Animal Kingdom: Essential NEET Facts

Largest phylum: Arthropoda (>80% of all animal species); defined by jointed appendages, chitinous exoskeleton, open circulatory system (haemocoel), Malpighian tubules, compound eyes
Second largest phylum: Mollusca; defined by mantle, radula, calcareous shell
Only pseudocoelomate phylum (NEET syllabus): Aschelminthes (Nematoda)
Only acoelomate triploblastic phylum: Platyhelminthes
Diploblastic phyla: Cnidaria and Ctenophora only
Radially symmetrical phyla: Cnidaria, Ctenophora, Echinodermata (adults); all others bilateral
Phylum with water vascular system: Echinodermata — operates tube feet for locomotion, feeding, respiration
Phylum with cnidocytes/nematocysts: Cnidaria — for defence and prey capture
Metameric segmentation: Annelida (true metamerism); Arthropoda $\frac{tagmatized}{modified segmentation}$
Excretory organs: Flame cells (Platyhelminthes) → Nephridia (Annelida) → Malpighian tubules (Arthropoda) → Kidneys (Vertebrata)
Chordata four features: Notochord, dorsal hollow nerve cord, pharyngeal gill slits, post-anal tail — all four must appear at some stage of life
Heart chamber progression: Fish = 2 → Amphibia/Reptilia = 3 → Crocodile/Aves/Mammalia = 4
Cephalopoda exception: Closed circulatory system (Octopus, Loligo, Sepia) — all other molluscs have open circulation
Crocodile exception: 4-chambered heart — only reptile with 4 chambers
Platypus exception: Oviparous mammal (lays eggs) — also Echidna; both are monotremes
Whale: Aquatic but is a mammal — breathes via lungs, has mammary glands
Penguin: Flightless but is a bird (Aves) — has feathers, 4-chambered heart
Limulus (king crab): Living fossil in Arthropoda; belongs to class Merostomata
Notochord retention: Urochordata (larval tail only) → Cephalochordata (throughout life) → Vertebrata (replaced by vertebral column)
Bioluminescence: Ctenophora — also present in some deep-sea organisms
Hirudin: Anticoagulant from Hirudinaria (leech) — prevents blood clotting in the host
Spongocoel: Central cavity of sponges lined by choanocytes that create feeding currents
Echinoderm symmetry shift: Bilateral in larvae → radial (pentamerous) in adults