Among the following organisms, which one does not belong to the class of other three?
Answer & explanation
Answer: (a) Crab
Mites, scorpions and spiders are all arachnids (class Arachnida), with four pairs of legs and no antennae. A crab is a crustacean (order Decapoda), so it is the odd one out. All four are arthropods, but the crab belongs to a different group within that phylum.
- ✓ (a) A crab is a short-tailed member of the crustacean order Decapoda in phylum Arthropoda. It is a crustacean, not an arachnid, so it does not belong with the other three.
- ✗ (b) Mites are arachnids. They are grouped with ticks, spiders and scorpions in class Arachnida.
- ✗ (c) Scorpions are arachnids and belong to class Arachnida along with spiders, mites and ticks.
- ✗ (d) Spiders are arachnids (class Arachnida). They are neither insects nor crustaceans.
Remember · Arthropoda splits into groups such as Insecta (6 legs), Arachnida (spiders, scorpions, mites, ticks; 8 legs) and Crustacea (crabs, prawns, lobsters). Crab is the odd one out here.
📘 Read it in NCERT: Class 9 Exploration, Ch 12 (practise this chapter)
Sources
- NCERT Class 9 · Exploration, Chapter 12 “Arthropods (arthro means limbs and poda means appendages) includes insects, crabs (Fig. 12.16f) and spiders. They have segmented bodies with different segments specialised for different functions.”
- Britannica: Arachnid ↗ · reference work “Some well-known types of arachnids (class Arachnida) include spiders, daddy longlegs (harvestmen), scorpions, mites, and ticks.”
- Britannica: Crab ↗ · reference work “crab , any short-tailed member of the crustacean order Decapoda (phylum Arthropoda)—especially the brachyurans (infraorder Brachyura), or true crabs”
Question: UPSC Civil Services (Preliminary) 2014, General Studies Paper I. Explanation: Minimalist IAS, checked 25 Oct 2026 (how we verify). ·
