Crustaceans belong to their own subphylum, Crustacea, within the Arthropoda phylum, which also includes insects and spiders.
Most, but not all, crustaceans are aquatic, and most have a hard outer shell.
The largest and best known crustaceans are the Decapoda, an order that includes crabs and lobsters. Other members of the crustacean family include shrimp, fairy shrimp, water fleas, wood lice, barnacles, and some types of plankton.
Although most often associated with salt water, many crustaceans, such as crayfish and various species of crabs, live in fresh water. Some crabs are amphibious and one species, the mangrove crab, can actually climb trees.
There are also terrestrial crustaceans, most of which live in damp areas. Some of them, for example, have been found living among damp leaves on the floors of tropical forests, and wood lice prefer rain-soaked wood.
In size, crustaceans range from almost microscopic plankton to the American lobster, which can reach a weight of more than 40 pounds, and the giant Japanese spider crab, which has a leg span of as much as 12 feet.