Biology · Animal Kingdom · NEET
Animals are classified using fundamental (deep, shared) body features, not by looks. Two animals can look totally different but share the same body plan. NCERT lists these features: (1) level of cell organisation, (2) body symmetry, (3) diploblastic or triploblastic (germ layers), (4) presence or type of coelom, (5) segmentation, and (6) presence of a notochord. Looks (like colour or size) can change a lot even inside one group, so they are not reliable for classification.
1) Levels of organisation - cellular (sponges), tissue (coelenterates), organ (flatworms), or organ-system (annelids onward). 2) Symmetry - asymmetrical, radial, or bilateral. 3) Germ layers - diploblastic (2 layers) or triploblastic (3 layers). 4) Coelom - coelomate, pseudocoelomate, or acoelomate. 5) Segmentation - body divided into repeating segments (metamerism). 6) Notochord - present (chordates) or absent (non-chordates). NEET asks these directly and in match-the-column questions.
It means how the cells of the body are arranged. In sponges, cells are just loose groups (cellular level). In coelenterates, cells form tissues (tissue level). In Platyhelminthes and higher, tissues form organs (organ level). In Annelids, Arthropods, Molluscs, Echinoderms and Chordates, organs form working systems (organ-system level). Higher level = more division of labour.
Diploblastic animals have only two germ layers - ectoderm (outer) and endoderm (inner) - with a non-living jelly called mesoglea in between. Example: coelenterates. Triploblastic animals have a third middle layer, the mesoderm, between ectoderm and endoderm. Examples: Platyhelminthes up to Chordates. The mesoderm is important because it forms muscles, the coelom lining, and many organs.
A coelom is a body cavity between the body wall and the gut wall that is lined by mesoderm. Animals with it are coelomates (annelids, molluscs, arthropods, echinoderms, hemichordates, chordates). If the cavity is present but not fully lined by mesoderm (mesoderm is in scattered pouches), it is a pseudocoelom (aschelminthes). If there is no cavity at all, the animal is acoelomate (platyhelminthes). NEET tests this a lot.
A notochord is a rod-like support structure formed on the dorsal (back) side during embryo development. It is derived from mesoderm. Animals that form a notochord are called chordates. Animals that never form it are non-chordates (porifera to echinoderms). This single feature is the top-level split in the animal kingdom.
Adult echinoderms are radially symmetrical, but their larvae are bilaterally symmetrical. This is a very common NEET trap. So if a question says 'adult echinoderm has bilateral symmetry' it is WRONG - adults are radial. Coelenterates, ctenophores and adult echinoderms all show radial symmetry; annelids, arthropods and chordates show bilateral symmetry.
Consider following features: (a) Organ system level of organisation (b) Bilateral symmetry (c) True coelomates with segmentation of body. Select the correct option of animal groups which possess all the above characteristics.
Radial symmetry is NOT found in adults of phylum _______.
Statement I: When any plane passing through the central axis of the body divides the organism into two identical halves, it is called radial symmetry. Statement II: In phylum Echinodermata, both adults and larvae are radially symmetrical. Choose the most appropriate answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT discusses six: levels of organisation, symmetry, diploblastic/triploblastic organisation, coelom, segmentation, and notochord. Circulatory type (open/closed) and digestive type (complete/incomplete) are extra sub-points under organ-system complexity.
Because it makes the biggest, top-level split: chordates (notochord present) versus non-chordates (notochord absent, from Porifera to Echinodermata). All other features are used to sort animals within these groups.
No. Sponges (Porifera) are mostly asymmetrical - no plane through the centre divides them into equal halves. They also have the simplest, cellular level of organisation.
A true coelom is lined by mesoderm. If the cavity is only partly lined (mesoderm in scattered pouches) it is a pseudocoelom, and if there is no cavity it is acoelomate.
Metameric segmentation (metamerism). The body is divided externally and internally into repeating segments, seen clearly in the earthworm (Annelida) and also in Arthropoda and Chordata.
