Male crab showing narrow foldedback abdomen

To get an idea of some of the wonderful ways in which the crustacean body is modified in detail, while the body plan itself is not modified at all, look at the set of drawings opposite by the famous nineteenth-century zoologist Ernst Haeckel, perhaps Darwin's most devoted disciple in Germany (the devotion was not reciprocated, but even Darwin would surely have admired Haeckel's draughtsmanship). Just as we did with the vertebrate skeleton, look at each body part of these crabs and crayfish, and see how, without fail, you can find its exact opposite number in all the rest. Every bit of the exoskeleton is joined to the 'same' bits, but the shapes of the bits themselves are very different. Once again, the 'skeleton' is invariant, while its parts are anything but. And once again the obvious - I would say the only sensible -interpretation is that all these crustaceans have inherited the plan of their skeleton from a common ancestor. They have moulded the individual components into a rich variety of shapes. But the plan itself remains, exactly as inherited from the ancestor.

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