Q. Among the following organisms, which one does not belong to the class of other three? (UPSC Prelims 2014)
Answer:
Crab
Notes: The correct answer is
[A] Crab. While all four organisms belong to the
Phylum Arthropoda (characterized by jointed legs and an exoskeleton), they are divided into different
classes based on their physiological structures.The Biological Classification:
- Crab (A): Belongs to the Class Crustacea. Crustaceans are primarily aquatic, have two pairs of antennae, and typically possess branched (biramous) limbs. They also breathe through gills.
- Mite, Scorpion, and Spider (B, C, D): All three belong to the Class Arachnida.
Key Differences: Crustaceans vs. ArachnidsTo understand why the crab is the "odd one out," we can compare their physical characteristics:
| Feature | Crustaceans (Crab) | Arachnids (Spider, Scorpion, Mite) |
| Class | Malacostraca / Crustacea | Arachnida |
| Number of Legs | Usually 5 pairs (10 legs) | 4 pairs (8 legs) |
| Antennae | Two pairs | None |
| Body Segments | Cephalothorax and Abdomen | Cephalothorax and Abdomen |
| Habitat | Mostly Aquatic (Water) | Mostly Terrestrial (Land) |
Detailed Analysis of the Arachnids:
- Spiders: Known for silk production and venomous fangs (chelicerae).
- Scorpions: Distinguished by their pedipalps (pincers) and a segmented tail with a venomous stinger.
- Mites: Tiny arachnids that often have a fused body structure where the cephalothorax and abdomen aren't clearly separated.