Subtopic 1 — Basis of Classification (~100w)

Classification of the Animal Kingdom uses six hierarchical criteria. Level of body organisation distinguishes cellular (Porifera), tissue (Cnidaria), organ (Platyhelminthes), and organ-system levels. Body symmetry separates asymmetrical, radially symmetrical, and bilaterally symmetrical animals. Germ layers classify animals as diploblastic (Cnidaria, Ctenophora) or triploblastic (Platyhelminthes onward). Coelom type divides phyla into acoelomates, pseudocoelomates, and coelomates. Metameric segmentation is present in Annelida and modified in Arthropoda. Presence of a notochord defines Chordata.

Subtopic 2 — Non-Chordate Phyla (~200w)

Porifera: cellular organisation, choanocytes, spicules. Cnidaria: tissue level, diploblastic, cnidocytes, polymorphism $\frac{polyp}{medusa}$. Ctenophora: diploblastic, bioluminescence, comb plates. Platyhelminthes: bilateral, acoelomate, flame cells, includes tapeworm and liver fluke. Aschelminthes: pseudocoelomate, complete gut, includes Ascaris and Wuchereria. Annelida: coelomate, metameric segmentation, nephridia, closed circulatory system, hirudin from leeches. Arthropoda: LARGEST phylum, jointed appendages, chitinous exoskeleton, open circulation, Malpighian tubules. Mollusca: SECOND LARGEST, mantle, radula, calcareous shell; Cephalopoda exception (closed circulation). Echinodermata: water vascular system, tube feet, $CaCO_{3}$ endoskeleton, radial symmetry in adults. Hemichordata: stomochord, represented by Balanoglossus.

Subtopic 3 — Phylum Chordata (~100w)

Defined by four features present at some stage: notochord, dorsal hollow nerve cord, pharyngeal gill slits, post-anal tail. Sub-phyla: Urochordata (notochord only in larva), Cephalochordata (notochord throughout life), Vertebrata (notochord replaced by vertebral column). Vertebrata is divided into seven classes showing progressive complexity in heart chambers, body covering, and reproductive strategies.

Subtopic 4 — Vertebrate Classes (~150w)

Seven classes span increasing complexity. Cyclostomata: jawless, parasitic, 2-chambered heart. Chondrichthyes: cartilaginous skeleton, placoid scales, no swim bladder, viviparous tendencies. Osteichthyes: bony skeleton, swim bladder, operculum, scales. Amphibia: 3-chambered heart, dual habitat, cutaneous respiration, metamorphosis. Reptilia: 3-chambered heart (4-chambered in Crocodilia), dry scaly skin, ectothermic. Aves: 4-chambered heart, feathers, pneumatic bones, air sacs, endothermic. Mammalia: 4-chambered heart, mammary glands, hair, mostly viviparous (monotremes are oviparous).

Subtopic 5 — Exceptions and Special Cases (~50w)

NEET-critical exceptions: crocodile (4-chambered heart in reptiles), platypus (oviparous mammal), Cephalopoda (closed circulation in molluscs), Limulus (living fossil arthropod), whale (aquatic mammal), echinoderm adults (radial symmetry from bilateral larvae).