CAPF AC 2020 Q14: Science & Technology – Biology
In which of the following phylum, would you place an animal with a bilaterally symmetrical, triploblastic body without a true internal cavity (coelom)?
- (a) Porifera
- (b) Platyhelminthes
- (c) Cnidaria
- (d) Annelida
Answer: (b) Platyhelminthes
Explanation
Phylum Platyhelminthes (flatworms)
How to think: use the three clues one by one.
Bilateral removes Porifera and Cnidaria.
No true coelom removes Annelida.
The question gives three clues: bilaterally symmetrical, triploblastic and without a true coelom.
Only Platyhelminthes has all three.
| Phylum | Symmetry | Germ layers | Coelom |
|---|---|---|---|
| Porifera (sponges) | Mostly asymmetrical | Cellular level, no true tissues | Absent |
| Cnidaria (jellyfish, Hydra) | Radial | Diploblastic | Absent |
| Platyhelminthes (flatworms) | Bilateral | Triploblastic | Absent (acoelomate) |
| Annelida (earthworm) | Bilateral | Triploblastic | True coelom |
Meaning of the terms:
| Term | Meaning |
|---|---|
| Triploblastic | Three germ layers: ectoderm, mesoderm, endoderm |
| Acoelomate | No body cavity |
| Pseudocoelomate | False cavity, as in roundworms (Aschelminthes) |
| Coelomate | True cavity lined by mesoderm |
Examples of flatworms: tapeworm (Taenia), liver fluke (Fasciola) and Planaria.
Answer: (b) Platyhelminthes.
Why each option is right or wrong
- (a) Incorrect. Porifera are mostly asymmetrical and have no true tissues.
- (b) CORRECT. Platyhelminthes are bilaterally symmetrical, triploblastic and acoelomate.
- (c) Incorrect. Cnidaria are radially symmetrical and diploblastic.
- (d) Incorrect. Annelida are bilateral and triploblastic but have a true coelom.
Key point
Platyhelminthes (flatworms): bilateral symmetry, triploblastic, acoelomate. Aschelminthes: pseudocoelomate. Annelida onwards: true coelom.
Remember
Flatworms = bilateral, triploblastic, no coelom.
Common mistakes
- Choosing Annelida, which matches the first two clues but has a true coelom.
- Mixing up acoelomate flatworms with pseudocoelomate roundworms.
- Forgetting that Cnidaria are diploblastic, with only two germ layers.
Exam tip
Educated guess: bilateral and triploblastic fit both flatworms and Annelida. The last clue, no coelom, leaves only flatworms.
Sources
- NCERT Class XI Biology — Animal Kingdom (basis of classification)
- NCERT Class IX Science — Diversity in Living Organisms