Science Puzzle
Which Animal Doesn’t Belong?
A spider, a crab, a beetle and an octopus. Not one of them has a backbone, so all four are invertebrates.
Yet three of them belong to the same major group and one is the odd one out. Which one, and what is the rule?
The Answer
The octopus. Spiders, crabs and beetles are all arthropods: animals with a hard external skeleton and legs built from hinged, jointed sections. The octopus is a mollusc, with a soft boneless body and flexible arms containing no joints at all. It can squeeze through a hole the size of its beak, which no crab could ever manage.
The tempting wrong answers show how easy it is to group by the wrong feature. Leg count separates spiders from beetles, but both are still arthropods. Living in water separates the crab, but plenty of insects and spiders live in water too, and the octopus lives in water as well. Biologists group animals by shared body plan and ancestry, not by habitat or by counting limbs.
Arthropods are staggeringly successful: they make up over three quarters of all known animal species. The jointed exoskeleton, it turns out, is one of the best designs evolution ever produced.
The principle: Classification by shared body plan. Animals are grouped by inherited structural features such as an exoskeleton and jointed limbs, not by superficial traits like habitat or leg count.