Animal Kingdom

Animals are sorted into phyla not by a single trait but by a fixed sequence of body-plan criteria, each of which splits the animal kingdom into large, natural groups before finer distinctions are made. The first criterion is the level of organisation. In sponges, cells are arranged as loose aggregates with no true tissues (cellular level); in coelenterates, cells performing the same function combine into tissues (tissue level); in Platyhelminthes, tissues combine into organs (organ level); and from Annelida onward, organs work together as organ systems (organ-system level). Independent of this hierarchy, a digestive system may be a blind sac with a single opening (incomplete, as in Platyhelminthes and Coelenterata) or a tube with a separate mouth and anus (complete, as in annelids onward), while a circulatory system may be open, with blood not always confined to vessels (arthropods), or closed, remaining within vessels throughout (annelids, chordates).

The second criterion is symmetry: sponges are largely asymmetrical (no plane divides them into equal halves); coelenterates, ctenophores and adult echinoderms are radially symmetrical (any plane through the central axis gives two equal halves); and Platyhelminthes onward are bilaterally symmetrical (only one plane, through the long axis, gives identical left and right halves).

The third criterion is the number of embryonic (germ) layers: coelenterates are diploblastic, with an outer ectoderm and inner endoderm separated by an undifferentiated mesoglea, while Platyhelminthes onward are triploblastic, adding a true mesoderm between the two.

The mesoderm's arrangement then defines the coelom: a coelomate has a body cavity fully lined by mesoderm (annelids, molluscs, arthropods, echinoderms, chordates); a pseudocoelomate has a body cavity that is not mesoderm-lined (Aschelminthes); an acoelomate has no body cavity at all (Platyhelminthes). Later criteria include segmentation (serial repetition of body segments, from Annelida onward) and the presence of a notochord, which marks the chordates off from every other phylum in the animal kingdom.

NEET frequently blurs pseudocoelomates with acoelomates: a pseudocoelomate such as Ascaris does have a fluid-filled body cavity; it is simply not lined by mesoderm, unlike a true coelomate. Only the acoelomate flatworms genuinely lack any body cavity at all.

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.