Cycloids have a "striking"[1] resemblance to crabs, and are thought to have inhabited a similar ecological niche, and to have been driven to extinction when crabs became widespread and diverse.[1] The largest members are over 6 centimetres (2.4 in) across the carapace.[2] Their gills are often preserved in three dimensions, and do not resemble those of other crustaceans.[2] Cycloid taxa differ in the number of walking legs, in the form of the mouthparts and in other significant ways.[