Biology · Animal Kingdom · NEET
They are two shapes of the same group of animals (Cnidaria). A polyp is sessile (fixed to one spot), cylindrical or tube-like, and its mouth points upward. Hydra and Adamsia (sea anemone) are polyps. A medusa is free-swimming, umbrella- or bell-shaped, and its mouth points downward. Aurelia (jellyfish) is a medusa. Simple way to remember: polyp is standing still, medusa is floating and moving.
Metagenesis means alternation of generations. It happens when one cnidarian shows BOTH the polyp form and the medusa form during its life cycle, one after the other. The polyp makes medusae by asexual budding, and the medusae make polyps by sexual reproduction. So the animal keeps switching between the two forms across generations. NEET's standard example is Obelia.
The polyp reproduces asexually (by budding). The medusa reproduces sexually (it makes eggs and sperm). This is the exact NCERT line: 'polyps produce medusae asexually and medusae form the polyps sexually'. NEET often flips this to trick you, so lock it: polyp = asexual, medusa = sexual.
No, and NEET tests this confusion. Metagenesis is the alternation between polyp and medusa forms in cnidarians (two adult body forms, sexual and asexual generations alternating). Metamorphosis is when a larva changes into an adult (like a tadpole becoming a frog). Different words, different phyla. Do not mix them up.
Only in cnidarians (Coelenterata). A common NEET trap says 'metagenesis is observed in helminths' — this is FALSE. Metagenesis belongs to Cnidaria, example Obelia. Helminths (flatworms, roundworms) do NOT show metagenesis. NEET 2021 tested exactly this wrong statement.
No. Only cnidarians that exist in BOTH forms show metagenesis. Hydra is only a polyp (no medusa stage). Aurelia is mainly a medusa. Obelia shows both, so Obelia is the example of metagenesis. Corals (which build calcium carbonate skeletons) are polyp forms.
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.
Match the following genera with their respective phylum: (a) Ophiura (b) Physalia (c) Pinctada (d) Planaria with (i) Mollusca (ii) Platyhelminthes (iii) Echinodermata (iv) Coelenterata.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Obelia. It shows both polyp and medusa forms in its life cycle, so it is the standard NCERT example of metagenesis (alternation of generations) in Cnidaria.
A jellyfish is a medusa. Aurelia is the NCERT example of the free-swimming, umbrella-shaped medusa form. Hydra, by contrast, is a polyp.
A polyp is cylindrical/tube-like and sessile (fixed), with the mouth facing up. A medusa is umbrella- or bell-shaped and free-swimming, with the mouth facing down.
Hydra and Adamsia (sea anemone) exist only as polyps. Corals are also polyp forms and secrete a calcium carbonate skeleton. These do not show a medusa stage, so they do not show metagenesis.
NEET frequently asks statement-based and match-the-column questions where metagenesis is falsely linked to helminths, or where polyp/medusa reproduction is swapped. Knowing polyp = asexual, medusa = sexual, and Obelia = example lets you eliminate wrong options fast.
