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Majidae (spider crabs; class Malacostraca, order Decapoda) Family comprising slow-moving, marine crabs, whose carapace is longer than it is wide. The chelipeds are usually not much longer than the other pereopods, and the body and appendages are equipped with hooked setae for the attachment of camouflage derived from pieces of seaweed, sponges, hydrozoans, bryozoans, and ascidian colonies. Some species lie among the spines of sea urchins and the tentacles of anthozoans, from which they derive a degree of protection.