🐾 Animal Kingdom (Master NEET Notes)

1. Basis of Classification

Classification is not random; it's based on shared fundamental features despite the diversity in form and structure.

1.1 Levels of Organisation

• Cellular Level: Cells are arranged as loose cell aggregates. No tissues. Ex: Porifera (Sponges).
• Tissue Level: Cells performing same function are arranged into tissues. Ex: Coelenterata & Ctenophora.
• Organ Level: Tissues grouped into organs. Ex: Platyhelminthes.
• Organ System Level: Organs associated into functional systems. Ex: Aschelminthes to Chordates.

1.2 Symmetry

• Asymmetry: Cannot be divided into equal halves through any plane. Ex: Most Sponges.
• Radial Symmetry: Any plane passing through the central axis divides the body into two identical halves. Ex: Coelenterates, Ctenophores, and Adult Echinoderms (Important!).
• Bilateral Symmetry: Only one plane divides the body into identical left and right halves. Ex: Annelids, Arthropods, etc.

[Image of radial and bilateral symmetry in animals]

1.3 Diploblastic and Triploblastic Organisation

• Diploblastic: Cells arranged in two embryonic layers: External Ectoderm and Internal Endoderm. An undifferentiated layer, Mesoglea, is present between them. Ex: Coelenterates.
• Triploblastic: A third germinal layer, Mesoderm, is present between ectoderm and endoderm. Ex: Platyhelminthes to Chordates.

1.4 Coelom (Body Cavity)

Presence or absence of a cavity between the body wall and the gut wall is very important.

• Acoelomates: Body cavity is absent. Ex: Platyhelminthes.
• Pseudocoelomates: Body cavity is not lined by mesoderm; instead, mesoderm is present as scattered pouches. Ex: Aschelminthes (Roundworms).
• Coelomates (Eucoelomates): Cavity lined by mesoderm. Ex: Annelids to Chordates.

1.5 Segmentation & Notochord

• Metamerism: Body is externally and internally divided into segments with serial repetition of at least some organs. Ex: Earthworm.
• Notochord: Mesodermally derived rod-like structure on the dorsal side during embryonic development.

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2. Phylum - Porifera (Sponges)

Commonly known as Sponges. Mostly marine and asymmetrical.

• Water Canal System: Water enters through minute pores (Ostia) into a central cavity (Spongocoel) and goes out through the Osculum.
• Choanocytes (Collar Cells): Line the spongocoel and canals. Unique to Porifera!
• Digestion is Intracellular.
• Skeleton made of spicules or spongin fibres.
• Hermaphrodite (Sexes not separate). Reproduction: Asexual (fragmentation) and Sexual (gametes).
• Examples: Sycon (Scypha), Spongilla (Freshwater sponge), Euspongia (Bath sponge).

3. Phylum - Coelenterata (Cnidaria)

Aquatic, mostly marine, sessile or free-swimming, radially symmetrical.

• Cnidoblasts/Cnidocytes: Present on tentacles and body. Contain stinging capsules (nematocytes). Used for anchorage, defense, and prey capture.
• Central gastro-vascular cavity with a single opening: Hypostome.
• Two Basic Forms:
  1. Polyp: Sessile, cylindrical. Ex: Hydra, Adamsia.
  2. Medusa: Umbrella-shaped, free-swimming. Ex: Aurelia (Jellyfish).
• Metagenesis (Alternation of Generation): Polyps produce medusae asexually and medusae form polyps sexually. Ex: Obelia.
• Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan), Meandrina (Brain coral).

4. Phylum - Ctenophora (Sea Walnuts/Comb Jellies)

• Exclusively marine, radially symmetrical, diploblastic.
• Comb Plates: 8 external rows of ciliated plates for locomotion.
• Bioluminescence: Property of emitting light. Well-marked in Ctenophores.
• Sexes are not separate. Only sexual reproduction.
• Examples: Pleurobrachia, Ctenoplana.

5. Phylum - Platyhelminthes (Flatworms)

• Dorso-ventrally flattened body. Endoparasites in animals.
• Bilateral symmetry, triploblastic, acoelomate.
• Hooks and Suckers present in parasitic forms.
• Flame Cells: Specialized cells for osmoregulation and excretion.
• High regeneration capacity seen in Planaria.
• Examples: Taenia (Tapeworm), Fasciola (Liver fluke).

6. Phylum - Aschelminthes (Roundworms)

• Body is circular in cross-section. Free-living, aquatic, or parasitic.
• Organ-system level, bilateral, triploblastic, pseudocoelomate.
• Alimentary canal complete with a muscular pharynx.
• Sexes separate (Dioecious). Females are often longer than males.
• Examples: Ascaris (Round worm), Wuchereria (Filaria worm), Ancylostoma (Hookworm).

[Image of Ascaris male and female]

7. Phylum - Annelida (Ringed Worms)

• Metamerically segmented body.
• Have longitudinal and circular muscles for locomotion.
• Parapodia: Lateral appendages in aquatic annelids (Ex: Nereis) for swimming.
• Nephridia: For osmoregulation and excretion.
• Circulatory system is Closed.
• Examples: Nereis (dioecious), Pheretima (Earthworm - monoecious), Hirudinaria (Blood sucking leech).

8. Phylum - Arthropoda (Largest Phylum)

Over two-thirds of all named species on earth are arthropods.

• Exoskeleton made of Chitin.
• Body consists of Head, Thorax, and Abdomen.
• Jointed appendages (Arthro = joint, poda = appendages).
• Respiratory organs: Gills, book gills, book lungs, or tracheal system.
• Circulatory system is Open.
• Excretion via Malpighian Tubules. Sensory organs: Antennae, eyes (compound/simple), statocysts (balance).
• Important Examples:
  • Economic: Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect).
  • Vectors: Anopheles, Culex, Aedes.
  • Living Fossil: Limulus (King crab).

9. Phylum - Mollusca (Second Largest Phylum)

• Terrestrial or aquatic, organ-system, bilateral, triploblastic, coelomate.
• Body covered by Calcareous shell. Unsegmented with Head, Muscular Foot, and Visceral Hump.
• Mantle: Soft, spongy layer of skin over the visceral hump. Space between hump and mantle is the mantle cavity (contains gills).
• Radula: File-like rasping organ for feeding in the mouth.
• Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Sea-hare), Dentalium (Tusk-shell), Chaetopleura (Chiton).

10. Phylum - Echinodermata (Spiny Skinned)

• Endoskeleton of calcareous ossicles. All are marine.
• Symmetry Shift: Adults are Radially Symmetrical, but Larvae are Bilaterally Symmetrical (NEET Fav!).
• Water Vascular System: Most distinctive feature. Helps in locomotion, capture/transport of food, and respiration.
• Excretory system is Absent.
• Examples: Asterias (Star fish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber), Ophiura (Brittle star).

[Image of sea star water vascular system]

11. Phylum - Hemichordata

• Previously a sub-phylum of Chordata, now a separate non-chordate phylum.
• Worm-like marine animals. Body: Proboscis, Collar, and Long Trunk.
• Respiration via gills. Circulatory system open type.
• Excretory organ: Proboscis gland.
• Examples: Balanoglossus, Saccoglossus.

12. Phylum - Chordata

Defined by four key features: 1. Notochord, 2. Dorsal tubular nerve cord, 3. Pharyngeal gill slits, 4. Post-anal tail.

Chordates	Non-Chordates
Notochord present.	Notochord absent.
Central nervous system is dorsal, hollow, single.	CNS is ventral, solid, double.
Pharynx perforated by gill slits.	Gill slits absent.
Heart is ventral.	Heart is dorsal (if present).
Post-anal tail present.	Post-anal tail absent.

[Image of chordate characteristics]

12.1 Sub-phyla of Chordata

1. Urochordata (Tunicata): Notochord present only in Larval Tail. Ex: Ascidia, Salpa, Doliolum.
2. Cephalochordata: Notochord extends from head to tail and persists throughout life. Ex: Branchiostoma (Amphioxus/Lancelet).
3. Vertebrata: Notochord replaced by a cartilaginous or bony vertebral column in adults.

13. Class - Cyclostomata

• Ectoparasites on some fishes. Elongated body with 6-15 pairs of gill slits.
• Mouth is circular and sucking without jaws.
• Body lacks scales and paired fins.
• Marine but migrate to fresh water for spawning. Larvae return to ocean after metamorphosis.
• Examples: Petromyzon (Lamprey), Myxine (Hagfish).

14. Fishes: Chondrichthyes vs Osteichthyes

Feature	Chondrichthyes (Cartilaginous)	Osteichthyes (Bony)
Mouth	Ventral	Terminal
Gills	No Operculum (gill cover)	Operculum present
Scales	Placoid	Cycloid/Ctenoid
Air Bladder	Absent (must swim constantly)	Present (buoyancy)
Fertilization	Internal; many viviparous	External; mostly oviparous

Examples Chondrichthyes: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray).

Examples Osteichthyes: Exocoetus (Flying fish), Hippocampus (Sea horse), Labeo (Rohu), Catla, Betta (Fighting fish).

15. Class - Amphibia

• Live in both aquatic and terrestrial habitats. Most have two pairs of limbs.
• Body divisible into Head and Trunk. Skin is moist (no scales).
• Tympanum represents the ear. Cloaca is a common chamber for Alimentary, Urinary, and Reproductive tracts.
• Respiration: Gills, lungs, or skin. 3-chambered heart (2 auricles, 1 ventricle).
• Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra, Ichthyophis (Limbless amphibia).

16. Class - Reptilia

• Creeping or crawling mode of locomotion. Mostly terrestrial.
• Body covered by dry and cornified skin, epidermal scales or scutes.
• Heart is usually 3-chambered, but 4-chambered in Crocodiles (NEET Hint!).
• Poikilotherms (Cold-blooded). Snakes and lizards shed scales as skin cast.
• Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus, Alligator, Hemidactylus (Wall lizard). Poisonous: Naja (Cobra), Bangarus (Krait), Vipera (Viper).

17. Class - Aves (Birds)

• Presence of Feathers and ability to fly (except Ostrich). Forelimbs modified into wings.
• Hind limbs have scales and are modified for walking/swimming.
• Endoskeleton is bony; long bones are hollow with air cavities (Pneumatic bones).
• Crop and Gizzard are additional chambers in digestive tract.
• Heart is 4-chambered. They are Homeothermic (Warm-blooded).
• Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).

18. Class - Mammalia

• Found in diverse habitats. Most unique feature: Mammary Glands (milk producing).
• Presence of Hair on skin. Two pairs of limbs.
• External ears or Pinnae are present. Different types of teeth in jaws.
• Heart is 4-chambered. Homoiothermic. Respiration by lungs.
• Examples:
  • Oviparous (Egg-laying): Ornithorhynchus (Platypus) - V. Important!
  • Viviparous: Macropus (Kangaroo), Pteropus (Flying fox), Camelus, Macaca, Canis, Felis, Elephas, Equus, Delphinus, Balaenoptera (Blue whale), Panthera tigris, Panthera leo.