Q.5 Radial symmetry is the characteristic feature of which of the following phyla?
Cnidaria exhibits radial symmetry among the given options.
Radial symmetry allows body parts to mirror around a central axis, unlike bilateral symmetry where only one plane divides the body into mirror halves. This feature suits sessile or drifting lifestyles in certain phyla.
Option Analysis
Arthropoda shows bilateral symmetry, with a segmented body divisible into left-right mirror halves, as seen in insects and crustaceans.
Mollusca displays bilateral symmetry, though some gastropods lose it due to torsion; examples include snails and squid.
Cnidaria features radial symmetry (or biradial in some), enabling division into mirror halves by multiple planes through the central axis, as in jellyfish and sea anemones.
Chordata has bilateral symmetry throughout, with a left-right mirror division, including vertebrates like humans.
Correct Answer
(C) Cnidaria stands out as the phylum with radial symmetry in this context.
Radial symmetry characteristic feature of Cnidaria phylum makes it a key topic for biology exams like CSIR NET. This symmetry type defines how organisms like jellyfish divide into mirror images around a central axis.
What Defines Radial Symmetry?
Body parts radiate from a center, cuttable into equal halves by planes through the axis—ideal for floating or attached lifestyles. Cnidaria exemplifies this, unlike bilateral phyla.
Cnidaria: The Radial Symmetry Phylum
Jellyfish, sea anemones, and corals show radial (or biradial) symmetry for efficient cnidocyte use in feeding/defense. Polyp and medusa forms both display this trait.
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Hydra: Simple radial polyp.
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Aurelia (jellyfish): Umbrella-shaped medusa.
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Corals: Colonial with radial polyps.
Why Not Other Options?
| Phylum | Symmetry Type | Key Examples | Reason Incorrect |
|---|---|---|---|
| Arthropoda | Bilateral | Insects, crabs | Segmented, left-right mirror |
| Mollusca | Bilateral | Snails, octopuses | Torsion may alter, but base bilateral |
| Chordata | Bilateral | Fish, humans | Notochord, left-right division |
Echinodermata (not listed) also shows adult radial symmetry, but Cnidaria fits the options.
Exam Relevance for CSIR NET
Questions test phylum traits: Cnidaria’s diploblastic, acoelomate, radial body aids identification. Bilateral dominates advanced phyla for mobility.
