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Decapods like crabs have exactly 10 legs. This crab has 5 pairs of legs

However, shrimps have 5 pairs of swimming legs and 5 pairs of walking legs. A total of 10 pairs. Why are they classified as decapods then? enter image description here

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Species are classified based on common ancestry. See https://en.m.wikipedia.org/wiki/Cladistics

Decapods aren't classified together because of the number of legs they have. That's where the order got its name, and it's one possible clue that they are related, but it's not why they are together.

Furthermore, swimmerets aren't legs. They're leg-like, but they aren't legs. Shrimp inherited the ten legs from the same common ancestor as other decapods, and they also share swimmerets with most other decapods, even crabs have them.

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