Biology · Animal Kingdom · NEET
The difference is the NUMBER of planes that give two equal halves. In radial symmetry, MANY planes (any plane through the central axis) each cut the body into two identical halves. In bilateral symmetry, only ONE plane cuts the body into equal left and right halves. NCERT says: radial = 'any plane passing through the central axis divides the organism into two identical halves'; bilateral = 'body can be divided into identical left and right halves in only one plane.'
A radially symmetrical animal has MANY planes of symmetry. Picture a wheel or a round cake: you can cut it top-to-bottom from many angles and each time you get two matching halves, as long as the cut passes through the central axis. This is why sea anemones and jellyfish look the same from every side. A bilateral animal has only ONE such plane.
Three groups are radially symmetrical: Coelenterata (Cnidaria), Ctenophora, and ADULT Echinodermata. Easy memory line: 'Coelenterata, Ctenophora, and adult Echinoderms are radial.' Remember 'adult' for echinoderms, because their larvae are bilateral. Sponges (Porifera) are usually ASYMMETRICAL, not radial.
This is the most common trap. In Echinodermata, the ADULT is radially symmetrical but the LARVA is bilaterally symmetrical. So a starfish (Asterias) adult is radial, but its larva is bilateral. NEET repeatedly tests this exact point, so always check whether the question says 'adult' or 'larva.'
A human is bilaterally symmetrical. Only one plane, running down the middle from head to toe, divides you into matching left and right halves. You cannot cut a human sideways or diagonally and get two equal halves. Most higher animals, including annelids, arthropods, molluscs and all chordates, are bilateral.
Symmetry is one of the fundamental features NCERT uses to classify the animal kingdom, along with body organisation, coelom and germ layers. It matters because a single question often combines symmetry with coelom or germ layers (for example, 'bilaterally symmetrical AND acoelomate' = Platyhelminthes). Knowing symmetry per phylum lets you eliminate wrong options fast.
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:
Radial symmetry is NOT found in adults of phylum _______.
Which of the following animals are true coelomates with bilateral symmetry?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Radial symmetry: any plane through the central axis makes two equal halves (like a wheel). Bilateral symmetry: only one plane makes equal left and right halves (like your body).
Coelenterata (Cnidaria), Ctenophora, and adult Echinodermata. Remember 'adult' for echinoderms because their larvae are bilateral.
No. Sponges (Porifera) are mostly ASYMMETRICAL, meaning no plane through the centre divides them into equal halves. Do not confuse asymmetrical with radial.
Echinoderms evolved from bilateral ancestors, so the larva keeps the bilateral form, but the adult develops secondary (five-part) radial symmetry. NEET tests this exact 'adult radial, larva bilateral' point often.
Yes, bilateral symmetry is linked to a head end and directional movement, so it appears in higher animals like annelids, arthropods, molluscs and all chordates, including humans.
