Crabs are 10-legged animals that walk sideways. There are almost 5,000 different species of crabs; about 4,500 are true crabs. Marine crabs breathe underwater using gills, which are located in a two cavities under the carapace. True land crabs have enlarged, modified cavities that act like lungs so that the land crabs can breathe air.
The sizes of crabs can range from a small marble to that of several meters. The smallest known crab is the PeaCrab, named for its size; while the largest crab is the famous Japanese SpiderCrab that has a walking leg span of as many as 3 to 4 m, and can go out to 8 m when outstretched.
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