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UPSC Prelims 2003 · Environment · Species Watch
Octopus is
- (a) an arthropod
- (b) an echinoderm
- (c) a hemichordate
- (d) a mollusk
Show the answer and explanation
Answer: (d) a mollusk
The octopus is a cephalopod mollusc, like squid and cuttlefish — it has lost its shell.
The idea
Molluscs are soft-bodied animals, most with a shell. They come in three main kinds: snails, two-shelled animals like oysters, and the cephalopods. In the cephalopods the shell has moved inside or disappeared, and the foot has become arms.
Snails and slugs → Bivalves: oyster, mussel, clam → Cephalopods: octopus, squid, cuttlefish
Why the others are wrong
(a) Arthropods have jointed legs and a hard exoskeleton.
(b) Echinoderms (starfish) have radial symmetry and tube feet.
The trap
“Mollusc” brings to mind snails with shells. Octopuses lost theirs.
Also know
An octopus has eight arms, three hearts and blue blood, which uses copper to carry oxygen. It is the most intelligent of the animals without a backbone. Arthropods are a different group: insects, spiders, crabs and prawns.
🔒 Cephalopods (octopus, squid, cuttlefish) = molluscs.