Nine phyla make up the nonchordate side of the Animal Kingdom, arranged from the simplest to the most complex body plan and united by the absence of a notochord at any stage of life. Moving from Porifera to Echinodermata, this sequence tracks the same organising criteria used across the kingdom — level of organisation, symmetry, germ layers, and the presence and lining of a body cavity — each phylum adding a level of complexity the one before it lacks.
- Porifera (sponges): cellular-level organisation, mostly asymmetrical, body wall perforated by pores (ostia) opening into a water-canal system driven by collar cells (choanocytes); mostly marine, sessile. E.g., Sycon, Spongilla, Euplectella.
- Coelenterata (Cnidaria): tissue-level organisation, diploblastic, radially symmetrical, bear stinging cnidoblasts and a single-opening gastrovascular cavity; exist as sessile polyps (Hydra) or free-swimming medusae (Aurelia), with reef-building corals among them (Gorgonia).
- Ctenophora: exclusively marine, also tissue-level and diploblastic, radially symmetrical; move using eight external rows of ciliated comb plates rather than stinging cells, and are commonly bioluminescent. E.g., Pleurobrachia, Ctenoplana.
- Platyhelminthes (flatworms): triploblastic, acoelomate, bilaterally symmetrical, dorsoventrally flattened; many are endoparasites bearing hooks or suckers (Taenia, Fasciola), while free-living Planaria is well known for regenerating lost body parts.
- Aschelminthes (roundworms): triploblastic, pseudocoelomate, body cylindrical rather than flattened, sexes separate, alimentary canal complete with mouth and anus. E.g., Ascaris, the filarial worm Wuchereria.
- Annelida: triploblastic, coelomate, body metamerically segmented into repeating rings, closed circulatory system, excretion by nephridia. E.g., Nereis, earthworm (Pheretima), leech (Hirudinaria).
- Arthropoda: the largest phylum in the Animal Kingdom; jointed appendages, a chitinous exoskeleton, and an open circulatory system in which blood (haemolymph) does not stay confined to vessels; includes insects (cockroach), crustaceans (prawn) and arachnids (scorpion, spider).
- Mollusca: the second-largest phylum; soft, unsegmented body with a head, muscular foot and visceral hump wrapped in a mantle that usually secretes a calcareous shell; respires through gill-like ctenidia. E.g., Pila, Pinctada (pearl oyster), Sepia, Octopus.
- Echinodermata: exclusively marine, with a spiny calcareous endoskeleton; coelomate and triploblastic; adults are radially symmetrical (larvae are bilateral) and move using a unique water vascular system. E.g., Asterias (starfish), Echinus (sea urchin), Holothuria (sea cucumber).
NEET commonly pairs Ctenophora with Coelenterata, since both are diploblastic, radially symmetrical, marine animals organised at the tissue level; the discriminating feature is that ctenophores entirely lack cnidoblasts and instead glide using external ciliary comb plates, whereas cnidarians sting using cnidoblasts and possess no comb plates at all.