by lungs, supplemented by air sacs
- Separate sexes, internal fertilisation
- Oviparous, direct development
Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture)

7. Class – Mammalia

Features:
- Found in various habitats: polar, desert, mountains, forests, grasslands, caves
- Adapted to flying (bats) or living in water (whales, dolphins).
- Milk-producing glands (mammary glands)
- Two pairs of limbs adapted for various activities
- Skin with hair, external ears (pinnae)
- Different types of teeth (heterodont dentition).
- 4-chambered heart
- Warm-blooded (homeothermic).
- Respiration by lungs
- Separate sexes, internal fertilisation
- Mostly viviparous (give birth to live young), with some exceptions (e.g., oviparous monotremes like the platypus).
- Direct development
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)

Chapter Summary:

Basic Features for Classification:
- Level of organisation
- Symmetry
- Cell organisation
- Coelom
- Segmentation
- Notochord
Porifera:
- Multicellular animals
- Cellular level of organisation
- Flagellated choanocytes
Coelenterates:
- Have tentacles and cnidoblasts
- Mostly aquatic, sessile or free-floating
Ctenophores:
- Marine animals
- Have comb plates
Platyhelminths:
- Flat body
- Bilateral symmetry
- Parasitic forms with suckers and hooks
Aschelminthes:
- Pseudocoelomates
- Parasitic and non-parasitic roundworms
Annelids:
- Metamerically segmented animals
- True coelom
Arthropods:
- Most abundant group
- Jointed appendages
Molluscs:
- Soft body with calcareous shell
- External skeleton made of chitin
Echinoderms:
- Spiny skin
- Water vascular system
Hemichordates:
- Worm-like marine animals
- Cylindrical body with proboscis, collar, and trunk
Phylum Chordata:
- Possess notochord throughout or during embryonic life
- Dorsal, hollow nerve cord
- Paired pharyngeal gill slits
Agnatha (Class Cyclostomata):
- Primitive chordates
- Ectoparasites on fishes
Gnathostomata:
- Two superclasses: Pisces and Tetrapoda
Pisces:
- Classes Chondrichthyes and Osteichthyes
- Have fins for locomotion
Chondrichthyes:
- Fishes with cartilaginous endoskeleton
- Marine
Tetrapoda:
- Classes Amphibia, Reptilia, Aves, and Mammalia
- Two pairs of limbs
Amphibia:
- Adapted to live on land and water
Reptilia:
- Dry and cornified skin
- Limbs absent in snakes
Poikilothermous Animals:
- Cold-blooded
- Fishes, amphibians, and reptiles
Aves:
- Warm-blooded
- Feathers and wings
- Hind limbs adapted for various activities
Mammalia:
- Mammary glands and hair
- Usually viviparous

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