Biology · Animal Kingdom · NEET
It means how much the cells of an animal cooperate. In a simple animal the cells are just loosely grouped and each cell works almost on its own. In a complex animal the cells team up into tissues, then organs, then whole systems. So the 'level' tells you how organised and complex the body is. For NEET, each phylum is fixed to one level, so you must memorise which phylum sits at which level.
Only Porifera (sponges) has the cellular level. NCERT says the cells in sponges are arranged as loose cell aggregates. There is some division of labour (different cells do different jobs), but the cells do NOT join to form true tissues. That is why it is the lowest level. Remember: cellular level = sponges only.
Tissue level (Coelenterata/Cnidaria and Ctenophora): cells that do the same job are arranged into tissues, but these tissues do not join to form organs. Organ level (Platyhelminthes / flatworms): tissues are grouped together to form organs, and each organ does one special job. So the key jump is: tissue level = tissues only; organ level = tissues combine into organs. Coelenterates stop at tissues; flatworms reach organs.
Aschelminthes (roundworms), Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata and Chordata. In these animals, organs join together to form functional systems (like a full digestive system or circulatory system). This is the highest level. NCERT lists Annelids, Arthropods, Molluscs, Echinoderms and Chordates as examples, and roundworms (Aschelminthes) also belong here - a common NEET point.
No and no. Sponges (Porifera) have NO true tissues - they are only at the cellular level. Cnidarians (Coelenterata) have tissues but NO organs - they are only at the tissue level. Mixing these up is the most common trap. Flatworms (Platyhelminthes) are the first phylum to reach the organ level.
NCERT notes that organ systems still vary in complexity. Two examples: (1) digestive system can be incomplete (one opening acting as both mouth and anus, as in flatworms) or complete (two openings - mouth and anus). (2) circulatory system can be open type or closed type. So even at the same level, animals differ. This directly leads into the open vs closed circulatory system topic.
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.
Read the following statements: (a) Metagenesis is observed in Helminths. (b) Echinoderms are triploblastic and coelomate animals. (c) Round worms have organ-system level of body organization. (d) Comb plates present in ctenophores help in digestion. (e) Water vascular system is characteristic of Echinoderms. Choose the correct answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Cellular level, tissue level, organ level, and organ-system level. They go from simplest (cellular, in sponges) to most complex (organ-system, in most higher animals).
Cellular level (Porifera/sponges) is the lowest. Organ-system level (annelids, arthropods, molluscs, echinoderms, chordates and roundworms) is the highest.
Tissue level. Their cells are arranged into tissues, but the tissues do not form organs. Ctenophora is also tissue level.
Platyhelminthes (flatworms). Here tissues group together to form organs, each specialised for one function.
NEET regularly asks you to match a phylum to its level, or to combine level with symmetry, coelom and segmentation in one question. Knowing the exact level for each phylum quickly earns marks and avoids the Mollusca and sponge traps.
