Some species have evolved "backwards" and straightened out their abdomens again.
The other large group are the Anomura (false crabs), with an ancestor that looked more like a lobster.However, at least four groups of Anomura – sponge crabs, porcelain crabs, king crabs and the Australian hairy stone crab – have independently evolved into a crab-like form in much the same way as the true crabs.
Like the true crabs, their compact bodies are more defensive, and can move sideways faster.This means "crabs" aren't a real biological group.But crabs aren't the exception.However, feathered wings and flight also evolved earlier in other groups of dinosaurs, including troodontids and dromaeosaurs.
Both groups evolved from the same common ancestor over 100 million years ago, the marsupials largely in Australasia and the Americas and the placentals elsewhere.
Squid, and octopuses, which are molluscs and more closely related to snails and clams, also evolved camera eyes with the same components.Eyes more generally may have evolved independently up to 40 times in different groups of animals.