The Animal Kingdom classifies all multicellular, heterotrophic eukaryotes without cell walls using criteria such as symmetry, germ layers, and the presence of a body cavity (coelom). Porifera are the most primitive, with cellular-level organisation and choanocytes that drive water currents through the spongocoel for filter feeding. Cnidaria are diploblastic with cnidocytes (nematocysts) and alternate between polyp and medusa forms, while Ctenophora are distinguished by bioluminescence and comb plates. Platyhelminthes are the first bilateral, acoelomate, triploblastic phylum, with flame cells for excretion, and include major parasites like Taenia and Fasciola. Aschelminthes are pseudocoelomate with a complete digestive tract, and include Ascaris, Wuchereria (filariasis), and Ancylostoma. Annelida are coelomate with true metameric segmentation, nephridia for excretion, and the leech Hirudinaria produces the anticoagulant hirudin. Arthropoda is the largest phylum, characterised by jointed appendages, a chitinous exoskeleton, and Malpighian tubules, while Mollusca (second largest) is defined by the mantle, radula, and a calcareous shell. Echinodermata adults show radial symmetry with a water vascular system powering tube feet, and Chordata are defined by four features: notochord, dorsal hollow nerve cord, pharyngeal gill slits, and post-anal tail. The seven vertebrate classes show heart-chamber progression from 2 (fish) to 3 (amphibia, most reptiles) to 4 (crocodile, aves, mammalia). Critical NEET exceptions include the crocodile's 4-chambered heart, the platypus as an oviparous mammal, and Cephalopoda having closed circulation unlike all other molluscs.