What animals are crabs descended from? More than one. It turns out that various earlier species evolved into crabs at least five different times. This process is called carcinisation, or the tendency for adaptations to push creatures into the shape of crabs. You might call it crabification, which is not really a word. And it's not just the shape of a crab that these different evolutionary lineages share.
“Some of the internal anatomical characters studied herein are structurally dependent on the external characters of a crab-like habitus. Since morphological coherence can also exist between internal anatomical structures, the coherence chains which can be traced back to the external characters of a crab-like habitus are relatively complex in some cases (indirect coherences).”
In other words, the internal organs are crabbish because of the crab shape, which makes the animals crabs no matter what they descended from. Read more about crabification at Popular Mechanics. -via Damn Interesting
(Image credit: Baeza, J. Antonio)