Animal Kingdom: Key Numbers, Data and Quantitative Facts

Arthropoda: Largest phylum — over 1 million described species (~80–85% of all known animal species)
Mollusca: Second largest phylum — approximately 100,000 species
Germ layers: 2 (ecto + endo) in diploblastic; 3 (ecto + meso + endo) in triploblastic
Heart chambers — fish (Cyclostomata, Chondrichthyes, Osteichthyes): 2 chambers (1 atrium + 1 ventricle)
Heart chambers — Amphibia: 3 chambers (2 atria + 1 ventricle)
Heart chambers — Reptilia (most): 3 chambers with incompletely divided ventricle
Heart chambers — Crocodilia, Aves, Mammalia: 4 chambers (2 atria + 2 ventricles)
Chordata defining features: 4 — notochord, dorsal hollow nerve cord, pharyngeal gill slits, post-anal tail
Vertebrate classes: 7 — Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, Mammalia
Major phyla in Animal Kingdom (NEET syllabus): 11 — Porifera, Cnidaria, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata
Oviparous mammals (monotremes): 2 — Ornithorhynchus (platypus) and Echidna (spiny anteater)
Choanocytes (sponges): Generate water currents through the spongocoel; 1 flagellum per cell
$CaCO_{3}$ or $SiO_{2}$ : Two types of mineral spicules in Porifera; Spongilla (freshwater sponge) has siliceous spicules
Wuchereria bancrofti: Transmitted by Culex mosquitoes; causes lymphatic filariasis (elephantiasis)
Taenia solium: Pork tapeworm; infects intestine; cysticercosis can affect the brain (neurocysticercosis)
Malpighian tubules: Found in Arthropoda; remove uric acid (nitrogenous waste) from haemolymph
Hirudin: Protein anticoagulant from Hirudinaria granulosa (Indian cattle leech) — Annelida
Urochordata notochord: Present only in larval tail; lost in adult (metamorphosis)
NEET weightage for Animal Kingdom: 3–4 questions per year (approximately 12–16 marks)