Crayfish, crawfish, or crawdads are freshwater crustaceans resembling small lobsters, to which they are probably closely related. They breathe through feather-like gills and are found in bodies of water that do not freeze to the bottom; they are also mostly found in brooks and streams where there is fresh water running, and which have shelter against predators. Most crayfish cannot tolerate polluted water, although some species such as the invasive Procambarus clarkii are more hardy. Some crayfish have been found living as much as 3 m (10 feet) underground.
Encyclopaedia Britannica (11th ed., vol. 7) (New York, NY: The Encyclopaedia Britannica Company, 1910)